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mas985
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Postby mas985 » Sun 03 Oct, 2010 22:25

I have only a couple of disagreements with what you posted last.

The classical definition of an open loop plumbing system in both the plumbing and hydraulic industries is a plumbing system that has ANY point where the water contacts the air and is at atmospheric pressure. This happens at the pool surface so by definition any pool plumbing with or without a VRV is an open loop plumbing system. A closed loop plumbing system is defined as being isolated from the environment where the water has no contact with the air which is not the case for a pool even one without a VRV. A section of pipe may be open or closed to the air but the entire pool system is always open. But I think we have wasted enough time on that point so let’s move on.

Allen G Myerson wrote:The flow rate, in gallons of water per minute, remains the same. It has to be the same as what is supplied. If the vacuum relief valve is open, you will get a turbulent mix of water and air. The water height will determine the velocity of the water air mix, but the flow rate remains the same.


I disagree with this statement as written. A pump’s flow rate is dependent upon the head loss (static and dynamic) that the plumbing system creates. So if a plumbing system has more head loss, the flow rate from the pump decreases. When the VRV opens and total head loss increases because of the loss of head gain on the other side, the pump flow rate will slow down. In fact, depending on the pump’s head curve it can slow down so much that the pump can become dead headed where the water will stop flowing at a certain height in the pipe. The pump will raise the water to maximum head of the pump curve and simply maintain that level. However, if the pump head curve is high enough, then water will continue to flow albeit at a slower rate but not the same rate as when the VRV was closed. Not only does an open VRV have more head loss, it will also have much less flow rate.

Third, static head is not dependent on the siphon effect but siphoning is dependent on static head differences. Static head can exist without siphoning but siphoning cannot exist without static head differences. So the canceling effect of static head has absolutely nothing to do with a siphon. A siphon cannot exist without a differnce in static head. Static head only has to do with the change in elevation, siphoning is not even involved since the two pipes are at the same height plus the system is on a pump which does not meet the criteria for the definition of a siphon where it must be gravity fed.

Other then that, I think we agree.


Mark
Hydraulics 101 ; Pump and Pool Spreadsheets ; Pump Ed 101
18'x36' 20k gallon plaster/gunite pool, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge Filter, Solar Panels, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater
Allen G Myerson

Solar Panel System

Postby Allen G Myerson » Sun 03 Oct, 2010 23:43

mas985
When the VRV opens and total head loss increases because of the loss of head gain on the other side, the pump flow rate will slow down.

The VRV prevents vacuum. Therefore, when there is a VRV, you will never get a head gain. There is no head gain to lose.

I was responding to this statement that you made:
mas985
This water height then determines the flow rate through the remaining part of the plumbing.

It seems like you were saying that the flow rate of the water returning to the pool would be determined by the height of the panels. If that is what you were saying, my point is that the flow rate can't be anything more than the flow rate being supplied. It seems like you were saying that the flow rate from the panels back to the pool could be higher than the flow rate from the pump to the panels.

mas985
So the cancelling effect of static head has absolutely nothing to do with a siphon. A siphon cannot exist without a difference in static head. Static head only has to do with the change in elevation, siphoning is not even involved since the two pipes are at the same height plus the system is on a pump which does not meet the criteria for the definition of a siphon where it must be gravity fed.

This is just not accurate. When there is not a VRV, the siphon effect does cause the static head to cancel. The weight of the water falling creates a lower pressure at the top, which equals the static head pressure. This siphon effect works whether there is a pump or not. In this case, the siphon effect is equal, which cancels the height difference.
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Postby mas985 » Mon 04 Oct, 2010 15:05

Allen G Myerson wrote: The VRV prevents vacuum. Therefore, when there is a VRV, you will never get a head gain. There is no head gain to lose.


My point was that when the VRV opens, you effectively lost some of the pressure gain from the non-VRV case because air entered the return pipe. However, this situation is not necessarily binary and can have some gradation of static head. You will only lose all of the pressure gain if all of the water is removed out of the return pipe and there is no elevation to the water level in the return pipe. Even a small amount of water will cause a pressure differential from the input to output of the pipe.

When the VRV opens, air enters the plumbing system but it also encounters head loss due to the friction loss through the VRV itself so the pressure on the inside of the plumbing at the location of the VRV does not immediately go to 0 PSI. This is much like a balloon deflating through a small hole. The entering air mixes with the flow from the pump and creates an air/water mixture which is lighter than water alone so the pressure differential across the return pipe starts to decrease as more air enters the pipe and the percentage of air increases. However, there will always remain a pressure differential between the ends of the return pipe as long as there is at least some water in the mixture creating weight at the bottom of the pipe. If the flow rate of the pump decreases, due to the extra head, enough such that water cannot collect at the bottom of the return pipe, then and only then will the pressure differential across the return pipe go to zero. For example, if the pump can provide enough flow rate so the air/water mixture remains at 50%, then half of the pressure differential between the ends of the pipe remain.

