SUCTION SPECIFIC SPEED LIMIT

6.1.9 The pump suction-specific speed shall be calculated in accordance with Annex A and, if specified, limited as stated on the data sheet. {API 610}

Suction-specific speed, S or Nss, an index number relating to a pump’s suction performance, is calculated at best efficiency point flowrate with the maximum diameter impeller at a given rotative speed and provides an assessment of a pump’s susceptibility to internal recirculation. It is defined by Equation (A.2): {API 610}

N_SS or S = N Q^0.5/ NPSH3^0.75

N             is the rotational speed, expressed in revolutions per minute;

Q             is the flowrate per impeller eye, expressed in cubic metres per second (US gallons per minute), equal to one of the following:

• total flowrate for single-suction impellers,
• one-half the total flowrate for double-suction impellers;

(NPSH3) is the net positive suction head required, expressed in metres (feet).

Suction-specific speed derived using SI units multiplied by a factor of 51.64 is equal to suction-specific speed in

USC units. The USC symbol Nss is sometimes used to designate suction-specific speed.

• Double-suction impellers require less NPSH than do single suction impellers and the NSS is calculated using one-half the GPM.
• NSS shall be 1900 (rpm,m3/m,m) or smaller for impplers with no inducer or 3000 or smaller for the impller with an inducer. {VDR Sazeh}
• Nss (for hydro carbon services) is preferably less than 12770 (metric) for single suction or 10640 (metric) for double suction. {VDR Sazeh}
• Suction specific Speed shall preferably be less than 11610 metric (9675 for double suction pumps) and shall not exceed 12770 metric (10640 for double suction pumps) on water services. {Kermanshah Spec.}
• GP 10-1-1 limits SSS to 12000. Values of SSS range from 7000 to approximately 13000 for commercially available pumps. Higher specific speed pumps have large inlet eye areas and are generally prone to internal recirculation when operated at flows away from the BEP. (Exxon Mobil}

Comments:

• The impeller identification number, specific speed and suction-specific speed shall be shown on the curves.
• The Vendor shall indicate, in the proposal, the value of the suction specific speed for the pumps proposed. Suction specific speeds, without inducer, shall be limited as indicated in the table below. {Total}

SUCTION SPECIFIC SPEED LIMIT

• Vendor shall specify Q at BEP for maximum impeller.
• All proposals shall clearly indicate the use of an inducer, if necessary. The use of an inducer shall be subject to Purchaser approval. {Total}