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  • Lamianr water flow measurement through a 6in pipe

    Discussion in 'The main mechanical design forum' started by bhollehday, Aug 15, 2013.

    1. bhollehday

      bhollehday New Member

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      Im measuring the Cv of a valve using an inline flow meter upstream of the valve. My question is when you are measuring the flow, ALL of the water inside the pipe is moving at the same flow rate no matter where you are taking the measurement in regards to the position of the valve. There might be a small difference in flow rate initially when closing the valve if the flow meter is far upstream of the valve, but the difference is negligible, specially at a line pressure around 30psi. Is this thinking correct.

      Assuming that the water is laminar and not compressible flow rate readings will be identical no matter where you are measuring it?

      With a much more dense fluid like oil would it would make more of a difference right?
       
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    3. vidgolob

      vidgolob Active Member

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      I dont know - flows tend to be turbulent to some extent. Laminar flows are generally considered more because they are easier to calculate and/or predict but that doesnt mean that you actually have laminar flow inside the tube. This is not likely if not impossible ( I am talking about actual laminar flow not something in between laminar and turbulent flow ) especially outside a laboratory or in theoretical cases.
      As far as measuring goes - pipe bends, t-pieces and other pipe elements all produce turbulence. The reading immediately after a pipe bend has to produce different results ( if not, something is wrong with your sensor ). But if you have a straight pipeline with no diameter changes at a constant temperature and pressure it is possible to have a pipe segment that gives you same measurement results no matter where you measure. The pipe segment should be as long as possible to help you achieve this.
      Dense liquids? Dont know... Anyone?
       
    4. vidgolob

      vidgolob Active Member

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      Sorry, I meant in practical cases.
       
    5. bhollehday

      bhollehday New Member

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      Ok I agree but what im trying to figure out is without any bends or fittings the flow should be the same at the valve as it is 20ft upstream. Say you have segments long enough after a bend or fitting to where the flow is unaffected by it both at the valve and 50ft before the valve. When the valve is shut off the reading at the valve will be the same as that 50ft before the valve, considering there is no disturbance from fittings or bends. I am taking differential pressure across the valve, and for accurate Cv purposes, Im questioning whether or not the flow meter should be as close to the valve as possible or if it will be the same as long as it is unaffected by fittings or bends.

      Another example....you have a straight 200ft line leading up to a valve. You start to turn the valve off or decrease the Cv. Will the measurement of flow ideally be the same at the valve as it is 200ft upstream?
       
    6. vidgolob

      vidgolob Active Member

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      Valve by itself should produce a slightly different measuring result - what I mean here is that the valve itself is a flow disturbance and measuring the flow immediately after the valve should produce slightly different results. If you place the sensor anywhere before the valve (upstream) you should be getting more or less the same result if the valve is completely opened. However after the valve ( downstream ) it is advisable to place the sensor at a ˝safe˝ distance.
      This distance should be:
      1) minimum -> L= 3 x pipe diameter
      2) optimum -> L= 5 x pipe diameter
      L -> length between the obstacle ( valve, bend, etc ) and the point of measurement. After this distance it is very common that the created turbulence ˝settles down˝.
      This is a standard recommendation for all flow measurements in the pipeline.
      As far as upstream goes -> I wouldnt place the sensor immediately before the valve ( maybe try the minimum safe distance here ). As mentioned - you should be getting more or less the same result if the valve is completely opened and probably when you close the valve completely after a short period of time ( after the kickback ). But there should be some fluctuations if you leave the valve partially opened.
       
    7. vidgolob

      vidgolob Active Member

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      So to answer your question - ideally ( theoretically ) it is possible. In an actual pipeline - I wouldnt bet my money on that...
       
    8. bhollehday

      bhollehday New Member

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      Thanks for the insight, I understand all of that. My question is referring to the flow rate upstream or before the valve. Taking a measurement 200ft before the valve will be the same as 50ft before the valve correct? What about if you start to close the valve, will the flow rate meters both 200ft and 50ft before the valve have the exact same reading or would there be lag between each meter? Not concerned with the disturbance the valve can create before or after, just whether or not the flow meters location inline differs with length. Again, not considering bends and fitting disturbances before the valve...
       
    9. Erich

      Erich Well-Known Member

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      If the flow is incompressible, the continuity equation assures you that the flow rate will be the same.
       
    10. bhollehday

      bhollehday New Member

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      Thank you sir, that's exactly what I need!! I was thinking along those lines, but couldn't relate it theoretically.

      And now I know theoretically fluids are usually always consider to be in-compressible, although in the real world all fluids are compressible to a certain point at a certain pressure, but what is that pressure for water at room temp? And is it so small it is negligible or is there a certain point to where waters compression should be considered and would cause a fluctuation between flow meters at different points before a closing valve?

      Reason I need to know is im doing Cv test across a ball valve at certain percent openings. I have a flow meter upstream and taking differential psi across valve. I want to know if Im getting innaccurate readings because my flow meter is not right before the valve and its disturbance to the water flow. Also what is the formula for were i can take accurate pressure readings before and after the valve where the valves disturbance is not affecting the readings? Thanks!!
       

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