One of the important aspects to keep in mind when installing a pump in a plant is NPSH (net positive suction head). There are two values of NPSH: NPSH available calculated by the plant engineer, and NPSH required which depends on the characteristics and performances of the pump. So why are these data so important?
The term is normally shortened to the acronym NPSHa where the ‘a’ denotes ‘available’. It is the result of the absolute pressure of a fluid at the inlet to a pump minus the vapour pressure of the liquid.
This is a term used by pump manufactures to describe the energy losses that occur within many pumps as the fluid volume is allowed to expand within the pump body. This energy loss is expressed as a head of fluid and is described as NPSHr (Net Positive Suction Head requirement) where the ‘r’ suffix is used to denote the value is a requirement. Different pumps will have different NPSH requirements depending on the impeller design, the impeller diameter, the flow rate, the pump speed and other factors. A pump performance curve will usually include a NPSHr graph so that the NPSHr for the operating condition can be established.
The essencial condition to have a good pump functioning is that NPSHa is higher than NPSHr to avoid cavitation.
Cavitation is the formation of gas bubbles when the pressure within a fluid falls below its vapour pressure. If a fluid which contains gas bubbles is allowed to move through a pump, it is likely that the pump will increase the pressure within the fluid so that the gas bubbles collapse. This will occur within the pump and reduce the flow of delivered fluid. The collapse of the gas bubbles may cause vibrations which could result in damage to the pipework system or the pump. This effect is known as cavitation.
Correct pump selection
In order to choose the correct pump size, it’s always important to know the NPSHa and check if it is suitable with the NPSHr of the pump. Once the relationship between these values is right the pump will work correctly and cavitation will not occur.