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Positive Displacement Pumps - Piston and Plunger Pumps

Updated on October 13, 2014

Piston Pump Types

Positive Displacement Pumps

Unlike Centrifugal Pumps, Positive Displacement Pumps are used where the desired flow is low and the pressure requirements are high. A Positive Displacement pump compresses the liquid that is sucked in and discharges them at high pressure according to the designed parameters.


Types of Positive Displacement Pumps

Positive Displacement Pumps are broadly classified into two categories based on their operating principles.

1) Reciprocating Pumps and

2) Rotary Pumps

Reciprocating Pumps

Reciprocating Pumps use the reciprocating action of a Piston or a Plunger to pump fluids. The priciple is almost like how a petrol engine works. There will be hollow cylinder where a Piston or Plunger will be moving to and fro and create suction and discharge strokes to discharge the fluid.

Types of Reciprocating Pumps

1) Piston Pumps &

2) Plunger Pumps.

Piston Pumps

Construction:

One or more Pistons move inside a Cylinder. Piston is a solid mass of Rod whose construction might vary from MS to SS depending upon the requirements. Surface of the Piston will be finely polished to minimize friction. The Cylinder through which the Piston moves is also has a finely polished surface to reduce friction.

The length of the Cylinder will be more than the Piston so that the complete movement of piston will allow some space remaining either in the suction side or at the discharge side. On either side of the Cylinder, there will be provision for Suction and Discharge. The opening will have Valve plates fixed over them that allow the Fluid to enter the Cylinder and get discharged out.

The Valve Plates act as non return Valves as well. Suction Valve Plates allow fluid only to enter and the Discharge Valve Plates allow Fluid only to exit the Cylinder. The plates are spring loaded. The tension of the spring is so designed that they will allow the fluid to enter or exit the cylinder as per our requirement.

The entire Cylinder and the Piston assembly are connected to the drive which normally is a Motor with a Crankshaft arrangement in between. Rotary motion produced by the Motor is converted into to and fro motion by the Crank Shaft.

There will be a provision for altering the rate of to and fro action by way of metering in most cases. Depending upon the required flow the metering mechanism will increase the action rate or decrease it and by altering the sped of the Motor. This facility may not be available in pumps that do not have an adjustable flow rate.

Piston rings and sealing mechanism will be in place between the piston and the cylinder so that there is no leak when the piston is moving with liquid inside.



Operating Principle of a Piston Pump

When the Motor is started, the piston first moves forward inside the cylinder. This movement disposes off any content that is already present in the cylinder. Then when the Piston returns back, it creates an empty space where there will be a vacuum that will pull the Valve Plate and through the opening created the fluid will rush into the Cylinder.

When the Piston again gets back into it's previous position, the space will no longer be empty but will be filled up with the fluid that needs to be pumped. As irrespective of whether there is anything in that space or not, Piston will move as per the design. This movement compresses the fluid and as the pressure increases above the set pressure of the Valve Plate in the discharge side, the Plate opens up and allows the Fluid to gush out through that opening with high pressure.

The same process is repeated and for every suction and discharge stroke a particular quantity of Fluid flows out.

Piston Pumps - Operating Procedure

  • Ensure that the discharge Valve is kept open. Do not try to operated the pump with the discharge valve closed as this is a positive displacement pump, it will blast out any restriction and might cause damage to the pump or the system or Manpower.
  • Open the Suction Valve.
  • Start the Pump.
  • Check for any excessive vibration or sound while the pump is running.
  • Check if all lubrication is done properly at regular intervals as suggested by the Pump Supplier.

Features and Application of Piston Pumps

  • Piston Pumps can create high pressures.
  • Flow created will be less when compared to a Centrifugal Pump.
  • Piston pumps can be used in places where high pressure is required and less flow is sufficient.
  • Flow adjustment is possible only when a metering mechanism is attached to the Pump.

Maintenance

  • As wear and tear is more in a Piston Pump, maintenance cost will be a higher than the Centrifugal pump.
  • Piston rings are subjected to extreme friction and pressure and so damage to the piston rings is inevitable. But by proper alignment and maintenance the failure rate can be reduced.
  • Suction and discharge plates are also likely to get damaged often.
  • If there is a reduction in pressure produced by the pump it is advisable to thoroughly check the pump for damages in Piston rings and Valve Plates.

Plunger Pump

  • Plunger Pump is almost similar to the Piston Pumps. But only difference is that instead of a Piston there will be Plunger moving forward and backward inside the cylinder.
  • Plunger pumps are preferred where more pressures are required. More than 150 Bar is also achievable with the Plunger Pump.

Advantages

  • Easy to operate.
  • No priming is required.
  • Cavitation almost never happens.
  • Precise flow rate and pressure can be achieved.

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