Pump FAQs

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      Rotodynamic Pumps

    • What items need to be included in regular pump maintenance, and what schedule should be followed? ▼

      Pumping systems can be complex, with many moving parts and subsystems that need to be regularly inspected and constantly maintained. Failure to frequently inspect pumping systems can lead to premature failure, losses in efficiency and increased operating costs. Therefore, it is recommended that a monitoring, maintenance and schedule be adopted, and it should include, at a minimum, the following: 

      • 1.When applicable, gland packings must be adjusted to maintain concentric alignment of the gland follower, and maintain specified leakage so that the packing and follower do not overheat.
      • 2.Check for any leaks from gaskets and seals. The correct functioning of the shaft seal must be checked regularly.
      • 3.Check bearing lubricant level, and verify if the hours run show a lubricant change is required.
      • 4.Check and verify that the duty condition is in the allowable operating region for the pump.
      • 5.Check vibration, noise level and surface temperature at the bearings to confirm satisfactory operation.
      • 6.Check that dirt and dust are removed from areas around close clearances, bearing housings and motors.
      • 7.Check coupling alignment and realign if necessary.

      Note: Additionally, installed auxiliary systems should be included in the maintenance plan so they are monitored and maintained to ensure they function properly.


      An inspection and maintenance log should be kept and problems that are identified should be reported immediately. A suggested guide for preventative maintenance for normal applications is given below. Unusual applications with abnormal heat, moisture, dust, etc., may require more frequent inspection and service.

      A maintenance plan should include required spare parts to keep on hand. A list of recommended spare parts will depend on normal supplier lead time when ordering parts; whether pumping equipment is used for “normal duty” or “severe duty;” and whether or not there is backup pumping while a unit is down for maintenance. Below is a suggested list of spare parts for pumping units. Note that the items listed for severe duty are in addition to the items listed for normal duty.

      For more information about how to maintain pumping systems, refer to HI standard, ANSI/HI 1.4 Rotodynamic Centrifugal Pumps for Manuals Describing Installation, Operation, and Maintenance at www.pumps.org.


      How does a centrifugal pump work?▼

      A centrifugal pump is a type of rotodynamic pump that uses bladed impellers with essentially radial outlet to transfer rotational mechanical energy to the fluid primarily by increasing the fluid kinetic energy (angular momentum) and increasing potential energy (static pressure). Kinetic energy is then converted into usable pressure energy in the discharge collector.  

      Figure 1 provides a cross section view of a centrifugal pump, which shows the use of a rotating impeller to add energy to the pumped liquid. The liquid enters the impeller axially at a smaller diameter, called the impeller eye, and progresses radially between the vanes until it exits at the outside diameter. As the liquid leaves the impeller, it is collected in a pressure container casing. One design referred to as a volute collects the flow and efficiently directs it to a discharge nozzle. Figure 2 highlights the discharge nozzle, which is shaped like a cone so that the high-velocity flow from the impeller is gradually reduced. This cone-shaped discharge nozzle is also called a diffuser. During the reduction in velocity in the diffuser, energy in the flow is converted to pressure energy. An optimum angle of seven to 10 degrees is used to most efficiently convert velocity energy to pressure energy.  

      Centrifugal pumps can have many drivers, but the most common is the electric motor. The motor provides the mechanical energy to pump shaft through a coupling. The radial and axial loads are carried by pump and/or motor bearings. Sealing of the pumped fluid can be done with compression packing or mechanical seals. Additionally, sealless designs are available with canned motors or magnetic drive couplings.

      For more information on centrifugal pumps, their construction and typical industries served, refer to the Rotodynamic Pump Certificate training modules at www.pumps.org.

      What are the applications of vertical turbine pumps? ▼

      Vertical turbine pumps are a type of rotodynamic pump that use radial or modified radial flow impellers in a vertical configuration. Vertical turbine pumps are typically multistage pumps with several levels of impellers encased in a bowl assembly. Vertical turbine pumps can further be classified as deep well turbine pumps, barrel or can pumps, and short set pumps.

      A deep well turbine is usually installed in a drilled well with the first stage impeller laying below the water level of the pump. These pumps are self-priming, typically a multistage assembly, and are primarily used to transport water. The multistage assembly of a deep well turbine pump can be seen in Figure 1 (below). Transporting water from deep wells to the surface is the primary application of these pumps. These pumps transport water to treatment plants, irrigation sprinklers, and to the faucets in our homes. Short set pumps operate very similarly to deep well pumps. Short set pumps will often operate in water pits and typically have a maximum length of 40 ft.

      Barrel or can pumps are pumps that are mounted to enclosed containers such as barrels, cans, etc. These pumps operate as booster pumps and are used in situations where inadequate suction is present. These pumps have a similar assembly to the other vertical turbine pumps using a multistage bowl assembly. Additional Net Positive Suction Head (NPSH) is created by these pumps by extending their shaft further into the fluid increasing suction head.


                  Another unique application of vertical turbine pumps is that the pumps can be run in reverse and be used as hydraulic turbines to generate power. When used in this application the suction nozzle becomes the outlet of the turbine and the discharge nozzle becomes the inlet of the turbine. The efficiency of the pump as a turbine is also comparable to the efficiency of the pump.


                  For more information on vertical turbine pumps refer to HI standards, ANSI/HI 2.1-2.2 Rotodynamic Vertical Pumps for Nomenclature and Definitions and ANSI/HI 2.3 Rotodynamic Vertical Pumps Of Radial, Mixed, And Axial Flow Types For Design And Application.

      What is a chopper pump? ▼

      Chopper pumps are centrifugal pumps with the capability to handle fluids with a high concentration of solids. Chopper pumps have a cutting attachment added made of hard materials of fixed and rotating elements that macerate solids before entry to the impeller that allows it to handle difficult materials. They cut solids so they pass through the pump more easily and flow out with the rest of the pumped fluid. 

      A chopper pump’s ability to handle solids gives them more flexibility in what they can pump; this characteristic makes them particularly useful in wastewater treatment plants. Wastewater treatment is split into primary treatment and secondary treatment. Primary treatment is the physical separation of float-able materials and insoluble solids from the wastewater. Secondary treatment is biological treatment of water using microorganisms to remove the remaining solids in the fluid. Both treatments contain solids in the pumped fluid and may require chopper pumps.

      Specific steps in the process include pumping scum, mixing the contents of the aeration basin and the anoxic zone, and pumping sludge. Image 1 shows an example of an aeration basin at a treatment facility where chopper pumps mix oxygen with wastewater to encourage the growth of microorganisms to break down solids. 

      A typical centrifugal pump impeller is more easily clogged by solids, which can halt pumping and cause damage to the system. In particular, stringy materials found in wastewater during the treatment process is especially troublesome to normal centrifugal pumps as the material can tangle the impeller. However, the chopper pump is more effective in dealing with this issue.


      For more information on the application of pumps in power plants, refer to HI Guideline Wastewater Treatment Plant Pumps at pumps.org.
      I am used to hearing the term “centrifugal” pump, but sometimes hear them referred to as “rotodynamic” pumps. Are centrifugal and rotodynamic terms that can be used synonymously? ▼

      Rotodynamic pumps are kinetic machines in which energy is continuously imparted to the pumped fluid by means of a rotating impeller, propeller or rotor. These pumps transfer mechanical energy to the fluid primarily by increasing the fluid kinetic energy. Kinetic energy is then converted into potential energy (pressure) in the discharge collector. The most common types of rotodynamic pumps are radial (centrifugal), mixed flow and axial flow (propeller) pumps, including pumps historically referred to as vertical turbine pumps. Radial, mixed and axial flow impellers are shown in Image 1. 

      As seen from the definition of a rotodynamic pump and Image 1, it is a term used to describe a larger group of pumps that includes centrifugal (radial flow) pumps, but also includes pumps of mixed and axial flow pumps and some other unique constructions. Centrifugal pumps are the most common type and the term is synonymous with radial flow impellers where the flow enters the impeller in line with the pump shaft, but discharges the impeller perpendicular to the pump shaft.

      Rotodynamic pump types are also commonly described by their general mechanical configuration as described in Image 2.


      So, to answer the question directly, a centrifugal pump is a type rotodynamic pump, and not all rotodynamic pumps are centrifugal pumps.

      For more information on rotodynamic pumps refer to ANSI/HI 14.1-14.2 rotodynamic pumps for nomenclature and definitions, and ANSI/HI 14.3 rotodynamic pumps for design and application at pumps.org.
      What are typical markets and applications for rotodynamic pumps? ▼

      Rotodynamic pumps are used in most all municipal, process, power, oil and gas markets and many more. The markets and applications are too many to list, but a few are provided for perspective.

      Water, Waste Water and Irrigation

      Modern living depends heavily on water supply, wastewater removal, and irrigation, which all require pumps which are among the oldest machines ever used. These needs constitute a large market for pumps and are the heart of civilization.


      The water that we take from the faucet gets to us by the action of pumps, which move the water through pipelines from the reservoir to our homes. Water may also come from deep wells which require vertical deep-well pumps to bring it to the surface.


      Water from the sink and tub is collected and treated before disposal. Pumps are needed to move the water to the treatment plant, as shown here, and during the treatment process.


      Water is also vital to the irrigation of farm products, shrubs and lawns. Pumps supply the water and pressure needed for sprinkler systems as shown or trenches to carry the water to rows of farm products.


      Oil & Gas, and Process Industry

      Manufacturers of liquid products that we use every day are a second major market for pumps. Making chemical products, for example, requires pumps for processing. Petroleum products, including gasoline, are derived from crude oil which must be processed at high temperatures and pressures with the aid of pumps.  Operating temperature for such pumps can be over 800°F.


      Electric Power Industry

      Electricity which we use to light our homes and drive electric motors is produced in large electric power generating plants. Electricity is generated by large turbines using steam from high pressure boilers. Many pumps are needed in this process and make up a third large market for pumps. These powerful high pressure pumps are used to feed water at high pressure to steam boilers (Image 1).


      Other pumps in power plants are necessary to remove the condensed steam from the bottom of massive steam condensers and send it to heaters which increase the water temperature. Heater drain pumps then move the water to the boiler feed pump.


      Cold water is needed to cool the spent steam and turn it back to water. These pumps circulate water from the cooling tower through the condenser and back to the cooling tower where it is cooled as it splashes down through the tower.


      Exhaust gas from the furnace in electric generating stations is scrubbed by water from pumps in order to remove solid particles.


      Mining and Metals Industry

      Mining and the refining of metals is a fourth major market of pumps. To facilitate deep pit mining, pumps are needed to remove ground water.


      After steel is produced in large ingots, it is heated and rolled into many different shapes.

      High pressure water from pumps removes scale, which appears on the outside surface of the steel sheets and rods.


      Other Industries


      Commercial construction of buildings is a fifth key market for pumps. Buildings require pumps for applications such as fire protection. These fire pumps are engine driven in case of an electrical power failure (Image 2).


      Heating and air conditioning in buildings requires pumps to circulate hot water for heating or chilled water for air conditioning (Image 3).


      There are many addition pump markets and applications that are too many to list, but one can get the idea of how common pumps are in our daily life, even though we may not be thinking about them on a regular basis.


      For more information on pump applications, and training on rotodynamic pump fundamentals refer to HI’s web based Rotodyanmic Pumps eLearning course at pumps.org.

      What are the benefits of multistage pumps? ▼

      Multistage pumps use multiple impellers plus diffusing element stages for developing higher head through the series addition of head from one stage to the next. Types of multistage pumps include the between bearing types, which consist of the axially split BB3 and the radially split BB4 and BB5. These pumps are typically used in applications for boiler feed, reverse osmosis, and other high pressure and temperature applications. Overhung impeller multistage pumps such as the OH7j, the OH1j and the OH13j are useful in low-flow, high-pressure applications and control hydraulic radial load through the use of diffusers.

      For multistage pumps, a low NPSH required (NPSHr) first stage can be added to supply the second stage. This is especially applicable when the second stage has a higher NPSHr than the first stage. Vertical and horizontal multistage pumps behave similarly to multiple single-stage pumps operating in series. This should be considered when designing a pumping system that has higher head requirements.

