Ron Astall of United Pumps challenges the concept of NPSH, or net positive suction head.
Does the term ‘NPSH’ make sense?
While NPSH, which stands for net positive suction head, is a critical concept in pump systems, I am, perhaps controversially, recommending we change the name of this long-accepted and critical parameter. I believe it is a flawed and misleading misprint.
People who only occasionally deal with pumps and piping systems can be excused for their confusion.
Why the confusion?
Consider two simple examples. In Figure 1, the pump has a positive liquid level 2.5m above the pump inlet. Does this mean the net positive suction head is 2.5m?
In Figure 2, the pump is situated 1.5m above the liquid level. Does this mean the pump has a net positive suction head of minus 1.5m?
Of course, these calculations are quite wrong. Experienced pump and system people may cringe, but I’ve been given NPSH values calculated this way many times. I’ve frequently had discussions along the lines of, “The liquid level is three metres above the pump, so I have roughly three metres of NPSHa”.
Why these calculations are wrong
NPSH is a parameter that tells us how close the liquid is to vapourising or boiling. Put simply, NPSH is the head margin above the liquid vapour pressure. It is the absolute pressure less the vapour pressure, then converted to head units.
NPSH must always be calculated using absolute pressures, which are never positive nor negative, they are absolute.
Levels and gauge pressures can be either positive or negative. The term “positive” embedded in net positive suction head implies, quite reasonably to many, that they can use levels and gauge pressures to calculate NPSH. This is why NPSH is often miscalculated.
NPSH, that well-known misprint
Let’s get rid of the word “positive”. The parameter we are calculating is the usable or “net” head margin, based on absolute pressures and heads. Let’s be bold and rename this parameter ‘net absolute suction head’ (NASH). It means the same thing and describes the parameter correctly.
In figure 1A and 2A, these calculations are completed again with absolute pressure/head values.
I am encouraging brave pump manufacturers to put NASHr on their performance curves and brave specifiers to advise the NASHa on their pump data sheets.
Why is NASH important?
Net absolute suction head (NASH) is a critical parameter that allows us to assess whether a pump will be affected by vapourisation of the pumped liquid at the inlet. Vapourisation (or boiling) can occur with any liquid if the local pressure drops below the vapour pressure. This phenomenon is called cavitation, and it is a pump wrecker.
NASHa, which is the available head margin over the liquid vapour pressure at the pump inlet, is to be calculated by the system designer.
NASHr is what the pump needs to avoid cavitation and must be advised by the pump manufacturer (currently called NPSHr in pump specifications).
Much conjecture on the best way to express this information abounds, such as “cavitation free” curves and NASHr3 curves etc. Further discussion on some of these aspects could run to thousands of pages and is beyond the scope of this discussion.
To avoid cavitation, system NASHa must always exceed pump NASHr.
Conclusions
NASH, which is the head margin above the liquid vapour pressure, is the absolute pressure less the vapour pressure, converted to head units.
From now on, think net absolute suction head (NASH) when assessing suction conditions. If you see the term NPSH, it really means net absolute suction head, so call it by its proper name.
Am I creating more confusion? Maybe. Hopefully this discussion will generate a better understanding of the physical meaning of this critical parameter.
Think of NASH as the parameter formerly known as NPSH.
This feature appeared in the Summer 2025/26 edition of Pump Industry.
