POSITIVE DISPLACEMENT PUMP AND ITS WORKING PRINCIPLE

Pumps and Wells

1. POSITIVE DISPLACEMENT PUMP AND ITS WORKING PRINCIPLE

Pump:
A pump is a device used to raise, compress, or transfer fluids. Pumps are used to move any substance which flows
or which can be made to flow. It is a device that adds energy to the fluid to enable it to move from one point to
another.
Positive Displacement Pump:
A positive displacement pump is one in which a definite constant volume of liquid is delivered for each cycle
regardless of the resistance to flow offered by the system of pump operation.
Classification:

Figure 1: Classification of PD pumps

Working Principle
At first, check valves in the suction and discharge ports allow flow in only one direction.



Then, during the discharge stroke, the piston moves to the right, seating the check valve in the suction line and
opening the check valve in the discharge line.

2. After that, during the suction stroke, the piston moves to the left, causing the check valve in the suction line
between the reservoir and the pump cylinder to open and admit water from the reservoir.

Finally, the volume of liquid moved by the pump in one cycle (one suction stroke and one discharge stroke) is
equal to the change in the liquid volume of the cylinder as the piston moves from its farthest left position to its
farthest right position.

Reciprocating Pump: It is based on based on two stroke principles: i) High pressure, high efficiency and
ii) Self-priming. Used mainly for handling slurries in plant processes and pipeline applications.
1.Piston Pump
2.Diaphragm Pump
3.Plunger Pump
Rotary Pump: Rotary pumps operate on the principle that a rotating vane, screw, or gear traps the liquid in
the suction side of the pump casing and forces it to the discharge side of the casing.
1.Gear Pumps
2.Lobe Pumps
3.Screw Pumps
4.Vane Pumps
Advantages of Rotary Pumps:
 They can deliver liquid to high pressures.
 Self - priming.
 Give a relatively smooth output, (especially at high speed).
 Positive Acting.
 Can pump viscous liquids.
Disadvantages of Rotary Pumps:
 More expensive than centrifugal pumps.
 Should not be used for fluids containing suspended solids.
 Excessive wear if not pumping viscous material.
 Must never be used with the discharge closed.