      Multistage pumps may also be useful for noise reduction. For pumps of the same power, an increase in the number of stages lowers noise levels compared to a single stage. Estimates for sound pressure reductions for increased stages at the same power level can be referenced in Image 2.


      For more information about valves and multistage pumping, refer to ANSI/HI 9.6.6 Rotodynamic Pumps for Pump Piping, ANSI/HI 14.1-14.2 Rotodynamic Pumps for Nomenclature and Definitions and ANSI/HI 14.3 Rotodyanmic Pumps for Design & Applications at pumps.org.

      What are essential spare parts to keep on hand for rotodynamic pumps? ▼

      In rotodynamic pumps, badly worn wear components can result in severe degradation of pump head and rate of flow, especially on small pumps. Therefore, it is important to monitor the condition of the equipment and be prepared to make repairs.  When making repairs, examination of wear patterns can provide valuable information that can help diagnose pump problems and determine their origin.  The list of recommended spare parts will depend on factors such as normal supplier lead time when ordering parts, whether pumping equipment is for use as normal duty or severe duty, and whether or not there is backup pumping while a unit is down for maintenance and component parts replacement.

      An example of spare parts for service handling abrasive liquids that may experience interrupted service is recommended as follows:

      • shaft sleeves,
      • bearings,
      • wearing rings,
      • packing or mechanical seals,
      • gaskets and O-rings,
      • coupling connectors and
      • an impeller.

      Recommended spare parts lists for other applications can be referenced in the standard ANSI/HI 14.4 Rotodynamic Pumps for Installation, Operation, and Maintenance Manuals. Items used in the maintenance of pumping equipment include replacement packing, lubricant, cleaning materials, touch-up coating, anodic protection, coolant, and mechanical seal barrier fluid.

      For more information about groundwater, wastewater and wet wells and replacement pump parts, refer to ANSI/HI 9.8 and ANSI/HI 14.4 at www.pumps.org.
    • Back to Top
    • Positive Displacement Pumps

    • What are some differences between simplex and duplex air-operated pumps? ▼

      For a duplex air-operated pump, compressed air is directed into the pump air chamber via the air distribution system. The compressed air is separated from the liquid by a diaphragm. The diaphragm, in turn, applies pressure on the liquid and forces it out of the pump discharge. While this is occurring, the opposite-side air chamber is depressurized and exhausted to atmosphere and liquid is drawn into the pump suction. The cycle repeats, thus creating a constant reciprocating action, which maintains flow through the pump. (See Figure

      A simplex pump operates in much the same manner except that no liquid is discharged during the recharge stroke of the diaphragm or bellows. A spring return is typically used to return the diaphragm and provide energy for the suction stroke. Intake and discharge valves direct flow into and out of the pumping chamber. It should also be noted that simplex pumps require a controller to cycle the compressed air/gas into the air chamber of the pump and to exhaust the air out. The controller can be mechanical or electrical, such as a repeat cycle timer and solenoid valve. The speed of the pump is set by the controller, and the maximum speed is determined by the manufacturer.

      To determine the maximum pumping speed on a duplex pump, increase the air supply while the pump rate of flow increases. When the rate of flow no longer increases, throttle back the air supply until the pump rate of flow starts to decrease. This point is the optimum pump cycle rate achievable under these system conditions. 

      For more information about air-operated pumps see ANSI/HI 10.1-10.5 Air-Operated Pumps for Nomenclature, Definitions, Application, and Operation.

      What are some common positive displacement pumps used in the wastewater industry?  ▼

      Positive displacement pumps provide a certain volume of liquid with every revolution of the shaft that is relatively independent of the differential pressure, which makes them different from rotodynamic or centrifugal pumps. In wastewater handling/treatment there are numerous applications for positive displacement pumps with solids handling or metering capabilities. As a result, water/wastewater treatment is the second largest positive displacement pump market.  There are many kinds of positive displacement pumps used in the wastewater industry, but images for four common positive displacement pumps used in the industry are provided, which include: peristaltic pumps, lobe pumps, circumferential piston pumps, progressing cavity pumps.


      Peristaltic pumps are the most common type of flexible member positive displacement pumps. One reason why peristaltic pumps are commonly used in the wastewater industry is that there is no need to worry about corrosion or contamination of parts because the pumping fluid is contained within the tubing. In addition, the simple design of peristaltic pumps makes them low cost and easy to service. These pumps can also pump high viscosity fluids with a high percentage of solids very well.

      Lobe pumps use lobes on the rotating elements within the pump casing to direct the pumped fluid. Since the lobes do not contact each other, lobe pumps can move large solids and slurry-laden media, making them useful in the wastewater industry. Two-lobed pumps tend to handle larger solids and high-viscosity fluids better than three-lobed pumps at the expense of increased pulsation. Another useful feature of lobe pumps is that rotor surfaces within the pump cooperate to provide continuous sealing.

      Circumferential piston pumps are well suited for a wide range of applications due to their relatively low speeds, low inlet pressure requirements and large cavities. In this kind of pump, each rotor has one or more wing lobe elements, called pistons. The fluid is carried from inlet to outlet in spaces between piston surfaces. What makes circumferential piston pumps suitable for the wastewater industry is that they are able to handle high viscosity fluids and have low shear at low speeds.

      Progressing cavity pumps unique design makes them very versatile and useful for handling difficult to pump fluids, such as highly viscous or abrasive fluids. Progressing cavity pumps are commonly used to pump raw sewage and sludges. In addition, progressing cavity pumps can pump shear sensitive fluids and fluids with large particles.

      Not pictured but also commonly used for disinfectant are controlled-volume metering pumps.  These deliver flow streams at adjustable, yet controlled, rates with a high degree of accuracy. These pumps are most commonly used for the injection of chemical additives, proportional blending of multiple components, or metered transfer of liquid additives to a variety of processes at measured rates. Metering pumps are found in virtually every facet of water and wastewater treatment (both municipal and industrial).

      For more information regarding pumps used in the wastewater industry, refer to the “Positive Displacement Pump Certificate” training modules by at www.pumps.org

      What types of positive displacement pumps are used in food and beverage processing? ▼

      Both positive displacement and rotodynamic pumps are used for food and beverage processing. A few positive displacement pumps that are used are:

      Controlled volume metering pumps, or simply metering pumps, pump fluid through reciprocating action. They are often used to pump a fixed amount of a mixture in situations where precision is crucial. These metering pump shave a stroke length adjustment so that the volume delivered can be precisely metered, which is important in many blending or mixing operations. In food processing, this may include the addition of dyes or other ingredients into the product.

      Flexible vane pumps are a type of rotary positive displacement pumps that have a broad range of uses, including food and beverage processing and chemical processing. The pump uses an elastomer rotating member with enlarged vane tips that form a pumping chamber in conjunction with a casing when the rotor is placed with the shaft centered in the substantially circular casing that incorporates an eccentric section. Discharge forcing action is accomplished as the vane bends in the eccentric section, effectively squeezing liquid from the discharge chamber. 

      Peristaltic pumps use a flexible tube compressed between one or more moving rollers or shoes and a fixed track. The roller or shoe compresses the tubing and pushes the fluid in front of the roller or shoe towards the discharge end of the tubing. For the food and beverage industry, a primary benefit is that the fluid contacts only the tubing. This is a benefit for food and beverage applications, as reactivity with the pump piping does not need to be addressed.


      For more information on positive displacement pumps and the markets they are used in, refer to HI’s Positive Displacement Pump eLearning Course at pumps.org.



      What information is available regarding starting torque requirements for a reciprocating power pump with a liquid bypass? ▼

      Application of reciprocating power pumps requires careful consideration of their starting and running torque demands. These affect the selection of driver motors, motor starters, engines, gear reducers, belts or chain drives, couplings and universal joints. The effect of such loads on an electrical distribution system requires thought, especially if the pump is large.

      For starting the pump with a liquid bypass, the operator manually opens a bypass valve, or a power-actuated dump valve programmed to open automatically, which bypasses the liquid during the start and the stop function. A check valve is employed in the pump discharge line, and it remains shut as long as the bypass (dump) valve remains fully or partially open (see Figure 6.47).

      The liquid pressure exerted on the plungers (or pistons) is largely that caused by liquid mass and friction as it passes through the bypass valve and piping. When correctly sized, the bypass valve and piping cause low back-pressure. Only a relatively small torque is required while bypassing the liquid to a tank. With liquid bypass, the total starting torque requirement is mainly related to the mechanical inertia of the pump, couplings, gears and motor rotor. These components are heavy, and substantial starting torque may be required. All the liquid in the pump suction line and in the bypass line must be accelerated from standstill to full liquid velocities.

      The torque needed to accelerate the entire mechanical hydraulic inertia system depends on the inertia of all the moving parts, including the liquid, the rate of acceleration and the total system friction. The rate of acceleration is very important, and the starting torque is directly proportional to it. In other words, peak torque is inversely proportional to the time duration of acceleration.

      A few pumps are provided with mechanical suction valve unloader devices. These devices reduce the total starting torque requirement to that needed to accelerate the mechanical items, such as pump crankshaft, gears, couplings, etc. They stop liquid pumping action by mechanically holding open the pump’s suction valves, allowing the liquid inducted into each liquid cylinder to be delivered back into the pump suction. Because no liquid pumping occurs, the driving machinery does not need to apply a torque to accelerate the liquid. Only the torque to overcome mechanical inertia and friction is needed during the start.

      After the pump and driver have reached full speed, the suction valve unloading devices are retracted, and normal pumping action then commences. At this point, the driver must supply additional torque to accelerate the liquid system and also to meet the total running torque requirement caused by discharge pressure.

      For more information about torque requirements for reciprocating power pumps, see ANSI/HI 6.1-6.5 Reciprocating Power Pumps for Nomenclature, Definitions, Application, and Operation.

      How can I determine the pump input power for a reciprocating pump? ▼

      Pump input power may be determined by transmission dynamometers, torsion dynamometers, strain gauge type torque-measuring devices, calibrated motors or other sufficiently accurate measuring devices.

      When applicable, readings of power shall be taken at the same time that rate of flow is measured.

      Methods of measurement of power input to the pump fall into two general categories:

      Those which determine the actual power or torque delivered to the pump and are made during the test by some form of dynamometer or torque meter;
      Those which determine the power input to the driving element, taking into account the driver efficiency when operating under specific conditions.
      When pump input power is determined by transmission dynamometers, the unloaded dynamometer shall be statically checked prior to the test by measuring the load reading deflection for a given torque; and by taking the tare reading on the dynamometer scale at rated speed with the pump disconnected. After the test, the dynamometer should be rechecked to assure that no change has taken place. In the event of a change of ± 0.5% of the power at the best efficiency point (BEP), the test should be rerun. An accurate measurement of speed within ± 0.3% is essential.

      The use of calibrated dynamometers or motors is an acceptable method for measurement of input power to the pump.

      Calibration of the torsion dynamometer should be conducted with the torsion-indicating means in place. The indicator should be observed with a series of increasing loadings and then with a series of decreasing loadings. During the taking of readings with increasing loadings, the loading is at no time to be decreased; similarly, during the decreasing loadings, the loading should be based on the average of the increasing and decreasing loadings as determined by the calibration.

      If the difference in readings between increasing and decreasing loadings exceeds 1%, the torsion dynamometer shall be deemed unsatisfactory. 

      Dynamometers shall not be employed for testing pumps with a maximum torque below one-quarter of the rated dynamometer torque.

      When strain gauge type torque measuring devices are used to measure pump input horsepower, they shall be calibrated, with their accompanying instrumentation, at regular intervals (see Figure 6.72). After the test, the readout instrumentation balance shall be rechecked to assure that no appreciable change has taken place. In the event of a change of ± 0.5% of the power at BEP, the test shall be rerun.

      Calibrated electric motors are satisfactory to determine the power input to the pump shaft. The electrical input to the motor is observed, and the observations are multiplied by the motor efficiency to determine the power input to the pump shaft.   

      Calibrated laboratory type electric meters and transformers shall be used to measure power input to all motors.

      For more information about this topic, see ANSI/HI 6.6 Reciprocating Pump Tests.

