other variable constant, we can write the following equation:
S = C Q 0.125N0.25
Solving for C for the best fit with the second Yedidiah graph (and it fits reasonably well) results in
Styp = 550 Q 0.125N.25 (13-5)
Where (in U.S. units):
S typ = 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:
S 2 = Suction specific speed of second pump
S 1 = Suction specific speed of reference pump
Q 2 = BEP capacity (per eye) of second pump
Q 1 = BEP capacity (per eye) of reference pump
N 2 = RPM of second pump
N 1 = RPM of reference pump
Note that if both pumps are the same pump, we can substitute from the Affinity Laws:
Q2/Q1 = N 2/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)
S n= 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
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.