Also, because a pump is dynamic and the flow rates are changing with the changing total head, you can get into a situation where the solar system will go through cycles of the VRV closing and opening so it oscillates back and forth between two states. This is often described as periodic bursts of bubbles out of the returns rather than a constant stream.

In the case of a constant stream of bubbles, it is likely that the there is an equilibrium of water and air in the return pipe and it is still creating at least some pressure differential in the return pipe so not all of the return gain is lost.

But the bottom line is that only under the situation where the pump cannot deliver enough flow rate to at least partially fill the return pipe, does the pressure differential in the return pipe go to zero and all of the static head disappears.

Allen G Myerson wrote: This is just not accurate. When there is not a VRV, the siphon effect does cause the static head to cancel. The weight of the water falling creates a lower pressure at the top, which equals the static head pressure. This siphon effect works whether there is a pump or not. In this case, the siphon effect is equal, which cancels the height difference.


The classical definition of a siphon is a device that moves fluid from one height to a lower height using ONLY gravity. In the case that you have outlined, the two pipes are of equal length, contain equal amounts of water and are at identical elevation so a siphon is not possible under those conditions. If you turn off the pump, water will remain in the pipes and there will be no siphoning of water from one pipe to other. That requires a pressure differential which can only be caused by difference in water height that each pipe is connected to. Also, plumbing systems with pumps are not usually referred to as siphons at least not in text books I have read. Also, I don’t think the term “siphon effect” does not accurately describe what is actually going on and can be misleading. The only force which creates static head is the force of gravity pulling downward on each of the columns of water. There are no other forces involved with static head. However, a difference in elevation from two bodies of water can create pressure differences which then force water movement like in the case of the siphon. But that doesn’t really apply in this case.

The water in each of the two pipes is exerting a downward force or pressure due only to the effect of gravity on the water. The common node of the two pipes has the same pressure and is lower than the pressure at the bottom of each of the pipes. It is the higher pressure on the bottom of the pipe which holds up the water in the pipes. If you separated and capped each pipe at the top, the pressure at the top and the bottom of the pipe would remain the same and water would still remain in the pipe so the net force at the common node between the two pipes is zero and the pipes are in equilibrium which means that neither side of the pipe is exerting force on the other. Therefore, without an external force, no water will flow. Now if a pump is attached to the setup, then the pump creates an imbalance in pressure which then causes water to flow.

Also, the concept of static head being “cancelled” is somewhat misleading. One side doesn’t really cancel the other it is just a case where the weight of the water is the same above both pipes so there is no difference in static head and it doesn’t add to the total head of the pump. In other words, the pump does not need to lift the water to a higher elevation. One way to make this distinction is to imagine two locations at the bottom of a lake and at the same depth, do the locations have the same pressure because one side cancels the other or is it simply because they have the same amount of water overhead? I believe it is later.

Perhaps I am being too picky about this but I think if the wrong terminology is used, there can a misunderstanding of exactly what is going on.
Mark
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Allen G Myerson

Solar Panel System

Postby Allen G Myerson » Mon 04 Oct, 2010 17:16

Example:

You have a 2-inch PVC pipe rise out of the water 23 feet straight up, turn 90 degrees and run 10 feet horizontally, turn 90 degrees back down 23 feet into the water. Assume that the pump is at the water level and has a check valve. Sea level ambient atmospheric pressure =14.7 psi absolute or 0 psi gauge. Flow rate = 48 gpm.

With no vacuum relief valve:
If the pump turns on, fills the entire pipe with water, and then turns off, the pressure at the check valve would be 14.7 psi absolute or 0 psi gauge. The pressure at the top would be 4.7 psi absolute or -10 gauge. When the pump runs, the only head loss would be due to the dynamic head loss. The total length of pipe would be 56 + 11.4 for the 90 degree fittings = 67.4 feet. The dynamic head loss would be 2.86 feet.

With a vacuum relief valve:
If the pump turns on, fills the entire pipe with water, and then turns off, the water would run out of the downward pipe and the pressure at the check valve would be 24.7 psi absolute or 10 psi gauge. The pressure at the top would be 14.7 psi absolute or 0 psi gauge. When the pump runs, the head loss would be due to the dynamic head loss up to the downward pipe + 23 feet for the static height. The total head loss would be 1.88 feet + 23 feet = 24.88 feet. Without a vacuum relief valve, the pump has to push against an additional 23 feet of head. The additional head is only partially offset by the subtracting dynamic head loss due to the water's flow through the downward pipe. The water's potential energy due to its height at the panels can only be used by the return trip.

As you can see, the difference is quite significant. The difference is 24.88 - 2.86 = 22.02 feet of head, which is almost equal to the height. You could increase the resistance of the downward pipe by 22.02 psi to close the vacuum relief valve, but it wouldn't make any difference to the total head loss that the pump has to push against

My primary point is that, when there is a VRV, the static head does not necessarily cancel. It only partially cancels unless the return head is unusually large. It seemed to me that some people were automatically assuming that the static head up and down automatically cancelled.
Allen G Myerson

Solar Panel System

Postby Allen G Myerson » Mon 04 Oct, 2010 18:36

The simplest and most obvious way that I can put this in non technical terms is that when there is not a vacuum relief valve, the falling water creates a suction in the top pipe, which allows it to "reach back" all of the way to the suction inlet and "pull" on the entire column of water with a force that exactly cancels the static head loss.