      What are the design features of untimed rotary screw pumps and in what applications can they be used? ▼

      Screw pumps are used in oil field, pipeline, refinery, marine, power generation, chemical, hydraulic systems, and general industrial applications for transfer, lubrication, injection, and hydraulics handling a wide range of fluids, such as fuel oils, lube oils and greases, asphalts, noncorrosive viscous chemicals, and high-pressure coolants.

      The untimed rotary screw pump is an axial-flow, multirotor, positive displacement design used in a wide range of applications in pumping clean to mildly abrasive viscous liquids. It is often a more efficient alternative than centrifugal pumps. The design may use two, three, four, or five screws. The most common configuration is the three-screw pump, which consists of a power rotor (drive screw) and two symmetrically opposed idler rotors (driven screws) that mesh within a close-fitting housing forming a succession of cavities to continuously convey fluid to the pump discharge.

      Untimed screw pumps are available with a double-ended flow path as illustrated in Figure or with a single-ended flow path as shown in Figure Timing is accomplished through rotor geometry. In a properly applied three-screw pump, there is no rotor contact because screws are supported radially in their bores and are hydraulically balanced or free to float on a hydrodynamic film created by the pumped liquid. In other untimed screw pump configurations, the screws may be supported in product-lubricated bushings.

      Units are commercially available in product families with flows to 1200 m3/h (5300 gpm) and discharge pressures to 310 bar (4500 psi). Applications cover a wide viscosity range from 2 to 220,000 cSt (33 to 1,000,000 SSU) and temperatures from below zero to 274°C (500°F). Because of the axial movement of the fluid and the compact diameter of the rotors, untimed screw pumps typically operate at motor speeds (two-, four-, and six-pole). Screw pumps operate with a minimum of noise, vibration, and fluid pulsation. Other characteristics important in many applications are their good suction capability and low shear rate. Untimed screw pumps are frequently found in installations where extended uninterrupted service life is required.

      For more information about rotary pumps, see ANSI/HI 3.1-3.5 Rotary Pumps for Nomenclature, Definitions, Application and Operation.

      Is it possible for air operated pumps to be submerged during operation? ▼

      Some air-operated pumps are designed to be submerged. Before submerging a pump, check the chemical compatibility of the material of the wetted and nonwetted parts of the pump with the liquid in which the pump is to be submerged. Check the manufacturer’s operation manual for a particular pump before trying to use it in a submergible application.

       When submerging a pump, a hose should be attached to the pump air exhaust with the exhaust piped above the liquid level to prevent the liquid from entering the pump (see Figure To learn more about air operated pumps, see ANSI/HI 10.1-10.5 Air Operated Pumps for Nomenclature, Definitions, Application, and Operation.

      What is the proper method for configuring pressure-measuring instruments during a rotary pump test? ▼

      The following precautions should be taken in forming orifices for pressure-measuring instruments and for making connections:

       a) The opening in the pipe shall be perpendicular to the wall of the liquid passage.

      b) The wall of the liquid passage shall be smooth and of unvarying cross section for a distance of at least the larger of two pipe diameters or 12 in (300 mm) preceding the orifice. All tubercles and roughness shall be removed with a file or emery cloth.

      c) The opening shall be of a diameter d from 0.125 to 0.25 in (3 to 6 mm) and a length equal to twice the diameter.

      d) The edges of the opening shall be provided with a suitable radius tangential to the wall of the liquid passage and shall be free from burrs or irregularities. Figures 3.6.9a and 3.6.9b show two suggested arrangements of taps or orifices in conformance with the above.

      e) Manometers, when used on very viscous products with wet lines interfacing directly or indirectly to the manometer, require larger inlet taps into the pipeline to avoid excessive lag time for fluid to stabilize in the measuring system.

      Where more than one tap or orifice is required at a given measuring section, separate connections, properly valved, shall be made. As an alternative, separate instruments shall be provided.

      Multiple orifices can be connected to an instrument, except on those metering devices such as venturi meters, etc., where proper calibrations have been made on an instrument of this form.All connections or leads from the orifice tap shall be tight. These leads shall be as short and direct as possible. For the dry-tube type of leads, suitable drain pots shall be provided and a loop shall be formed of sufficient height to keep the pumped liquid from entering the leads. For the wet-tube type of leads, vent cocks for flushing shall be provided at any high point or loop crest to ensure that there are no leaks.

      For additional information regarding rotary pump test procedures, see ANSI/HI 3.6 Rotary Pump Tests.

      In what ways are mechanically coupled and hydraulic coupled disk diaphragm pumps different from one another? ▼

      A mechanically coupled disc diaphragm liquid end (see Figure contains a flexible, round diaphragm, clamped at the periphery, which is in direct contact with the process liquid being displaced. This type of design is inherently leak free.

      The diaphragm material is typically a fluoropolymer, elastomer, or fluoropolymer-elastomer composite. A connecting rod is connected directly to the diaphragm. The diaphragm is not pressure balanced as the process pressure is acting on one side of the diaphragm and atmospheric pressure is acting on the other side. This results in higher stress levels in the diaphragm and therefore these pumps are typically used for lower pressure applications. In operation, the process liquid is admitted through the suction check valve as the diaphragm/connecting rod assembly moves away from the wet end. As the diaphragm/connecting rod assembly moves towards the wet end, the suction check valve closes and the discharge check valve opens discharging liquid.A hydraulic coupled disc diaphragm liquid end (Figure contains a flexible, single or double configuration diaphragm, clamped at the periphery, and is in direct contact with the process liquid being displaced. This type of liquid end design is inherently leak free. The diaphragm material is typically a fluoropolymer, elastomer, or fluoropolymer-elastomer composite. Liquid end designs featuring flexible metallic diaphragms are available and used in applications where severe operating conditions prohibit the use of fluoropolymer or other elastomers.

      In operation, the diaphragm is moved by a hydraulic fluid, which is displaced by a reciprocating plunger or piston. The stresses in the diaphragm are minimal, as the process pressure acting on one side of the diaphragm is balanced by the hydraulic pressure acting on the opposite side. The process liquid is admitted through the suction check valves as the diaphragm moves rearward. As the diaphragm moves towards the wet end, the suction check valve closes and the discharge check valve opens discharging liquid. Liquid end designs of this type may include provisions such as contour plates, springs, or diaphragm positioning hydraulic control valves (Figure to ensure the diaphragm does not move beyond its elastic limits.For additional information regarding various controlled-volume metering pumps, see ANSI/HI 7.1-7.5 Controlled-Volume Metering Pumps for Nomenclature, Definitions, Application, and Operation.

      What principles drive the pumping action in a controlled-volume metering pump? ▼

      The pumping action is developed by a reciprocating piston. This reciprocating motion develops a flow profile represented by a sine wave. Actual rate of flow is determined by the following formula:

                    Rate of flow - Displacement x Cycles per unit of time x Volumetric efficiency     

      Figure 7.3.1a illustrates how the rate of flow from a reciprocating, controlled-volume metering pump is minimally affected by changes in discharge pressure.

      Figure 7.3.1c shows rate of flow versus stroke-length setting for a controlled-volume metering pump at a given pressure and stroking speed. The curve is linear. The curve is not necessarily proportional in that 50% stroke setting may not equal 50% flow. This is because the curve may not pass through zero on both axes simultaneously. By measuring flow at two rate-of-flow settings, plotting both points, and drawing a straight line through them, other rates of flow versus stroke length settings can be accurately predicted.

      The relationship between theoretical and actual rate of flow of a controlled-volume metering pump is illustrated in Figure 7.3.1d. The difference between the theoretical and actual curves represents the volumetric efficiency (VE), the ratio of the actual pump delivery per stroke to theoretical displacement, expressed as a percent.For more information on controlled-volume metering pumps, see ANSI/HI 7.1-7.5 Controlled-Volume Metering Pumps for Nomenclature, Definitions, Application, and Operation.


      What piping installation recommendations are important to consider for rotary pumps? ▼

      Because rotary pumps are designed with close running clearances, clean piping is a must. Dirt, grit, weld bead or scale, later flushed from an unclean piping system, will damage and may seize the pump. Figure illustrates pipe-to-pump alignment considerations.

      Piping should be installed on supports independent of the pump. Supports must be capable of carrying the mass of the pipe, insulation and the pumped fluid. Supports may be hangers, which carry the mass from above, or stands, which carry the mass from below. 

      Clamps or brackets may be used to secure piping to existing columns. Supports must allow for free movement of the piping caused by thermal expansion or contraction. Supports should be installed at intervals that uniformly and amply support the piping load, precluding contact with adjacent piping and equipment. Pipe strains or stresses transmitted to the pump by improper piping support systems may cause distortion, wear or binding of the rotary members as well as excessive power requirements. 

      Piping systems that contain expansion joints must be designed so the expansion joint is not exposed to more motion than accounted for in its design. Expansion joints or flexible connectors should not be used to compensate for misaligned piping.

      Threaded joints should be coated with compounds compatible with, but not soluble in, the pumped liquid. End users working with Teflon-taped joints should be careful to prevent shredded pieces of Teflon from entering the piping system. Piping should start at the pump and work toward the source of supply and the point of discharge. Shutoff valves and unions (for pumps with tapped ports) are recommended to facilitate future inspection and repair. Reducers are preferred to bushings when a change in pipe size is necessary. Avoid unnecessary restrictions in the pipeline, such as elbows, sharp bends, globe or angle valves, and restricted-type plug valves.

      Users should predetermine pipe size by taking into account the required flow rate; minimum or maximum velocities; the fluid viscosity at the lowest pumping temperature; the length of the piping system, including valves, strainers and other restrictions; and the elevation of the pump with reference to supply and discharge points.

      For additional information regarding rotary pump test procedures, see ANSI/HI - 3.1-3.5

      What methods are recommended for measuring a controlled-volume metering pump’s flow rate, and what should be taken into account to ensure an accurate measurement? ▼
      The following methods can be used to measure flow rate for a controlled-volume metering pump:

      1. Rate-of-flow measurement by weight: Measurement of rate of flow by weight depends on the accuracy of the scales used and the accuracy of the measurement of time. A certification of scale calibration should be part of the test record, or, in the absence of certification, the scales should be calibrated with standard weights before or after the test. Time intervals for the collection period must be measured to an accuracy of one-quarter of 1 percent.

      2. Rate-of-flow measurement by volume: This method involves measuring the change in the volume of a tank or reservoir during a measured period of time. The tank or reservoir can be positioned on the inlet or discharge side of the pump as long as all flow into or out of the tank or reservoir pass through the pump. When establishing reservoir volume by linear measurements, users should consider the geometric regularity (flatness, parallelism, roundness, etc.) of the reservoir surfaces as well as dimensional changes resulting from thermal expansion or contraction, or distortion resulting from hydrostatic pressure of the liquid. Liquid levels should be measured using hook gauges, floats, and vertical or inclined gauge glasses. In some locations and under some circumstances, evaporation and loss of liquid by spray may be significant, having a greater effect than thermal expansion or contraction. Users should either allow for such loss or prevent the loss altogether.

      3. Rate-of-flow measurement by direct reading meters: For this method, the only suitable meters are direct read devices such as paddle meters, magnetic flow meters and other accepted process measurement instruments.

      Table 1 shows acceptable fluctuations of test readings and instrument accuracy.
      For additional information, refer to ANSI/HI 7.6 Controlled-Volume Metering Pumps for Test.
      What procedures should be followed to ensure proper testing of controlled-volume metering pumps? ▼

      Uniform procedures for the setup and testing of controlled-volume metering pumps and for recording the test result data are available in ANSI/HI 7.6 Controlled-Volume Metering Pumps for Test.

