Net Positive Suction Head
A limiting factor on suction performance of a centrifugal pump is the Net positive Suction Head, NPSH, required at the intake of the pump to avoid cavitation.
Cavitation occurs when the Net Positive Suction Head Available at the suction system at this point, is less than the Net Positive Suction Head Required by the pump. Cavitation is the formation of pockets of bubbles of vapour at points within the pump where the NPSH falls below the vapour pressure of the liquid, and the subsequent collapse of the bubbles as they flow into an area of higher pressure. Cavitation has the following effects on the performance of the pump:
1. A drop in the total head developed, resulting in a reduced systems flow rate.
2. A drop in the efficiency of the pump.
3. A rumbling or crackling noise together with vibration can result in mechanical failure of the pump.
4. Excessive wear of the impeller usually occurs.
The collapse of bubbles is also referred to as the implosion of bubbles causing the erosion of the impeller.
The formula for calculating pump installations are as follows:
NPSHa = Hatm – Hvap + Zs – Hi – Hfs for gravity feed
NPSHa = Hatm – Hvap - Zs – Hi – Hfs for suction lift
The NPSHa must be carefully evaluated for all duties above sea level, including gravity feed to ensure that it is not less than the NPSHr.
High temperatures associated with high Absolute Vapour Pressure will reduce the value Hatm – Hvap and subsequently the value of NPSHa.
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