Positive displacement pump

A positive displacement pump makes a fluid moved by trapping a fixed amount of fluid and forcing or displacing that trapped volume into a discharge system.

Some positive displacement pumps used an expanding cavity on the suction side and decreasing cavity on the discharge side.

Positive displacement pump types

Types of positive displacement pump

 A positive displacement pump can be further classified according to the mechanism used to move the fluid.

There are three types.

1. Rotary type positive displacement pump.

2. Reciprocating type positive displacement pump.

3. Linear type positive displacement pump.

1.Rotary type positive displacement pump :

Nature of positive displacement pump requires the close clearance between rotaring pump and the outer edge making it rotate at a slow and steady speed.

If rotary pumps are operated at high speeds the fluid cause erosion, which eventually causes enlarged clearance that liquid can pass through, which reduces efficiency.

Rotary displacement pump falls into three main types.

1. Gear pump 

2. Screw pump 

3. Rotary vane pump 

2.Reciprocating type positive displacement pump

Reciprocating pumps move the fluid using one or more oscillating pistons, plunger or membranes (diaphragms), while valves restrict fluid motion to the desired direction.

In order to suction take place, the pump must pull the plunger in an outward motion to decrease pressure in the chamber. Once the plunger pushes back it will increase the pressure chamber and the inward pressure of plunger will then open the discharge valve and release the fluid into delivery pipe at a high velocity.

Typical reciprocating pumps are.

1. Plunger pump 

2. Diaphragm pump

3. Piston pump

3. Linear type positive displacement pump

In linear type positive displacement pump the fluid displacement takes place linearly (in straight line).

Typical positive displacement pumps are

1. Rope pump 

2. Chain pump