two-speed with solar panels

[cleancloths]

mas985 - that was an excellent explanation and it would be great if that could be made a sticky or a FAQ for the board. You did a much better job of simply stating what I was trying to say. Thanks.

[joenj]

Just one foggy thought left for me. When my sand filter pressure goes up, needs a backwash, the panels do not stay full with water. You start to see bubbles in the returns and can hear the vacuum break opening/closing. This is my audiable alarm to backwash!

Is this an indication how a low speed pump would work on my pool? With a low speed at 1/2 gpm, is there enough flow to keep the vacuum break closed, with a clean filter? Right now I have a 1hp pump and the panels' highest point is 20'.

Joe

[mas985]

Each pool is unique in operation and your results may vary. First, at low speed the GPM is 1/2 of the high speed value not 1/2 GPM. So if your normal flow is 70 GPM, at low speed it will be 35 GPM.

When a filter gets dirty, the pressure ahead of the filter goes up and the pressure after the filter goes down. So, the problem you are experiencing could be due to this pressure decrease when the filter is dirty. You do not have enough pressure to fully prime the solar pipes. It sounds like you are waiting too long before cleaning your filter.

Another reason could be that your vacuum release valve is not operating properly but difficult to prove. Normally, they only require a few PSI to close and negative PSI (vacuum) to open. So if it is not closing and you have enough pressure after the filter (hard to determine) then it could be a malfunctioning valve.

Of the two, my guess would be the first problem. So the solution is to keep your filter clean. With a clean filter, there should be no problem with low speed but I would need to know the pressure readings that you currently have.

Pump model, HP, Service Factor
Clean Filter Solar Off PSI
Clean Filter Solar On PSI
Dirty Filter Solar Off PSI
Dirty Filter Solar On PSI

With these numbers, I can assess if a two speed would work for you.

[joenj]

Mark,

I meant 1/2 gpms of the high speed pump not 1/2 gpms total. My bad for not clarifing that. I was guessing a low speed rate may act like a dirty filter.

I understand your explanation, and believe the dirty filter is the problem.

My system is a Fafco Sun Saver 5- 4x12s panels.

pump- Waterway svl56 1hp 1.25sf Clean Filter Solar Off PSI 14psi
Clean Filter Solar On PSI 16psi(17psi at first, then drops to 16 when all air is purged)
Dirty Filter Solar Off PSI 20psi, time to backwash
Dirty Filter Solar On PSI I need to verify this reading.

Thanks for looking at my numbers.

P.S. I agree with Cleancloths with the STICKY recommendation.

Joe

[mas985]

If the highest point of your solar panel is 20 ft above the pump, you will need at least 9 PSI to get to the top of the panels, 2 PSI of friction loss and another 2 PSI or so to close the valve for a total of 13 PSI. By my estimates, you will have about 12 PSI after the dirty filter or 1 PSI at the top of the panel which may not be enough to close the valve. I suggest cleaning after 4 PSI rise instead of 6.

As for two speed, as long as the pressure does not drop below zero PSI when switching to low speed, the valve should stay closed. But this does require a proper prime at high speed so all of the air is purged from the system. If not, when the pump is switched to low speed, the valve may open.

[semenzato]

Thanks for the excellent explanation. It's quite clear and I agree with everything except one part: the high-speed priming to fill the system. To be fully honest, I think that either what you say is wrong, or there is some factor at play that we haven't discussed.

It all boils down to this. To keep the valve closed, the pressure near the valve must be positive. I believe that pressure and flow completely describe the state of the system near the valve. To be concrete, suppose we have 2 psi and 40 GPM.

Suppose there is a diverter at the top that when turned, instead of sending the water to the panels, it sends it into a wide pipe going straight up for 4.6ft, open at the top. The pipe is wide so dynamic loss is negligible. We divert the flow. What happens?

In my book, the water goes up the open pipe and overflows, at an almost identical rate. The pressure near the valve is still about 2 psi (2 psi x 2.31 = 4.6 ft) and the flow is about the same. At the pump, there is no way of telling if the water is going through the panels or falls out of the pipe extension.

So it would seem that our pump (at low speed) is capable of lifting water to the valve, actually a bit higher than that, with plenty of flow. This pump, according to you, is not able to prime the system. Why? Sure, there is a period of transition as the pipe fills up. But the pressure at the pump can never be higher than when the pipe is full. Surely there is dynamic head, but that is also always less than when the pipe is full.

Then there is no need to prime at higher speed or power.

Joe's (joenj) contribution is revealing and made me realize that in fact there is one more variable we can play with: the jet diameter.

Just one foggy thought left for me. When my sand filter pressure goes up, needs a backwash, the panels do not stay full with water. You start to see bubbles in the returns and can hear the vacuum break opening/closing. This is my audiable alarm to backwash!

As Joe's filter gets more and more clogged, the after-filter pressure drops, the pressure at the top drops, finally becomes negative (or near there, depending on how the valve is built) and the valve opens. What happens then? The water on its way down (which is "responsible" for the negative pressure) flows at a higher rate that the pump can supply, and air is sucked in. As the amount of air in the pipe increases, there is less water in the pipe, consequently less weight. The suction drops (that is, pressure increases), and the valve closes again. The frequency of opening/closing depends on the hysteresis of the valve, and possibly other factors.

But the flow of the water on its way down can be reduced by narrowing the jet outlets. If you can live with the lower flow, that will make the system work again.

This also means that as you get close to zero pressure at the top, the dynamic loss on the down pipes is no longer relevant when computing the savings due to a lower flow. On the up pipes, it still is. (Sorry, I should explain this more clearly, but it's dinner time.)

I should say that it's VERY helpful to be able to have this kind of detailed discussions. I cannot thank enough all those involved.

[mas985]

I deleted my original response because I realized that I could make this real simple.

If you look at the head curve of a 3/4 HP two speed pump, you will notice that at 0 GPM it reaches a maximum head of about 20 feet or so. This means that with a pipe 20 ft in the air, no water will come out of the top, 0 GPM. There is a good chance you will burn up the pump in the process too.

If the vacuum valve requires 2 PSI to close, then the maximum head loss possible is 15 feet (20-5). If your roof is lower than 15 ft, you may have a chance at closing the vacuum valve. If not, you are out of luck. Its that simple.

Even if the valve does close, you need quite a bit of flow to push the air downwards to completely prime the pipe. So in reality, your are limited by much less than 15 ft.