      The test procedures cover the following:

      • functional testing of production units
      • mechanical integrity test at rated speed and specified pressure
      • rate of flow and mechanical integrity at rated speed and specified pressure
      • verification of performance to the manufacturer’s specifications


      Optional testing for consideration (based on specific application or criticality of service) includes:

      • steady-state accuracy
      • linearity
      • repeatability
      • net positive suction head or net positive inlet pressure

      The items the pump test may use include, but are not limited to, the following: 

      • Factory or purchaser furnished driver
      • Drive motor of proper voltage or appropriate design to operate metering pump
      • Safety relief valve to protect the pump from overpressure
      • Closed tank or open sump, properly sized for the pump being tested
      • A discharge pressure gauge suitable for measuring not more than two times the complete range of pressures being tested, located as close as possible downstream of or near the pulsation dampener
      • If required, dampening devices, such as pulsation dampeners, needle valves or capillary tubes may be used to dampen out the pressure pulsations at the discharge pressure gauge.
      • For variable speed applications, a means for measuring input speed to the pump should be provided and should be suitable for measuring the complete range of speed over the turndown range.
      • A means for measuring pump rate of flow in liters/hour (gallons/hour), or other agreed-upon units

      Test setups that do not conform with respect to intake structure, piping and measuring equipment may not duplicate test facility results. 

      For additional information on controlled-volume metering pump testing, refer to ANSI/HI 7.6 Controlled-Volume Metering Pumps for Test

      What is the difference between a Newtonian and non-Newtonian fluid, and how do different fluid types impact the performance of rotary pumps? ▼

      The impact of the fluid type on a rotary pumps is generally viscous related. The viscosity of a fluid is the tendency of the fluid to resist an internal shearing force. The viscosity of the fluid being pumped affects the net positive inlet pressure required (NPIPR) and will impact the pump flow slip.

      A fluid is Newtonian when the ratio of shear stress to shear rate is a constant for all shear rates, is independent of time, and zero shear rate exists only at zero shear stress (see Figure 1). Most mineral oils at temperatures above the cloud point (the temperature at which the oil begins to appear cloudy), solvents and water approximate this condition and are considered Newtonian fluids. The viscosity of these fluids is independent of rate of shear.

      A non-Newtonian fluid will change viscosity with changes in the rate of shear applied to the fluid and/or the length of time at shear.

      Several types of non-Newtonian fluids are defined in Figures 2 through 6. When the ratio of shear stress to shear rate increases as shear rate increases, reversibly and independent of time, a fluid is said to be dilatant (see Figure 2). Highly concentrated pigment-vehicle suspensions—such as paints, printing inks and some starches—are dilatant fluids. The apparent viscosity of these fluids increases as the rate of shear increases. Some dilatant fluids solidify at high rates of shear.

      When the ratio of shear stress to shear rate is constant for shear rates above zero, it is independent of time, but when shear occurs only for shear stress above a fixed minimum greater than zero, a fluid is termed plastic. A plastic fluid, such as putty or molding clay, is characterized by a yield point. This means that a definite minimum stress or force must be applied to the fluid before any flow takes place.

      When the ratio of shear stress to shear rate decreases as shear rate increases, reversibly and independent of time, and zero shear rate occurs only at zero shear stress, a fluid is pseudo-plastic. Many emulsions, such as water-base fluids and resinous materials, are pseudo-plastic fluids. Their apparent viscosity decreases with increasing shear rates but tends to stabilize at high rates of shear.

      A fluid is thixotropic when the ratio of shear stress to shear rate decreases and is time-dependent in that this ratio increases back to its “rest” value gradually with lapse of time at zero shear rate and stress, and decreases to a limit value gradually with lapse of time at constant shear rate. Most greases, drilling mud, gels and quicksand are thixotropic fluids when the apparent viscosity of these materials decreases for an increasing rate of shear and for an increasing length of time at shear.

      When the ratio of shear stress to shear rate is constant for all shear rates at any given instant of time, but increases with time, a fluid is rheopectic. Some greases are intentionally manufactured to have partial rheopectic properties that facilitate pumping in a stable condition; however, upon shearing in a bearing, the grease builds up to a higher apparent viscosity.

      Additional information about fluids and the effect of viscosity on pump and system performance may be found in ANSI/HI 3.1-3.5 Rotary Pumps for Nomenclature, Definitions, Application, and Operation.

      What are the key considerations for upstream (suction) piping for single and multiple control-volume metering pumps? ▼

      Because of the characteristic pulsating flow of metering pumps (see Figure, where peak flow rates can reach three times the average flow, operators must carefully consider suction piping to ensure that it can deliver adequate fluid to the pump inlet

      • The piping must accommodate the peak demands of the pump throughout its full range of operation, as well as prevent offgassing of liquids with high vapor pressure or dissolved gases. This can be accomplished by appropriately increasing the diameter of the suction piping and connections or by adding additional accessories to increase the flow of liquid to the inlet of the pump
      • Long lengths of pipe, elbows, tees, strainers, valves and other accessories installed in the suction piping can decrease the net positive inlet pressure available (NPIPA) to an unacceptable level
      • The pulsating flow in the system suction piping creates a pulse pressure that typically subtracts from system suction pressure. For example, a pump operating at 150 strokes per minute and 71 gallons per hour drawing water from 10 feet of half-inch schedule 40 pipe could subtract 10 pounds per square inch from the pump’s supply pressure. For shorter runs of pipe with minimum restrictions, a general rule of thumb is to increase one pipe size above the discharge piping, or two pipe sizes above the pump’s suction connection. Long piping runs with multiple bends, elbows, restrictions and/or higher-viscosity liquids require larger size piping
      • The most frequent reason for technical support calls to pump manufacturers regarding problems with pump performance is suction piping that cannot supply the demands of the pump. To ensure adequate flow to the inlet of the pump, refer to NPIPA calculations outlined in ANSI/HI 7.8-2016.

      When connecting more than one pump head to a single suction manifold, operators must consider a number of issues. Suction manifold piping must accommodate peak demand of the pumps throughout its full range of operation, as well as prevent offgassing of liquids with high vapor pressure or dissolved gases. This is accomplished by appropriately increasing the diameter of the suction manifold and pump head connections or by adding accessories to increase flow of fluid to the pump inlet. The most important consideration is whether the multiple heads are part of a multiplex pump (multiple heads connected to a single motor or driver) or if the heads all have independent motors or drivers. In most cases, multiplex pumps are connected to a common manifold piping arrangement, driving these recommendations. The other option is to connect each pump head to its own suction line connected to the supply source. 

      In a multiplex where a single driver is used, if the pumps all operate from the same gear set or with multiple gear sets operating at the same stroking speed, industry best practices suggest that the drive mechanism for each individual pump head should be run out of phase (i.e. duplex pumps, 180 degrees; triplex pumps, 120 degrees; and so on). This staggers the peak suction requirements of the individual pumps. Running several pumps from the same driver (motor) with proper phasing of the drive elements to ensure this timing during all running conditions and the best suction flow characteristics as they relate to piping design.

      When independent pumps, each with its own driver, are connected to a common suction line, or if the application requires multiplex pump heads operating in phase, operators must make allowance in the piping design in case all pumps simultaneously demand peak suction flow.

      For more information on piping guidelines for control-volume metering pumps, refer to the new standard ANSI/HI 7.8 Control-Volume Metering Pump Piping Guideline.

    • Back to Top
    • Pump Systems

    • How does motor load impact the total pump system efficiency? ▼


      Motors, along with pumps, drives, control valves, and piping, are critical components of pumping systems. It is extremely important to understand the relationship between motor efficiency and load in order to maximize the efficiency of pumping systems and reduce costs.


      The relationship between motor efficiency and load can be seen in Figure 1, which shows typical motor part-load efficiency as a function of the full load. Generally, at 50% load, a given induction motor will operate very close to its rated efficiency. Above 50% of the full load, the motor will operate at nearly the same percent efficiency. However, reducing the load on a motor below 50% causes the motor efficiency to drop more significantly, which therefore reduces the efficiency of the entire pumping system. The effect operating a motor at reduced load varies depending on the motor horsepower size, as illustrated in Figure 1. Motors that that have a higher horsepower size, such as the larger 75-100 horsepower motors, experience a less drastic reduction in efficiency when operated below 50% of the full load. For smaller motors, such as the 0-1 horsepower motors, the effect of dropping below 50% of the full load is much more drastic.

      For additional information related to pump system efficiency refer to HI’s online eLearning pump systems assessment certificate training program at www.pumps.org

      How do I calculate the three-phase input power to a motor? ▼

      To calculate the three-phase input power to a loaded motor, three parameters need to be taken into consideration:

      1. the RMS voltage (the mean line-to-line of 3 phases),
      2. the RMS current (mean of 3 phases), and
      3. the power factor as a decimal

      In the field these parameters can typically be measured directly by electricians using hand-held instruments or determined from the motor characteristic curve (power factor).  With these three parameters, the three-phase power input power to a motor can be calculated by using 



      Additionally commercial power meters are available that can be installed that measure all these parameters directly including power factor to directly measure the three phase power input to an electric motor.

      For more information related to the calculation of electrical input power in the field refer to HI’s Pump Systems Assessment Body of Knowledge at www.pumps.org
      What should I consider when installing a VFD in a pumping system? ▼

      Variable Frequency Drives, or VFDs, are electronic devices that control the rotational speed of an AC electric motor by controlling the frequency and voltage of the electrical power supplied to a motor. When properly applied this reduces the stress on and the energy consumed by the pumping system.  As mentioned, VFDs and their controls are crucial elements to the reliability and energy consumption of a pumping system. VFDs can provide controlled starting and stopping of the pumps and through proper feedback control change the speed of a pump to match the system requirements. 

      Because a VFD controls the speed of the motor, the system can accommodate varying operating conditions, making the entire system more versatile (see Figure 2). These control function can include flow control, level control, pressure control, temperature and control.  Additionally VFDs can provide detection of a prevention of upset conditions such as cavitation, pump deadhead, and dry running adding to their benefits.


      Despite all of the advantages of incorporating a VFD into a pumping system, special consideration needs to be taken when programming and installing these devices. There are many different kinds of pumps with different torque requirements, so a VFD must be programmed to provide the correct amount of torque for the given pump system.  For example positive displacement pumps have a constant torque load as speed is reduced, which requires special considerations compared to a rotodynamic pumps that is a variable torque load.  Another issue that needs to be considered when programming a VFD is that the speed range increases the chance of a pump forcing frequency interacting with a natural frequency of the system; a phenomenon known as resonance.

      These are a small portion of the considerations for use of VFDs and variable speed pumping, specific information on installation considerations for VFDs along with other useful information regarding VFDs and variable speed pumping can be found in the HI Guidebooks, “Variable Frequency Drives: Guidelines for Application, Installation, and Troubleshooting” and “Application Guideline for Variable Speed Pumping” found at www.pumps.org

      How can I determine if my pumping system is a good candidate for improved energy efficiency and how do I quantify it?▼

      Inefficiencies in pumping systems can take many forms. Below is a list of common indicators that may signify a pumping system contains inefficiencies:

      • Existence of flow control valves that are highly throttled
      • Existence of bypass line (recirculation) flow regulation
      • Batch type processes in which one or more pumps operate continuously
      • Frequent on/off cycling of a pump in a continuous process
      • Presence of cavitation noise either at the pump or elsewhere in the system
      • A parallel pump system with the same number of pumps always operating
      • A pump system that has undergone a change in function, without modification
      • A pump system with no means of measuring flow, pressure, or power
      • A pump system with frequent failures and high maintenance costs

      Pump system assessment and optimization is the process of identifying, understanding and effectively eliminating unnecessary loses while reducing energy consumption, improving reliability, and minimizing the cost of ownership over the economic life of the pumping system. The first step in system optimization is performing and assessment of the system and documenting the indicators mentioned above during the system pre-screening. If a pumping system contains one or more of the above indicators, it is recommended that the system undergo further levels of assessment.

      Since a pumping system will operate at the intersection of the pump curve and system curve, the next step is to thoroughly evaluate the pumping system in question to determine the baseline system requirements or system curve.  This is what the system requires to meet it purpose, not what the current state of the system is.  For example, later in the assessment it may be determined that the system is operating at an excessively high pressure or rate of flow and the pump could be resized or controlled in a more efficient manner to meet the system requirements.

      Once the pumping system requirements have been determined, system operating data should be acquired. Data can be acquired with permanently installed process transmitters or portable instruments, which measure system flow rates, system pressures and power consumption. The data collected should be done in a manner so that pump flow and head can be evaluated against its performance curve and it should be collected over an appropriate amount of time to represent the normal operating condition. For example, systems with varying or seasonal loads may require long-term data logging equipment.  After sufficient data is collected, it should be compared to the measured rates of flow/head and the required rates of flow/head. This may reveal an imbalance between measured and required conditions, which is evidence of an inefficient system.

