is my PRV shot?

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by tbdub, Oct 19, 2009.

  1. tbdub

    tbdub New Member

    Messages:
    10
    Location:
    Southern IL
    I have a PRV in a separate meter pit out by the road next to my 3/4" water meter pit. I live in a rural area and this is common practice out here. The pressure in the water main is well over 80 psi so we have a PRV.

    I have 1-1/2" PVC run to my house, which is about 250 ft away. Inside the house i reduce to 1" CPVC and have a pressure guage mounted near the building control valve.

    the gauge read about 50 psi, which isn't bad but i wanted to set it at 60. I went to the road and had my wife watch the gauge while i adjusted the PRV. I opened the cold water on a LAV. I adjusted the bolt and she told me the pressure got up to 100 psi. i turned it the other way and got it down to 60. I came back up to the house and turned the LAV off....the gauge now read 75 psi...

    what gives? A couples of things got me here.

    1. why was i able to get over 75 psi out of the PRV? i believe mine is a WATTS 25AUB-Z3 which says limit is 75.
    2. why did the LAV being open drop the pressure 15 psi? Shouldn't the PRV regulate dynamic pressure AND static pressure? I understand friction loss from the road could be a factor, but its 1-1/2" PVC which shouldnt have much friction loss in 250'.
  2. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    Actually you have MORE friction loss in bigger pipes.... why? Because you have more surface area. But you need slower flow rates to supply the same amount of water.

    1. Is your house LOWER then the PRV station at the road?
    2. Sort of and not really.

    You should be adjusting the PRV at a static pressure. Close EVERYTHING and then adjust the PRV. The way you were trying to do it is a bit goofy because you seem to be trying to set a PRV for a running pressure and they're not designed for that.

    A PRV works on downstream pressure. When the pressure downstream is strong enough it forces the valve closed. So as long as you have a faucet open your pressure is going to be lower then what you set the PRV too. When your PRV closes your system pressure is always gonna go up. 15psi seems like a bit much however I would attribute that to friction loss. And if your house is above the the road I would attribute some of it to head pressure loss.

    I hope this helps!
  3. tbdub

    tbdub New Member

    Messages:
    10
    Location:
    Southern IL
    I understand that more surface equals more friction, but with 1-1/2" pipe I will lose less PSI per 100ft than with 3/4" pipe. From my meter to the PRV i shouldn't lose much at all. 2 GPM in 1-1/2 is .09 PSI loss per 100 ft. 0.09*2.50 = .225 psi. Yes, my house is above the meter, but since my guage is in the basement i would say the rise is 6ft. max. 6ft*.433 = 2.6 psi. So, I am saying i should only see about 3 psi loss with one LAV open on the cold side only, not 15 psi.

    I initially adjusted the PRV with everything closed (static) and I got it up to 100 psi. I had to open the LAV to release the built up pressure to get it down to the 60 level that I wanted. I was fully expecting the gauge to be a few psi higher when i shut the LAV off but not 15 psi higher. I stopped adjusting when i reached 60, expecting to shut the LAV off and the guage read 65 at the most. If I have 100 psi in the main, then that PRV should be able to let more flow in to keep up with the demands of a single LAV.

    So, if my PRV isn't bad because it is allowing me to adjust pressure, then maybe the strainer on the front end needs cleaned because it is restricting flow from the main?
  4. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,253
    Location:
    Cave Creek, Arizona
    Prv

    The pressure should only have a nominal change, if anything, until such time as the flow approaches the valves maximum capacity. There should be NO change with just a lavatory faucet running. And, if the valve is operating properly, there should also be no "overrun" when the faucet is closed.
  5. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    That's not right at all! The velocity falls by the square of the diameter ratio. Enlarge the pipe diameter and the pressure drop falls for the same flow rate. The flow rate is typically volumetric or mass. It is not velocity. You are confusing velocity with flow.
  6. Terry

    Terry Administrator Staff Member

    Messages:
    14,770
    Location:
    Bothell, Washington
    Larger pipes reduce friction loss.

    The pipe is not the problem.
    You did right by using the larger pipe for the 250 feet distance.

    A PRV will adjust all the way up to the highest incoming pressure.
    If you run the setting all the way up, you are using line pressure.
    It's when you adjust the setting down, that you start reducing the pressure.

    After adjusting the PRV, I open a faucet up and release a bit of water, then recheck to see where the gauge reads.

    Clockwise higher pressure, counter clockwise, (anti-clockwise in England), lower pressure.

    http://www.terrylove.com/watersize.htm
  7. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    You guys are telling me that if I have a 2" line and a 1" line with the same flow rates the 2" line will have less friction loss then the 1" line?

    The 2" line has 4x the surface area as the 1" line. I know this is not what the thread is about but I think you guys may be confused about what I was saying.... or I"m confused :D
  8. Runs with bison

    Runs with bison New Member

    Messages:
    891
    Location:
    Midwest
    Yes. Water flowing through 1" Sch. 40 steel pipe at 10 gpm has a pressure drop of 2.99 psi/100 feet. In 2" Sch. 40 the same flowrate yields 0.108 psi/100 feet. (Source: Flow of Fluids through Valves, Fittings, and Pipe; Technical Paper No. 410, Crane, 1988, pages B-14,15.) The pressure drop rises to approximately the fifth power of the ID ratios.

    Surface area does not determine the pressure drop in a system. (Wetted perimeter is a factor when determining an equivalent hydraulic radius, but that's a relative subtlety.) The primary factor is velocity. Other factors are things such as two phase flow, viscosity characteristics of the fluid, density, etc. Pipewall roughness is also a factor.
  9. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    Thank you for that great explanation! Again I had velocity and flow rate a bit mixed up in my head.
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