Suction Specific Speed (Part Two)


Written by:
Terry Henshaw, P.E.

Although a correction for a speed difference would seem to have value, the author is unaware of any speed adjustment now made in suction specific speed evaluations. The following is therefore offered for consideration.

Most process pumps, at least in the United States, are tested and rated at 3,550 rpm. Because NPSHR is measured at 3,550 rpm, S is calculated for that speed. To normalize all suction specific speeds to 3,550 rpm, the following equation, from Equation 13‑3, can be used:

S3550 = S                                   (13-4)

     Where:

     S3550 = Suction specific speed, normalized to 3,550 rpm

     S = Suction specific speed established by testing (NPSHR for a 3 percent head drop)

     N = Test speed, rpm

       Example:

       Test speed = 1,770 rpm; S = 8,500

S3550 = 8500 = 11,000

Effect of Capacity on Suction Specific Speed

Yedidiah (2) plotted (on log‑log paper) values of S versus Q for several hundred pumps made by 10 companies, all running at 1,750 rpm. The scatter of points showed about a 2:1 variation in S for all capacities from 30 gpm to 4,000 gpm. He drew a single line though the approximate center of these points and measured the slope at about 0.18, indicating S=KQ0.18, but stated that this exponent would be 0.125 "under ideal conditions."

Evaluating the same set of points, this author measures a slope of about 0.15, acknowledging that a slope of 0.125 would also fit the points satisfactorily.

Effect of Capacity and Speed on Suction Specific Speed

Yedidiah (2) also stated that as a function of speed, S "should" vary as S=AN0.25.

If we let the capacity exponent be 0.125, and the speed exponent 0.25, and if we assume that these exponents apply while holding the other variable constant, we can write the following equation:

S = C Q0.125N0.25

Solving for C for the best fit with the second Yedidiah graph (and it fits reasonably well) results in

Styp = 550 Q0.125N.25                         (13-5)

     Where (in U.S. units):

     Styp = Suction specific speed of a "typical" pump

     Q = Pump capacity* at BEP, GPM

     N = RPM of pump

*If the impeller is double‑suction, Q in the above equation is one-half the BEP capacity of the pump.

This equation can be used to calculate S for a "typical" pump, realizing that published performance data may show a value 40 percent above or below the "typical" value.

A more useful purpose may be to further "normalize" suction specific speed. Equations 13‑3 and 13‑4 provide for normalizing S for a particular pump to a common speed, such as 3,550 rpm. Equation 13‑5 can be massaged to provide normalization of speed and capacity for different pumps, resulting in Equation 13-6:

S2=S1(Q2/Q1)0.125(N2/N1)0.25 (13-6)

     Where:

      S2 = Suction specific speed of second pump

      S1 = Suction specific speed of reference pump

     Q2 = BEP capacity (per eye) of second pump

     Q1 = BEP capacity (per eye) of reference pump

     N2 = RPM of second pump

     N1 = RPM of reference pump

Note that if both pumps are the same pump, we can substitute from the Affinity Laws:

Q2/Q1 = N2/N1

Equation 13‑6 reduces to Equation 13‑3, confirming the 0.375 exponent.

If we choose to "normalize" S to 3,550 rpm and to, say, 1,000 gpm (per eye), Equation 13‑6 becomes:

Sn = S                      (13-7)

     Where:

     Sn= Suction specific speed, normalized to 1,000 gpm and 3,550 rpm

     S = Suction specific speed established by testing (NPSHR for a 3 percent head drop)

     Q = BEP capacity of pump (per eye), GPM

     N = Test speed, RPM

 

References

1. Lobanoff, Val S. & Ross, Robert R., Centrifugal Pumps: Design & Application, Gulf Publishing, Houston, Texas, 1985.

2. Yedidiah, S., "Factor Pump Size into NPSH Comparisons," Power, June 1973.

 

Pages


See also:

Upstream Pumping Solutions

© Copyright Cahaba Media Group 2014. All Rights Reserved. Privacy Policy