      To learn more about assessing pumping systems and a new certification program for Pump System Assessment Professionals (PSAP), refer to the PSAP Body of Knowledge Guidebook and the PSAP Certification Study Guide at www.pumps.org/psap
      How do I use the information on a pump curve to select a pump for my system? ▼

      A pump curve (sometimes called a performance curve) is a graph that shows the total head, power, efficiency and net positive suction head (NPSH) where a 3 percent head loss occurs (NPSH3) plotted against rate of flow. These curves contain extremely important data that pump users need to analyze and interpret for proper pump selection and efficient operation.

      There are three main types of pump curves supplied by the pump manufacturer:

      1. The selection chart shown in Image 1 (also known as the range chart or the family curves)
      2. The published curve shown in Image 2
      3. The certified curve

      The certified curve is different from the selection chart and published curve because it is for the specific pump and impeller trim purchased and not the general product line. Often it will include the acceptance test standard and acceptance grade that the pump was tested against.  

      The selection chart shows the various pump sizes available for a given manufacturer’s pump line and speed. The desired head and flow rates are entered on the curve, and the pumps that overlap the area are valid choices to consider for selection. The selection chart is useful in developing a short list of pumps for consideration. For example, if the application called for a pump running at a nominal 1,800 revolutions per minute (rpm), that could provide 1,000 gallons per minute (gpm) at 100 feet of total head, the chart shows that 5 × 6 × 11 and 6 × 8 × 11 size pumps overlap on the selection chart and will likely be the two best sizes to evaluate further.


      Although the published curve may seem confusing, a lot of critical information can be extracted from this pump curve. If you understand the charts, you will benefit from the data they offer. Remember:

      • The Y axis (vertical) on this curve is the head in feet and meters, and the X axis (horizontal) is the capacity (flow rate) in m3/h and gpm.
      • Each of the downward sloping blue lines is called a head capacity curve.
      • Each of the numbers above the head capacity curves to the right of the Y axis represents different impeller diameters. As can be seen, the total head is reduced when the impeller diameter is reduced.
      • The numbers in the circles above the topmost head capacity curve are the pump efficiency, and the lines stemming from these circles are lines of constant efficiency. The triangles that contain a number and word “NPSH” are constant lines of NPSH (in feet) that the system must supply for the pump to operate with a 3 percent head loss. NPSH margin above this value is required for the pump to operate at the published head (refer to ANSI/HI 9.6.1).
      • Finally, the diagonal lines that run through the head capacity curves signify lines of constant pump input power.

      Using the selection chart to narrow down the appropriately sized pump for the duty point of 1,000 gpm and 100 feet of head, the manufacturer’s published curves can be referenced to help determine the best pump for an application. Image 3 shows an example of the published curve for a 5 × 6 × 11 pump running at 1,770 rpm. A significant amount of information can be derived from the manufacturer’s pump curve for this application, including the following:

      • The impeller diameter that meets the duty point falls between 10 and 10.5 inches.
      • The pump is 85 percent efficient at the rated point and 86 percent efficient at the best efficiency point (BEP).
      • At the rated point, the shaft power will be between 25 horsepower (hp) and 30 hp; however to ensure a non-overloading condition at the end of curve, a 40-hp motor may be required.
      • NPSH3 is between 9 and 10 feet at the duty point.

      Note that data displayed on a manufacturer’s pump curve are based on 68 F or 20 C water. If a liquid other than water will be pumped, information on the manufacturer’s published curve must be adjusted for the liquid density and viscosity, which affects the head, flow, efficiency and pump input power.

      For more information regarding pump curves, refer to the Hydraulic Institute’s Pump System Assessment Certificate program at www.pumps.org.
      What is an eddy current drive? ▼

      An eddy current drive, also known generically as a magnetic drive, eddy current clutch or magnetic clutch, is an electro-mechanical variable speed  drive that uses a constant speed motor as the input. The eddy current drive is installed between the motor and the driven equipment. Torque is transmitted to a variable speed output in proportion to a direct current (DC) applied to a set of coils on the output’s rotor.  This method of varying speed only reduces the speed of the driven load, which is different than variable frequency drives and DC drives, which vary the speed of both the motor and driven load by electronically altering the input power to motor.

      Horizontal eddy current drives can either be flange-coupled to the motor or may be separately mounted in a shaft-coupled configuration.

      In a typical vertical configuration, shown in Image 3, an ordinary induction motor is mounted atop the eddy current drive. The eddy current drive is built within a stationary frame of fabricated or cast steel, with a lower flange and shaft similar to that of a vertical motor.


      For both configurations, within the frame are two concentric rotating members: the drum, sometimes called the ring, and the output rotor, sometimes referred to as the magnet or field.

      The drum is a cylinder of magnetically permeable steel coupled directly to the motor shaft, and so rotates at constant speed, determined by the speed of the motor. The variable speed magnetic rotor is mounted concentrically within the drum and is separated by bearings, which maintain an air gap between the inner diameter of the drum and the outer diameter of the magnet to a specified tolerance. The outer surface of the magnet consists of a series of magnetic poles having alternating north and south poles, which are magnetized at varying strength by a low-power DC current, referred to as excitation current.

      For more information on eddy current drives and other variable speed drives, refer to HI’s Application Guideline for Variable Speed Pumping at www.pumps.org.
      What is the difference between a grid coupling and a gear coupling? ▼

      Flexible couplings can be broken down into elastomeric and metallic types. Both grid and gear couplings fall into the lubricated category of metallic couplings as outlined in Image 1.


      Grid style couplings are often used in medium- to heavy-duty applications and use a spring-style metallic element with looping segments that interlock with the hub teeth. An example of a grid coupling is shown in Image 2. The grid element or spring is made of high tensile alloy steel. The grid element transmits torque and accommodates some misalignment and shock loading in the system. The metallic grid coupling design allows for a more power-dense coupling solution in comparison to elastomeric designs. Because of the relative movement between mating metal surfaces, the grids and teeth must be lubricated as specified by the manufacturer. Typical grid couplings consist of two hubs, two seals, two gaskets, a grid element, and either an axially split or vertically split (perpendicular to the shaft) cover.


      Gear couplings come in many different designs, but all transmit torque and accommodate some misalignment through gear teeth. A few examples of gear couplings are shown in Images 3 and 4. Each hub has gear teeth cut around the outside diameter, with the hub connecting to a sleeve with mating gear teeth cut into the inner diameter. These products also require lubrication due to the relative movement of mating metal surfaces and have seals between the sleeve and hubs.


      Gear couplings are often used in applications where high torque and balance are required. In some cases, gear couplings are the only solution with enough torque capacity to accommodate demanding applications. There are many custom options available with gear couplings in addition to those described here. This type of coupling can be configured as a flex-flex, flex-rigid or rigid-rigid style gear coupling. A flex-flex configuration uses two flexible style halves, with one half consisting of a flexible hub and a matching flexible sleeve. A flex-rigid style uses one flexible style half and one rigid half. A rigid-rigid gear coupling uses two rigid style halves. Flexible style grid halves also have seals to retain lubrication, and both styles typically incorporate a gasket between the mating flange faces.

        For more information on couplings used with pumps refer to HI’s Flexible Coupling Basics Guidebook at pumps.org.
      What is the difference between a pressure gauge, pressure switch and pressure transducers? ▼

      System pressure measurement is one of the most important variables to measure and control in a pumping system. It provides information about system pressure and the pump/system operation, and it can be used to control the system in an efficient and reliable manner.

      The simplest form of pressure measurement is with a pressure gauge. A pressure gauge is a pressure indicating device that provides a local readout of pressure to monitor the operation or condition of a process by locally indicating a quantity of pressure. There are many types of pressure gauges, but the most common is the analog indicating bourdon tube type. 

      A pressure switch is next on the list. It may have a digital display, but the display is not the primary objective of a switch. A pressure switch is a device that, after the deviation of a physical pressure, opens or closes a set of contacts. The pressure setting can typically be adjusted and is used to open or close a valve or turn a pump on or off based on the physical pressure.


      A pressure transducer or transmitter is an electronic device used in combination with a pressure sensor and voltage-to-current signal conditioner to generate a proportional output to the pressure it is reading such as 4–20 mA or 1-5 volt (or other type) output signal. This proportional pressure signal can be feed to a distributed control system or other control to continuously control the operation of a pumping system. A typical application for control with a pressure transmitter and controls system is the modulation of a control valve or modulation of pump speed through a variable speed drive to maintain system pressure set point. 


      Pressure gauges, switches and transducers can read, indicate and/or control based on positive, negative (vacuum) or differential pressures.


      Tables 1 and 2 provide a summary of the capabilities and relative cost of each type of device and considerations for selection and measurement.



      Table 1. Relative capabilities of pressure sensing devices

      Type Pros Cons Relative Cost
      Pressure gauge Simple, local read out No transmission and no control $
      Pressure switch Simple and adjustable On/off operation control only $
      Pressure transducer/transmitter Continuous monitoring and controllability Cost $$



      Table 2. Important selection and measurement considerations

      Selection Considerations Measurement Considerations

      • Accuracy

      • Range

      • Repeatability

      • Product compatibility

      • Operating conditions

      • Mounting method

      • Installation environment

      • Cost

      • Approval rating

      • Calibration and maintenance requirements

      • Connectivity

      • Service environment, history

      • Calibration

      • Instrument range

      • Accuracy

      • Overpressure capability

      • Physical location, setup

      • Process connection point

      • Accounting for sensing element elevation

      • Proper instrument line fill and vent



      For more information on instrumentation and measuring of pump system variables, refer to HI’s Pump Systems Assessment Professional Study Guide at pumps.org/psap.

      What are the different kinds of pumps used at power plants ▼

      Many different kinds of pumps are used in power plants to fulfill a wide range of applications. Boiler feed pumps are crucial for the operation of power plants. These pumps are typically multistage and deliver feedwater to boilers. An example of a multistage boiler feed pump can be seen in Image 2. Feedwater is then turned into steam, which is what turns the turbines generating power at the power plants. To assist in the pumping of feedwater, boiler feed booster pumps are also present upstream of the feed pumps. The booster pumps increase the suction pressure of feedwater fulfilling the NPSH requirement of the main boiler feed pump.


                  Condesate pumps collect saturated water from the condenser hotwell and pumps it either to a deaerating heater or back into the boiler feed pump. Condesate pumps operate at extremely low suction pressures. For this reason these pumps are often located at the lowest level of any pump in the power plant and installed in a suction can that is below ground level. The velocity of the flow entering the condesate pipe is kept to a minimum to limit frictional losses in the piping to keep the elevation difference as the only contribution to NPSH available (NPSHa).  

                  Condenser circulating pumps deliver cool water from freshwater sources near the power plant and pump it through the condenser to condense exhaust steam from the turbine. These pumps can be either located in dry pits or wet pits. If they are in wet pits, then they are a vertical design.

      Boiler circulating pumps pump water through a boiler helping to increase the effectiveness of the boiler. The water is at the same temperature and pressure as the boiler and the pump operates at a low total head requirement only needing to overcome the friction in the tubes of the boiler. However, these pumps operate at very high temperatures and suction pressures and are welded into the boiler itself.

                  Heater drain pumps are necessary in power plants to pump condesate from closed heaters back into the feedwater system.   Similar to condensate pumps they have very little NPSHa.

                  For more information on the application of pumps in power plants, refer to the HI Guideline Power Plant Pumps—Guidelines for Application and Operation and the Rotodynamic Pump Certificate training modules at pumps.org.
      How can I measure the power consumed by a pump in the field to compare to the manufacturer’s performance curve? ▼

      Monitoring a pump system’s performance through power measurements is a great way to evaluate the condition of equipment over time. As noted in the question, pump manufacturers typically publish the power required by the pump as a function of flow rate at a given rotational speed, which does not incorporate losses from other components of the system such as intermediate gears, motors or variable speed drives. The input power to the pump is determined by measuring the torque and rotational speed at a given flow rate, or by measuring the electrical input power to a motor that has been characterized so that the motor input power can be correlated to motor output power (pump input power). Image 1 shows a typical performance curve for a rotodynamic pump. Note that power and efficiency are specific to the pump and not inclusive of the motor used.


