PUMP SPECIFIC SPEED
By PumpWorks Engineering Team / March 31, 2021
SUCTION SPECIFIC SPEED
Design engineers find composite measurement parameters useful for selecting solutions to engineering problems. These measurement parameters are dimensionless numbers that provide excellent design tools.
Fluid engineers use Reynold’s Numbers, while heat transfer engineers use Prandtl and Nusselt numbers. Pump engineers use two closely related dimensionless parameters: specific speed and suction specific speed.
Both parameters relate pump speed and pump capacity. Specific speed utilizes total pump head, and suction specific speed incorporates Net Position Suction Head (NPSH) required.
How to Calculate Specific Speed
Nss = n * Q1/2 / H3/4
N = shaft speed [RPM]
Q = pump capacity @ max impeller diameter [GPM]
H = head @ BEP [ft] for double suction impellers the Q value used would be 50% of capacity
Suction Specific Speed
For Suction Specific Speed, Nsss head [H] is replaced by NPSH required.
Nsss = n * Q1/2 / NPSHR3/4
A review of the units associated with both Nss & Nsss will show that these parameters are NOT truly dimensionless. Yet both have been treated as dimensionless and most literature refers to them as dimensionless. This inconsistency in no way lessens their effectiveness and their historical value.
Both parameters become dimensionless only when the gravity acceleration constant (g) is used with US customary units of [ft/sec²]. The inconsistency of units, i.e. Q = [GPM] and H = [ft] rather than Q = [ft³/min] further adds to the potential confusion.
Nss = n * Q1/2 / g * H3/4
The same substitution for suction specific speed would be required.
The omission of the gravitational constant in no way lessens the utility of these two very important design parameters. In concept, they remain dimensionless even if they are not.
PumpWorks Helps Customers Understand Pump Specific Speed
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