NPSHA in the system is calculated as :
NPSHA = Terminal Pressure in the vessel (in guage)
(+) Static Head of fluid above pump centre line (see note).
(+) Atmospheric Pressure
(-) Vapour Pressure of liquid at pumping temperature
(-) Friction loss in suction piping up to pump centre line consisting of the following : Entrance and exit losses
Loss in suction strainer
Loss in control valves, instruments, exchangers etc. if any Line losses
Note : a) The height of liquid in the vessel should be taken to be at the vessel bottom tangent line. b) Height of the pump centreline is calculated as follows :
HPC = HPB + F where
HPC = height of pump centreline above finished floor level
HPB = height of pump centreline above pump baseplate
F = height of pump foundation above finished floor level
HPB is estimated from the following table based on typical vendor information : Pump Flow Rate Height of Pump Centreline
(m3/hr) above Base Plate HPB (mm)
upto 200 450
200 - 300 650
300 400 750
The height of foundation F is typically taken at 300 mm.
Calculate NPSHA carefully considering all Conditions i.e. start-up, original fill of lines, winter operation, all control valve pressure drops, summer operation, piping conditions, exit and entrance losses. Provide NPSHA at 1 meter over worse conceived NPSHR curve by the manufacturer. In general, NPSHR of pumps should be considered as 3.0 meters, though the exact requirement, will be specified by the pump manufacturer. In case of submersible pumps, the available submergence should be specified. NPSH is a function of flow, it should always be determined at design capacity regardless of the total head required. It is not uncommon for NPSHR curve to turn upward at low flows. Low flow effects can often be amplified by selecting too large a pump which forces re-circulation within the pump. When liquids at their bubble points are pumped from closed vessels, NPSHA is only the static liquid head above the pump centre...
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