PRV and GPM?

Discussion in 'Irrigation' started by irrigatorsllc, Jun 15, 2006.

  1. irrigatorsllc

    irrigatorsllc LawnSite Member
    Posts: 13

    :hammerhead: In what way does a PRV affect GPM?
     
  2. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 48,003

    About the same way it affects sunspots. The two aren't related. As already stated on the other thread you started, there is a friction loss in a PRV, which is figured separately from its pressure-reducing function.
     
  3. irrigatorsllc

    irrigatorsllc LawnSite Member
    Posts: 13

    How do you figure friction loss for a PRV. Sorry for the questions, I tried to search this subject but found nothing.
     
  4. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 48,003

  5. Valveman

    Valveman LawnSite Member
    Posts: 53

    Most friction loss charts for PRV's are for the valve locked in the fully open position. This is a little misleading in that when using a pilot operated PRV the diaphragm chamber is always under a little pressure. You have to open up enough water to get the pressure below the set point of the PRV before you can read actual friction loss between an upstream and a downstream gauge. At this point, most pilot operated PRV's have about 15 PSI of friction loss as long as your flow rate is not excessive for the size of valve.
     
  6. irrigatorsllc

    irrigatorsllc LawnSite Member
    Posts: 13

    :dizzy: I am such an idiot when it comes to the PRV and understanding how it works. I have 160 psi that I want to restrict down to 90psi. Should I just buy the BR4, hook up after the main, and calibrate it then with a pressure guage, then continue on. I know its not rocket science, but for some reason my brain just will not comprehend the instruction. I even went here: http://www.watts.com/pro/divisions/watersafety_flowcontrol/learnabout/learnabout_wprv.htm to try and understand how to choose the right PRV, el no comprehendo.
     
  7. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 48,003

    The folks at Wilkins said those charts assume you have at least a 50 psi pressure differential between your set point and the supply pressure. Then you apply the friction loss chart. They claimed that was a standard way to chart PRV friction losses.
     
  8. Valveman

    Valveman LawnSite Member
    Posts: 53

    The Chart you guys are looking at is for Reduced Pressure Fall off not friction loss. When it says 10 PSI fall off at 20 GPM that means if the valve is set to hold 100 PSI while flowing 1 GPM that you will only have 90 PSI while flowing 20 GPM. Friction loss is about 25 PSI for this type of valve but, you will not see friction loss until the flow rate increase to the point that you are below the pressure setting of the PRV. Then you will have 25 PSI more on the linlet of the PRV than you have on the discharge of the PRV.
     
  9. Critical Care

    Critical Care LawnSite Bronze Member
    Posts: 1,654

    Okay... someone show the math on this.
     
  10. Wet_Boots

    Wet_Boots LawnSite Fanatic
    Posts: 48,003

    The chart still works like a friction loss chart, in terms of calculation. The Wilkins book supplies nothing on friction loss, and I would have assumed it is part of the 'fall-off' number. I didn't learn about the 50 psi differential 'requirement' until I phoned Wilkins and asked why I wasn't getting the charted performance on a three-tier PRV ladder I had set up on a factory line. Their regulator catalog mentions 'a small compensation must be made in the sizing selection' when there isn't that 50 psi (or more) difference between supply pressure and set point.

    For most excess-pressure situations, I would be looking at using a regulating master valve, since those don't have the '50 psi requirement' ~ but in the case of 160 psi static, a BR4 is a good way to go.
     

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