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Suction Specific Speed
Appendix G Suction Specific Speed GARR M. JONES
Suction specific speed is a value that is computed by the relationship
S¼
n(QBEP )0:5 NPSHR0:75
Where: n ¼ speed, rpm. ¼ flow rate, m3 =min (gal/min) at the best efficiency point. QBEP NPSHR ¼ net positive suction head required, m (ft), at the best efficiency point. For double suction pumps, Q is divided by 2 to obtain suction specific speed. ANSI/HI 1.3, Section 1.3.4.1.15 states: ‘‘For pumps of normal design, values of ‘S’ (Suction specific speed) vary from 7000 to 14,000 (metric) (8000 to 12,000) (US customary units).’’ A similar statement appears in the companion standard for vertical pumps, ANSI/HI 2.3. Suction specific speed has been used by pump designers to judge the stability of a particular pump design. Discussions with experienced pump design engineers indicate that suction specific speeds outside the recommended range are possible but only at the loss of stability outside a region within 5% of the BEP. Process Industry Practices Specification PIP RESP003H [1] limits suction specific speed to 11,000 (US customary units) unless the purchaser gives prior approval. Brennan [2] discusses some information on suction specific speed and its uses as a means of identifying cavitation characteristics of various pump types. Pump designers recommend the use of the suction specific speed charts in ANSI/HI 1.3 and ANSI/HI 2.3 and a limiting value to establish maximum speeds for pumps in purchase orders and specifications. Ebara Pump System Engineering Handbook [3] recommends a similar approach, but contains expanded recommendations on limiting suction specific speeds. Based upon one extreme experience, the authors recommend that suction specific speed be used as a means of evaluating candidate pumps and that the above limitations be placed in specifications or procurement documents. Additionally, when factory tests are made, an NPSHR test at the BEP should be required so that suction specific speed can be checked against contractual limits.
G.1
G.2
Appendix G Suction Specific Speed
G-1. References 1. PIP RESP003H Specification for Horizontal Centrifugal Pumps for Water Service, Process Industry Practices (February 2004). 2. Brennan, C.E. Hydrodynamics of Pumps, Concepts NREC and Oxford University Press, White River Junction, VT., Chapter 5 (1994). Available on the Internet at http://caltechbook.library.caltech. edu/archive/00000022/01/pumps.htm 3. Ebara Pump System Engineering Handbook, Ebara Corporation (1997).
Suction specific speed is a value that is computed by the relationship
S¼
n(QBEP )0:5 NPSHR0:75
Where: n ¼ speed, rpm. ¼ flow rate, m3 =min (gal/min) at the best efficiency point. QBEP NPSHR ¼ net positive suction head required, m (ft), at the best efficiency point. For double suction pumps, Q is divided by 2 to obtain suction specific speed. ANSI/HI 1.3, Section 1.3.4.1.15 states: ‘‘For pumps of normal design, values of ‘S’ (Suction specific speed) vary from 7000 to 14,000 (metric) (8000 to 12,000) (US customary units).’’ A similar statement appears in the companion standard for vertical pumps, ANSI/HI 2.3. Suction specific speed has been used by pump designers to judge the stability of a particular pump design. Discussions with experienced pump design engineers indicate that suction specific speeds outside the recommended range are possible but only at the loss of stability outside a region within 5% of the BEP. Process Industry Practices Specification PIP RESP003H [1] limits suction specific speed to 11,000 (US customary units) unless the purchaser gives prior approval. Brennan [2] discusses some information on suction specific speed and its uses as a means of identifying cavitation characteristics of various pump types. Pump designers recommend the use of the suction specific speed charts in ANSI/HI 1.3 and ANSI/HI 2.3 and a limiting value to establish maximum speeds for pumps in purchase orders and specifications. Ebara Pump System Engineering Handbook [3] recommends a similar approach, but contains expanded recommendations on limiting suction specific speeds. Based upon one extreme experience, the authors recommend that suction specific speed be used as a means of evaluating candidate pumps and that the above limitations be placed in specifications or procurement documents. Additionally, when factory tests are made, an NPSHR test at the BEP should be required so that suction specific speed can be checked against contractual limits.
G.1
G.2
Appendix G Suction Specific Speed
G-1. References 1. PIP RESP003H Specification for Horizontal Centrifugal Pumps for Water Service, Process Industry Practices (February 2004). 2. Brennan, C.E. Hydrodynamics of Pumps, Concepts NREC and Oxford University Press, White River Junction, VT., Chapter 5 (1994). Available on the Internet at http://caltechbook.library.caltech. edu/archive/00000022/01/pumps.htm 3. Ebara Pump System Engineering Handbook, Ebara Corporation (1997).