Reciprocating Pump Theory and Working Principle

There is a piston that moves to and fro inside a cylinder. This movement of the piston inside the cylinder sucks and discharges water or other similar liquids. When the piston moves in the forward direction the reciprocating pump discharges or delivers water. On the other hand, when it moves backward, a section of the water from the underground takes place. In other words, when the piston moves backward it traps the water.

Construction of a Reciprocating Pump

The working, as well as the construction of the pump, is quite simple.

Reciprocating Pump


Suction Pipe

The suction pipe is a cylindrical metallic pipe. In connects the inlet of the pump to the source of the water or liquid.

Suction Valve

The inlet portion of the reciprocating pump consists of a suction valve. Because of this suction valve once the liquid comes in the pump it can’t return to the source again. That means this suction valve traps the water coming inside the cylinder of the pump.

Pump Cylinder

During the backward movement of the piston in the cylinder, the water comes inside the cylinder. During the forward stroke, water comes out through the outlet of the pump.

Piston or Bucket

The main purpose of the piston is to create positive and negative pressure inside the cylinder of the reciprocating pump during its to and fro movements.

Delivery Valve

The outlet of the pump consists of a delivery valve to prevent the backflow of the delivered water through the outlet of the pump. The delivery valve remains closed during the backward stroke. And it remains open during the delivery stroke of the piston.

Tie Rod

Normally a prime mover operates a reciprocating pump. The piston is connected with the help of lever and gear arrangements with the prime mover motor. So, the angular motion of the motor creates a linear motion to the piston in the cylinder. During rotation of the gear fitted with the prime mover motor, the piston moves to and fro to create strokes.

Terms used in the Reciprocating Pump

Suction Head

The suction head is the vertical distance between the level of the water source and the midpoint of the cylinder of the reciprocating pump. Capital Hs indicates the suction head of the pump in our figure.

Delivery Head

The distance between the top point of the delivery pipe and the midpoint of the pump cylinder is the delivery head. Hd is the delivery head shown in our figure.

Static Head

This is the vertical distance between the level of the liquid source to the maximum height up to which the water has to be lifted. Obviously this is the scalar sum of suction head and delivery head. Therefore,

Gross Head

During the operation of the reciprocating pump, the movement of the piston creates pressure inside the cylinder. Obviously this created pressure must be more than the static head of the pump. Otherwise, the pump cannot lift any liquid up to the static head height. This is because during the movement of the liquid from source to the delivery point there will be frictional losses. And obviously, due to this frictional losses, there will be some loss of head in the system. So, the pump has to work against these resistances to lift the liquid up to the delivery point. Therefore, the pressure inside the pump created by the piston must be more than the static head H. So, the total head or gross head is the sum of the static head and loss head, that means,

Loss Head

As we told already in our just previous paragraph that due to friction in the cylinder and pipelines of the pump, there will be some loss of head. And the loss head is the net measurement of these losses.


Working Principle of a Reciprocating Pump

As we already discussed during the construction details of the pump, when the gear wheel of the prime mover motor rotates the rotational motion of the prime mover gets converted into the linear motion of the piston inside the cylinder.

For every half-cycle of the rotation of the gear wheel or crank wheel, there will be one forward stroke of the piston. And during the very next half-cycle of the gear wheel, there will be a backward stroke in the piston.

When the piston moves backward, it creates a low-pressure zone inside the cylinder. Due to the pressure difference between the cylinder and the suction pipe, the suction valve opens. As a result, the water enters the cylinder to fill the partial vacuum.  This is the reason we refer to this movement of the piston as the suction stroke.

Now for the next half-cycle of the prime mover, the piston moves forward or inward as shown in our figure.

In that case, due to the forward movement of the piston, the volume inside the cylinder reduces. As a result the pressure inside the cylinder increases. Hence, the pressure inside the cylinder becomes higher than the delivery head. Due to this pressure difference, the delivery valve opens and water comes out from the outlet and flows through the delivery pipe to the delivery point. Since during this forward movement of the piston, water gets delivered to the delivery point, we refer to this stroke as the delivery stroke of the reciprocating pump. This is the simple working principle of a reciprocating pump. You see it is quite simple and straight forward.

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