Positive Displacement Pumps - Basic Principle

POSITIVE DISPLACEMENT PUMPS

PUMP
Rotodynamic

Positive displacement

Centrifugal
Rotary

Reciprocating

Axial flow

Mixed flow
Gear

Piston

Lobe

Diaphragm

Turbine

Sliding Vane

Screw

Plunger


Positive displacement pumps physically entrap a quantity of
liquid at the suction of the pump and push that quantity out
the discharge of the pump.
A definite volume of liquid is delivered for each cycle of
pump operation.
The positive displacement pump delivers liquid in separate
volumes with no delivery in between,
Positive displacement pumps can be grouped into three
basic categories based on their design and operation. The
three groups are reciprocating pumps, rotary pumps, and
diaphragm pumps.


Principle of Operation:
Check valves in the suction and discharge ports allow flow
in only one direction.

Section
Discharge


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.


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.


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 pumps
Reciprocating positive displacement pumps are generally
categorized in four ways:
1. Direct-acting or Indirect-acting.
2. Simplex or Duplex .
3. Single-acting or Double-acting .
4. Power pumps.


Direct-Acting and Indirect-Acting Pumps:
Direct-acting pumps have a
plunger on the liquid (pump) end
that is directly driven by the
pump rod (also the piston rod or
extension thereof) and carries
the piston of the power end.


Direct-Acting and Indirect-Acting Pumps:
Indirect-acting pumps are
driven by means of a beam
or linkage connected to and
actuated by the power
piston rod of a separate
reciprocating engine.


Simplex and Duplex Pumps:
A simplex pump, sometimes referred to as a single pump, is
a pump having a single liquid (pump) cylinder.
A duplex pump is the equivalent of two simplex pumps
placed side by side on the same foundation.


The driving of the pistons of a duplex pump is arranged in
such a manner that when one piston is on its upstroke the
other piston is on its down stroke, and vice versa. This
arrangement doubles the capacity of the duplex pump
compared to a simplex pump of comparable design.


Single-Acting and Double-Acting Pumps:
A single-acting pump is one that takes a suction, filling the
pump cylinder on the stroke in only one direction, called the
suction stroke, and then forces the liquid out of the cylinder
on the return stroke, called the discharge stroke.
A double-acting pump is one that, as it fills one end of the
liquid cylinder, is discharging liquid from the other end of the
cylinder.
On the return stroke, the end of the cylinder just emptied is
filled, and the end just filled is emptied.


Single-Acting and Double-Acting Pumps:
One possible arrangement for single-acting and doubleacting pumps is shown :


Power Pumps:
Power pumps convert rotary motion to low speed
reciprocating motion by reduction gearing, a crankshaft,
connecting rods and crossheads.
Plungers or pistons are driven by the crosshead drives. Rod
and piston construction, similar to duplex double-acting
steam pumps, is used by the liquid ends of the low
pressure, higher capacity units.


Power Pumps:
The higher pressure units are normally single-acting
plungers, and usually employ three (triplex) plungers. Three
or more plungers substantially reduce flow pulsations
relative to simplex and even duplex pumps.
Power pumps typically have high efficiency and are capable
of developing very high pressures. They can be driven by
either electric motors or turbines.


Power Pumps:
They are relatively expensive pumps and can rarely be
justified on the basis of efficiency over centrifugal pumps.
However, they are frequently justified over steam
reciprocating pumps where continuous duty service is
needed due to the high steam requirements of direct-acting
steam pumps.


Power Pumps:
In general, the effective flow rate of reciprocating pumps
decreases as the viscosity of the fluid being pumped
increases because the speed of the pump must be
reduced.
In contrast to centrifugal pumps, the differential pressure
generated by reciprocating pumps is independent of fluid
density. It is dependent entirely on the amount of force
exerted on the piston.


Piston/Plunger Pump:
A tight-fitting piston in a closed cylinder or a loose-fitting
plunger acting as a displacer are familiar versions of the
common reciprocating pump.


Piston/plunger pumps have
the following characteristics:
• Capable of almost any
pressure, and of large flow
capacity.
• Capable of almost any
pressure, and of large flow
capacity.


Piston/plunger pumps have
the following characteristics:
•

•

NPSH requirements for these
pumps are more complex than
for rotary or kinetic pumps due
to the pulsed nature of the
suction.
Are expensive in large sizes.
• Easily controlled by stroke
adjustment or variable speed.


Advantages include the
following:
•

The ability to develop high
pressures in a single stage.
•

•

High reliability.

The ability to develop high
pressures in a single stage.


Disadvantages include the
following:
•

The necessity of slow speed
operation

•

The necessity of slow speed
operation


Diaphragm Pump:
Fluid is transferred by the pressure of a diaphragm that
flexes to form a cavity that is filled by liquid.
A diaphragm pump has the
following characteristics:
•Transfers virtually any liquid.
•Designs can handle high
temperatures.
•Designs can handle high
temperatures.


Diaphragm Pump:
•Is infinitely adjustable in capacity
and discharge pressure by
regulating the movement of the
diaphragm.
•Can be flexed by either an air
supply or a reciprocating plunger.
•Is used for pumping chemicals,
glue, ink, solvents, fat, grease, and
dirty water.


Diaphragm Pump:
•Is limited to low flow and head
application due to the design of the
flexible diaphragm.

PUMP
Rotodynamic


Centrifugal
Rotary

Reciprocating

Axial flow

Mixed flow
Gear

Piston

Lobe

Diaphragm

Turbine

Sliding Vane

Screw

Plunger
Blow case
pump


Blow case pump:
This is a special configuration
of a positive displacement
pump .
It consists of two pressure
chambers that are alternately
filled with liquid.
When a chamber is filled, air or
steam is forced into the
chamber.


Blow case pump:
This causes the contents to be
discharged into the system.
The two chambers alternate in
this action, resulting in a fairly
constant discharge.
It is popular for pumping hot
condensate.
?Why
because there is no heat loss, and
flashing liquid can be transferred.

PUMP
Rotodynamic


Centrifugal
Rotary

Reciprocating

Axial flow

Mixed flow
Gear

Piston

Lobe

Diaphragm

Turbine

Sliding Vane

Screw

Plunger
Blow case
pump
Open-screw
pump


Open-screw pump:
This is an example of a pump
configuration that does not
conform to the classical forms
discussed in the preceding
sections.
An open-screw pump consists
of a U-shaped channel into
which a rotating screw fits
tightly (minimal clearance).


Open-screw pump:
Modern forms of this pump are
usually quite large.
This pump is used extensively
in waste water plants for
moving contaminated water,
and in irrigation channels for
lifting large volumes of water.


Open-screw pump:
An open-screw pump is well
suited for this purpose as there
is little chance of down time.
The large sizes with closely
fitted screws are reasonably
efficient.


Open-screw pump:
One version surrounds the
screw within a large tube and
the whole assembly is then
rotated.
All bearings are thus outside of
the liquid and there is no liquid
leakage.

Positive Displacement Pumps - Basic Principle

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Positive Displacement Pumps - Basic Principle