As discussed in last chapter, pumps are hydraulic machines that convert mechnical energy to hydraulic energy, which is in the form of pressure energy. If the mechanical energy is converted into hydraulic energy by sucking the liquid into a cylinder in which a piston is reciprocating (moving backward and forward), the pump is called reciprocating pump. In this pump the reciprocating action exerts a thrust on the liquid and increases its pressure energy.
(Remember, in a centrifugal pump the convertion of energy is by centrifugal action whereas in a reciprocating pump it is by reciprocating action)
1. Suction pipe
Similar to that of a centrifugal pump. Receives water from the sump and transfers to the cylinder.
2. A cylinder with a piston, piston rod, connecting rod and a crank
The cylinder is usually circular in cross section. The diameter of piston is same as that of the cylinder so that it fits inside the cylinder. The piston rod connects the rear end of the piston to the connecting rod. The piston rod always moves to and fro. The other end of connecting rod is attached to outer side of the crank. The crank is circular in shape and is rotated by an electric motor.
3. Delivery pipe
It receives water from the cylinder and delivers it to the water at the upper tank or outlet.
4. Suction valve
It is attached to the suction pipe. Similar to that in a centrifugal pump, it is a one way valve or non-return valve. It allows water to move upwards only. But unlike the centrifugal pump, it is not always open when the pump is working.
5. Delivery valve
It is attached to the delivery pipe. It is also a one way valve or non-return valve.
As said the centrifugal pump consists of a piston moving to and fro. The movement of piston is obtained by connecting the crank by means of a connecting rod. The crank is rotated by an electric motor. When the crank is at A, the piston is at the extreme left position in the cylinder. As the crank rotates from A to C, it pulls the piston and the piston moves from left to right. The movement of piston from left to right creates a vaccum in the cylinder. The sump of water on which atmospheric pressure is acting is at a higher pressure than the pressure inside the cylinder. Thus the water is forced into the suction pipe and it opens the suction valve and enters the cylinder.
After the crank completes its 180o, it moves back from C to A. Thus it pulls the piston from right to left. Thus due to increased pressure of water in the cylinder, it closses the suction valve and opens the delivery valve. Thus the water is pushed to the delivery pipe to the overhead tank.
Thus in every rotation of the crank, a cycle of suction of water and delivering it to the overhead tank takes place.
Reciprocating pumps can be classified based on:
I. According to water on one side or both side of piston
a. Single-acting pump
This is the one discussed above.
b. Double acting pump
In a double- acting pump the water is acting on both sides of the piston. Thus there are two suction pipes and two delivery pipes. When there is suction stroke on one side of the piston, there is delivery stroke on the other side. Thus for one revolution of crank there are 2 suction strokes and 2 delivery strokes. The main advantage of this type of pump is that the discharge and work done is almost twice as that of the single-acting pump.
II. According to number of cylinders provided
a. Single cylinder pump
b. Double cylinder pump
c. Triple cylinder pump