You are getting a resonant oscillation due to some sort of variable-space and the pipe run topography. The variable space allows for energy storage to support the oscillation.
It's rather like a capacitor in a tuned electrical circuit.
It can be remedied by changing something - anything - that shifts the system resonance point to somewhere that fails to resonant.
Things to consider:
The valve's gate may be loose in it's channel. A gate valve can commonly suffer from this. The gate then rattles providing the initial source for the oscillation.
The pump can run at a speed or have an impeller pitch that drive the problem.
There could be air trapped in the system that provides the energy storage.
There could be a flexible section, like a hose, that can expand and contract to store the energy.
There could be an accumulator or anti-water hammer unit that stops water hammer but causes this problem.
A long stretch of straight pipe can exacerbate the issue.
What you need to do is 'change things' in the hope of moving the system's resonance point somewhere the system has no excitation frequencies at or remove the driving source,(like a loose valve gate).
Examples:
Put a valve on the other end of the piping and partially close it.
Add inline direction changing fittings.
Change to a different type of valve.
Change pump speed.
Change pump type.
Remove anti hammer accumulator(s).
Remove flexible sections.
Add flexible sections.
Increase pipe diameter.
Reduce pipe diameter.
Add a nozzle after the valve.
Change pipe diameter in a section.
You get the point. Scan my list and do what you think is easiest first and work you way thru the system until it stops. Any of several of the above suggestions or any of ones you can think of will likely solve your problem.
Do let us know what worked as this is a common problem and we love to hear the solutions.
Keith Cress
kcress -
