Pumps - Suction Specific Speed

Suction Specific Speed - N_ss - can be useful when evaluating the operating conditions on the suction side of pumps. Suction Specific Speed is used to determine what pump geometry - radial, mixed flow or axial - to select for a stable and reliable operation with max efficiency without cavitation. N_ss can also be used to estimate safe operating ranges.

Suction Specific Speed is a dimensionless value and can be calculated as

N_ss = ω q^1/2 / NPSH_r^3/4                                (1)

where

N_ss = Suction Specific Speed

ω = pump shaft rotational speed (rpm)

q = flow rate capacity (m³/h, l/s, m³/min, US gpm, British gpm) - for the pump at Best Efficiency Point (BEP)

NPSH_r = Required Net Positive Suction Head - for the pump at BEP (m, ft)

Suction Specific Speed can be compared with Specific Speed but instead of using the total head for the pump the Required Net Positive Suction Head (NPSH_r) is used.

N_ss have the same value for geometrically similar pumps. As a rule of thumb the Specific Suction Speed should be below 9000 (calculated with US gpm) to avoid cavitation and unstable and unreliable operations. Empirical studies indicates that safe operating ranges from Best Efficiency Points (BEP) are more narrow at higher Suction Specific Speeds.

Specific Suction Speed - N_ss - Pump Calculator

This calculator can be used to calculate the Suction Specific Speed for a pump.

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Note! When comparing pumps and their documentation - be aware of the units used.

Convert between Imperial units (gpm) and Metric units (m³/h, l/s)

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

Example - Actual NPSH_a vs Required NPSH_r

For a pump with flow 1000 gpm, head 500 ft, speed 3000 rpm and a maximum acceptable Suction Specific Speed 9000 - the Required Net Positive Suction Head can be calculated by modifying (1) as

NPSH_r = (ω q^1/2 / N_ss)^4/3

          = ((3000 rpm) (1000 gpm)^1/2 / (9000))^4/3

          = 23 ft

The calculated Required Net Positive Suction Head - NPSH_r - is the minimum head required for proper operation. It is common to add a safety margin when calculating the Actual Net Positive Suction Head - NPSH_a - for the installation.

NPSH_a = NPSH_r S_r                             (2)

where

S_r = safety ratio 

For the example above we assume the safety ratio S_r to be 1.5 (50%). Actual NPSH_a with the safety margin can then be calculated to

      NPSH_a = (23 ft) (1.5)

                = 34.5 ft

Note that if the head at the suction side of the pump is lower than the required NPSH_a - the head can be increased by

• increasing the dimensions on the suction pipes
• shorten the suction pipes
• remove or reduce the number of components like valves or filters in the suction pipes
• increase the static pressure in the system
• lower the pump (increasing the static pressure in the pump)   

Example - Suction Specific Speed

For a pump the actual Net Positive Suction Head - NPSH_a - available in the process line is determined to 20 ft. For a pump with rotational speed 1750 rpm and flow rate 500 US gpm - the operating Suction Specific Speed with this NPSH_a can be estimated to

N_ss = (1750 rpm) (500 gpm)^1/2 / (20 ft)^3/4

       = 4138

• well below the limit 9000 to avoid cavitation

We can use the same safety margin as in the example above and calculate NPSH_r by modifying (2) to

 NPSH_r = NPSH_a / S_r    

           = (20 ft) / (1.5)

           = 13.3 ft

Double Suction Type Pumps

For a double suction pump the flow at the inlet is divided by two. Using a double suction pump is one way of meeting system NPSH and obtaining a higher head.