RECIPROCATING PUMPS: Discharge through a Reciprocating Pump, Work done by Reciprocating Pump, Slip of a Reciprocating Pump and Negative Slip of the Reciprocating Pump

RECIPROCATING PUMPS

The mechanical energy is converted in to hydraulic energy (pressure energy) by sucking the liquid in to a cylinder in which a piston is reciprocating, which exerts the thrust on the liquid and increases its hydraulic energy (pressure energy) the pump is known as reciprocating pump.

A single acting reciprocating pump consists of a piston, which moves forwards and backwards in a close fitting cylinder. The movement of the piston is obtained by connecting the piston rod to crank by means of a connecting rod.

The crank is rotated by means of an electric motor. Suction and delivery pipes with suction valve and delivery valve are connected to the cylinder.

The suction and delivery valves are one way valves or non-return valves, which allow the water to flow in one direction only. Suction valve allows water from suction pipe to the cylinder which delivery valve allows water from cylinder to delivery pipe only.

When the crank starts rotating, the piston moves to and fro in the cylinder. When the crank is at A the piston is at the extreme left position in the cylinder. As the crank is rotating from A to C (i.e. from 𝜃 = 0 𝑡𝑜 1800 ) the piston is moving towards right in the cylinder. The

movement of the piston towards right creates a partial vacuum in the cylinder. But on the

surface of the liquid in the sump atmospheric pressure in acting, which is more than the

pressure inside the cylinder. Thus the liquid is forced in the suction pipe from the sump. This

liquid opens the suction valve and enters the cylinder.

When crank is rotating from C to A (i.e. from 𝜃 = 1800 𝑡𝑜 3600 ), the piston from its extreme right position starts moving towards left in the cylinder. The movement of the piston towards the left increases the pressure on the liquid inside the cylinder more than atmospheric pressure. Hence the suction valve closes and delivery valve opens.

The liquid is forced in to the delivery pipe and is raised to the required height.

SLIP OF RECIPROCATING PUMP

Slip of a pump is defined as the difference between the theoretical discharge and actual discharge of a pump.

The actual discharge of pump is less than the theoretical discharge due to leakage. The difference of the theoretical discharge and actual discharge is known as slip of the pump.

Negative Slip of the Reciprocating Pump:

Slip is equal to the difference of theoretical discharge and actual discharge. If actual discharge is more than the theoretical discharge, the slip of the pump will become –ve. In that case the slip of the pump is known as negative slip.

Negative slip occurs when the delivery pipe is short, suction pipe is long and pump is running at high speed.