Reciprocating Pump

Classification of reciprocating pump

(1) According to the water being in contact with one side or both sides of the piston:

          (a)Single acting pump          (b)Double acting pump(2) According to number of cylinder:            (a) Single cylinder pump            (b) Double cylinder pump             (c)Triple cylinder pump.

Reciprocating pumpThe reciprocating pump is a positive displacement

pump and consists of a cylinder, a piston a suction valve, a delivery valve, a suction pipe, a delivery pipe and crank and connecting rod mechanism operated by a power source e.g. steam engine, I.C. Engine or an electric motor.

Positive Displacement Pumps, unlike a centrifugal pumps, will produce the same flow at a given speed (RPM) no matter the discharge pressure.

A Positive Displacement Pump must not be operated against a closed valve on the discharge side of the pump because it will continue to produce flow until the pressure in the discharge line are increased until the line bursts or the pump is severely damaged

Working principleIn reciprocating pumps the mechanical action causes the

fluid to move using one or more oscillating pistons, plungers etc.

During the suction stroke the piston moves left thus creating vacuum in the Cylinder. This vacuum causes the suction valve to open and water enters the Cylinder.

During the delivery stroke the piston moves towards right. This increasing pressure in the cylinder causes the suction valve to close and delivery to open and water is forced in the delivery pipe.

Air vesselAir vessel is a closed chamber containing

compressed air at the top and liquid at the bottom.

Uses:1. It provides uniform discharge from pump.2. The chances of cavitation or separation are

considerably reduced.3. A considerable amount of work is saved as

frictional resistance.4. The pump can run at higher speed and

provides higher discharge.

Use of air vessel

Discharge through a pump per second is given as

Where A= the cross sectional area of the piston in

L= the stroke of the piston in mN= the speed of crank in rpm

3

3

for single acting pump60

2Q= for double acting pump

60

ALN mQ s

ALN ms

2m

Work done by reciprocating pump per second is given as

for a single-acting pump

for a double-acting pump

Power required driving the pump for a single-acting pump

for a double-acting pump

(Where ρg = weight density of liquid in N/m3)

( )60 s d

gALNh h

2( )

60 s d

gALNh h

( )60 1000 s d

gALNh h kW

2( )

60 1000 s d

gALNh h kW

Slip & Co-efficient of dischargeSlip = Volume swept/stroke – actual

discharged/strokeThe value of slip is generally positive. However in practice sometimes delivery valve

opens before suction stroke is completed, thus delivering a greater volume of water than actually swept by the piston. Hence the slip will be negative in such a case.

Co-efficient of dischargeactual discharge/stroke

volume swept/strokeact

dth

QC

Q

Percentage slip= th act

th

Q Q

Q

Pressure head due to acceleration (ha) in the suction and delivery pipes is given as

Where

2 cos for suction pipesas

s

l Ah r

g a

2 cos for discharge pipedad

d

l Ah r

g a

length of suction/discharge pipe

a= cross section area of suction/discharge pipe

l

Work done by the pump per second due to acceleration and friction in suction and delivery pipes

2 2for single-acting

60 3 3s d fs fd

gALNh h h h

2 2 2for double-acting

60 3 3s d fs fd

gALNh h h h

Indicator diagram

Indicator diagram with acceleration and friction head effect

Operating characteristic curve

Head

Discharge

Input power

Efficiency

Discharge(ideal)Discharge(actual)

Input

power Efficiency

Speed N=constant