Reciprocating pump - construction and working | The Mechanical post
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Today at The Mechanical post we'll see about what is reciprocating pump? its construction, working, advantages, disadvantages and applications. So let's start with the definition of reciprocating pump.

Reciprocating pump is a mechanical device which is used to deliver (or pump) the liquid with the help of a piston which reciprocates inside an enclosed cylinder.

Working principle 


The reciprocating pump works on the principle that when a piston moves to and fro in an enclosed cylinder, it causes suction and compression of the liquid resulting in suction at the inlet and delivery at high pressure at the outlet.

Construction of reciprocating pump


The basic components of a single acting reciprocating pump are:

Plunger / piston 

Unlike centrifugal pump which has an impeller, the reciprocating pump has a piston or plunger and it reciprocates in cylinder due to which suction and increase in pressure takes place.

Piston rod

piston rod connects the connecting rod to the piston and transfers reciprocating motion to the piston from the connecting rod. As shown in the figure below.

Connecting rod

Connecting rod is used to connect the piston rod to the Crank.

Crank

The  crank is connected to a prime mover which maybe an electric motor an engine. This crank is then connected to the connecting rod as shown in the figure. Thus the rotary motion of the crank is converted into the reciprocating motion of the piston rod.

Suction pipe 

It is the pipe which connects the reservoir to the inlet cylinder of the pump as shown in the figure.

Delivery pipe

The delivery pipe is used for delivering the pressurized liquid coming out from the pump to its destination.

Suction and delivery valve

These are non-return valves and allow liquid to enter from one direction only. one valve is fitted at the inlet of the cylinder where as the other is fitted at the outlet of the cylinder as shown in the figure. This ensures that the water or the liquid doesn't flow backwards.

Strainer

A Strainer is attached at the beginning of the suction pipe so as to prevent entering of any foreign material or particles.

Reciprocating pump - construction and working | The Mechanical post
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Working of single acting reciprocating pump.


Crank of reciprocating pump is connected to prime mover like an electric motor or an engine. Crank and connecting rod convert rotary motion to reciprocating or to and fro motion. If A is the initial position of Crank, from A to C plunger starts moving from left to right. During this vacuum is created in between cylinder and piston as delivery valve is closed and suction pipe is in reservoir. When the crank reaches the position C it is the extreme right position of the plunger. This stroke is called as suction stroke. During suction stroke suction valve opens and there will be flow of liquid from the sump in the suction pipe and hence to the cylinder.

When the crank starts rotating from C to D to a the plunger starts moving from the right to left or its return stroke. During this this stroke the plunger applies pressure on the water entered in the cylinder. Hence the pressure of water increases from vacuum to positive value. Due to high pressure of liquid in cylinder, delivery valve opens and the water is raised by the plunger rushes in the delivery pipe and is finally delivered to its destination. The discharge of this pump is not continuous as shown in figure below.

Reciprocating pump - construction and working | The Mechanical post
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Slip in reciprocating pump


Slip in reciprocating pump is defined as the difference between the theoretical discharge and the actual discharge of the pump. The actual discharge of the pump is less than the theoretical discharge due to small amount of leakage.

Work done by reciprocating pump


A single acting reciprocating pump has one suction stroke and one delivery stroke for one rotation of crank. The liquid is delivered by the pump during delivery stroke only.

Let,
R = radius of crank
L = length of stroke = 2R
A = cross sectional area of the cylinder
N = speed of pump in r.p.m 
    = N/60 r.p.s

Then,

Stroke volume = A × L

Discharge of pump per second (Q ) = stroke volume ×  no of revolutions per sec
                                                         = A × L × N / 60

Therefore,

Weight of water discharged per sec (W)= w × Q

Work done by the pump = Weight of water discharged per sec × Total height through which the water is lifted.
                                        =  W × ( Hs + Hd)

Advantages of reciprocating pump



  • No priming required.
  • Can be used to deliver liquid at higher heads i.e. the pump has high pressure output.
  • It has higher efficiency as compared to centrifugal pump.
  • Starting torque is less.


Disadvantages of reciprocating pump

  • High maintenance cost due to more wear and tear of the parts.
  • As compared to centrifugal pump it is heavy and bigger in size.
  • Initial cost is high.
  • Flow rate of the liquid delivered is low.
  • Requires more floor area and heavy foundation.

Applications of reciprocating pump



  • Used as feed pump for small boilers.
  • For pumping industrial fluids.
  • Oil drilling operations.
  • Hydraulic jack.
  • Fire fighting application.
  • Fuel injection in automobile engines.

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