What is a Positive Displacement Pump? | Types of PD Pumps

A pump is a mechanical machine used to transport liquid from one area to other. Pumps have multiple types, and the positive displacement pump is the most famous one. A positive displacement pump is also known as a PD pump. A positive displacement pump transfers fluid by using a reciprocating component such as piston, diaphragm, rotor, or plunger. This article explains the working, types, and applications of positive displacement pump.

What is a positive displacement pump?

A positive displacement pump pumps a liquid by frequently trapping a constant volume of the liquid and mechanically directing it through the system. The PD pump has cyclic pumping movement. This type of pump uses a diaphragm, roller, gears, screw, plunger, or piston to the fluid. 

 Working of Positive displacement Pump

A positive displacement pump trapeses a specific amount of the fluid and uses a piston, screw, or gear to pressurize the fluid. When the fluid is pressurized according to the requirements, it discharges through the outlet valve and delivers to the desired area.

Positive displacement pump working

What are the types of Positive displacement Pumps?

The positive displacement pump has the following major and most common types:

  1. Reciprocating Pump
  2. Rotary Pump

1) Reciprocating Pump

A reciprocating pump is a type of positive displacement pump in which a diaphragm, piston, or plunger moves forward and backward to transfer the fluid. The piston, diaphragm, or plunger moves continuously reciprocates inside the compression chamber. Therefore, it is known as a reciprocating pump. 

When pumping the fluid, the cyclic operation of the reciprocating pump generates pulses, which accelerate the fluid during the compression stroke and decelerate it during the suction stroke. Due to the reciprocating operation of the pump, it generates vibrations that can damage equipment. These pulses may also be reduced by using two diaphragms, pistons, or pistons (one for the suction stroke and the other is for the compression stroke).  

The reciprocating pumps have the following major types:

  1. Diaphragm Pump
  2. Piston Pump
  3. Plunger Pump

i) Piston Pump

A reciprocating pump in which a reciprocating piston is used to pump the fluid is known as a piston pump. The piston pumps are the most commonly used type of reciprocating positive displacement pumps.

This pump has a piston. The piston’s first stroke generates a cavity inside the pump. Due to the generation of a cavity, a pressure difference produces between the pump’s internal and external sides. When this pressure produces, the suction valve opens, and discharge valve shuts, and the desired amount of liquid enters the pump chamber (suction process).

As the liquid enters up to a specific level, the suction and discharge valves close, and the piston starts to compress the liquid (compression process). During this pressurization phase, the piston increases the liquid pressure by reducing its volume. After pressurization, the discharge valve opens, and liquid transfers to the desired area.

A water hand pump is a most common example of a piston pump. The piston pumps also have two types:

  1. Single-acting piston pump: In this pump, only a single side of the piston uses to suck and compress the fluid.
  2. Double-acting piston pump: In this pump, both sides of the piston are used. One side uses to suck the fluid while the other uses to compress it.

ii) Plunger Pump

A positive displacement pump that uses a plunger to transfer the liquid is known as a plunger pump. This pump works in the same way as a piston pump.

The main difference between a plunger pump and a piston pump is that, in a plunger pump, the volume of the fluid depends on the size of the plunger, while in a piston pump, it is dependent on the cylinder size.

A seal is used to surround the plunger to stop the leakage and to continue proper pumping. The plunger pump seal is easier to maintain than the piston pump seal. This is because the piston’s continuous upward and downward movement in a cylinder may loose or damage the seal. 

iii) Diaphragm Pump

A reciprocating pump that uses a flexible diaphragm or membrane instead of a plunger or piston to transfer fluid is called a diaphragm pump. As the membrane expands, the fluid is introduced into the pumping chamber. As the membrane compresses the fluid, the fluid pressure increases, and it discharges.

Read Also: Working of Multistage Pump

2) Rotary positive displacement pump

A positive displacement pump that transfers fluid by using a rotating element (such as screws or gears) is known as a rotary pump. This pump has two or more gears or screws instead of pistons or plungers.

As the gears or screws rotate, they generate a cavity at the suction side of the pump so that the fluid can enter from the storage tank to the pump. As the fluid enters, it is trapped between the teethes of the gears or screws. The rotation of the gear or screw increases the pressure of the fluid and pushes it toward the outlet valve.

The rotary positive displacement pump has the following major types:

  1. Gear Pump
  2. Screw Pump

i) Gear Pump

A rotary pump that uses gears to pump fluid is called a gear pump. The gear pump most commonly has two gears. Out of these two gears, one is known as a drive gear, and the second is known as a driven gear. The gear pumps have the following two main types:

  1. External gear pump
  2. Internal gear pump
i) External Gear Pump

This type of positive displacement pump has two gears (driver and driven gears). These gears mesh with each other. An electric motor or generator uses to drive the driver gear.

When the driver gear moves the driven gear, a vacuum produces inside the pump, and fluid flows from the storage tank to the pump. When the fluid comes into the pump, it is surrounded by the driver and driven gears. With the movement of the gears, the fluid flows from the inlet side to the outlet side and discharges.

In the gear pump, the fluid can’t go back from the center to the suction side due to the interlocked gears.

The main difference between the external gear pump and the internal gear pump is that, in an external gear pump, both the gears have the same size while the internal gear pump doesn’t have the same size gears.

ii) Internal Gear Pump

This positive displacement pump also works in the same way as the external gear pump. However, this pump contains gears of different sizes.

The gears of the gear pump need proper lubrication for an efficient operation. These gears are lubricated by the working fluid. Therefore, if you run dry to your gear pump, it may be failed or be damaged. These pumps are best suitable for pumping high viscosity fluids such as oils.

2) Screw Pump

A rotary pump that uses rotary screws instead of gears to pump fluid is called a screw pump. The screw pump also has two screws. These pumps work on the same principle as the gear pump.

Difference between the Centrifugal Pump and Positive Displacement Pump

Centrifugal PumpPositive Displacement Pump
It doesn’t work efficiently at high viscosity.The fluid viscosity doesn’t affect it.
Flow rate changes with the pressure change.Flow rate is not affected by the pressure change.
These pumps require low maintenance.They need high maintenance.
It requires priming.It doesn’t need priming.
It has a low installation cost.It has a high installation cost.

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