Note! When comparing pumps and their documentation - be aware of the units used.
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
NPSHr = (ω q1/2 / Nss)4/3
= ((3000 rpm) (1000 gpm)1/2 / (9000))4/3
= 23 ft
The calculated Required Net Positive Suction Head - NPSHr - is the minimum head required for proper operation. It is common to add a safety margin when calculating the Actual Net Positive Suction Head - NPSHa - for the installation.
NPSHa = NPSHr Sr (2)
where
Sr = safety ratio
For the example above we assume the safety ratio Sr to be 1.5 (50%). Actual NPSHa with the safety margin can then be calculated to
NPSHa = (23 ft) (1.5)
= 34.5 ft
Note that if the head at the suction side of the pump is lower than the required NPSHa - the head can be increased by
For a pump the actual Net Positive Suction Head - NPSHa - 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 NPSHa can be estimated to
Nss = (1750 rpm) (500 gpm)1/2 / (20 ft)3/4
= 4138
We can use the same safety margin as in the example above and calculate NPSHr by modifying (2) to
NPSHr = NPSHa / Sr
= (20 ft) / (1.5)
= 13.3 ft
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.
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