What is a single-acting pump?

Short Answer:

A single-acting pump is a type of reciprocating pump in which the liquid is acted upon by the piston on only one side. It means the suction and delivery of the fluid take place once in every revolution of the crankshaft. During the backward stroke, the liquid is drawn into the cylinder, and during the forward stroke, it is discharged.

This pump delivers liquid only during one stroke of the piston, so the flow is not continuous but pulsating. It is simple in construction, easy to maintain, and mostly used where low discharge and high pressure are required, such as in hand pumps and small hydraulic systems.

Detailed Explanation :

Single-Acting Pump

A single-acting pump is a type of reciprocating pump where only one side of the piston is used for pumping the liquid. The piston moves back and forth inside the cylinder, creating suction and discharge alternately. The other side of the piston remains unutilized and is open to the atmosphere or crankcase. This pump is widely used for small-scale applications where continuous discharge is not required but high pressure is needed.

Working Principle

The single-acting reciprocating pump works on the principle of positive displacement. It means a definite quantity of liquid is moved in each stroke. The working is divided into two main strokes — the suction stroke and the delivery stroke.

  1. Suction Stroke:
    • The piston moves backward due to the rotation of the crankshaft.
    • A vacuum is created inside the cylinder as the piston moves away from the cylinder head.
    • This vacuum causes the suction valve to open, and liquid from the sump or tank enters the cylinder through the suction pipe.
    • During this time, the delivery valve remains closed to prevent backflow of liquid.
  2. Delivery Stroke:
    • As the crank continues to rotate, the piston now moves forward toward the cylinder head.
    • The suction valve closes, and the pressure inside the cylinder increases.
    • When the pressure exceeds the delivery pressure, the delivery valve opens, allowing the liquid to flow into the delivery pipe.
    • Once the piston completes its stroke, the cycle repeats.

In this type of pump, discharge occurs only during the forward stroke, so the flow is intermittent or pulsating. The flow rate depends on the speed of the crankshaft and the size of the cylinder.

Main Components

  1. Cylinder:
    The main body in which the piston moves back and forth. It is made of strong metal to handle high pressure.
  2. Piston:
    The piston moves inside the cylinder, performing suction and discharge actions alternately. It is connected to the crankshaft through a connecting rod.
  3. Crank and Connecting Rod:
    These parts convert the rotary motion of the shaft into reciprocating motion of the piston.
  4. Suction Valve:
    A one-way valve that allows liquid to enter the cylinder during suction and closes during discharge.
  5. Delivery Valve:
    A one-way valve that opens during discharge and closes during suction to prevent backflow.
  6. Suction and Delivery Pipes:
    The suction pipe draws liquid from the source, and the delivery pipe sends it to the desired location.

Working Cycle Explanation

When the crankshaft rotates, it drives the connecting rod, which moves the piston backward and forward. During the backward movement, the pressure in the cylinder drops, causing the suction valve to open and allowing the liquid to enter. During the forward movement, the liquid is compressed, and when pressure rises above delivery line pressure, the discharge valve opens, and the liquid is forced out.

Thus, suction occurs during one half of the cycle, and delivery happens during the other half. The discharge is not continuous but occurs in pulses because liquid is delivered only during one stroke of the piston. This makes the flow uneven, which can be reduced by using an air vessel.

Advantages

  • Simple in design and easy to operate.
  • High-pressure generation with low discharge.
  • Compact and suitable for small-scale applications.
  • Capable of handling viscous or thick liquids.
  • Reliable and requires less maintenance.

Disadvantages

  • Flow is pulsating and not continuous.
  • Produces vibration and noise during operation.
  • Efficiency decreases if air leaks into the system.
  • Not suitable for large discharge rates.
  • Requires priming before starting in some cases.

Applications

  • Hand pumps used in rural water supply.
  • Hydraulic systems for operating presses and jacks.
  • Boiler feed pumps in small steam plants.
  • Chemical and oil industries where high pressure is required for small discharge.
  • Testing equipment for pressure testing of pipes and tanks.
Conclusion

A single-acting pump is a simple and effective type of reciprocating pump used where low discharge and high pressure are required. It operates with a single piston face performing both suction and discharge alternately. Though it delivers liquid only during one stroke, its sturdy construction and easy maintenance make it highly useful for domestic, agricultural, and industrial purposes. It is an excellent example of a positive displacement pump suitable for small-scale applications requiring reliability and precision.