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By PumpWorks Engineering Team / March 31, 2021

pump 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

PumpWorks has the expertise to manufacture any type of pump for any type of application. Our team is here to help you find the pump solutions to meet your exact needs. We offer ANSI process pumps, API 610 pumps, remanufactured pumps and more!

Call PumpWorks at 855.979.9139 or contact us online today to learn more.


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