Reciprocating Pumps Basics

Engineers are generally very familiar with operating principles, performance curves and selection criteria for centrifugal pumps but the training and knowledge around the operating principles of reciprocating pumps is not as common.

Unlike Centrifugal pumps, Reciprocating Pumps have a much stronger interaction with the system within which they sit. This is due to the pressure pulsations they generate. If we think about any linear reciprocating motion of a piston, at some point the velocity of the piston is zero as it changes direction at the top and bottom of its stroke.

This means that the pressure pulsations are much larger in a reciprocating machine than in a centrifugal machine.

Centrifugal pump engineers are very familiar with selecting pumps from a performance range using performance curves that map their operating range. A reciprocating pump’s flow is determined by the volume of its cylinders and the rate at which the piston can move fluid through into the system. The pump is a fixed volume machine.

If you want to change the flow of a reciprocating pump the easiest way to do this to change the rate at which the pistons move the liquid through the pumps valves into the system. As reciprocating pumps are fixed volume machines they do not have a performance curve in the way a centrifugal pump do (Fig 1). Closing the discharge valve increases the pressure but does not change the flow at all.

Reciprocating pumps run at a much lower speed than centrifugal pumps. By increasing the speed there can be compromises to the packing and sealing life of the machine. Balancing speed and plunger size is the key to reliability plunger and sealing life.

Fig 1 Reciprocating Pump Fixed Volume. Flow is determined by Stroke, Area and Speed

In general a reciprocating pump is much more efficient that a centrifugal pump. Reciprocating machines are generally applied on low flow/high head service. Centrifugal pumps can have efficiencies as low as 20% on low flow high head service. A reciprocating machine can have efficiencies over 90% on the same service. Considerable power savings can be made by using reciprocating machines.

Many users of reciprocating pumps are considering replacing their machines with diaphragm pumps due to environmental emission regulations. Whilst reciprocating machines have a packing system that leaks it is possible to both mitigate and collect this leakage allowing the pump to meet environmental regulations. Modifying stuffing box arrangements is much less expensive than replacing the machine.

Another significant difference between the performance of centrifugal pumps and reciprocating pumps is how NPSHR is determined.

With a centrifugal pump the NPSHR is determined when the head has dropped 3% under reducing suction pressure.

For a reciprocating pump 3% capacity drop is the criteria for defining NPSHR (Fig 2). NPSHA is reduced until the capacity drop has exceeded 3 percent. The NPSH that was available at the 3 percent reduction is established, by definition, as the NPSHR. There a number of components within the machine that can be modified to improve NPSHR. This can be a much simpler modification than that required by a centrifugal machine where complete impeller design is the most common option for improving the machine NPSHR.

It is becoming more important for engineers to understand the performance of reciprocating pumps. There is a temptation to apply the knowledge acquired on centrifugal machines and

apply this to the reciprocating word of pumps. This can lead to real difficulties and reliability.

Fig 2 NPSHR of a Reciprocating Pump based on 3% capacity reduction problems.