Pumps - Suction Specific Speed

Suction Specific Speed Calculator

Subction Specific Speed (Nss) is used to determine pump geometry selection and evaluate operating conditions on the suction side of pumps.

Formula:

Nss = ω × q^(1/2) / NPSHr^(3/4)

Where:

• ω = pump shaft rotational speed (rpm)
• q = flow rate capacity at Best Efficiency Point (BEP)
• NPSHr = Required Net Positive Suction Head at BEP

Safe Operating Range: Nss should be below 9000 (US gpm units) to avoid cavitation.

Unit Conversions:

• Nss (US gpm) = 1.63 × Nss (metric l/s) = 0.86 × Nss (metric m3/h)
• Nss (metric l/s) = 0.614 × Nss (US gpm)
• Nss (metric l/s) = 0.67 × Nss (British gpm)

Additional Calculations

Calculate Required NPSHr from Target Nss

If you want to find the Required NPSHr for a maximum acceptable Suction Specific Speed:

NPSHr = (ω × q^(1/2) / Nss)^(4/3)

NPSHa vs NPSHr with Safety Margin

The Actual Net Positive Suction Head (NPSHa) should include a safety margin:

NPSHa = NPSHr × Sr

Where Sr (safety ratio) is typically 1.5 (50% safety margin) or higher for critical applications.

To Increase Available Head on Suction Side:

• Increase suction pipe dimensions
• Shorten suction pipes
• Remove or reduce components (valves, filters) in suction lines
• Increase static pressure in the system
• Lower the pump installation (increases static pressure)