What is the difference between NPSHa and NPSHr?

Let’s start with what NPSHa and NPSHr mean. NPSHa stands for Nett Positive Suction AVAILABLE and NPSHr stands for Nett Positive Suction Head REQUIRED.

Now, what is the difference between NPSHa and NPSHr? Well, firstly NPSH is a measure of the pressure experienced by the fluid at the suction of the pump. This is always quoted in metres (or feet) rather than as a pressure because head is a fluid independent property. Keep in mind that a pump will always lift liquid to the same height regardless of the fluid's respective densities.

NPSHa (available) is a property of the system and is calculated by the system designer giving a value of the pressure on the suction side of the pump.

NPSHr (required) is a property of the pump. This is calculated by the manufacturer as the point where cavitation occurs.

It is good practice to have a safety margin between 0.5m and 1m.

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