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!

[mas985]

Just a couple of comments and questions. Regarding your numbered points.

#3 - That should happen immediately. Are you saying it doesn't?

#4 - If true, any suction would happen at the top of the panels near the VRV, not at the filter. It is the low pressure that opens the VRV but pressure at the filter should be higher because of the weight of the water? Is the equipment much higher than the pool?

Even if the VRV was very restrictive and only allowing a small amount of air into the system, it just means the water would slowly drain from the panels. That shouldn't allow an oscillation to be set up. My VRV is quite restrictive as well but I never experienced what you are. So I wouldn't expect that to be the issue.

Do you have a solar valve that shuts off when the pump shuts off?

Do you have air in the pump basket when running?

When the pump is running, is there air in the filter? This could be causing the oscillation.

Can you describe the solar setup in detail?

Height of the panels above equipment

Height of equipment above pool

Height and location of VRV

Pictures of equipment might help too.

[semenzato]

>> #3 - That should happen immediately. Are you saying it doesn't?

The check valve closes quickly, yes, but not "immediately", unless that means "a short time". The pump slows down and stops, and the water has to change direction. It could be a fraction of a second.

>> #4 - If true, any suction would happen at the top of the panels near the VRV, not at the filter.

Any suction near the VRV will produce a lower pressure at the filter.

>> It is the low pressure that opens the VRV but pressure at the filter should be higher because of the weight of the water? Is the equipment much higher than the pool?

You're right, statically the pressure at the filter on the panels side cannot be lower than on the pool side. Dynamically, I don't know.

The equipment is actually lower than the pool, say 8 feet or so below the pool surface. The panels are something like 15 feet above the surface.

>> Even if the VRV was very restrictive and only allowing a small amount of air into the system, it just means the water would slowly drain from the panels. That shouldn't allow an oscillation to be set up.

Yes, I would expect that too. But the oscillations are there and I am trying to explain them.

>> My VRV is quite restrictive as well but I never experienced what you are. So I wouldn't expect that to be the issue.

>> Do you have a solar valve that shuts off when the pump shuts off?

Yes, but at this moment I removed the valve actuator, so the solar valve doesn't change position when the pump shuts off. The loop remains the same.

>> Do you have air in the pump basket when running?

No. Any air in there gets purged quickly.

>> When the pump is running, is there air in the filter? This could be causing the oscillation.

Probably. When the pump starts, there is air in the panels, and it is forced into the filter. Eventually it all flows out into the pool and bubbles away, but there could still be air in the filter housing in many cases, and you are right that this could cause the oscillation.

>> Can you describe the solar setup in detail?

Pump (at -8 feet) --> filter --> check valve --> solar valve --> solar panels (at +15 feet) --> SWG --> pool --> back to pump.

>> Height of the panels above equipment

23 ft

>> Height of equipment above pool

negative 8 ft

>> Height and location of VRV

At the top of the highest and furthest away panel.

>> Pictures of equipment might help too.

I can send those when I get home, but I am not sure they'll help much. It's just the usual stuff.

Thanks a lot!

[mas985]

Any suction near the VRV will produce a lower pressure at the filter.I disagree. As I stated before, the VRV is much higher than the filter so the water above the filter will create higher pressure at the filter than at the VRV. This is static head. At the instant the pump shuts off and right before the VRV opens, the pressure at the VRV is ~ -6.5 PSI and the pressure at the filter is ~ +3.5 PSI (pool level is 0 PSI). As air enters the VRV and the water drains, the pressure at the head of the water is related to the height of the water.


Probably. When the pump starts, there is air in the panels, and it is forced into the filter. Eventually it all flows out into the pool and bubbles away, but there could still be air in the filter housing in many cases, and you are right that this could cause the oscillation.The panels are plumbed after the filter so air should not be forced backwards into the filter. When priming, all air in the panels should be forced out of the panel return and then go forwards out the pool returns. Do you have a filter air purge valve? When the pump is running and when you open it, does air come out?

Also, when the equipment pad is well below the pool level, it is a good idea to install the solar valve and check valve on the side of the house above pool level instead of at the pad. Unless that is they way it is done now, that could help. The pad water pressure being higher than atmospheric could be causing an issue with the draining water. But that is the only thing I can think of causing the issue unless the filter accumulates air when the pump is running.

[semenzato]

>>> I disagree. As I stated before, the VRV is much higher than the filter so the water above the filter will create higher pressure at the filter than at the VRV. This is static head. At the instant the pump shuts off and right before the VRV opens, the pressure at the VRV is ~ -6.5 PSI and the pressure at the filter is ~ +3.5 PSI (pool level is 0 PSI). As air enters the VRV and the water drains, the pressure at the head of the water is related to the height of the water.

That's fine, thanks. I was making a feeble attempt at analyzing the dynamic behavior, but I am far from being sure on any conclusion I have made.

>>> The panels are plumbed after the filter so air should not be forced backwards into the filter. When priming, all air in the panels should be forced out of the panel return and then go forwards out the pool returns. Do you have a filter air purge valve? When the pump is running and when you open it, does air come out?

Now that you mention it, you're right, I am not so sure that there is any air in the filter housing at this point. I get air occasionally in there under other circumstances.

I don't have a filter air purge valve. I wish I could install one, but I didn't come with the filter housing, and I didn't find it as a possible accessory.

>>> Also, when the equipment pad is well below the pool level, it is a good idea to install the solar valve and check valve on the side of the house above pool level instead of at the pad. Unless that is they way it is done now, that could help.

No, right now everything is below the pool level, and I don't think it is feasible to change that---I would have to run many pipes a long way.

The only thing I could do is install another check valve higher up. Who knows, maybe it would subtly alter the dynamics and prevent the oscillation.

>>> The pad water pressure being higher than atmospheric could be causing an issue with the draining water. But that is the only thing I can think of causing the issue unless the filter accumulates air when the pump is running.

Thanks!

[mas985]

No, right now everything is below the pool level, and I don't think it is feasible to change that---I would have to run many pipes a long way.Actually shouldn't have to run anymore than you are now (2). The solar valve would just bridge the two pipes running up the house in a H configuration. The check valve would just be before the H on the supply side. Similar to below. The only difference is the pump and filter is lower than what is shown in the picture. But you only need a supply pipe and a return pipe to and from the house with the diverter valve. Also, they don't show the check valve but you should have that right before the diverter.

http://www.solarsouthwestflorida.com/wp-content/uploads/2012/12/Solar-Pool-Heater-Diagram.jpg

[semenzato]

Yes, the picture corresponds to what I have now, except that pump, filter, and diverter are about 8 ft lower. Also, as you said, I have a check valve between filter and diverter.

I also have an additional check valve on the heated water return, near (4) in the picture, probably to prevent backflow into the panels when the diverter is in the non-solar position. But since i am leaving the diverter in the solar position, that shouldn't matter.

Thanks!

[mas985]

So what I was suggesting was to move the solar valve and both check valves so all are above water level. That should not require any additional piping. It is basically moving the H ~9' higher.

[semenzato]

The house is on a hill, with the main floor level with the pool. The equipment is in a room in a floor below. There is no easy way of moving the position of those valves.

This didn't use to be a problem. I am not sure when or how it started. I made small changes here and there. For instance, I added a separate loop, with two diverter valves, to reuse the same pump, filter, heater, and chlorinator, with a hot tub, so some of the pipes changes. Evidently some of those changes were enough to create the oscillation.

[mas985]

OK then pictures might be useful.