      If a user is interested in trending the condition of the overall system (pump, motor, gear, etc.), then measuring the electrical input power to the motor or variable speed drive at a given flow rate and rotational speed is sufficient. However, if the focus is monitoring the condition of just the pump and comparing it to the manufacturer’s pump curve, at a given flow rate and rotational speed, there are two options:

      • 1.Measure the torque of the pump shaft and its rotational speed.
      • 2.Measure the motor electrical input power and incorporate the losses of the motor. If a variable speed drive or other piece of equipment is used in the system, the losses of these components also need to be considered. 

      When assessing equipment in situ, most of the time it is not practical to directly measure the torque on the pump shaft (although it can be done). Therefore, the second option above is typically the most straightforward approach for in situ measurement of the pump input power. The second option requires measuring the electrical input power to the motor and then correlating the motor input power to the output power. Image 2 is an example of a characterized motor curve (sometimes called a calibrated motor), and this is used to correlate the motor input and output powers. 


      The power meter can be installed in the switch gear to measure the motor input power, or it can be calculated based on several individual measurements with use of the characteristic motor performance curve. As shown in the motor input power equation (Equation 1), motor current and voltage must be measured and power factor must be determined from the characteristic curve to calculate motor input power. To convert kilowatts (kW) to horsepower (hp), divide kW by 0.746. Once the motor input power is measured or calculated the output power can be determined from the characterized motor performance curve.

      eq1                     (Equation 1)


      I      =   RMS current in amperes (A) (meter reading)

      V     =   RMS volts (meter reading)

      PF  =   power factor (motor characteristic curve)

      C    =   1 for single-phase current

      =   2 for two-phase four-wire control

      =   1.73 for three-phase current


      Because the data displayed on manufacturers’ pump curves are typically based on 68 F (20 C) water at given rotational speed, two additional points to consider are:

      • 1.Is the process water-like (i.e. same density and viscosity)?
        • If the process fluid is not similar to the water test, the liquid density must be corrected, which affects pump input power linearly with respect to the water density. If the viscosity differs from the manufacturer’s test, the head, flow, efficiency and pump input power will be affected. ANSI/HI 9.6.7 Rotodynamic Pumps – Guidelines for Effects of Liquid Viscosity on Performance should be consulted to adjust the performance for liquid viscosity.
      • 2.Is the in situ test speed the same as the manufacturer curve?
        • The manufacturer’s performance curve is typically represented at a constant speed. The pump performance will change with pump speed according to the affinity rules. The equations relating the rotodynamic pump performance parameters of flow rate, head, and pump input power, to speed are known as the affinity rules as outlined in Equation 2.

      eq2                        (Equation 2)



      Q2 = rate of flow at desired speed

      Q1 = rate of flow at original speed

      n2 = desired pump speed

      n1 = original pump speed

      H2 = total head at desired speed

      H1 = total head at original speed

      P2 = pump input power at desired speed

      P1 = pump input power at original speed

        For more information on assessing the performance of pump systems, refer to the Hydraulic Institute’s Pump Systems Assessment Professional (PSAP) Certification Program at pumps.org/PSAP.
      How much will pump efficiency be affected with the addition of a variable speed drive? ▼

      It is a misconception that adding a variable speed drive (VSD) to a pump will increase its efficiency. When considering the wire-to-water efficiency of the pump, motor and VSD, each component that is added lowers the wire-to-water efficiency at a respective flow rate because each component has losses associated with it. Image 1 illustrates this concept showing the pump efficiency being the greatest, then the wire-to-water efficiency decreasing at the respective flow rate when the motor and VSD are added to the extended pump product.



      The advantage that the VSD brings to the picture is that it can control the speed of the pump to meet the requirements of the system, which can reduce power consumed by less efficient controls that is not an essential requirement of the process. Additionally, the VSD can be used with on-off controls so the pump operates at a minimum speed where the specific energy consumption is optimized. Based on this, the VSD potentially increases the entire pump system efficiency by eliminating wasted head across control valves and wanted flow through bypass vales, and in some instances allows the pump to operate closer to its best efficiency point (BEP).

      Image 2 illustrates two examples where the gray box represents power that is essential to the system and the orange box represents power that is not essential to the system. The orange box shows how much energy is wasted by throttling control in the first image or bypass control in the second image. In each case the reduced speed pump curve can satisfy the operating flow without the orange wasted control power. The wasted control power typically outweighs the slight decrease in wire-to-water efficiency of the extended product. The two examples in Image 2 show systems with all friction head, and it is understood that the potential variable speed energy savings will decrease when static head is introduced to the system curve.

      For more information on variable speed pumping and the application and efficiency considerations, refer to HI’s Application Guideline for Variable Speed Pumping at pumps.org.
      We are updating our building cooling system. It was mentioned that the update would include primary-secondary pumps, what does this mean? ▼

      Primary-secondary (P-S) pumping is incorporated into hydronic system designs for a variety of applications. The most frequent application of P-S pumping is used in chilled water systems where there is a need to have fairly constant flow through the chiller evaporator and variable flow through the distribution system. P-S pumping also allows a designer to break up very large, complicated systems into smaller, more manageable subsystems. P-S pumping can also provide different supply water temperatures to each secondary loop without mixing valves.


      The fundamental operating principle of P-S pumping lies in the interconnection of the piping loops. Image 1 is a piping schematic where the two pumping loops share a small section of piping, referred to as the common pipe. The common pipe is located between two standard tees that are spaced approximately three pipe diameters in length apart. If the pressure drop in the common pipe is minimized, the flow rate in each loop will be independent of the other.


      Image 1: Piping schematic of two pumping loops sharing a common pipe


      For more information on pumps used in building services, refer to the Pump Application Guideline for Commercial Building Services, which will be published in spring 2019, at pumps.org. 

      How can a single variable speed pump be used to control temperature in an HVAC system? ▼

      In HVAC systems, there is an opportunity to vary the flow based on heating/cooling demand. The pumping system is designed to supply flow and pressure at the highest system demand. Without control, the pump operates at full speed. This can waste energy and cause wear on itself and other system components.


      Operating at full speed when not required creates an artificial demand for cooling in chilled water systems due to higher developed head and flow through the distribution system. Image 2 shows that a temperature input from downstream of a heating/cooling load can be used to slow the pump to the minimum flow required to satisfy the system downstream temperature requirements.


      In the case of a variable or staged heating boiler or chiller system, a controller is used to optimize the heating/cooling equipment and the pump flow so the minimum total energy is used to meet the system demand at the heat exchanger. 


      As illustrated in Image 3, differential temperature can also be used to control the speed of a pump to deliver the flow rate that will maintain a set point differential temperature across a load. This control requires two temperature sensors.

         jan2019.2             jan2019.3

                       Image 2 - Temperature control                                       Image 3 - Differential Temperature Control


      For more information on controlling pumps, refer to Application Guideline for Variable Speed Pumping at pumps.org.
      Why are synchronous motors used for the large pumps in my facility but not small pumps? ▼

      A type of electric motor used in pumping applications is synchronous. As the name suggests, the rotation of the shaft is synchronized with the frequency of the supply current. This is different than the most common pump driver, which is the induction motor that has a shaft that rotates at a speed less than the synchronous pole speed.

      Synchronous motors have a unique and merited position as the most efficient electrical drive in the industry for slow speed applications. As shown in Image 3, synchronous motors can be applied over a wide range of power and speed but are the preferred driver for low speed and high horsepower applications. They perform with great economy while converting electrical power to mechanical power.


      Image 3. Synchronous motor general areas of application


      For more information on pumping systems, refer to the newly published guidebook “Pump System Optimization: A Guide for Improved Energy Efficiency, Reliability, and Profitability” at pumps.org. 

      Our commercial building has many pumps, but I am not clear on what fluids are being handled.  What types of fluids are being pumped/handled? ▼

      Fluid streams within a commercial building are generally water based but sometimes include chemicals for disinfecting.  The water based fluids may be characterized as follows:

      •         Potable (Domestic) water

      •         Non-potable clean water

      •         Wastewater

      •         Grey water

      •         Glycol and water mixtures


      Potable (Domestic) water is city or well water that has been treated and is safe for human consumption.


      Non-potable clean water contains minimal or very small solids; however, it is not suitable for human consumption.


      Wastewater may include household and commercial solids, large solids, stringy material, sanitary waste, plastic scraps, food waste, sticks, leaves, abrasive materials, and other inorganic and organic solids. Become more prevalent in wastewater flows, are personal wipes, towels, cleaning cloths, and household cleaning materials that are marketed to be discarded through the sewer system. These materials, in addition to the stringy materials and rags, can bind together to create a large mass that can lead to clogging issues in the wastewater pump and associated piping.  Rotodynamic pumps that specifically designed to deal with solids, and minimize clogging are referred to as solids-handling pumps.


      Grey water can be water from a sink or bath that is kept separate from wastewater from toilets and sewer systems. It may also be harvested rainwater that is collected in a cistern.


      Glycol and water mixtures have properties that differ from water. The viscosity is increased, the boiling point is increased, and the freezing point is lowered. Glycol is typically added to hydronic systems to prevent the liquid from freezing at temperatures below the freezing point for plain water.  Propylene and Ethylene Glycols are typically used in commercial buildings. Most glycol manufactures recommend that the water mixed with the glycol be treated to prevent minerals commonly found in city water supplies from reacting with the glycol.


      The fluid streams within buildings may contain one or more of these characteristics. The properties of these fluid streams create different considerations that need to be addressed when selecting a pump for the intended service. The pump user should identify and communicate to the pump manufacturer the nature of the fluid for each specific application to ensure reliable operation.

        For more information on fluids pumped in commercial buildings and pumping considerations, refer to HI’s new guidebook Pump Application Guidelines for Commercial Building Services at pumps.org.
      What viscosity oil should be used with rolling element bearings? ▼

      The viscosity and type of lubricant used will depend on the bearing design and service. Rolling element bearings (Image 1) in process pumps are lubricated by grease, mineral oil or synthetic oil. When selecting a lubricant, it is important to consider that the primary purpose of oil, or the oil constituent of grease, is to separate the rolling elements and raceway contact surfaces, lubricate the sliding surfaces within the bearings and provide corrosion protection and cooling.


      Image 1. Rolling element bearings (Image courtesy of Hydraulic Institute)


      Viscosity is an important property of a lubricant, which supports the function of a lubricant in reducing friction and wear. A protective oil film is required to perform these functions. The three basic oil film conditions are referred to as:

      • 1.Full film denotes the presence of enough lubricant to ensure complete separation of the moving surfaces. Also known as hydrodynamic full film.
      • 2.Elastohydrodynamic (EHD), a hydrodynamic film formed by applied pressure or load. Predominantly found in rolling element bearings.
      • 3.Boundary layer is sometimes referred to as thin film lubrication and is usually the result of insufficient lubricant supply. Although lubrication is present, there is not enough to prevent metal-to-metal contact.

      Oil or grease base viscosity selection is influenced by load, temperature and speed. If an application involves high speeds, low loads and low temperatures, then a low viscosity lubricant is adequate. Conversely, if low speeds, high loads and high temperatures are the parameters, then a high viscosity lubricant should be chosen.


      Oil in process pumps are typically an ISO grade 32, 46, 68 or 100. These numbers relate to the kinematic viscosity in centistokes. The oil can be hydrocarbon oil, although synthetic oils are used for specific lubrication applications. The viscosity of synthetic oil is less sensitive to temperature changes, and more widely used when temperature fluctuations exist. If temperature also exceeds 100 C (212 F), a synthetic is recommended because the oxidation rate of mineral oil accelerates at higher temperatures.


      Use of the correct viscosity lubricant for the speed, temperature and loads ensures the development of a full oil film between rotating parts. The optimum fluid film is sufficiently thick to allow the rolling elements to function without metal-to-metal contact with minimum friction. The functionality of the lubricant is negatively affected when the incorrect viscosity is used.

