setting PRV and Expansion tank

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

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  1. Jun 9, 2019 #41

    SGkent

    SGkent

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    opinions always welcome. :)

    Wilkins sets theirs to 50 psi. The final PRV PSI is an arbitrary thing to the individual household based on stories, etc.. Wilkins, and I think Watt too have graphs where the GPM going thru the PRV can be compared to pressure loss across the PRV. Meaning that a PRV set at 50 PSI will drop up to 12 - 17 PSI depending on load. Because here water at say 40 PSI is quite weak and the washer takes forever to fill, that was actually the basis of my original question that started this thread - do I set it to say 60 and expect it to be 50 on demand etc. But in spending quite some time reading Watts and Wilkins documents there are a couple places where it says the PRV's are set closed. The second part of the equation was where to set the Expansion Tank - at the closed PRV value or the average operating PSI under load, The answer was to match the PRV closed value. Then there was a pissing match over whether to remove the expansion tank and drain it to check it, or to just turn the water off and open a faucet to relieve pressure before checking it. Some were concerned that the water in the pipe would greatly skew the pressure. No need to discuss all that again.
     
  2. Jun 9, 2019 #42

    Diehard

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    Good summary of events.:)
    FWIW, the PRV's pressure loss when flowing is called the Fall-off Pressure.
    For the ones not familiar, here's a typical residential PRV's fall-off pressure at various flows.

    For example, a 3/4" at 10 gpm would lose about 10+ psi.
    flow vs fall-off pressure.jpg
     
  3. Jun 9, 2019 #43

    frodo

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    what you need to know to install a prv is
    max inlet pressure, .... of the system, and the max design pressure of the valve you choose
    minimum ........... outlet pressure, of the system , and the minimum design pressure rating of the valve
    max flow rate......what is the max flow rate of the system you are working on
    min. flow rate ..... what is the minimum flow rate of the prv rated at
    max temp.......... will the valve handle the temp of the system

     
  4. Jun 10, 2019 #44

    Diehard

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    It was stated above that, "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."
    Are there any others that believe this statement that the tanks air will always be the pre-charged pressure above the system pressure? Or am I misunderstanding this?

    Of course the 10 lbs isn't correct but that's beside the point.
     
  5. Jun 10, 2019 #45

    SGkent

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    If the pressure were ambient on both sides the bladder would be in a relaxed state. When water pressure was added that pressure would push it until the pressure on the air side was equal to the water side. That would over stretch the bladder and when it ruptured early the factory would have a warranty claim. By adding a small pressure that helps protect the bladder against the damage from the guy who just puts it in without charging it first. When the pressure in the PRV is equal to the water pressure then the bladder is in a relaxed state again so it not only has more room to move but it also lasts longer.
     
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  6. Jun 10, 2019 #46

    Diehard

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    Yes! I knew it was a false statement but I was trying to be polite and testing others.;)
     
  7. Jun 10, 2019 #47

    SGkent

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    did I pass? :)
     
  8. Jun 10, 2019 #48

    Diehard

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    It's has been pretty obvious that you knew what was going on. Wasn't meant for you.

    I like the new avatar!
     
  9. Jun 10, 2019 #49

    SGkent

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    :)

    Here is a new one for me. Water Company came by a while ago. I had written them about the water hammer (documented in another thread here). Anyway they said that sometimes the PRV gets overloaded with hammer from the water system itself when the water meter on the opposite side of the street is directly across from another one. Meaning if the house across the street tapped in 180 degrees from mine then their system can send hammer spikes directly across the main pipe into my system and cause it to do weird things. Nice to know. May have to get one of those two needle gauges.
     
  10. Jun 11, 2019 #50

    Diehard

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    Assumming they have a PRV also, I wonder how the shock wave travels back through PRV?
     
  11. Jun 11, 2019 #51

    SGkent

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  12. Jun 11, 2019 #52

    Diehard

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    Yes, I am somewhat familiar with an hydraulic pressure wave but was speculating on the ability to carry backwards through a PRV.
    pressureregulator.jpg
     
  13. Jun 11, 2019 #53

    PlumbGate

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    This discussion is all well and good but realistically, expansion tanks are over-rated overreach by governmental entities. They were never even used until recently. And I suspect many of them are not installed correctly with the correct charge, I know mine was not. And I also suspect most of them are never serviced once they are installed and slowly leak down until they are doing nothing. My house never even had one until I replaced my water heater a few years ago. They guy took it out of the box and hooked it up he didn't check any pressure anywhere. I know it was almost out of air when my pipes started howling (I made a post here) and filled it myself. I guessed at the pressure using a laundry sink next to the hot water heater. No way in hell I was going to remove it just to confirm the water pressure. If it does ever fail I would remove it and leave it out. Amazing all these years people lived without these but now they are a necessity.
     
  14. Jun 11, 2019 #54

    frodo

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    right on point
     
  15. Jun 11, 2019 #55

    SGkent

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    Frodo - very few homes in here have PRV's because they were not required. The water pressure is like 90 PSI so I put one in years ago. I am speculating that in the scenario the water engineer described, one house has a PRV and one does not, or both do not. The shock wave coming from the house with no PRV overwhelms the house with or without the PRV. All I known is that he saw what I had, looked at the whole system and commented that they see it occasionally and when it occurs they have to move one of the service connections on the main to stop it.
     
  16. Jun 11, 2019 #56

    frodo

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    a shock wave is caused by when a fast acting valve in the system closes and causes a ''wave''
    or..when fast moving water is stopped suddenly
    this is a typical event in water piping, the ''shock wave'' dissipates it does not carry on mile after mile in a piping system.
    to carry this out further. look at the cross sectional image Diehard posted of a PRV.
    a shock wave will not transfer through the prv.
    I also suspect a ''shock wave will not transfer through a curb stop and water meter
    it [shock wave] will only continue in an open pipe,,then it dissipates


    on PFV's.
    prv's are pushed by prv manufactures, a simple expansion loop will serve the same purpose and does not require a purchase or maintenance.
    The water pipes supplying a water heater are typically 6' long. these pipes are an expansion loop in every sense of the word,,for a residential setting
     
  17. Jun 11, 2019 #57

    Diehard

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    Yeah you don't need one if you don't have any type of backflow prevention device upstream or if you do, you don't mind relying on your P&T relief valve to handle any elevated pressures.
    So you're saying that you would leave it out and live with the howling noise you have already experienced when it wasn't helping?
     
    Last edited: Jun 11, 2019
  18. Jun 11, 2019 #58

    Diehard

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    I'm completely with you on the first comments, but lost you on PRV(pressure reducing valves) vs expansion loops. Are you implying that an expansion loop takes care of over pressurization?
    Or am I misreading you again?
     
  19. Jun 11, 2019 #59

    PlumbGate

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    I never had howling until the expansion tank was installed.
     
  20. Jun 11, 2019 #60

    Diehard

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    Oh, I see.
    Did you ever experience occasional P&T relief valve discharge?
    Do you have a PRV that incorporates a small thermal expansion by-pass? They apparently help but cannot be relied upon for higher rates of thermal expansion.
     

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