Positive Displacement Pump – Definition, Types, Working

Positive displacement pump (PDP) is a type of pump in which a moving fluid is captured in a cavity and then
discharges that fixed amount of fluid. The displacement of fluid takes place by
some parts like plunger, piston, diaphragm etc. some of these pumps have
expanding cavity at the suction side and a decreasing cavity at the discharge
side. The liquid is sucked at the inlet side when the cavity expands and
discharges it when the cavity decreases.
In order to understand it
more clearly, let’s take a syringe which has a piston inside it. When the
piston is drawn outward the cavity starts expanding and water starts to enter
into the syringe cylinder. As the piston is pressed inward the cavity keeps on
decreasing and the liquid inside the syringe experiences a force that makes the
water to escape out of the syringe. So what is the conclusion of the
illustration? We have seen in that the water is displaced from the syringe
during suction and discharge stroke of the piston.

Types 

Types of Positive Displacement pump
By considering the
mechanism used to move the fluid it is divided into three major types
1. Rotary Type Positive
Displacement
2. Reciprocating Type
Positive Displacement
3. Linear Type Positive
Displacement

1. Rotary-Type Positive
Displacement Pump

In this pump the fluid is
moved by the use of a rotary part. It is the rotation which displaces the fluid
from reservoir to the discharge pipe.
The common example of
these types of pumps are: internal gear, screw pump, flexible vane or sliding
vane, flexible impeller, circumferential pump, helical twisted roots etc.
The rotary type positive
displacement pumps can be classified again into three main types
1. Gear pumps:  In
this pump, the fluid is moved in between two rotating gears. The liquid is
pushed between these two gears as it rotates.
2. Screw pumps: these
pumps consist of two screw type rotor turning against each other. When the two
screw rotates it sucks the water from the inlet and pump it to the outlet.
3. Rotary vane pumps: It
is similar to scroll compressors. It consist of cylindrical rotor having vanes
on it which is encased in a similar (i.e. cylindrical type) shaped housing.
When it rotates the vanes on the rotor traps the fluid in between the rotor and
casing and discharges it through outlet.

2. Reciprocating Types
Positive Displacement Pump

In these pumps, there is a
reciprocating part (which moves backward and forward) to pump the water. The
reciprocating parts may be of plunger, piston or diaphragm type. It contains
valves, inlet valves and outlet valves. The inlet valves open and outlet valves
remain closed during suction of liquid. And inlet valves remain closed and
outlet valves gets open during discharge of the liquid.

Reciprocating pump

As shown in the diagram above,
as the piston moves to the right, the cavity expands and the water is sucked
into it. And when the plunger moves to the left, it pushes the liquid which
makes the discharger valve opens and water starts discharging through the
cavity.
  • Some reciprocating pump
    has an expanding cavity at inlet side and a decreasing cavity at the outlet
    side. The Liquid is sucked into the cavity as it expands and discharges when
    the cavity starts decreasing.
  • The reciprocating pump’s
    category ranges from simple (with one cylinder) to Quad (four) or even more.
    Most of their types have duplex (two) or triplex (three) cylinder.
  • It can powered by manually, by steam or air, or through a belt driven by an engine.
  • It is most commonly used in 19th century as boiler feed water pump. Now days it is used to pump highly viscous fluids like heavy oils and concrete, and also used in special applications where fluid is needed at low flow rates against high resistance.
They can be either single
acting type or double acting type.
1. Single Acting
Reciprocating pump:
In
this, suction takes place in one direction motion of the piston and discharge in
other direction.
2. Double Acting
Reciprocating pump:
 Suction and discharge takes place in both directions.
Typically reciprocating
pumps can be classified as
1. Plunger pumps: A
plunger is used for pumping water.
2. Piston: It has piston
for pumping fluid.
3. Diaphragm: It works
same as plunger pump But it has diaphragm for suction and discharge of liquid.

Linear Type Positive
Displacement Pump

In this, the fluid
displacement takes place linearly (i.e. in straight line). Rope pumps and chain
pumps are the examples of these types.

For Better Explanation About Types Watch the Video Given Below:



The List of pumps working
on the principle of positive displacement are:
  1. Rotary lobe pump
  2. Progressive cavity pumps
  3. Rotary gear pumps
  4. Piston pumps
  5. Diaphragm pumps
  6. Screw pumps
  7. Gear pumps
  8. Hydraulic pumps
  9. Rotary vane pumps
  10. Peristaltic pumps
  11. Rope pumps
  12. Flexible impeller pump
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For more about Pumps Visit: https://en.wikipedia.org/wiki/Pump

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