        When viscosity is too low, it can cause mixed or boundary lubrication with resulting heat generation and component wear. Excessively high viscosity will prevent metal-to-metal contact, but will generate unnecessary heat and loss of power as the rolling elements are forced to plow through the oil film.

      When is grease used as a bearing lubricant? ▼

      The use of grease is primarily limited to lower-horsepower pumps where the parameters are in size and speed range of rolling element bearings. Grease is usually lithium, with a normal viscosity of 100 centistokes. This typically has a maximum operating temperature of 121 C (250 F), but it is usually limited to a service temperature of 93 C (200 F). To prevent the loss of grease and minimize the entrance of containments, shielded bearings may be used. Shielded bearings are usually limited to a maximum operating temperature of 52 C (125 F).


      Advantages to grease lubrication include:

      • less sensitive to level pump installation
      • simplified maintenance

      Disadvantages to grease lubrication include:

      • limited heat dissipation possible
      • overpressurization during regreasing can damage bearing shields, therefore follow manufacturer procedures to prevent overpressurization
      • grease used with sealed bearings can have higher frictional heat leading to accelerated oxidation of the grease
        For more information on lubrication of rolling element bearings, refer to HI’s free white paper: Proper Lubrication Methods for Bearings, at pumps.org
      What effect does pumping a high viscosity fluid have on the pump system? ▼

      When a highly viscous liquid such as a heavy oil is pumped by a rotodynamic pump, the performance can be significantly changed in comparison to performance with water, due to increased losses. This performance reduction can be estimated by applying correction factors for head, rate of flow and efficiency to the performance with water. These correction factors determine the head and efficiency curves for the pump when handling viscous liquids.


       Viscous correction factors are defined as follows (CH for head, CQ for flow, and Cη for efficiency):


      These correction factors are used to correct the water performance to viscous performance. The ANSI/HI 9.6.7 Rotodynamic Pumps – Guideline for Effect of Liquid Viscosity on Performance establishes these correction factors based on empirical data for viscosities up to 3,000 centistokes (cSt), but allows the procedures to be used up to 4,000 cSt with increased uncertainty. The correction factors are based on the “B” parameter calculation which considers the viscosity of the fluid, pump total head, flow rate at best efficiency and the speed of rotation.

      Image 1 is an example of water test data for total head, power and efficiency with respect to flow rate corrected for the viscous liquid that will be used in the process. Note that head, flow and efficiency will decrease with the increased viscosity, and the pump input power will have a corresponding increase.


      Image 1. Example performance chart for single-stage rotodynamic pump (Image courtesy of Hydraulic Institute)

      For positive displacement (PD) pumps, this same relationship is not applicable. The physics of how PD pumps and rotodynamic pumps operate is different. PD pumps will behave differently than rotodynamic pumps handling viscous liquids. Rotodynamic pumps have an impeller that increases the velocity of the liquid to induce flow, but PD pumps capture a volume of liquid and transfer it directly every revolution of the shaft. In PD pumps, a certain amount of volume of liquid will leak (called slip) to a lower pressure region. Increased viscosity can reduce the amount of slip, which can result in increased output flow per shaft revolution, and an increase the volumetric efficiency of the PD pump. This increase in volumetric efficiency is offset by other mechanical and friction losses. The PD pump manufacturer should be consulted about the predicted or actual viscous performance characteristics.

      For more information on the viscous performance of rotodynamic pumps, refer to ANSI/HI 9.6.7 at pumps.org.
      What are some important considerations when converting an old packed pump to one with a mechanical seal, in the chemical process industry?  ▼

      The selection of seal chamber design and seal arrangement is based on requirements to maintain a proper environment for the mechanical seal. Factors to consider include normal operation, start-up and shut-down conditions, and standby. Venting of vapors is important where the connection must be at the highest point to ensure a vapor free condition for the mechanical seal. Various pumps are equipped with seal flush and piping plans based on applications and pump types. Refer to Hydraulic Institute Mechanical Seals for Pumps, Application Guidelines for complete information on this topic.


      ASME/ANSI B73.1, Specification for Horizontal End Suction Centrifugal Pumps for Chemical Process; B73.2 (Vertical In-Line); and ISO 3069/DIN 24 960 Type C specify several seal chamber dimensions for general service pumps. New pump designs or upgrades should comply with those dimensions to maximize the choices of commercially available seal designs already developed and tested for those dimensional envelopes.


      There has been an effort to upgrade pumps originally designed for packing to accommodate mechanical seals; however, the interchangeability in the conventional stuffing box originally designed for packing results in a compromised mechanical seal design (Image 1).  Generally, there is inadequate removal of seal-generated heat, greater wasted space and overhang, vulnerability of seal damage under off-design pump operation, vulnerability of seal damage form abrasives, and seal installation difficulties with dual arrangements.


      Image 1 - Single mechanical seal mounted in a conventional process pump stuffing box


      Improvement 1 – Enlarged Cylindrical Bore


      An enlarged cylindrical bore seal chamber offers some solutions to the shortcomings of the conventional stuffing box. A typical single seal installed in an enlarged cylindrical bore seal chamber is illustrated in Image 2. This seal chamber design eliminates the wasted space between the seal and the bottom of the seal chamber, provides more radial clearance over the seal OD, and can provide more axial distance outside of the seal chamber for dual seal arrangements.


      The enlarged cylindrical bore design offers an advantage in regards to the thermal system as well.

      If the radial clearance above the seal is increased to a point where the seal chamber acts more like a large fluid reservoir than an annulus, and the pump throat is enlarged so that fluid can move in and out of the pump casing, then a single seal can perform satisfactorily with no flush.


      Image 2 – Single mechanical seal mounted in an enlarged cylindrical bore process pump seal chamber


      Improvement 2 – Enlarged Tapered Bore


      Enlarged tapered bore seal chambers offer the same solutions as enlarged cylindrical bore seal chambers plus they provide static self-venting and allow more complete process fluid drainage during pump shutdown. A typical single seal installed in an enlarged tapered bore seal chamber is illustrated in Image 3.  Enlarging the seal chamber bore as shown in Image 3 with a tapered bore results in the low seal face and seal chamber temperatures. This type of seal performance will yield extended seal life. Tapering the seal chamber bore also encourages self-venting of any gases produced within the seal chamber to prevent them from detrimentally affecting seal operating performance.


      Image 3 – Single seal mounted in an enlarged tapered bore process pump seal chamber

      For more information about seal chambers for chemical process pumps, refer to HI’s Mechanical Seals for Pumps, Application Guidelines at www.pumps.org.

      How does discharge piping impact pump performance and reliability? ▼

      A pump’s outlet (discharge) piping flow characteristics normally will not affect the performance and reliability of a pump, with a few exceptions. In some cases, sudden valve closures can cause excessively high pressure spikes (surge or water hammer) to be reflected back to the pump, possibly causing damage to the pump, system piping or components.

      When the flow of liquid is suddenly stopped, the liquid tries to continue in the same direction. In the area where the velocity change occurs, the liquid pressure increases dramatically due to the momentum force. As it rebounds, it increases the pressure along the pipe near it and an acoustic pressure wave is formed.

      This is analogous to the waves on a pond after a stone is dropped in. The waves radiate outward in all directions. As the wave travels further from the center, its energy is spread over a larger and larger area and it will dissipate and eventually die out. In a pipe, the acoustic pressure wave can only travel along the pipe. The potential magnitude of the pressure wave can be tremendous, so in systems where there may be a sudden closure of a check valve or sudden stopping of the pump, a transient flow analysis may be required.

      Additionally, the discharge piping of a pump system is a primary source of friction. This will impact the pump system curve, which is a representation of the head required to deliver a specific flow through the system (i.e., the head required from the pump). System curves have a static head (hs) component that is made up of elevation difference or pressure difference between the source and the destination. The static component of the system curve is typically independent of the flow velocity in the piping system, however, the friction head (hf) component is dependent on the flow velocity in the piping system. Since the friction head component is dependent of the velocity in the piping, the discharge piping size is a leading contributor to friction head, along with fittings and valves in the system.

      Image 1 shows a typical pump system curve with the static and frictional components. Take note that the frictional component increases with a squared relation to flow rate (i.e., velocity), and the curve becomes steeper when a valve is throttled (i.e., more friction). This is important because the pump will operate at the intersection of the pump curve and the system curve so if the system requires additional or less head—pressure—than the pump was selected for, the pump may operate at low flow rate or high flow rate, which impacts the pump’s overall reliability.


      IMAGE 1: Pump curve and system curves showing static head and friction head (Images courtesy of Hydraulic Institute)
      How do throttling valves affect the system curve? ▼

      Passive valves, or throttling valves, have a fixed position and do not change their loss coefficient in response to other system changes. These are manually set and do not react to pump and system changes. Since these passive valves are fixed over a range of system flows, they affect the system curve as they are changed.

      Changing the valve position will also change the system curve. As the valve is closed, the system curve will become steeper due to the increased loss across the valve. If the pump curve remains unchanged, then with the new system curve, one can predict a new operating point for the pump and flow rate through the system. Image 1 depicts a passive throttling valve and its effect on the system curve. The pump will now need to produce more head to overcome the increased system loss and will move back to a point where it will balance the flow rate and head of the system, in this case at a lower flow rate.


      IMAGE 1: Pump curve and system curves showing static head and friction head (Images courtesy of Hydraulic Institute)

      Different than passive valves, control valves are not considered part of the system curve. In this case, the control valve actively changes its position and loss, reacting to pump and system changes, to maintain its set point and constrains the system to be at a set flow rate, pressure, level or temperature. The active control valve is not part of the system curve because they are continuously varying their head loss (hcv) and do not have a unique value over the range of system flows. They are, however, represented on the pump versus system plot as the difference between the head produced by the pump at the set flow rate and the head loss of the system also at that flow rate.


      Image 2 shows an example of a pump versus system curve with a flow control valve. In this case, changes to the system curve (e.g., tank level changes, other devices opening or closing) would result in the loss across the control valve (hcv) changing. This would not change the operating point of the pump (unless the control valve fails) because the flow rate in the system is being set by the control valve.


      IMAGE 2: Active control valve moves operation point from pump/system curve intersection

      For more information about pump piping and system valves, refer to ANSI/HI 9.6.6 Rotodyanmic Pumps for Pump Piping and Guidebook Pump System Optimization: A Guide for Improved Energy Efficiency, Reliability, and Profitability at pumps.org.

      What factors affect pump efficiency?▼

      Perhaps the most important factor is to select a pump that is properly sized for the normal system operating conditions, because off design flow rate will result in lower operating efficiency. Assuming proper hydraulic selection, the first consideration is type or design of the pump. Focusing on rotodynamic pumps, there are many types with different configurations and features to meet specific service conditions—e.g., paper stock, sewage, slurries, etc.—all of which have compromises in design needed to address special applications. This will result in compromise efficiencies compared to clean liquid pumps (Image 1).


      IMAGE 1: Pump efficiency factors (Image courtesy of Hydraulic Institute)


      In addition to the design compromises made for specific applications, there are other items that will affect the attainable pump efficiency. For example, in vertical turbine diffuser-type pumps (due to hydraulic losses at the inlet and discharge of the pump), the number of stages will affect the attainable efficiency of the bowl assembly. A single-stage attainable efficiency could be as much as six points below the bowl efficiency when four or more stages are used because the entrance and exit losses do not repeat with each stage. When reviewing a pump curve for this type of pump, it will likely include an efficiency de-rating for low stage counts. An additional consideration for these types of pumps is that the overall efficiency will be less than the bowl assembly efficiency due to hydraulic losses in the column piping, discharge elbow losses and bearing losses. Because of the variability in this style pump, this correction needs to be analyzed for each application.


      Pump type and design is only the beginning. Another consideration is the pump’s specific speed.  Specific speed is an index for the pump design that relates to how the flow enters and exits the impeller. There is an optimal specific speed to reach maximum attainable efficiency that may vary depending on the pump design. However, it may not be possible to select a pump at the “optimal specific speed” because the application will generally dictate the specific speed for the pump. Pumps with a low specific speed discharge flow in a radial direction. Radial flow pumps (low specific speed) produce relatively higher head and lower flow rates. On the opposite end, pumps with higher specific speed, have flow enter and exit the impeller (propeller) in the same axial direction. Axial flow pumps (high specific speed) produce relatively higher flow rate and lower head.

