Mixed flow devices have an intermediate specific speed. High specific speeds represent axial flow devices - delivering high flow at low head. Low specific speeds represent radial machines - delivering high head at relatively low flow. Specific speed is common in pump design - it conveys information about the type of pump or impeller that will be required. Therefore the user is urged to take caution and determine how the manufacturer of their pump determined specific speed. This will result in the calculated specific speed being less by a multiplying factor of, or approximately 0.707 times less. There are alternative methods used to calculate specific speed that include dividing the flow rate (Q) in the above equation by 2 if the impeller is of a double suction design. H Rated = head per stage in meters (feet) Q Rated = Total pump flow rate in cubic meters per second (US gallons per minute) Ω Rated = Rotational speed in radians per second N Rated = Rotational speed in revolutions per minute There is a conversion factor of 51.6 between the two versions. In such cases it should be made clear what unit system is involved - for example Ns = 4000 (gpm units) or Ns = 77.5 (metric units). Note : Specific speeds using this definition are often presented as 'dimensionless' numbers - but the value returned depends on the unit system used. This equation is listed in both ANSI-HI 14.3.1.3.1-2019 ANSI/HI Standard 14.3-2019, American National Standard for Rotodynamic Pumps - for Design and Application, 2019, published by Hydraulic Institute, New Jersey, USAĪnd API 610 11th Edition: ANSI/API 610 11th Edition - Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries, 2010, published by American Petroleum Institute, 1220 L Street, NW, Washington DC, USA More commonly, a dimensional form is used. The dimensionless form of specific speed is defined as: It is possible to use this formula with a trimmed impeller, or at operating conditions away from BEP, but the number will no longer have the same meaning.

It is important that specific speed be calculated for maximum impeller diameter, and at the pump's BEP, considering head per stage. Pumps of identical specific speed will, in general, have similar behavior at the rated conditions. Specific speed is an index which can be used to characterize the behavior of a pump.

Specific speed ns in english units