A reciprocating power pump, as depicted in Figure 1, is a displacement machine. It has characteristics that are different than a centrifugal pump. Therefore, the system required for a displacement pump is different than that required for a centrifugal pump, and the operating procedures are also different. For instance, it is common practice to start a centrifugal pump against a closed discharge valve. Starting a displacement pump against a closed discharge valve can damage it. Preferably, a displacement pump should start against negligible discharge pressure. Starting a centrifugal against negligible pressure can damage it.
Figure 1. A triplex power pump with springs on both the suction and discharge valves and a threaded connection between the plunger and crosshead extension
Therefore, to properly select, apply and operate a power pump requires knowledge of its unique characteristics. Some of these characteristics will be presented by stating common myths, misconceptions or misunderstandings and hopefully dispel some of them in the following list. Since some are not really full-blown myths, just misconceptions or misunderstandings, they will be called “MM.”
Power pumps are self priming.As was learned from operators of pressurized-water nuclear power plants, when a power pump ingests a slug of gas while running, one or more pumping chambers can become “vapor-locked” and cannot expel the gas and regain prime. It is necessary to reduce the pump discharge pressure to near (or below) suction pressure to allow the pump to reprime, and if the pump is running at a high speed with the necessary strong valve springs, priming may be more difficult.
If the first power pump chamber becomes primed, the remaining chambers will automatically become primed.If this were a multistage centrifugal pump, the statement would be true because all impellers are in series. However, in a multiplex power pump, the plungers (pumping chambers) are in parallel, so one chamber can become primed while the other chambers remain “vapor-locked.”
To prime a power pump, it is necessary to disassemble the liquid end.A project engineer once told me that, when starting a new processing plant containing new power pumps, to prime each pump, he would remove each discharge valve cover, remove each discharge valve, fill each pumping chamber with liquid and then reassemble the pump. He was pleased to learn that this laborious process was unnecessary. If the design of the system allows the operator to start the pump against negligible discharge pressure, most power pumps will prime all pumping chambers. Disassembly is not required.
If the pump is driven by a variable speed driver, a bypass line is not required.As discussed above, priming a power pump requires reducing the discharge pressure to near suction pressure, and this is typically accomplished by opening a valve in a bypass line, which connects back to the suction vessel. Such a line is illustrated in Figure 2. A variable speed driver does not eliminate the need for a bypass line.
Figure 2. The system for a power pump should include a bypass line for startup and capacity control.
The motor driver for a power pump needs to have a high starting torque.This misconception arises from the difficulty experienced when starting a power pump against a high discharge pressure. For many reasons, the pump should be started against negligible discharge pressure. The requirement for a high starting torque driver is thus eliminated.
A power pump can be run backward satisfactorily.Although the liquid end will pump the same regardless of the direction of the crankshaft rotation, running a power pump backward can result in a hot power end, power end knocking, reduced lubrication to the crossheads and bearings, and shorter packing life because of the resulting oscillations of the plungers.
Small, high-speed pumps produce lower pulsations than large, low-speed pumps.It has been stated that a smaller pump, running faster, for the same capacity, will produce less pulsation in the suction and discharge piping. That is not true because, as seen in Figure 3, the liquid velocity variation produced by a triplex pump, for example, is typically 25 percent of the average velocity, whether the pump is large or small, running fast or slow. The acceleration of the liquid in the piping, being the rate of change of velocity, is therefore larger for a small, high-speed pump than for a large, low-speed pump when both pumps have the same capacity and the same size piping.
Figure 3. The velocity variations in both suction and discharge piping for the typical triplex pump are independent of pump size and speed.
All power pumps require pulsation dampeners.There are installations, with triplex and quintuplex pumps, that operate satisfactorily without pulsation dampeners. These installations are usually characterized by low-speed pumps and short, large-diameter suction and discharge piping.
A discharge pressure below the pump rated pressure indicates a problem with the pump.Unlike with a centrifugal pump, the discharge pressure of a displacement pump is established by the system. If there is low discharge pressure with a power pump, the cause is probably in the discharge system.