Lessons · Engineering · Cavitation: the margin that stops it
The margin that stops a pump destroying itself
Cavitation is vapour bubbles forming at the pump inlet and collapsing violently inside it. It is prevented by keeping the available net positive suction head (NPSH) above what the pump requires. Available comes from the installation; required comes from the pump.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
A cavitating pump sounds like gravel and wears out its impeller in weeks. It is almost always an installation problem -- the pump was mounted too high, or the suction line is too long or too narrow -- and almost never solved by changing the pump.
How to think about it
Compare two numbers with a margin between them: available must exceed required, not equal it. If it does not, work on the suction side -- lower the pump, shorten and widen the suction line, reduce the lift -- because that is where available comes from.
Worked example
Available is a property of the installation.Suction pressure, lift, losses, vapour pressure.
Required is a property of the pump.From its curve, and it rises with flow.
Available must EXCEED required, with margin.Equality is not a design.
Fix the suction side, not the pump.That is where the available figure comes from.
Your turn
Which of the two NPSH figures is a property of the installation rather than the pump?
The figure comes from the installation
Solve one, graded on the server
The trap
Buying a bigger pump to cure cavitation. A larger pump usually requires MORE suction head, so the same installation cavitates harder.