Working principle of reciprocating pump
Introduction
Reciprocating pumps create and displace a volume of liquid by
the action of a reciprocating element.
The pump consists of cylinder, piston or plunger and drive arrangement for to
and fro motion of the piston. These pumps are called positive displacement pump
as they delivers volume of the fluid filled in the cylinder irrespective of the
delivery head and develops higher
pressure as compared to centrifugal pumps. However, liquid discharge pressure
is limited only by strength of structural parts. A pressure relief valve and a
discharge check valve are normally required
for reciprocating pumps.
Reciprocating pump is used in milk homogenizer in dairy industry in
order to develop high pressure. Reciprocating
pumps can be further
classified into two types, as given below.
i)
Piston Pumps
ii)
Diaphragm Pumps
Piston Pump
Hand pump is a simplest
form of piston pump used in villages
for lifting water from the tube well. Though,
these pumps are replaced by sub-mersible electrically operated pumps even in villages. The piston pump is
one of the most common reciprocating pumps for a broad range of applications
prior to the development of high speed centrifugal pumps. Reciprocating pumps
are used in low flow rate applications with very high pressure.
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Hand pump
Working of reciprocating pump
A single acting reciprocating pump having piston which moves forwards
and backwards in a close fitting cylinder. The
movement of piston in the
cylinder is obtained by connecting the piston rod to crank by means of connecting rod. The
crank is rotated by means of an electric motor. The suction and delivery valves
are suitably placed which are one way valves.
As the crank
rotates from A to C (θ = 0° to θ = 180°), the piston moves towards right in the cylinder. This creates partial vacuum
in the cylinder. As the pressure on the water surface is higher, the
liquid enters the cylinder through suction valve. The movement of piston from C
to A (θ = 180° to θ = 360°), closes the suction valve and opens delivery valve
and the liquid is forced in the delivery pipe.
Working and main parts
of reciprocating pump (single acting)
The reciprocating pumps are further classified as single
acting and double acting. In case of
single acting pump as shown in the pump delivers one effective discharge stoke
per one revolution of the crank (i.e. one suction stroke and one delivery
stroke). Double acting reciprocating pump has
suction valve and delivery valve on both the sides of the piston.
Working of double acting
piston pump
The single acting pump discharges water only on its
forward stroke while the double acting pump discharges on its return stroke as
well.
Capacity of reciprocating pump
The capacity (Q) of a single acting piston or plunger
pump is proportional to its displacement per unit time. The displacement is the calculated capacity
of the pump, assuming 100% hydraulic
efficiency, and is proportional to the cross sectional
area of the piston (A), the length of its stroke (L), the number of cylinders
(n), and the speed (N) of the pump (r.p.m. of the crank).
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It is obvious that the rate of discharge will be double neglecting the volume of piston rod in case of double acting pump.
The actual discharge under field conditions will be less
as compared to theoretical discharge. The difference between actual discharge
and theoretical discharge of the pump is called slip of a pump. It is expressed
as % slip which is given by:
The ratio (r) is shown to be (c+d)/d where d is the volume
displaced by the piston or plunger and c is the additional volume between the
discharge and suction valves. The smaller is this ratio, the better the
volumetric efficiency. Expressed mathematically as:
VE = 1 - (P * b * r) – S
Where, P is pressure, b is the liquid’s compressibility
factor, r is the volume ratio, and S is slip. The compressibility factor for
water is quite small but at pressures greater than 10,000 PSI it does become a
factor. Although there is no cylinder wall around the plunger at the bottom of
its stroke, it still displaces fluid equal to its own volume. The actual
capacity of the pump is given by:
Qact = Qth * VE
Power requirement for reciprocating pump
The work done by reciprocating pump is given
as under:
work done/second = (weight of water lifted/second) *(total height through which water is lifted)
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 downstroke, and vice versa. This
arrangement doubles the capacity of the duplex pump compared to a simplex pump
of comparable design.
Triplex pump
A positive-displacement reciprocating pump that is
configured with three plungers. Generally milk homogenizer are of triplex type.
Triplex pump
Diaphragm Pumps
Diaphragm pumps are reciprocating positive displacement pumps that employ a flexible membrane instead of a piston or plunger to displace the pumped fluid. They are truly self priming (can prime dry) and can run dry without damage. They operate via the same volumetric displacement principle described earlier.
Diaphragm pump
Were its operation any simpler, it would compete with
gravity. The upper portion of the figure shows the suction stroke. The handle
lifts the diaphragm creating a partial vacuum which closes the discharge valve while allowing liquid to enter
the pump chamber via the suction valve. During the discharge stroke the
diaphragm is pushed downward and the process is reversed.
You will note that, unlike pistons and plungers,
diaphragms do not require a sealing system
and therefore operate leak free. This feature does, however, preclude the
possibility of a double acting design.
If nearly continuous flow is required, a double-diaphragm or duplex pump is usually
employed. The figure below is a cross section of an air operated,
double diaphragm pump.
The double diaphragm pump utilizes a common suction and
discharge manifold teamed with two diaphragms rigidly connected by a shaft. The
pumped liquid resides in the outside chamber of each while compressed air is
routed to and from their inner chambers. In the figure, the right hand chamber
has just completed its suction stroke and, simultaneously, the left chamber
completed its discharge stroke. As would be expected, the suction check is open
so that liquid can flow into the right chamber and the discharge check of the
left chamber is open so that liquid can flow out. Except for the double chamber
configuration, its operation is just like the double acting piston pump seen
earlier. The difference, of course, resides within the inner chambers and the method in which the reciprocating motion is maintained. This is accomplished by an
air distribution valve that introduces compressed air to one diaphragm chamber
while exhausting it from the other. Upon completion of the stroke the valve
rotates 90 degrees and reciprocation occurs.
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