What is specific speed of a pump?

Short Answer:

The specific speed of a pump is a dimensionless number that indicates the speed at which a geometrically similar pump would operate to deliver one unit of discharge against one unit of head. It helps in comparing different types of pumps and selecting the most suitable one for a specific application.

In simple words, specific speed shows the relationship between speed, discharge, and head of a pump. It is an important parameter for classifying pumps into categories such as radial flow, mixed flow, and axial flow types depending on their performance and operating characteristics.

Detailed Explanation :

Specific Speed of a Pump

The specific speed of a pump is an important characteristic used to compare the performance of pumps having different sizes, capacities, and designs. It gives a measure of the speed at which a pump of similar shape and efficiency would run if it were designed to deliver a discharge of 1 cubic meter per second against a head of 1 meter.

It helps engineers to identify the type of flow through the pump and to choose the correct pump for a given set of conditions. The specific speed is not the actual speed of a pump, but rather a calculated value derived from its operating conditions.

Definition

The specific speed (Ns) of a pump is defined as:

where,

  •  = specific speed of the pump (dimensionless)
  •  = speed of the pump in revolutions per minute (rpm)
  •  = discharge of the pump in cubic meters per second (m³/s)
  •  = head developed by the pump in meters (m)

This formula provides a way to compare pumps of different sizes and types based on their performance characteristics.

Physical Meaning

Specific speed represents the speed of a geometrically similar pump that would produce a discharge of 1 m³/s under a head of 1 m.

  • A low specific speed indicates that the pump is suitable for high head and low discharge, typical of radial flow pumps.
  • A high specific speed indicates suitability for low head and high discharge, typical of axial flow pumps.
    Thus, it helps in classifying the type of pump and understanding its flow characteristics.

Classification of Pumps Based on Specific Speed

Pumps are classified into three major types depending on their specific speed value:

  1. Radial Flow Pump (Ns = 10 to 80):
    • In this type, the liquid flows perpendicular to the pump shaft.
    • The pump develops high head but has a low discharge.
    • Commonly used for deep well or boiler feed applications.
  2. Mixed Flow Pump (Ns = 80 to 160):
    • The liquid flows both radially and axially through the impeller.
    • These pumps produce a moderate head and discharge.
    • Suitable for irrigation and water supply systems.
  3. Axial Flow Pump (Ns = 160 to 500):
    • The liquid flows parallel to the pump shaft.
    • They produce high discharge but low head.
    • Commonly used for flood control, drainage, and circulation systems.

This classification helps in selecting the most efficient pump for a given task.

Importance of Specific Speed

  1. Pump Selection:
    Specific speed helps engineers choose the right type of pump for required discharge and head conditions.
  2. Design and Performance Comparison:
    It allows the comparison of performance between geometrically similar pumps, even if they differ in size.
  3. Efficiency Prediction:
    Pumps with specific speeds close to the optimal range have better hydraulic efficiency and performance.
  4. Flow Type Indication:
    It identifies the nature of the flow inside the impeller — radial, mixed, or axial.
  5. Helps in Standardization:
    Manufacturers use specific speed as a design reference for creating pump families with consistent performance.

Example Calculation

Suppose a pump runs at 1000 rpm, delivers 0.05 m³/s of water, and develops a head of 25 m.
We can calculate specific speed using the formula:

 

Hence, the specific speed of the pump is 20, which falls under the radial flow type category.

Graphical Representation (Conceptual)

If we plot the head, discharge, and specific speed, we can observe that as the specific speed increases:

  • The head decreases.
  • The discharge increases.
  • The direction of flow changes from radial to axial.

This trend is used in pump design and performance mapping.

Factors Affecting Specific Speed

  1. Speed of the Pump:
    Increasing speed increases specific speed, leading to higher discharge but lower head.
  2. Head Developed:
    Higher head decreases specific speed as the denominator  becomes larger.
  3. Discharge Rate:
    Increasing discharge raises specific speed, making the pump suitable for higher flow rates.
  4. Fluid Properties:
    Although specific speed is dimensionless, viscosity and density of the fluid may slightly influence actual performance.
Conclusion

The specific speed of a pump is a key dimensionless parameter that helps in analyzing, designing, and selecting pumps for different industrial applications. It relates the speed, discharge, and head of a pump in a single number and determines whether the pump is radial, mixed, or axial flow type. By understanding specific speed, engineers can ensure better efficiency, proper selection, and reliable operation of pumping systems in practical use.