What is slip in reciprocating pumps?

Short Answer:

Slip in reciprocating pumps is the difference between the theoretical discharge and the actual discharge of the pump. It occurs because of leakage of liquid through valves, piston rings, or other clearances during operation. This means the pump delivers less water than it ideally should.

Slip can be expressed either as an absolute difference or as a percentage of theoretical discharge. Usually, slip is positive, but in some cases, especially when the suction head is very low or delivery head is high, negative slip may occur, meaning the actual discharge is slightly higher than the theoretical value.

Detailed Explanation :

Slip in Reciprocating Pumps

In a reciprocating pump, the theoretical discharge is the amount of liquid that the pump should ideally deliver per revolution of the crank, assuming there is no leakage or loss. However, in actual working conditions, due to mechanical clearances, valve leakage, and friction losses, the actual discharge is always slightly less than the theoretical value.

This difference between theoretical and actual discharge is called slip. It is a very important performance parameter used to determine the efficiency and accuracy of a reciprocating pump.

Definition of Slip

Slip can be defined as:
Slip = Theoretical discharge – Actual discharge

It shows how much of the expected discharge is lost due to leakage or other losses. A smaller slip indicates better performance and efficiency of the pump.

Slip is usually expressed in two forms:

  1. Absolute Slip (in m³/s or L/min):
    It is the actual quantity of fluid that leaks or fails to be delivered per second.
  2. Percentage Slip:
    It is the ratio of slip to theoretical discharge, expressed as a percentage.

Theoretical Discharge

The theoretical discharge of a reciprocating pump can be calculated using the formula:

Where:

  •  = Theoretical discharge (m³/s)
  •  = Stroke length of the piston (m)
  •  = Area of the piston (m²)
  •  = Speed of the crank (revolutions per minute)

This represents the total volume of water displaced by the piston per unit time if there were no losses or leakages.

Actual Discharge

The actual discharge (Qₐ) is always slightly less than  due to leakage through valves, packing glands, and other parts of the pump.

Hence, slip directly shows how much discharge has been lost in the process.

Causes of Slip

Several factors cause slip in reciprocating pumps:

  1. Leakage through Valves:
    The suction and delivery valves may not close completely, allowing water to flow backward, reducing actual discharge.
  2. Leakage through Piston Rings:
    Worn-out or loose piston rings may allow fluid to pass through, especially during high-pressure strokes.
  3. Clearance Losses:
    Small gaps between moving parts like piston and cylinder allow minor fluid leakage.
  4. Air Leakage:
    If air enters the suction pipe or cylinder, it compresses and reduces the effective volume of liquid delivered.
  5. Wear and Tear:
    Continuous operation causes mechanical wear, increasing the leakage path and reducing efficiency.

Types of Slip

  1. Positive Slip:
    When actual discharge is less than theoretical discharge, slip is positive.
    It is the most common condition and occurs under normal working situations.
  2. Negative Slip:
    In some special cases, the actual discharge becomes greater than theoretical discharge.
    This happens when:

    • The delivery pipe is short and delivery head is high.
    • The inertia of the water column causes additional water to be forced out during the suction stroke.
    • There is high-speed operation, resulting in momentum-driven extra discharge.
      Under these conditions, the pump seems to deliver more water than expected, resulting in negative slip.

Effect of Slip on Pump Performance

  • Efficiency: A large slip reduces the volumetric efficiency of the pump.
  • Flow Rate: Slip decreases the actual discharge, making the pump deliver less fluid.
  • Maintenance Requirement: Excessive slip indicates worn-out parts that require replacement.
  • Accuracy: Pumps with low slip provide more accurate and reliable performance in pressure-sensitive applications.

The volumetric efficiency (ηᵥ) of a reciprocating pump is related to slip by the formula:

Hence, lower slip means higher volumetric efficiency.

Methods to Reduce Slip

  1. Proper Maintenance: Regular inspection and replacement of worn-out parts like piston rings and valves reduce leakage.
  2. Accurate Alignment: Ensuring correct fitting of piston and cylinder helps minimize clearance.
  3. Air-tight Suction Line: Preventing air entry increases suction efficiency and reduces slip.
  4. Use of Non-return Valves: High-quality valves ensure tight sealing during operation.
  5. Lubrication: Reduces wear and maintains close contact between moving parts.

By maintaining these conditions, slip can be minimized and pump performance improved.

Conclusion

Slip in reciprocating pumps is the difference between theoretical and actual discharge caused mainly by leakage, friction, and air entry. It serves as an important measure of the pump’s efficiency. Normally, slip is positive, indicating some loss, but under certain conditions like high speed and short delivery pipes, negative slip may occur. Reducing slip through proper design, maintenance, and operation improves the overall performance, reliability, and life of the reciprocating pump.