POSITIVE DISPLACEMENT PUMPS - Positive displacement pump transmits water from one place to another. A positive displacement pump has an expanding cavity on the suction side and an enclosing cavity on the discharging side. In the pump, the mechanical devices which displace the liquid are such as a plunger, pistons, gears, cams, etc. These devices create a vacuum. In every cycle of operation, the volume of the liquid being pumped remains constant. The pump provides an approximately constant volume of liquid at a fixed speed, despite changes in the counter pressure or head.

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Positive Displacement Pump

its definition, types and working

engineeringinsider

Whenever we hear the word “pump”, we often imagine a device transmitting water or any other liquid from a source to the target place.

Positive displacement pump is also a similar device that transmits water from one place to another.

A positive displacement pump has an expanding cavity on the suction side and an enclosing cavity on the discharging side.

In the pump, the mechanical devices which displace the liquid are such as a plunger, pistons, gears, cams, etc. These devices create a vacuum.

In every cycle of operation, the volume of the liquid being pumped remains constant.

The pump provides an approximately constant volume of liquid at a fixed speed, despite changes in the counter pressure or head.

Types of Positive Displacement Pump

1.        Rotary Pumps

2.        Reciprocating Pumps

Rotary Pumps -

In this pump, the movement of liquid is obtained by the mechanical action.

The rotation of a sealed mechanism of intermeshing rotating parts, such as gears or cams, within the pump casing produce this mechanical action.

The gears or the cams rotate in opposite directions.

The water enters the pump through the suction side and is trapped in these rotating parts and is, then discharged, with force, through the discharging side.

This kind of pump is helpful where discharge is not greater than 40 liters per second.

There are three types of Rotary Pumps:

§     Gear Pumps: It is the simplest type of rotary pumps that consists of two gears. Rotating gears trap the water and then eventually discharge the same as the teeth of the gears mesh and go around again.

§     Screw Pumps: It consists of two screws that rotate in opposite directions. The screws are mounted on shafts that run parallel to each other. The turning of the screws and the shafts draws the water and then discharge the same water at a constant speed.

§     Moving Vane Pumps: This pump consists of a cylindrical rotor enclosed in a similarly shaped casing. With the movement of the rotor, water flows in and then flows out.

Reciprocating Pumps:

This type of pump consists of a piston or a plunger that acts as a rotating part of the pump.

With the intake stroke, the suction or the inlet valve opens up and allows water to come inside.

During this, the outlet or the discharging valve remains closed. 

During the discharging stroke, the outlet valve opens up, thus allowing the water to flow out while the inlet valve is closed.

The reciprocating valve can be either single or double acting:

§     Single Acting Reciprocating Pump: In this pump, only forward stroke of the piston discharges

§     Double Acting Reciprocating Pump: In this pump, both the forward stroke and the return stroke of the piston discharge the water.

 

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