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Determine the Rate of Change of Pressure Across a Valve

  1. Feb 25, 2015 #1
    Hello, I am Primarily a Software Engineer, but have some background knowledge from my A-Level and Uni days.
    However, I have the following problem.

    I'd like to find out the rate of change in pressure across a valve.
    Here are the things I know about the system:

    Cv at 100% open = 0.004 (I believe it's the US coefficient)
    Pressure drop across valve can be made up as this is variable within the system, lets assume 500psi for this example.
    Assuming it's pure water at 25°C with a specific gravity of 1 (because I don't want to get into all that mess).

    Also, am I right in thinking that if the Cv is 0.004 at 100%, then the Cv at 50% will be 0.002?

    Any help would be greatly appreciated.

  2. jcsd
  3. Feb 25, 2015 #2
    Empirical data is your best option, especially for something transient like this. Why do you need the transient data?
  4. Feb 25, 2015 #3
    I agree, empirical data would be a great commodity to have. Unfortunately, I'm writing the code to control the valve in an office in the West Midlands and the site is in Aberdeen.

    I need to control the valve to maintain a constant psi/sec drop. I was going to do a simple, "if drop is too big, close valve a bit, else open valve a bit" logic. But I thought it would be nicer to try and do it properly.
  5. Feb 25, 2015 #4
    Hmm. sounds like a cool problem. Maybe write a basic predictor-corrector method to estimate the derivative? If the derivative exceeds some value, then reduce flow to XX, re estimate derivative, repeat?

    Wish I got cool projects like that.
    It also just occurred to me that you might be able to utilize optimization methods. For constant flow rate you want the derivative to equal 0, so you can set this up as a minimization problem.
  6. Feb 25, 2015 #5
    Also, where is the flow rate being measured? This is important so you don't get erroneous readings due to turbulent fluctuations.
  7. Feb 25, 2015 #6
    Valve data is typically published by the manufacturer. Different types will vary greatly and most are far from linear.

    Do have any other data for your valve?
  8. Feb 25, 2015 #7
    I'm not actually measuring the flow rate, I'm measuring the pressure. Thus I can determine the current psi/sec drop. I then need to alter the control valve to maintain a certain drop. I can determine the flow rate from the pressure drop across the valve using the flow coefficient, but I'm having trouble finding the rate of change in pressure from that.

    From what I can gather, the flow coefficient is in units of "gallons per minute per psi". If I have the pressure drop across the valve in psi, can I find the pressure drop per minute?
    I have a hunch that I'll need the volume of the system, in which case I'll ask them to fill it up and measure how much water comes out. But ideally, I want to make commissioning as quick as possible.

    You can probably tell I'm not much for fluids and mechanics. I'm more electronically minded.
  9. Feb 25, 2015 #8
    Aha! I've found a graph that tells me the flow coefficient at different open levels. It's a 30VRMM valve, if anyone wants a look.

    So that answers one question, thanks!
  10. Feb 25, 2015 #9
    How you are measuring the pressure will determine the correlation between pressure reading and flow rate. For example, if you use a Pitot static tube you can equate your reading to the dynamic pressure. If you have a sensor at the bottom of the tube, you can say the pressure reading will be the hydrostatic less dynamic. You will need the surface area of the inlet and outlet to determine the flow rate. As montoyas7940 stated, valve curves are usually published to customers.
  11. Feb 25, 2015 #10
    Rate of flow is proportional to the Sqrt of Dp.

    Psi/sec? What do you mean?
    Dp across the valve is just Dp across the valve. Do you need to control the rate at which Dp changes as you open or close the valve?
  12. Feb 25, 2015 #11
    I should read more carefully.
    I would suggest a flow element for Dp instead of measuring Dp across the valve. Then use a flow control valve for flow control.
    To maintain a constant flow, if Dp is high across the element then close the valve a little.
  13. Feb 25, 2015 #12
    The pressure is measured using a 4-20mA sensor, it's a hydrostatic test, no air bubbles or anything. The transmitter located before the control valve, there is no transmitter after the control valve, so I'm assuming it to be 0psi (ignoring atmospheric pressure).
  14. Feb 25, 2015 #13
    Yes, I know the change across the valve. I want to know, if I open the valve x%, what will the pressure be after X seconds. That way I can just open the valve to the required %, rather than guessing and inching it open. I'd rather not guess when there could be 23000psi in the system!
  15. Feb 25, 2015 #14
    If the transmitter is only before the control valve, how will you regulate the flow? You need at least 1 upstream and downstream of the valve. It may not be feasible, but it would be best to have the downstream sensor 10 diameters from the outlet. Turbulent fluctuations will gives you oscillating readings which will require you to take a time average before any calculations are executed. The best you can do at this is point is estimate the incoming mass flow rate, then correlate that with your valve curve. Keep in mind that valve curve you have is only applicable to the ranges it was tested for. If you have an anomalous event you should not rely on it.
  16. Feb 25, 2015 #15
    I am sorry, I keep thinking in terms of mass flow rates, you can equate mass flow to pressure as was discussed earlier. Im used to thinking in those terms, sorry.
  17. Feb 25, 2015 #16
    You wont find that transient data. Maybe the vendor has a specific test that examined that, but almost always curves are for steady state. Nobody here can tell you how long it will take to equalize. Call the suppler and see if they can help.
  18. Feb 25, 2015 #17
    Ok, scratch most of that other stuff. You are measuring static pressure and then controlling the rate of pressure reduction/increase in a hydrostatic test vessel.
  19. Feb 25, 2015 #18
    Yes, that probably makes things a lot simpler, that might have been worth mentioning before! :confused:
    Sorry everyone!

    Although, I will seek your advice should my client ever want to automate Gas testing!
  20. Feb 25, 2015 #19

    jim hardy

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    Gold Member

    rate of change in pressure vs time or pressure vs valve position?
    if versus position ignore remainder of this post.

    Water being incompressible and dense and having inertia
    must be accelerated or decelerated as your valve changes its Cv.
    In a long straight pipe full of water , the inertia edit: mass of that long column of water has inertia that'll manifest itself as transient pressure opposing the change in flow - just like inductance for us electricals. In control systems it can wreak havoc.

    So i think you might need to know something about the system in which your valve resides

    Late entry : I see this is a manually adjusted needle valve - probably not an issue.
    Last edited: Feb 27, 2015
  21. Feb 25, 2015 #20
    I agree with Nala the Husky (my sister also has a dog named Nala).

    You don't have enough info about the system. You might ask the test facility to provide empirical data based on their experience.
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