When there is a VRV, the falling water cannot create a suction in the top pipe. Therefore, it cannot "reach back" and "pull" on the preceding water column. However, the water has still gained a certain potential energy due to its height. This potential energy can only be used to overcome the dynamic head loss due to the water's flow from the top pipe back to the pool. This means that the static head loss is only partially recovered. The static head loss recovery is limited to the dynamic head loss due to the water's flow from the top pipe back to the pool.

Yes, I know that water cannot be "pulled". However, it is a perfectly valid conceptual device just as when someone says that water is sucked through a straw or pipe.
Allen G Myerson

Solar Panel System

Postby Allen G Myerson » Tue 05 Oct, 2010 01:32

We don't try to make a closed loop where the water falling back down pulls the feed water up creating a syphon effect meaning the pump doesn't have to do any work. No No No. That would create negative pressure in the solar collectors and a negative pressure (a vacuum) can collapse pvc plumbing when it gets hot and hot it will get. http://www.h2otsun.com/pools/index.html
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mas985
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Postby mas985 » Tue 05 Oct, 2010 12:47

Allen G Myerson wrote:Yes, I know that water cannot be "pulled". However, it is a perfectly valid conceptual device just as when someone says that water is sucked through a straw or pipe.

My primary objection is in your use of the concept that suction is pulling on the water. Suction does not and cannot pull on water, period. You keep saying you know that and yet you continue to do so which I really don't understand. Read through any physics book or for that matter any elementary science book and the act of sucking on a straw is explained as simply as the higher pressure pushing on lower pressure which moves the fluid. The same forces are at work for the sealed vertical pipe except the water weight is creating the pressure differential instead of the mouth sucking on the straw. I really think you would better serve the forum by explaining the physics in proper terms rather than propagating a myth.

BTW, I would stick to educational text books when looking for the proper definition of a siphon rather then a commercial web site. They are using the term incorrectly as well.

In reality, it is the solar system WITH a VRV at an elevation lower than the top most point of the plumbing which can siphon water out of the panels. The case without a VRV or even with a VRV at the top most point of the panels, does not qualify as a siphon because gravity can not move the fluid up and over a high point. Only the case were a VRV is installed at a point lower than the solar high point can a siphon exist. in this case when the VRV opens, water will be siphoned away from the VRV up and over the top of the solar panels. The act of siphoning is the higher air pressure pushing up on the lower water pressure thereby moving the water up and over the high point.
Mark
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Allen G Myerson

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Postby Allen G Myerson » Tue 05 Oct, 2010 19:05

mas985
My primary objection is in your use of the concept that suction is pulling on the water. Suction does not and cannot pull on water, period. You keep saying you know that and yet you continue to do so which I really don't understand.

OK, you are right about this. It's easier than trying to explain it the correct way, but that's no excuse. I always get aggravated when I see science principles being explained improperly, especially on TV shows that should know better. I will make the effort to explain things correctly, and not take shortcuts that are not scientifically accurate. Thanks for helping so many people with your advice.
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Postby chem geek » Tue 05 Oct, 2010 20:14

Likewise Allen. Thank you for your contributions. I hadn't considered what was going on with my solar panels with regard to looking at static head due to the vacuum relief valve and not having too little pressure (i.e. too much less than atmospheric) near the top of the panels so you helped me out there. I was hoping you and Mark would see eye-to-eye since it seemed to me you were both saying pretty much the same thing, though not necessarily in the same (technically accurate) way.
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Postby mas985 » Wed 06 Oct, 2010 12:39

Allen,

Thank you for understanding my point of view. You have been both open minded and polite in the discussion which is sometimes a rarity on these forums. I didn't expect this to go this far but I am glad that we came to an agreement. To a fault, I often find debate stimulating, engaging and I usually come away with even deeper understanding of the scientific principles involved.


Also, Richard,

There have been several threads about this subject over at TFP mostly around solar and 2 speed/variable speed pumps but about also about air in the returns caused by the VRV opening although you may have missed some of the postings. Some have have experienced an open VRV due to a dirty filter, so too much head loss on the supply side can also be a problem which leads me to think that may be your issue with the check valve.

Also, some have asked if they can run on low speed with solar and in the past, someone would chime in and say that the static head loss is canceled so it should work ok. But in fact, static head still exists and affects the pressure at the VRV but simply does not add to total head with respect to the pump. This is why I don’t like to use the word cancelled for static head because some have interpreted that to mean the static head has disappeared all together when it really hasn’t. This slight nuance is often missed and can cause misinterpretations in what is really happening.

I could go on and on ad nauseum about this topic but I think it is time to get off my soap box now.
Mark
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18'x36' 20k gallon plaster/gunite pool, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge Filter, Solar Panels, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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