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Is high pressure differential always a bad thing?

  1. Jan 26, 2010 #1
    Hi all, was thinking about this the other day. Is high pressure differential always a bad thing?
    Please correct me if i am wrong, but from the way I understand, high pressure differential is bad because if you have a high pressure differential across a piece of equipment (valve, pump, fitting etc...), it means that some of the pressure energy are probably converted into heat and vibrations, which will effect your equipment lifespan.
    But for something like strainers/filters, wouldn't a high pressure differential be actually a desired characterisitc?
     
  2. jcsd
  3. Jan 26, 2010 #2

    FredGarvin

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    A high delta P across a filter or a strainer is how you tell if they are clogged and not operating properly. So no, you do not want a high delta P across them. Think of it this way, if you take a big drop across something, that is wasted energy. There are times you want a significant delta P, for example, an orifice plate. You need to have a somewhat sizeable drop across the plate to have meaningful pressure measurements for your flow calculations.
     
  4. Jan 26, 2010 #3

    brewnog

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    A high pressure differential across a turbocharger is what makes it work....
     
  5. Jan 26, 2010 #4

    FredGarvin

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    Good point Brews.
     
  6. Jan 26, 2010 #5

    russ_watters

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    A high pressure differential across a control valve is necessary for precise control.
     
  7. Jan 26, 2010 #6
    Can you please elaborate on that? I thought that it's the other way around, that the high delta P is the side effect of the control valve when the valves are in a throttling position?
     
  8. Jan 27, 2010 #7

    russ_watters

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    When you close a control valve, the pressure drop goes up, yes (obviously). But what if it didn't go up much? If you close a valve 99% of the way and the pressure drop doesn't go up noticeably because the valve is way too big, you won't have changed the flow much either. Then in the last 1% of valve travel, you'll completely shut off the flow. If your valve position accuracy is 1%, your flow control accuracy is plus or minus abouot 100%.
     
  9. Jan 27, 2010 #8
    pressure differential also is what keeps may solenoid valves closed. If you turn them around, they leak, because the pressure lifts the seat.
    pressure differential is also what makes pumps pump.

    dr
     
  10. Jan 27, 2010 #9

    FredGarvin

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    I'm not so sure of that. Can you provide an example? In my work, the piloted solenoid valves I have seen use the upstream system pressure to keep the valve closed or to provide opening motive force. I can't say I have ever heard of one that relies on the delta P for this. Most solenoids are limited on the delta P they can take across them. Maybe I am just looking at them in different way.

    EDIT: I guess, after thinking about it a little more, there is the need for the low side to set up your force balance in the valve. So you are correct. Never mind.
     
  11. Jan 27, 2010 #10

    FredGarvin

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    You will be forced to run a valve that is too big in that last few percent open range which will also probably lead to very high velocities through the valve, which lead to high losses and possible cavitation and flashing in the valve. The controlability will be shot because very small valve movements will have large effects. Not a good place to be indeed.
     
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