oscillation in pool loop with above-roof solar panels

[semenzato]

When I am using the solar panels on the roof and the pump shuts off, I hear annoying and possibly damaging noises for 20-30 minutes after shut off. The loop has a transparent check valve after the filter, and I see that every 3-4 seconds it is being pushed shut with a fairly loud bang (hammer effect), then it reopens.

I am suspecting some odd dynamics that have to do with the vacuum relief valve. The system had a very old valve and I changed it recently, but that seems to have worsened the problem (faster banging). I noticed that the new valve has a surprisingly low air flow, probably lower than the old valve. (I noticed this by sucking on the valve before I installed it.) I am thinking that this may be happening:

1. When the pump is turned off, the check valve is still open.
2. Water starts flowing down from the panels in both directions: back towards the filter, and forward towards the pool.
3. the water moving back eventually moves fast enough to close the check valve.
4. the inertia of the water moving forward produces a suction which is not relieved quickly enough by the valve. So it pulls water from the filter, and the valve is reopened, etc. etc.

Note that the air that the valve lets into the pipes acts as a "spring", so now you have a two-degree differential equation (inertia + spring, same as inductance and capacitance) plus some non-linear effects (the valve) and this is enough to create an oscillator.

This keeps happening until enough air has entered through the valve, so I am thinking that a different, faster valve may solve the problem. But how do I find out which valve would be good enough? Those valves are unreasonably expensive ($50 or so), so trial-and-error is not good. Could I use a check valve instead? Will it seal well enough?

I was also thinking of using a few irrigation vacuum relief valves, which are a lot cheaper.

Any suggestions as to the correctness of my diagnosis, and possible solutions?

Thanks!