mechanical engineering

Specific speed is a number characterizing the type of impeller in a unique and coherent manner. Specific speed are determined independent of pump size and can be useful comparing different pump designs. The specific speed identifies the geometrically similarity of pumps.

Specific speed is dimensionless and are expressed as
Ns = ω q1/2 / h3/4 (1)
where
Ns = specific speed
ω = pump shaft rotational speed (rpm)
q = flow rate (m3/h, l/s, m3/min, US gpm, British gpm) at Best Efficiency Point (BEP)
h = head rise (m, ft)
Note! When comparing pumps and their documentation be aware of the units used.

Typical values for specific speed - Ns - for different designs in US units (gpm)

• radial flow - 500 <>
• mixed flow - 2000 <>
• axial flow - 7000 < Ns < 20000 - typical for propellers and axial fans
  

Convert between US units (gpm) and Metric units (m3/h)

• Ns (US) = 1.63 Ns (metric)
• Ns (Metric) = 0.614 Ns (US)
• Ns (Metric) = 0.67 Ns (British)
  

Example - Specific Speed for a Pump
A pump has a capacity of 1500 gal/min at 100 ft of head and is rotating at 1760 rev/min. Specific speed can be expressed as:
Nsd = (1760 rev/min) (1500 gal/min)1/2 / (100 ft)3/4
= 2156 - a typical centrifugal pump

Standards of design and dimensional specifications are necessary to bring unity to centrifugal pumps. Standards are provided by organizations like

• ISO - International Standards Organizations
• API - American International Institute
• ANSI - American National Standards Institute
• DIN - Deutsches Institut f�r Normung
• NPFA - National Fire Protection Agency
• BSi - British Standards institute
  

Some commonly used centrifugal pumps standards

• ANSI/API 610-1995 - Centrifugal Pumps for General Refinery Service - Covers the minimum requirements for centrifugal pumps, including pumps running in reverse as hydraulic power recovery turbines, for use in petroleum, heavy duty chemicals, and gas industry services. The pump types covered by this standard can be broadly classified as overhung, between bearings, and vertically suspended.
• DIN EN ISO 5199 - Technical specifications for centrifugal pumps
• ASME B73.1-2001 - Specification for Horizontal End Suction Centrifugal Pumps for Chemical Process - This standard covers centrifugal pumps of horizontal, end suction single stage, centerline discharge design. This Standard includes dimensional interchangeability requirements and certain design features to facilitate installation and maintenance. It is the intent of this Standard that pumps of the same standard dimension designation from all sources of supply shall be interchangeable with respect to mounting dimensions, size and location of suction and discharge nozzles, input shafts, baseplates, and foundation bolt holes
• ASME B73.2-2003 - Specifications for Vertical In-Line Centrifugal Pumps for Chemical Process
• BS 5257:1975 - Specification for horizontal end-suction centrifugal pumps (16 bar) - Principal dimensions and nominal duty point. Dimensions for seal cavities and base plate installations.