      Pumpage is also an important consideration. The viscosity of the pumped fluid will have a detrimental effect on efficiency for rotodynamic pumps. For rotodynamic pumps, the performance as tested on water is corrected to the viscous performance using ANSI/HI 9.6.7 Rotodynamic Pumps – Guideline for Effects Liquid Viscosity on Performance. While this is not an exhaustive list, these considerations are important when estimating the efficiency of your pump.

      Is it really worth buying the more expensive, efficient pump over a cheaper, less efficient pump?▼

      The initial purchase and installation cost of a new pumping system is a small portion of the lifecycle cost of the pump, which typically lasts 15 to 30 years. It is the routine operating costs of energy, maintenance and other recurring expenses that make up the primary components of total life cycle cost (LCC) as illustrated in Image 1 for a typical installation.


      IMAGE 1: Example of life cycle cost for a typical pumping system (Images courtesy of Hydraulic Institute)

      For example, say the initial purchase and installation cost of a 100 horsepower (hp) pump was $30,000. The 100 hp pump running at full load with a cost of electricity of eight cents per kilowatt hour ($0.08/kWh), operating continuously would total $52,000 per year. In just one year, the energy cost is more than double the installed cost. Over a typical lifecycle of at least 15 years, the energy cost totals to  $780,000. In this example, the installed costs account for only 4 percent of the lifetime energy costs.


      To compare a more efficient pumping system’s annual cost, we will assume some round numbers and focus on just the pump input power (excluding the motor) for calculation simplicity. Start with a pump that is 70 percent efficiency and compare it to an identical pump that is 75 percent efficient. This change lessens the pump input power required. This difference can be calculated using the following formula.



      From this calculation we observe a power savings of 7 hp. To calculate the lifetime cost savings of 7 hp, use the following formula.


      It is important to also note that that pump operating efficiency is often lower than the rated condition due to oversizing, problems with control valves, poor installation and other factors that result in the pump operating away from the best efficiency point or rated condition. Therefore, it is beneficial to precisely analyze the system requirements, optimize the control systems, and when justified by a life cycle cost analysis, invest money upfront for a more efficient pumping system.

      What are common problems that cause excess pump system vibrations?▼

      Pump vibration is related to the pump’s rotational and hydraulic forces, and the dynamics of the pump’s rotor and structure. Typical forced vibration from the pump are related to the speed of rotation in rotations per minute (rpm) and multiples of rpm, such as: 



      IMAGE 2: Illustration of forced vibration due imbalance and vane pass occur at 1 X RPM, and N X RPM


      Some causes for these forced vibrations are misalignment of the pump and driver shaft, imbalance, bent shafts, damaged impeller vanes and many others. The vibration sources can come from system related issues such as cocked or damaged bearings,  and inadequate piping supports and other sources of excessive nozzle loading.

      Vibration can increase in severity when improper installation and foundation exits, such as improper grouting, inadequate tie-down bolts, inadequate baseplate material, inadequate baseplate rigidity and the lack of proper internal locking of the baseplate to the foundation

      Another consideration is that resonance can occur. Resonance is a condition where of the forced vibration frequency aligns with a structural or rotor natural frequency resulting in amplified vibration. In these cases, even low forced vibration can result in an amplified vibration that is unacceptable. When resonance results in undesirable vibration, the frequency of the forced vibration will need to be changed or avoided, or the natural frequency of the system will need to be modified. 

      For more information on optimizing pumping systems and pump vibration refer to HI’s Pump System Optimization Guidebook, ANSI/HI 9.6.8 Rotodynamic Pumps – Guideline for Dynamics of Pumping Machinery, and ANSI/HI 9.6.4 Rotodynamic Pumps for Vibration Measurements and Allowable Values at pumps.org

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    • General Pump Standards

    • When selecting various wetted parts for pumps, how much consideration should I give to the temperature of the liquid being pumped? ▼
      The handling of liquids at temperatures below 32°F (0°C) or above 250°F (120°C) usually requires careful selection of the materials and corresponding attention to construction details. Many materials are of the nature where high or low temperatures will affect the corrosion-resistance and/or the physical properties of these materials. Therefore, it is important that the temperature of the liquid being pumped is considered. 

      For material selections acceptable in the temperature range involved, the applicable codes and practices of the industry in which the pump is to be used should be consulted. Selection of materials for pumps operating at low temperatures should be made only after each component and its function has been considered. Many materials change from tough to brittle behavior with a decrease in temperature.

      As a starting point in the selection of a suitable ferric steel for low-temperature service, the user may consider a heat-treated, fine-grain, low-carbon alloy steel with low phosphorus, nickel and molybdenum, and of moderate hardness. This usually offers better notch toughness at low temperatures than other ferric steels. 

      Consideration should also be given to the austenitic stainless steels and to bronzes for possible use in low-temperature pumping applications. Austenitic stainless steels, fully annealed, show improving toughness with decreasing temperature and exhibit no transition point. Most bronzes and all aluminum alloys are not embrittled at low temperatures and may also serve for this type of service, if otherwise suitable for the application.

      Other considerations, such as cost, corrosion-resistance, availability, erosion-resistance, hardness, toughness and fatigue strength must be carefully considered before the final selection of materials for high-temperature or low-temperature services is made. It should also be noted that various other factors should be considered when selecting materials for wetted pump parts such as: the user’s experience, the manufacturer’s experience, the expected pump life (such as temporary or long-term use), intermittent or continuous duty, pumping hazardous or toxic liquids, and the condition of the liquid.

      For more information on wetted parts materials selection refer to ANSI/HI 9.1-9.5 Pumps – General Guidelines for Types, Definitions, Application, Sound Measurement and Decontamination.
      What are some adverse effects that can occur from mechanical processes during pumping operations? ▼
      There are numerous mechanical processes during pumping operations, not typically duplicated in laboratory, chemical, or physical analyses, that may affect equipment material. These include crevice corrosion, velocity effects, and thermal and hydraulic shock.

      Stainless steels and some other alloys rely on an oxide coating to generate a corrosive-resistant film. Pump designs that have tight crevices, or fits, do not permit the continued replenishment of this oxide film and may, therefore, corrode in these areas even though the material is basically compatible with the liquid being pumped. Certain liquids, such as seawater and halide salt solutions, are more likely to promote crevice corrosion.

      Most wetted surfaces in a pump, such as internal impeller and diffuser passages, and wearing ring, balancing drum and balancing disk clearances, are subject to relatively high liquid velocities. This velocity and its scouring effect on corrosion deposits, soft-base metals, and oxide films can, and often does, adversely affect corrosion rates. Therefore, laboratory static submergence tests for determining corrosion rates of specific liquid/material combinations can, at best, only be considered as a general guide to material selection.

      Certain pumping situations may subject the pump to thermal or hydraulic shock, which may preclude the use of brittle materials such as cast iron. Such shock may occur in startup on high-temperature applications, when pumps are handling flammable liquids that could leak or catch fire, or when water hammer shock results from sudden valve closure.

      For more information about special effects from mechanical processes during pumping operations, see ANSI/HI 9.1-9.5 Pumps – General Guidelines.
      How can I reduce the level of extraneous noise when measuring sound levels throughout a pumping system? ▼
      Separation of the various sound sources may require special measurement techniques or may not be possible. Therefore, consideration must be given to reducing the level of extraneous noise. The following precautions in the test set-up should help accomplish this:

      a) Valves: select low-noise type valves. Use two or more throttling valves in series to reduce the differential pressure across the valve. Locate valves as remote from the pump as possible. Avoid putting them between the microphone and pump. Cover noisy valves with an acoustical barrier;

      b) Piping: use pipe and fittings, sized 1:1. Avoid situations which cause a change in velocity. Use straight runs of pipe from the pump to the supply tank. Cover noisy piping with an acoustical barrier material;

      c) Gears: avoid high ratio reducers and increasers. Cover test gears with an acoustical enclosure or barrier material;

      d) Test foundations/bases: use rigid foundations and bases to support the pump. Avoid using large, flat, thin material which can vibrate and radiate noise.

      For more information on similar topics, see ANSI/HI 9.1-9.5 Pumps – General Guidelines for Types, Definitions, Application, Sound Measurement and Decontamination.
      What are the best materials to use in pump construction to minimize part corrosion? ▼
      Pumps are produced using a wide variety of materials. Factors that must be considered in the selection of materials for wetted pump components include user’s experience, life cycle costs, regulatory agency requirements (i.e. limits on lead content in bronzes that contact drinking water), required pump life, duty cycle (operating hours per period), corrosive and/or erosive properties of the fluid, hazardous nature or toxicity of the fluid, the potential for cavitation and the potential for contamination of the fluid. Corrosive and/or erosive properties of fluids may vary with temperature, concentration of chemicals or solids, the properties of the solids, velocity, and the extent of entrained gasses. 

      Some of the more frequently used materials are listed below.
      • Bronze-fitted pump: The casing is made of cast iron, and the impeller and impeller rings are made of bronze. This combination is commonly used for fresh water at ambient temperatures. 
      • All bronze pump: All parts of the pump in direct contact with the pumped liquid are made of manufacturer’s standard bronze. This type of pump is often used for pumping seawater. 
      • All iron pump: All pump parts in direct contact with the pumped liquid are made of ferrous metal (cast iron/ductile iron, carbon steel or low-alloy steel). This pump is commonly used in hydrocarbon services and some chemical applications.
      • Stainless-steel fitted pump: The casing is made of materials suitable for the service. The impellers, impeller rings and shaft sleeves (if used) are made of corrosion-resistant steel with suitable properties for the specific application. This type of pump is also used in hydrocarbon and chemical services. 
      • All stainless-steel pump: All pump parts in direct contact with the pumped liquid are made of corrosion-resistant steel with suitable properties for the specific application. This pump is commonly used in chemical applications. 
      • Rigid polymers/composites: All pump parts in direct contact with the liquid are made of rigid polymers or composites (plastics), either as coatings or as structural material. This pump type is commonly used in chemical services.

      For more information on materials of construction for pumps, see ANSI/HI 9.1-9.5 Pumps—General Guidelines.
      What should end users consider in the selection of wetted parts for pumps based on the temperature of the liquid being pumped?
      The handling of liquids at temperatures below 32 F (0 C) or above 250 F (120 C) usually requires careful selection of the materials and corresponding attention to construction details. The corrosion-resistance and/or physical properties of many of these materials are affected by high or low temperatures. For this reason, end users must consider the temperature of the liquid being pumped.
       For material selections acceptable in the temperature range involved, consult the applicable codes and practices of the industry in which the pump will be used. Selection of materials for pumps operating at low temperatures should be made only after each component and its function have been considered. Many materials change from tough to brittle behavior with a decrease in temperature.
       As a starting point in the selection of a suitable ferric steel for low-temperature service, the user may consider a heat-treated, fine-grain, low-carbon alloy steel with moderate hardness and low phosphorus, nickel and molybdenum. This material usually offers better notch toughness at low temperatures than other ferric steels.
       Users should also consider the austenitic stainless steels and bronzes for possible use in low-temperature pumping applications. Austenitic stainless steels, fully annealed, show improving toughness with decreasing temperature, and they exhibit no transition point. Most bronzes and all aluminum alloys are not embrittled at low temperatures, and they may also serve in this type of service, if otherwise suitable for the application.
       Other considerations, such as cost, corrosion-resistance, availability, erosion-resistance, hardness, toughness and fatigue strength, must be carefully considered before the final selection of materials for high- or low-temperature services can be made. Also note that other factors should be considered when selecting materials for wetted pump parts, including the user’s experience, the manufacturer’s experience, the expected pump life (such as temporary or long-term use), whether or not it is intermittent or continuous duty, pumping hazardous or toxic liquids, and the condition of the liquid.
       For more information on wetted parts materials selection, refer to ANSI/HI 9.1-9.5 Pumps – General Guidelines for Types, Definitions, Application, Sound Measurement and Decontamination.
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