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    Discussion in 'The main mechanical design forum' started by Elb, Sep 16, 2016.

    1. Elb

      Elb Member

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      Hello!
      I'am new in the forum.
      I m currently working on a project to test some valves ( pvr, check valves).
      I'm trying to design the hydraulic circuit. I have a centrifugal pump, I also have a variable frequency drive. for this, I don't know if i should connect my check valve directly to the pump and use the VFD to control the rotation of the pump. or stay ultimately on a fixed speed and use instead a valve in my circuit to control flow.

      I would appreciate it if you guys have some idea for building this kind of bench tests..
      Thank you for your time.
       
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    3. PWASS

      PWASS Well-Known Member

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      Hi

      What fluid are you using, I guess water if you're proposing to use a centrifugal pump.

      What type of tests do you want the rig to perform?
       
    4. Elb

      Elb Member

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      Thank you for your answer.
      I'm using for the moment water, but we want to use it also for some other fluids whose viscosity can reach 10-20 mm2/s. I think i should change the pump?

      I want to test my check valves: to see if they open at the pressure (fixed for them to open) and also measure the flow that passes through them.

      I should also mention that i'm working on low pressure 0 to 10 bar.
       
    5. PWASS

      PWASS Well-Known Member

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      I suggest you use a fixed displacement pump which will deliver a higher flow rate than you require.

      Use a relief valve to adjust the pressure, start at as near to zero pressure as you can get, the check valve should not open.

      Gradually increase the pressure until the check valve opens, measure the flow rate with a flow meter in the exhaust line.

      Continue to increase the pressure and plot a pressure vs flow graph, it should have a square relationship, to achieve twice the flow you'll need 4 times the pressure.
       
    6. Elb

      Elb Member

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      Thank you again Sir for your answer.
      I drew a circuit to make sure I understand what you said.
      test.PNG
      Why the pressure relief valve is used? is it used to help more increase the pressure after my check valve opens?

      My second remark is: how would the flow would change here? even if the pressure increases my pump will still deliver the same flow ( since no fluid passes through the PRV).

      Is this what you did mean Sir? Or should i put a restriction instead of a PRV to help increase the pressure and in the same time have a flow on the two lines?
       
    7. PWASS

      PWASS Well-Known Member

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      Your circuit is correct.

      The pump must be able to supply a flow rate much higher than the expected maximum flow rate of the check valve.

      Assume the opening pressure of the check valve is 0.5 Bar, set the relief valve lower than this, all fluid will pass through the relief valve.

      Gradually increase the pressure, the check valve will start to open at 0.5 Bar, some fluid will pass through the check valve, the rest will pass through the relief valve.

      Continue to increase the pressure, the fluid flow rate through the check valve will increase and the flow rate through the relief valve will increase.

      If you plot pressure vs flow, you should get a graph in which the pressure jumps up to 0.5 Bar before any flow starts, then the flow rate will increase as the pressure increases.

      The increase in pressure over 0.5 Bar vs flow rate is called the pressure drop of the check valve, this will change when using different fluid viscosities.
       
    8. Elb

      Elb Member

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      The reason I thought this won't work was, that I thought once the PRV is open, the pressure will stay fixed at the pressure (at which we set it to open). and I understand this this is not correct from what you said.

      For example, if my check valve opens at 6 bar, At what pressure do I need to set the relief valve? say that i set it at 1 bar, the pressure will still build up to my 6 bar to open my check valve? or should I set it at a higher pressure?

      Thank you
       
    9. PWASS

      PWASS Well-Known Member

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      The check valve inlet pressure will be what the relief valve is set at.

      If your check valve opens at 6 Bar, initially set the relief valve lower, say 5 Bar.

      Gradually increase the relief valve setting, once the pressure reaches 6 Bar, your check valve should start to open allowing flow the pass through. There will be a difference in pressure between your check valve starting to open and being fully open, this is a function of the spring characteristics.

      As you gradually continue the increase the relief valve setting more flow will pass through your check valve.
       
    10. Elb

      Elb Member

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      Thank you Sir. This helped me a lot

      I also wonder what would be the advantage of the PRV instead of a restriction for example that will permit the flow but help build pressure ( then I don't have to adjust the PRV setting?)
      I saw in a similar circuit that they used a back pressure valve but still trying to figure out how it works.
       
    11. PWASS

      PWASS Well-Known Member

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      The PRV will provide a stable pressure independent of flow rate changes.

      A restrictor valve would work but it would not provide a stable pressure because the pressure would change dependant upon the flow passing through it.
       

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