setting PRV and Expansion tank

Discussion in 'General Plumbing Help' started by SGkent, Jun 4, 2019.

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  1. Jun 7, 2019 #21

    Mikey

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    What Frodo said. I'm wondering, if we look at the ET diaphragm as a hydraulic piston, it's maybe 110 sq in in area, and it's got a water column at 1.55 psi pushing on it, so the total force is 175 lb. Getting above my pay grade now, but it seems to me that that has to affect even a perfect gauge measuring the force in there. Be all that as it may, what Frodo said...
     
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  2. Jun 7, 2019 #22

    SGkent

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    whatever. I do understand the physics. The original time it was set up it was empty. BTW the air in the bladder is also pushing back on the size of the bladder. So if I put 50 psi into the air side of a 1' bladder (.79 sq ft) and 50 psi into the side of a 4' bladder (12.57 sq ft), is the 4' bladder under 628.5 psi (pounds per square inch) because it is larger? No. Or is the 1' bladder only under 39.50 psi because it is only .79 sq ft? No. It is still under 50 PSI. (The answer is that the PSI means Pounds per Square Inch. The size of the bladder doesn't matter because we are measuring PSI and not dead weight. In a perfect world it would be empty but for 1.55 psi I am not going to worry. The water pressure gauge tee'd into the system isn't even that accurate.
     
    Last edited: Jun 7, 2019
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  3. Jun 7, 2019 #23

    frodo

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    8265a50fa20049525b45960ab6d4dd75fa3e4decbf9b398da0bfaf36e4fdf632.gif


    YOU
    have missed the whole point
    which is
    the extra water in the tank, diminishes the size of the tank capacity
    as an expansion vessel
    with diminished capacity you are negating the ability of the tank to act as an expansion tank
     
    Last edited: Jun 7, 2019
  4. Jun 7, 2019 #24

    Diehard

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    This threads title is going to be changed to "How to make a simple hydraulics problem confusing." :eek:

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  5. Jun 7, 2019 #25

    frodo

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    what happen to the top of the little blue pyramid? was it stolen?
    did the rectangle get it? I want it returned
     
  6. Jun 7, 2019 #26

    Diehard

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    Evaporation!
     
  7. Jun 7, 2019 #27

    frodo

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    expansion tanks are used to provide a cushion for the expansion of water when it is heated. Without this cushion of air, the pressure in the hot water system would exceed the setting on the pressure relief valve. This will cause the pressure relief valve to open and discharge water to relieve the pressure.
     
  8. Jun 7, 2019 #28

    Diehard

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    Was this meant for this thread?o_O
     
  9. Jun 7, 2019 #29

    SGkent

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    sigh. what extra water? The area below the bladder diaphragm has air in it. The area above it has zero water pressure other than 43" of water above it. 43" of water is equal to 1.6 PSI. The gauge on the expansion tank water line is a 10% gauge. That means at 50 PSI the PRV can actually be anywhere between 45 PSI and 55 PSI. The 1.6 pounds, which is less than the pressure gauge can be accurate when the PRV is adjusted, is not worth removing and draining the water side of the expansion tank. In other words maybe I can set the expansion tank within 1% of 50 psi with my Wiki air gauge when it is off and drained, but I can't set the PRV to 1% of 50 PSI. It will be somewhere between 45 psi and 50 psi when that gauge reads 50 psi. 1.6 psi is trivial compared to the PRV error. Frodo - that is just how physics works. Even Tolkien would agree with that one.

    The correct answers were: set the PRV to a fully closed system with no water running. Set the PRV to match with it at zero water pressure, empty if possible especially if the PRV is in a basement with lines in the attic, or the house is 2 story. Preferred PRV pressure is between 50 - 55 PSI for most one story homes. Factory sets them usually at 50 PSI.
     
    Last edited: Jun 7, 2019
  10. Jun 7, 2019 #30

    Diehard

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    BTW...You may be thinking the "Maximum Acceptance Value" of an expansion tank, which is not applicable to your example.

    EDIT: Additionally, FWIW, if you have the exact same pressure on the water side as the air side of the diaphragm, theoretical there would be no water entering the tank.
     
