Piston Pump

Written by Matt Milbury. Posted in Pump

A piston pump, also known as a plunger pump, is one of the oldest styles of pumps that is still in use. Its design is very simple. A piston moves in and out of a cylinder. The fundamental difference between a plunger pump and a piston pump is in a plunger pump, the piston (or plunger) moves through a stationary packed seal and is pushed into the fluid. For piston pumps, the piston moving up and down contains the seal. For both types of pumps, the pistons' movement outward decreases the volume available in the cylinder increases, and fluid enters through the one-way inlet valve. As the piston moves inward, the volume available in the cylinder decreases, the pressure of the fluid increases. The fluid then is forced out through the outlet valve. As the piston moves inward, the volume increases, creating a void which is filled with fluid from the inlet valve.

Unlike a centrifugal pump, a power pump does not develop pressure; it only moves the fluid. Outlet pressure is determined by the downstream process piping or piping system. If a valve closes during operation or if flow somehow becomes blocked the flow will be delivered independent of the pressure in the discharge piping system. Also, unlike a centrifugal pump, a piston or plunger pump will not move on its curve due to pressure. The line will continue to pressure until the pressures exceed the design limits of the pump or piping and failure will occur. For this reason, all piston pumps must have discharge pressure relief devices to limit the pressure in the piping system and avoid pump or piping failure.

The pumping rate constantly varies from no flow (at the point at which the piston changes direction) to a maximum when the piston is approximately halfway through its stroke. The inconsistent flow can be reduced by allowing fluid on both sides of the piston to even out the rate on the pump outlet. This is known as a double acting piston pump. A single acting piston pump has fluid on one side of the piston.

  • Another way to even out flow, is by adding additional cylinders. The amount of fluid that is moved through the pump is dependent on many different variables.
  • Changing the speed of the pumpChanging the number & size of pistonsChanging the stroke length
  • Piston and plunger pumps are expensive, but they are extremely reliable and durable.

Negative Characteristics

  • Vibrations of pump or piping
  • Mechanical failures, wear, erosion, alignment
  • High horsepower requirements, high motor current, torsional oscillations
  • Temperature extremes, thermal cycling


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