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Pipe sizing near centrifugal circulators

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steamfitter
steamfitter Member Posts: 156
As i understand it, circulator outlets are not supposed to determine the size of the connecting piping and valves.
I have seen on more than one occasion where the piping was reduced to the pump outlet size to accommodate the valves (check, balancing, shut-off) and strainer. I'm guessing the installer figured they would save money. There must be a considerable difference bet, let's say, a 6" & a 10" check valve.
My question is: how will it affect the system? What if the difference is maybe one pipe size, would it cause issues?

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  • hot_rod
    hot_rod Member Posts: 22,341
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    You would need to determine the actual required GPM at the pump outlet.

    Then any valve, pipe, or device connected to the pump discharge would be sized to handle that flow rate.

    Most all valves and devices have what is called a Cv rating. That number refers to the amount of gpm that flows thru the device with a 1 psi pressure drop.

    Ideally the valve or device would be sized with a Cv close to the actual flow. Tables show you how much pressure drop any device has as flow increases.

    Here is a Excels sheet we use. In the last box add a Cv value then change flow rates to see how pressure drop changes.

    In this example I put a 3 Cv valve with 7 gpm, notice the pressure drop.

    It is not unusual to see a smaller diameter valve in a pipe for example. Often check valves are a size or two smaller than the pipe they are in. As long as that valve Cv meets the actual flow.

    i'd be glad to share this Excel sheet.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jamie Hall
    Jamie Hall Member Posts: 23,536
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    I might add that many many valves in the larger sizes are reduced port, which is no problem except with saturated steam. The real trick with changing pipe size like that -- or with reduced port valves, where it should be built in (you get what you pay for) is to ensure that the transition between sizes is smooth. Then you shouldn't have that much extra head loss to worry about.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Henry
    Henry Member Posts: 998
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    Pump and boiler connection sizes are irelevant. The flow rate determines the pipe size. A good tool to use is from Bell& Gossett: SystemSyzer Calculator. There are two wheels.
    Scale 1 Temperature Difference and MBH capacity relaintionship. Scale 2 & 3 Pipe and tubing, Friction loss and velocity relationships. Flip it over, and you have:
    Scale 4 Pipe lenght and pressure drop relationships.
    Scale 5 Unknown pressure drop and Cv relaintionships, System curve relati0onship
    If you want to find out how many GPM a pump is giving to a boiler: Check the D T, turn the dail to with that D T to the net output of the boiler, then look at GPM scale.
    Example a KBN501 470,000 BTU net, the boile shows 25F D T=
    40 GPM on the dial.
    The tool is free from B & G.
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,735
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    Best practice is to size all components to the flow that the piping circuit requires.

    40 gpm would normally require 2" pipe so isolation valves etc should be 2".

    Some specialties like Triple duty or balancing valves are sized smaller to accurately measure flow. Suction diffusers could be sized smaller. Follow the recommendations of the pump MFG you are using to size these items.

    As a general rule 10% under or over flow is considered ok. Smaller valves may push you below of the flow you need.

    Can't make a generalized statement. Everything has to be calculated
  • hot_rod
    hot_rod Member Posts: 22,341
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    For pipe sizing up to 2", you want to stay between 2-4 fps velocity.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Henry
    Henry Member Posts: 998
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    The System sizer shows 2 to 5 FPS.
  • steamfitter
    steamfitter Member Posts: 156
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    Thank you all for the valuable information!
    I now see that it is possible to have different valves and even suction diffusers that are smaller than the pipe line size.
    I had been aware of the pipe sizing relationship to flow rate and velocity and the fact that outlet size of pumps and equipment should not affect pipe size design.
    However, I was negligent in thinking that the valves connected to the pump had to be the same size as the piping and the reducers would only be at the pump inlet and discharge outlets.
    Thanks for the clarification.
  • Harvey Ramer
    Harvey Ramer Member Posts: 2,239
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    One thing to watch out for is that the pressure drop of the piping and components leading from the x-tank to the inlet of the pump, do not drop the pressure below the rated NPSH of that particular pump. Failure to do so will make for a short lived pump life.