Why Pump Power is Higher than Expected

Maybe you actively monitor amperage and notice your trend lines are rising. Or perhaps your motor overheats and trips or it cannot generate enough torque to start. These are all signs that your pump is drawing more power than it should.

To diagnose what is causing the problem, the best place to start is with the definition of pump input power:

In other words, pump input power is a function of:

• flow rate (Q) in gallons per minute,
• total head (H) in feet,
• the specific gravity (s) of the liquid moving through the system, and
• the pump efficiency (η). 

Specific gravity is a relative density term compared to the density of water at standard temperature that is often used with US customary units. Therefore, if the liquid is water at ambient temperature the specific gravity will be 1.0 or very close to it.  The 3960 term is a constant used to convert units and results in power units of horsepower (hp).  

Since pumps are not 100 percent efficient, they will always require more horsepower than the system. So, if pump efficiency (η) is 50 percent, the motor will need to provide twice the hydraulic power.  If you need to calculate the power into the motor, then the motor efficiency would need to be included as well.

Knowing the efficiency of your pump and the variables in the equation tells you where to look for the answer to your power problem: 

• Greater head and flow than expected
• Higher liquid density (specific gravity)
• Lower pump efficiency than baseline 

Let’s hit them one by one.

Liquid properties (Specific gravity and viscosity). First it is important to understand and confirm the liquid properties before moving forward with additional troubleshooting steps. Higher specific gravity (lower temperature or different liquid composition), will increase the power proportionally.  For example, a specific gravity of 1.1 will require 10% more power than a specific gravity of 1.0. Higher liquid viscosity will result in a lower pump efficiency and corresponding higher power.  Similar to specific gravity viscosity could be higher if the liquid temperature is lower or the liquid composition has changed.  If your pump motor is overheating or tripping at night or in the early morning when it is cold outside, low temperatures may be increasing specific gravity and viscosity. To understand the effect of viscosity on performance, it can be found in ANSI/HI 9.6.7.  

Head and flow. System head and flow rate need to be compared to the design conditions and pump curve. Start by reviewing the expected design points, confirm the impeller diameter and find the pump curve supplied with the pump.  A flowmeter and pressure gauges on the inlet and outlet of the pump will be required to make a comparison to the pump curve flow and total head. If the data gathered results in a flow rate and head that matches a point on the pump curve, then you should review if the corresponding power point on the pump curve is more than the motor rating.  There are instances where the motor may not be rated for full coverage of the pump curve. If you are operating with a variable frequency drive, it is important to confirm the motor speed matches the pump curve speed. 

Pump efficiency. If the flow and head point is below the pump curve or on the pump curve and the power is high, then there may be a hydraulic or mechanical problem with the pump.  Internal wear could be resulting in volumetric losses and reduced efficiency.  Mechanical losses could be due to higher-than-expected losses in the seals, packing or bearings, or due to mechanical contact of the rotating components.  Mechanical contact could be due to distortion of components, foreign materials in the pump, or wear.  If your pump was recently rebuilt, check the alignment and nozzle loads. Nozzle loads, for example, could create enough of a load to distort the pump casing so that it makes contact with the impeller and also cause coupling misalignment. For smaller pumps, check that seal friction or packing friction, which prevents leaking, is not set so tightly that that its causing excessive friction. 

You can learn more about trouble shooting pump power problems in ANSI/HI 14.4 Rotodynamic Pumps for Installation, Operation, and Maintenance. To explore ways to understand the effects of viscosity, see ANSI/HI 9.6.7 Effects of Liquid Viscosity on Rotodynamic (Centrifugal and Vertical) Pump Performance.

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