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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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