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  • relief valve on an RP backflow preventer is cycling non-stop after water hammer

    So here's the setup: The backflow preventer is a 1 1/4 Watts RP on the hot water line feeding two industrial washing machines. After the RP and the machines is an arrestor, about 6 feet away. About 5 feet before the RP is a wash sink that is causing the problem. If someone opens or closes the sink tap quickly the water hammer is causing the RP relief valve to cycle open and closed rapidly and it won't stop until you close and open the preventer's first shut off valve. The problem has temporarily been fixed by closing the shut off to the sink part way, the reduced pressure doesn't create enough hammer to set off the RP.

    The RP has had everything replaced which it needed anyhow but it made no difference to the problem. How can I solve this? Should there be an arrestor between the sink and the RP?

    Thanks for your help!

  • #2
    Re: relief valve on an RP backflow preventer is cycling non-stop after water hammer

    It sounds like the shock aressortors might be causing a rebound affect and creating the surge.

    I would think a check valve installed after the rp device might help isolate the pressure from rebounding. It might also cause a clanking noise unless You use a quiet check, spring loaded check.

    You could try removing the shock aresstors and see if that fixes the problem too.

    Rick.
    phoebe it is

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    • #3
      Re: relief valve on an RP backflow preventer is cycling non-stop after water hammer

      Why would you have an rpz in that location? My guess would be because of an automatic chemical pump on the washers?

      The problem is when you open the sink valve it drops the pressure before the rpz (making the pressure beyond the rpz higher) and it dumps, sounds like it is working correctly. The washers have a built in air gap to prevent back siphon out of the tub. Who is requiring the rpz in that location?

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      • #4
        Re: relief valve on an RP backflow preventer is cycling non-stop after water hammer

        A lot of municipalities are requiring an rp on all newer commercial work
        No, it's not rocket science, it's plumbing and unlike rocket science it requires a license.

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        • #5
          Re: relief valve on an RP backflow preventer is cycling non-stop after water hammer

          Originally posted by MoJourneyman View Post
          A lot of municipalities are requiring an rp on all newer commercial work
          But why would you not put it on the incoming water line where it enters in to the building.

          The solenoid valves on the washers is what is going to be the culprit in this matter. When they slam shut it creates a water hammer effect, this makes the pressure on the downstream side of the rpz higher, thus opening the rpz and allowing it to dump.

          Maybe install a larger expansion tank to absorb the water hammer. I know when we install a rpz on an antifreeze loop sprinkler system code also calls for an expansion tank. If that was not in the loop the rpz would dump every time the antifreeze warmed up and expanded, causing a higher pressure on the downstream side of the rpz.

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          • #6
            Re: relief valve on an RP backflow preventer is cycling non-stop after water hammer

            Thanks for the help so far I really appreciate it.

            The reason for the RP there is basically the area I'm in has a VERY strict cross connection control program. The building does have an 8" RP on the line into the building but any other hazard type still requires it's own protection. As you can imagine it annoys a lot of people but those are the rules we have to live with.

            I get how opening the sink valve drops the pressure and causes the relief valve to dump but is it normal for it to cycle open and closed continuously after that? The system has been working fine for about 4 years. It was only recently when one of the two washing machines broke down that this started. I'm assuming it's related but don't know for sure.

            As Rick said above, it seems like there's some sort of rebound that's causing the pressure to fluctuate rapidly. I just have no idea how to stop it.

            thanks again.

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            • #7
              Re: relief valve on an RP backflow preventer is cycling non-stop after water hammer

              I would think that the simple solution is to stop or otherwise dampen the spike between the RP and the problem fixture. I would use some sort of air ballasted diaphragm tank such as would be used in a well or hot water expansion system.
              ~~

              ... it was plumbed by Ray Charles and his helper Stevie Wonder

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              • #8
                Re: relief valve on an RP backflow preventer is cycling non-stop after water hammer

                Have you broken down the RPV to check for debris? Perhaps something stirred up by the shut down when the washer was down for maintenance and got lodged in one of the RPV's internal check valves.

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                • #9
                  Re: relief valve on an RP backflow preventer is cycling non-stop after water hammer

                  Originally posted by Plumbus View Post
                  Have you broken down the RPV to check for debris? Perhaps something stirred up by the shut down when the washer was down for maintenance and got lodged in one of the RPV's internal check valves.
                  I have. It needed repairs anyhow so everything was replaced inside and it was thoroughly cleaned and inspected. I can actually recreate the problem with the RP on the cold line (it's not as easy to set it off due to a slightly different set up) so I'm fairly certain it's not an issue with the RP but rather the system.

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                  • #10
                    Re: relief valve on an RP backflow preventer is cycling non-stop after water hammer

                    There are two reasons to use backflow preventors, contamination control and isolation control.
                    In one case you are looking to block contaminating a whole water distribution system and the
                    other is protecting those on the property.

                    It sounds like there are a couple possible causes that could trigger this condition that are present in the system.
                    ---------------
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                    ---------------
                    “If I had my life to live over again, I'd be a plumber.” - Albert Einstein
                    ---------
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                    • #11
                      Re: relief valve on an RP backflow preventer is cycling non-stop after water hammer

                      A faulty rpz device might be the reason it did not give problems for the past 4 years.

                      As for the cycling open / closed continually, the downstream pressure past the no. 2 check is still above the pressure before the no. 1 check.

                      Maybe a small relief valve downstream of the rpz.

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                      • #12
                        Re: relief valve on an RP backflow preventer is cycling non-stop after water hammer

                        Throw a couple hammer arrestors on the sink and a thermal expansion tank on the outlet of the rp. Cheap fix I agree it would be the solenoids on the washers. My guess anyways get a guy with his cross connection control.ticket to check it out legally its suppost to be checked once a year bye a certified guy.
                        Last edited by seanny deep; 01-22-2013, 10:56 PM.

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                        • #13
                          Re: relief valve on an RP backflow preventer is cycling non-stop after water hammer

                          Thanks everyone, this has been really helpful, it gives me a few things to consider and try.

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                          • #14
                            Re: relief valve on an RP backflow preventer is cycling non-stop after water hammer

                            #1, you cannot put a check valve downstream of the RP. It will have to be upstream. Also you should have water hammer arrestors on the faucet, that will eliminate the water hammer (pressure surge which is causing the dumping).
                            Sam

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