Allen G Myerson wrote:If there is no vacuum, they can't cancel each other out. You're still adding the static head; you're just doing it on the return side.
Forgive me for butting in here but I would like to clarify this statement, it seems contradictory and not quite correct. First of all pressure is relative and what a vacuum is defined as depends on what you define as 0 PSIR. At sea level, 14.7 PSIA is defined as 0 PSIR. So what is a vacuum at sea level may not be a vacuum on top of a mountain. The atmospheric pressure only determines the pressure that the vacuum release valve opens.
I don't think that the statement is at all contradictory, or incorrect. I think that it is obvious that I was referring to local gauge pressure, where vacuum means a pressure less than 0 gauge, which is 14.7 psi absolute. In a closed loop system that is less than 32 feet high, the static height up and down cancel due to the vacuum created by the falling water and the siphon effect..
Second, not to be picky but technically ALL pool systems with or without a vacuum release valve are open loop systems since the pool itself is exposed to the air.
This is not correct. As long as the loop is airtight, it is closed. There is definitely a difference between a closed loop and one that has a vacuum relief.
Next is the concept of “pulling” water. In the past, there has been some debate on if water is pulled over a siphon or is pushed over a siphon. Most physicists now agree water is pushed over a siphon and Bernoulli’s equation fully explains the flow rate of water with pressure differentials and changes in height. Also, a siphon will still siphon if both tanks are under the same positive pressure and the top of the siphon is under positive pressure albeit less than the tanks. The pressure differentials are relative and it does not depend on a vacuum.
Of course water cannot be "pulled". It is just a conceptual device that can be used to make things easier to explain. When someone sucks water through a straw, they are just lowering the pressure at the top of the straw enough for the ambient atmospheric pressure to push the water up the straw. If there is a vacuum relief valve, then a siphon cannot work. If there is no vacuum relief, then a siphon can work. This is the difference between an open loop and a closed loop.
As long as the vacuum release valve remains closed air is kept out of the pipe, both the open loop and close loop systems behave identically.
This is the key point. If
the vacuum relief valve remains closed, then the return head has to be greater than the static head and the static head will cancel out. However, if one decides not to worry about closing the vacuum relief, then the static head does not cancel. The potential energy of the water coming from the roof can only be used for the trip from the panels to the pool. If the head loss of the plumbing from the panels to the pool is less than the height of the top of the panels, then the difference is lost and the static head does not cancel.
Where things get tricky is when the pressure is insufficient to keep the vacuum release valve closed and air gets sucked into the return pipe. At this point the system effectively breaks into two pieces that are somewhat independent. The first part is the pump overcoming the static head loss going up the pipe as well as the dynamic head loss in the pipe to the vacuum release valve. At this point the pressure is 0 PSIR and the pump only needs to overcome the pressure to this point. After this point, the system behaves as a gravity fed plumbing system. Water falls down the return pipe through the air and the return pipe may still be partially filled creating some head gain.
This agrees with what I have been saying.
This water height then determines the flow rate through the remaining part of the plumbing.
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.
So in summary, static head gain and loss can occur with or without a vacuum release valve and the plumbing system behaves identically as long as the vacuum release valve stays closed.
This agrees with what I have been saying. My point is what happens when the vacuum relief opens due to the return trip having less head loss than the height of the panels.
Solar off: Going from out of the filter to the pool, you have the equivalent of 100 feet of straight 2-inch PVC. Flow rate 48 gpm. The head loss is 4.24 feet.
Solar on, with a vacuum relief valve, and allowing the vacuum relief to remain open:
Going from out of the filter to the panels, you have the equivalent of 100 feet of straight 2-inch PVC. Flow rate 48 gpm. Head loss through the panels = 2 feet. Height to the top of the panels = 20 feet. From the panels back to the filter pad you have the equivalent of 100 feet of straight 2-inch PVC. And then, going to the pool, you have the equivalent of 100 feet of straight 2-inch PVC.
With the solar on, you have 4.24 feet of dynamic head to the panels + 2 feet of dynamic head through the panels + 20 feet of static head, which equals 26.24 feet of total head pressure that the pump has to push against.
From the panels back to the pool, you have the equivalent of 4.24 feet + 4.24 feet = 8.48 feet of head. Since this is less than the static head, you don't have to count it. Therefore, of the 20 feet of static head potential energy, you only use 8.48 feet and the rest (11.52 feet) is lost. This is a case where the static head going up was only partially offset by the static head coming down.