This document is intended to aid engineers who do not solely deal with pumps understand the concept of Net Positive Suction Head (NPSH). The document sets out important characteristics that need to be understood when considering the suction requirements of a pump to meet a given duty. Understanding these concepts in the pump selection stage will overcome problems later which are difficult to fix.

A fundamental law in the engineering world is mass must always be conserved it is therefore the case that what comes out of a pump must first be sucked into a pump. It must be acknowledged that though there is a large amount of control over the discharge of a pump, many natural phenomena affect the liquid flow into the pump. It is therefore important the following factors are considered when specifying a pump:

1. Pipe Restriction

This is a factor that the plant design team will have some control over as it is dependent upon the pipe diameter, pipe fittings, valves, bends, etc. These factors need to be considered as there will be a certain head loss of the fluid as it passes through these.

2. Vapour Pressure

This is a physical characteristic of any fluid, and varies with the fluid being pumped. The vapour pressure is the pressure at which the liquid will vaporise to gaseous form.

3. Pump NPSH Requirement (NPSHR)

This figure is the necessary amount of energy required (measured in metres) in the liquid at the pump inlet to overcome the internal losses/resistances within the pump and provide sufficient internal pressure to avoid cavitation. These losses are caused by the flow of liquid through the pump suction passage and the shock loss which occurs at the impeller blade. The NPSHR is calculated by the pump manufacturer and will vary depending on the size and speed of the pump.

4. Available NPSH (NPSHA)

The available NPSH (NPSHA) is the amount of energy (measured in m) available to the fluid at the pump inlet after the factors from section 1 have been taken into account. For a pump to run the NPSHA value must be greater than the NPSHR value. The NPSHA value will govern the amount of suction lift which may be attained with a given pump or the amount of static suction head which is required above pump suction to ensure correct operation.

5. The Head Equation

Pump manufacturers will always quote in metres liquid column when talking about head values, to convert values of pressure and velocity to metres liquid column the following equations should be used;

Pressure Head =

Velocity Head =

So the total head equation is given by;

Total NPSH =

This equation takes in the total head at the pump inlet. For NPSHA calculations if a vessel is located at a height z above the pump centreline this is also added in. The following examples are designed to give a better understanding of the head equation and NPSH.

6. Three examples are shown to give a clearer understanding of NPSH.

Example 2 shows how temperature dependent the suction head calculation can be. In example 1 with the water we have 8.26m of suction lift however in example 2 to overcome the increase in vapour pressure with the water at 95ºC we require an additional 0.06m of head for the pump to operate correctly as the atmospheric pressure does not give enough NPSHA to overcome the increase in vapour pressure.

If this extra head requirement was not accounted for the water would vaporize within the pump and lead to cavitation.

In this case the Dickow SC 6565 will be suitable as from the pump curve at 16m3/h it has an NPSHR of 1m.