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Help please - how does a valve work ?

  1. Dec 6, 2011 #1
    help please -- how does a valve work ??

    how does a valve work ??
    when in a uniform cross section pipeline a valve is closed it reduces d opening due to which the area decreses and as area decreases flow should increase
    A*v=constant
    so how does it work ?
    please explain mentioning the upstream and downstream pressure ,flow and other parameters.

    and also tell whether in a pipeline which has a some fluid at certain pressure if we bifurcate the line into 2 equal lines wat will happen to the pressure and flow in each line??
    please recommend some books on this matter
     
  2. jcsd
  3. Dec 7, 2011 #2

    Mech_Engineer

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    Re: help plz

    This looks like homework to me, what are your initial thoughts on this?
     
  4. Dec 9, 2011 #3
    Re: help please -- how does a valve work ??

    Why put effort into answering your questions if you are not putting any effort into asking them. Respect that people here are offering help. It is a privilage. That was a helpful suggestion.

    With that said, it does sound like homework, in which case this belongs in the HW section. Also, you have to explain what you mean by flow. Do you mean flow velocity or flow capacity (i.e fps or gpm).
     
  5. Dec 9, 2011 #4
    Re: help please -- how does a valve work ??

    i am sorry.I meant Flow Capacity
    What i want to ask is : "what happens when the opening of a flow control valve decreases?
    how does this reduces flow? can this be explained by bernoulli's theorem?
    and also whether in a pipeline which has a some fluid flowing at certain pressure if we bifurcate the line into two equal lines what will happen to the pressure and flow in each line??
     
  6. Dec 9, 2011 #5
    Re: help please -- how does a valve work ??

    Flow capacity is reduced across a valve. Think of it as a resistor (if you know a thing or two about electronics) impeding the flow. I'm sure you could use bernoulli, but this has a lot to do with oriface flow theory than standard Bernoulli's equations. The velocity of water through a partially closed valve will increase, but the volume of water that passes through per second will decrease as compared to a fully open valve. It has a lot to do with the energy "contained" in the system.

    Pressure and capacity are very closely related. Look at a pump curve. If the pump has to overcome a small amount of head, it pumps at a high capacity because the energy imparted on the water makes the volume of water move quickly as it is unimpeded by obstructions.
    But if we close a valve halfway, we get a ton of resistance to the flow. This means a great deal of energy must go into speeding up the flow of water through the opening in the valve (as seen in A*v=A*v). Because of this, the capacity is lower, the volume of water moves more slowly through the system.

    This is a basic view of the system and isn't 100% accurate physically. But I don't want to get into that. There are hundreds of books and millions of articles dedicated to this very topic.
     
  7. Dec 9, 2011 #6

    turbo

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    Re: help please -- how does a valve work ??

    @OP: Have you spent any time studying valves? What are the flow characteristics of a gate valve, a globe valve, butterfly, etc? Engineering is complex, and fluid flow is particularly complex. If you can save a percent or two in energy costs in an industrial setting, you can be a hero to the plant manager/production manager.

    Valve selection in an industrial setting can be critical to process control, because how can you possibly make your process-flow respond reliably to your input (generally computer-driven these days) when you don't have valve/actuator pairings that will give you the control that you need?

    You probably need some pretty advanced books on engineering to get non-trivial answers to your initial questions.
     
  8. Dec 9, 2011 #7
    Re: help please -- how does a valve work ??

    Also. Basically yes. The pressure upstream is greater than downstream due to losses of energy. Therefore there is reduced flow capacity.
     
  9. Dec 9, 2011 #8
    Re: help please -- how does a valve work ??

    thanks
    Can we say that that on closing the valve the velocity of the fluid increases whereas the flow decreases.??

    i am sorry but i was unable to get this point :
    "Pressure and capacity are very closely related. Look at a pump curve. If the pump has to overcome a small amount of head, it pumps at a high capacity because the energy imparted on the water makes the volume of water move quickly as it is unimpeded by obstructions.
    But if we close a valve halfway, we get a ton of resistance to the flow. This means a great deal of energy must go into speeding up the flow of water through the opening in the valve (as seen in A*v=A*v). Because of this, the capacity is lower, the volume of water moves more slowly through the system. "

    and also please answer my other question :
    and also whether in a pipeline which has a some fluid flowing at certain pressure if we bifurcate the line into two equal lines what will happen to the pressure and flow in each line??

    please do recommend some good books on fluid flow
     
  10. Dec 9, 2011 #9
    Re: help please -- how does a valve work ??

    The velocity momentarily increases across the valve gate. It does not necessarily (or actually) speed up after the valve. A*v=constant does not hold for valve systems as there are energy losses associated with the valves.

    If you are unable to get that point, then you have to do some more studying. Read up on "head" in water piping systems. Not necessarily pump head, but that would work as well. There is a nifty handbook out there called "Control Vavle Handbook". It's super long, but very informative.

    As for your second question I have no idea what you are asking. First off, piping media doesn't flow at certain pressures. You can have a 100psi system flowing at 10 gallons per minute and a similar 100psi system that flows a 1000gpm.
    Secondly, you don't mention what type of pressure. Is this a closed loop system under pressure regulation? Is it an open system in which the only "pressure" in the pipe is the head pressure? Is this compressed gas? These things all kind of matter.
     
  11. Dec 9, 2011 #10
    Re: help please -- how does a valve work ??

    but A*v =constant is simple material balance this should apply everwhere
    how does it concern energy balance>???

    for the pipe system
    i will reframe my question :
    what happens to the discharge pressure when we operate to compressors in parallel working on same capacity
     
    Last edited by a moderator: Dec 9, 2011
  12. Dec 9, 2011 #11

    berkeman

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    Re: help please -- how does a valve work ??

    (Moderator note -- thread edited and moved to Homework Help / Engineering)
     
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