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A pump with capacity of 100 m3/h and pump head of 75 m, base on suction drum elevation, fluid condition and suction piping condition, the calculated available Net Positive Suction Head (NPSHa) is 2m. This NPSHa shall be verified with NPSH required (NPSHr) by selected pump.How would you know if NPSHa is sufficient before the pump vendor provide the information on pump required Net Positive Suction Head (NPSHr) ?
Horizontal axis : Specific speed (Ns)
Vertical axis : Thoma Sigma number, Sigma = NPSHr / H, where H=pump head
Incline line : Suction specific speed (Nss)
All points above the pink line are indicates pump operate in safe region.
From above example, the calculated pump specific speed (Ns) is about 996. From Thoma cavitation curve, with Ns=996 vertically upward cross the pink limiting line, read horizontal to the left, the Thoma sigma number is about 0.04.
Pump required Net Positive Suction Head (NPSHr) is 0.04 x 75 = 3m (more than NPSHa of 2m). This indicated that the pump potential cavitate under normal operation as NPSHa less than NPSHr. Quick action to rectify the drum elevation & pump suction piping design may required to avoid potential future change.
Above pink limiting line was based on suction specific speed, Nss = 11000. In many event, pump manufacturer may design pump with normal Nss of 8000-8500.
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