    Last edited: Jun 7, 2019
  11. Jun 7, 2019 #31

    frodo

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    this is very true
    so....tell me. why are we needing to recharge the bladder?
    could it does not have the required psi ?
    so,,to follow that logic
    if it does not have the required psi in the bladder, that means it is not equal pressure on both sides
    of the bladder.
    and if it is low in air pressure than the required area needed for system expansion is not being meet because it has been replaced by waa waa and the possibility of a leaking pop off is possible

    here is what it boils down to, the factory recommendation is to test, charge or re charge the ET with the tank being empty of water.
    The suggestion was made that this could be done by releasing the pressure on the house piping.
    and opening a valve to drain.
    I AGREE that would be acceptable it seems to be a common sense approach.
    IF the placement of the ET was such that the water in the tank was not trapped
    The water IS trapped.
    Debating the amount of the trapped water, calculating, drawing graphs and beating the issue to death does not change the fact, the tank is not empty a point that was specifically pointed out and cautioned by the manufacture
    So, I have made my recommendation, the OP can do as he wishes.
    enjoy, good luck, have fun,
    cheers,
    DSCN5293.JPG
     
  12. Jun 8, 2019 #32

    SGkent

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    Thanks Frodo - still can't see the trapped air everyone is claiming. The bladder is shipped empty of water. When in operation it has air on one side and water on the other. If there is no leakage of water into the air side, once the water is shutoff to the house, and the faucets open, then the only weight on the water side is the 43" of water which is 1.6 PSI. So if one had a perfect 0% error gauge on both sides they could set the PRV to 50 PSI before turning the water off then set the tank to 51.6 PSI once the faucets in the house are opened. What trapped water is everyone concerned about? I just don't see it. The valve at the tank is in the open position always. It has one purpose only - if the tank goes bad I would use it to shut the tank off and swap it rather than turn the whole house off. Are you thinking that is where I am shutting the water off - no. It is being turned off at the service entry.
     
  13. Jun 8, 2019 #33

    SGkent

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  14. Jun 8, 2019 #34

    Diehard

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    It sounds like they may not be able to follow your logic. Simple enough, you're just adjusting the air pressure to compensate for what little head (43") of water is on it. All this stuff about area of bladder, etc. has absolutely nothing to do with the task of equalizing the pressures between the air side and the water side.

    Hard to follow you.
    You say, "this is very true" but then go on questioning it. The answer to your question, I believe, is that it has been found that over time the air pressure can gradually decrease and should be recharged.

    You say, "if it does not have the required psi in the bladder, that means it is not equal pressure on both sides of the bladder." What's this have to do with the price of eggs? Why do you even say this? This is obvious and no ones ever said otherwise. In fact the task at hand was obtaining equal pressures.

    etc., etc., etc.
     
  15. Jun 9, 2019 #35

    SGkent

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    you are correct and I have chosen to ignore that rather than remove the unit from the wall, and then have to purge the whole house again over 1.6 psi when the water pressure gauge can be off by 10% mid-scale or 5 PSI at an indicated 50 PSI. I would lay odds you don't remove your expansion tank every six months to trim the pressure.
     
  16. Jun 9, 2019 #36

    frodo

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    you are correct, I do not remove my tank
    It has a hose bib between the tank and the isolation valve
    close valve, open hose bib. done deal
     
  17. Jun 9, 2019 #37

    SGkent

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    but either there is water in the tank which weighs something or you installed it upside down per the manufacturer's instructions. same argument everyone gives me. :)

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  18. Jun 9, 2019 #38

    frodo

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    sideways, not upside down

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  19. Jun 9, 2019 #39

    MoNs00n

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    Sorry to butt in but two items discussed bothers me.
    #1
    The sole purpose of the expansion tank is to give wat to thermal expansion thus saving the water heater from catastrophic failure (exploding) under pressure.
    The expansion tank should have no effect on water hammer.
    The factory sets the psi at I believe 10 lbs so that no matter what psi the system is fed it will always be 10 lbs above that pressure.
    If it were to fall below system pressure the bladder would close in and block it's own opening.
    #2
    Typically pressure reducing valves are set to 55 or 60 psi but can be adjusted and can safely be set at 70 pounds.
    The only important fact that needs attention would be the maximum psi not the minimum.
    Night time psi when the public system is not being used is when the psi climbs to unsafe pressure.
    I have seen systems exceed 150 lbs over night.
    You can buy a guage at home depot that has two needles that will give a max pressure over a period of time.

    As far a hammering goes your best bet would be to install hammer arrestors at sink supply lines and even at water heater if you wish.
    You can add as many and wherever you like until the hammering stops.
    Hope this helps.
     
  20. Jun 9, 2019 #40

    frodo

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    you are not butting in, it is an open forum. sit down, grab a cold one and express your opinion
     

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