two-speed with solar panels

[mas985]

OK it looks like I failing misserably at explaining this so I will try again. Let's start with fundamentals and then I will answer your questions below.

Head loss in a pump and plumbing system consists of several factors.

Static head: This is the net elevation change from the suction port to the return port. In a water filled pipe that goes up 100 feet and loops back to the ground, there is 0 static head. There is only static head in plumbing systems such as water towers where the water ends up at a different elevation. However, there can be temporary static head while priming piping that goes up to your roof. Once the pipe is filled, the static head gets cancelled out.

Dynamic head: This head loss is due to the water movement inside the pipes and is sometimes called friction loss. A pool's plumbing system will experience dynamic head loss on both the suction side of the pump and return side of the pump since water is moving through pipes on both sides.

So total head loss = Static Head + Suction Dynamic Head + Return Dyanmic Head

Return dynamic head can be estimated from a filters PSI reading with this equation:

Return Dyanamic Head = 2.31 * Filter PSI

Suction dyanmic head can be estimated from a vacuum guage at the input to the pump, if you have one, with this formula

Suction Dynamic Head = 1.13 * Inches Mercury (vacuum guage reading)

Also, it is important to understand the pump affinity equations, when going from high speed to low speed:

RPM Low Speed = RPM High Speed / 2
GPM Low Speed = GPM High Speed / 2
Dynamic Head Loss Low Speed = Dynamic Head Loss High Speed / 4
Static Head Loss Low Speed = Static Head Loss High Speed (this is why high speed is required for priming)
Filter PSI Low Speed ~ Filter PSI High Speed / 4

So I think most of the confusion is due to the priming of the solar panels. Initially, the panels will have air in them and no water. So when the pump is first turned on, it must first lift the water to the top of the roof before the static head can be cancelled out. It is this intial lift which requires the the high speed of the pump. Once the water has been lifted to the roof, it will start to drop on the other sided of the panels and that is when the static head gets cancelled out. At this point, you can immediately switch over to low speed and the flow will continue with a drop in PSI but not so much as to trip the vacuum valve.

If you turn off the pump, allow the panels to drain and then turn on low speed, there will not be enough power to reprime the panels. However, if you switch over fast enough, there is not time for the panels to drain so low speed will be ok. Automatic controllers have this feature which runs the pump at high speed for 3 min and then switches to low speed automatically.

So with the numbers you gave me,

Filter PSI no Solar High Speed = 16 PSI
Filter PSI with Solar High Speed = 23 PSI

Solar is adding 7 PSI.

using the pump affinity equations at low speed you will get

Filter PSI no Solar Low Speed = 4 PSI
Filter PSI with Solar Low Speed = 5.75 PSI

Note that 5.75 PSI is the equivalent of 14 ft of static head which is not enough to prime onto a two story roof. Perhaps a one story but I doubt it.

On to the questions:

OK. But can this work if the high point in the loop is higher than 13 feet over the pool water level, and there is a valve at that point that opens at negative pressure? (Taking atmospheric pressure as the origin.)

Yes, because after the pipe is filled on the pump's high speed, there is no more static head loss and you are left with only dynamic head loss which should be less than 14 feet when the pump is at low speed.

If you think this works, then it means that the pump at low speed generates enough head to get the water all the way to the top, and there is no need for the high speed.

As I explained before, you must prime the pipes with high speed before switching over to low speed. Low speed will probably not have enough head to overcome static head but does have enough for the dynamic head without static head so the panels must be filled with water before switching to low speed.

More specifically: you say the low-speed dynamic head is 13ft, or 5psi (by the way, 13ft is closer to 6psi).

No, because I also included suction head. 5 PSI * 2.31 + 1.3 in.mg. * 1.13

So on the way up to the panels, the pressure drops, both because of friction and height.
Yes

What's the pressure in the pipe at the top? Is it 2psi? 1psi? Or -1 psi? (The reference point is atmospheric pressure, about 15psi at sea level, so -15psi is the lowest value.) If you give me a positive number (or even 0 as a limit case), I'll tell you that in this case 5psi is enough to get the flow started, not just maintained. If you don't agree, please explain why. If you give me a negative number, I'll tell you that that's not possible, because the valve will open. If you don't agree, please explain why.

Again, this depends on what stage of priming and high or low speed. Pressure will drop the further away from the pump you get until the pool which is 0 PSI.

So let's assume that the pipe is fully primed with the high speed. If we assume 2/3 of the 7 solar PSI loss is from the filter to the vacuum valve, then the PSI drop is about 4.5 and the PSI at the vacuum valve is 18.5 PSI. The rest is lost in the remaining plumbing on the way to the pool return.

For low speed, the calculation is similar. But all the PSIs are 1/4 the value at high speed so the PSI at the vacuum valve will be 4.25 PSI. This is more than enough to keep the valve closed.

To ease your mind, people with pools and two speed pumps have been operating on low speed with solar for years so no worries.

I am sure it doesn't hurt to have the higher speed available. I still don't see why it's necessary. I agree completely that the two-speed pump makes sense if there is a siphon effect. But the valve prevents that effect. What's wrong with this reasoning?

Again, you are ignoring the intial priming of the pipes. Head loss is high while the pipes are filling with water which requires high speed. After the pipes are filled, then you switch to low speed. When the pump is off, all the water drains back out again so the next time you turn on the pump, you must use high speed again.

By the way, I just took readings of the pressure at my filter. It's 16 psi without solar. When solar kicks in, the reading rises to 24 psi, then drops down to 23 psi. That's well over the static head for the panels, so I am positive I could run at much less power and flow.

Remember that at low speed, the PSI is only 5.75 or 14 feet of static head.

Hope this helps and let me know if I am still not clear.

[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.

[semenzato]

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.

I completely agree with this. 100%.

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.

I see. If the flow is too low, you could end up with air permanently stuck in the pipe. This I believe. In fact, if you make the pipe wide enough, that will happen at any flow. You probably need a turbulent air-water mix to remove all the air. Which implies some minimum flow.

But why is it bad to have air permanently stuck in the pipe? Mmmm... maybe I see now. With air stuck in the pipe, the potential energy of the water as it falls down is wasted instead of contributing to the suction. So you don't get the pressure reduction at the top. Ah ha.

So that's the issue. You want to get rid of the air from the pipe to make the system work more efficiently. And that requires the high speed. Now I believe you.

However, if the pressure near the top is relatively low, then you don't care, because in any case you never want to have negative pressure. So the 1-speed pump works fine, even with air in the pipe, as long as it barely makes it over the hump. Which is probably not a healthy situation.

I think maybe I will stick to my 3/4 HP pump!

Thanks again!