Three Keys to Getting the Most from Your Reciprocating Pump
Reciprocating pumps date back more than 2,000 years, but it takes skill and knowledge to operate them efficiently. Unlike other pumps that provide steady fluid pressure, the stroke of a reciprocating pump produces a pulsating flow with a peak up to three times greater than its average flow.
Reciprocating pumps date back more than 2,000 years, but it takes skill and knowledge to operate them efficiently. Unlike other pumps that provide steady fluid pressure, the stroke of a reciprocating pump produces a pulsating flow with a peak up to three times greater than its average flow.
For that reason, it is no surprise that the most common root cause of reciprocating pump performance problems (based on technical support calls to pump manufacturers) involves suction piping that cannot supply the peak flow demands of the pump. Here are some tips on how to attack piping and other issues surrounding reciprocating pumps.
Beware of vapor pressure. Vapor pressure is the hidden enemy of reciprocating pumps. To understand why, consider water. Water boils at 212°F at atmospheric pressure. But in Denver, the Mile High City, where atmospheric pressure is lower, it boils at 203°F. The lower the pressure, the lower the temperature at which a liquid will vaporize. The downstroke of a reciprocating pump that occurs after it expels a fluid creates a negative pressure to recharge the pump. Depending on its temperature and composition of the fluid, this negative pressure could pull gasses out of the fluid. Off-gassing may prevent the pump chamber from sucking in all the fluid it needs. It also leads to the formation of bubbles that eventually implode, causing vibration and that erode in the pump and piping.
Know your NIPIPA to control off-gassing. The pulsating flow in the system suction piping creates a pulse pressure that typically subtracts from system suction pressure. This is measured by Net Positive Inlet Pressure Available, or NPIPA, which is the absolute pressure at the inlet (including acceleration pressure as a negative) of the pump above the absolute vapor pressure. Long lengths of suction pipe, elbows, tees, strainers, valves, and other accessories can decrease NPIPA to unacceptable levels and lead to poor performance and off-gassing. Consider, for example, a pump operating at 150 strokes per minute and supplying 71 gallons per hour. A 10-foot length of half-inch piping could subtract 10 psi from the supply pressure. To avoid such sharp falloffs, increase pipe diameter and limit the length of pipe, elbows, tees, strainers, valves, and other accessories installed in the suction piping wherever possible.
Design with negative pressure in mind. The pulsating flow in the system suction piping creates a pulse pressure that typically subtracts from system suction pressure. One way to avoid a large pressure drop is to size pipes properly. The longer and smaller the pipe diameter, the greater the pressure drop caused by the acceleration of fluid surging through it. On the other hand, shorter, wider pipes will have less impact on pulsation pressure and produce less of a pressure drop. The rule of thumb for shorter pipe runs with few restrictions is to increase one pipe size above the suction connection. For long piping runs with multiple bends, elbows, restrictions, and/or higher viscosity liquids require larger size piping.
To learn more about proper operation of reciprocating pumps, check out these Hydraulic Institute standards.
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