famiri,
Thanks for the question… Here is the response I received:
On the face of it his problem is a water hammer one, but not the type associated with long pipes. As one pump stops (and I presume it’s a centrifugal type), reverse flow can flow through it once the impellor slows down enough, which can be rapidly if its moment of inertia is small, which it probably is. With a significant pressure rise over the pumps, there’s a big driving force for reverse flow, so reverse flow can build up rapidly. He put non-return valves on the outlets, which is necessary for a parallel pair of pumps. However he used swing check valves, which are the ideal way to create water hammer – the reverse flow builds up significantly before it drives the valve shut, when the slam stops the flow suddenly, hence the spike pressure and probably noise in the system. There’s a chance that repeatedly doing this will lead to a mechanical failure.
How does he fix it? He has two alternatives:
1. Use motors with much higher moment of inertia to prevent the reverse flowrate building up much before the valve closes. Ideally fit them with flywheels. However these are not readily available. The only one I installed we had to make specially for the job.
2. Use a better non-return valve. There are valves made specially to counteract this problem. They are spring-assisted or fluid padded and close gently before any significant reverse flowrate can build up, hence the problem is avoided. The detailed design isn’t simple, but manufacturers will help select he right one. Suggestions: Goodwin (http://www.checkvalves.co.uk/en/products/nozzle-check) , or Val-matic http://www.valmatic.com/checkvalves.html
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