SPECIFIC SPEED (6.1.9)
Specific speed is an index that represent and specify the shape of impeller, number of pump stage and optimum efficiency of pump.
Index relating flow, total head and rotational speed for pumps of similar geometry. {API 610}
- The pump Ns shall be calculated in accordance with Annex A. {API 610}
- It is recognized that very low-Ns pumps might not be able to reach flowrates beyond 105 % to 110 % of BEP. In such cases, the expected flow limitations shall be indicated on the proposal curves (see 10.2.4). {API 610}
Specific speed, ns, is an index number relating to a pump’s performance at best efficiency point flowrate with the maximum diameter impeller and at a given rotative speed. Ns is defined by Equation (A.1): {API 610}
NS= N Q0.5 / H 0.75
N is the rotational speed, expressed in revolutions per minute;
Q is the total pump flowrate, expressed in cubic metres per second (US gallons per minute); use Q/2 for double suction pumps
H is the head per stage, expressed in metres (feet).
- Higher Ns means pump with low head and high capacity plus lower rotational speed.
- Lower Ns means pump with high head and low capacity plus higher rotational speed.
Centrifugal pumps are generally divided into three classes depending on the Ns. The classes are as follows: Centrifugal or Radial-Vane, Mixed-flow, and Axial-flow. {Exxon Mobil}
Ns derived using SI units multiplied by a factor of 51,64 is equal to Ns in USC units.
Comments:
- The pump Ns shall be calculated in accordance with Annex A. {API 610}
- The impeller identification number, Ns and Nss shall be shown on the curves. {API 610}
- Vendor shall specify Q at BEP for maximum impeller.
- Ns shall be within 9 < Ns < 150 or 500 < Ns < 8000.