NPSH in pump

NPSH Calculation:

In addition to the total head, capacity, power and efficiency requirements, the condition at the inlet of a pump is critical. The system on the inlet side of the pump must allow a smooth flow of fluid to enter the pump at a sufficiently high pressure to avoid cavitation.

This is called the Net Positive Suction Head, generally abbreviated NPSH. Pump manufacturers supply data about the net positive suction head required by their pumps (NPSHr) for satisfactory operation. When selecting a pump it is critical the net positive suction head available (NPSHa) in the system is greater than the net positive suction head required by the pump. A simplified way to look at NPSHa. is to imagine a balance of factors working for (static pressure and positive head) and against (friction loss and vapor pressure) the pump.

The value of NPSHa in the system is dependent upon the characteristic of the fluid being pumped, inlet piping, the location of the suction vessel, and the pressure applied to the fluid in the suction vessel. This is the actual pressure seen at the pump inlet. It is important to note, it is the inlet system that sets the inlet condition and not the pump.

 It is calculated as follows:

 NPSHa = Pa ± hs - hfs – Pvp 

Where:

Pa = Pressure absolute above fluid level (bar).

Hs = Static suction head (m).

Hfs = Pressure drop in suction line (m).

Pvp = Vapour pressure (bar a). 

Net Positive Suction Head -NPSHr 

The definition of Net Positive Suction Head required (NPSHr) is the reduction in total head as the liquid enters the pump. NPSHr is experimentally determined by several methodologies. One procedure is operating the pump under study with clear water while incrementally reducing NPSHa by throttling a suction valve. The on-set of cavitation is then observed and recorded at controlled flow rates.

How do we accurately determine the on-set of cavitation? It is an approximation at best, but has been officially defined as corresponding to a 3% drop in total developed pump pressure, i.e., discharge head. Obviously there is sufficient cavitation that is already occurring to produce this 3% reduction in discharge pressure. The actual start of cavitation is known as incipient cavitation and it always occurs well before the point of NPSHr is reached.

An important consideration with respect to the evaluation of NPSHr is that of the liquid’s vapor pressure. Pump performance curves plot NPSHr versus flow; these data points are experimentally determined by conducting tests utilizing water. If in the practical application the fluid being pumped exhibits a vapor pressure exceeding that of water, it should be intuitively obvious that NPSHr values provided on the pump manufacturer’s standard performance curve cannot be considered reliable. Practically, in order to compensate for system variations and incorrect analytical assumptions.

NPSHa >> NPSHr

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NPSH in pump