Pressure and a Manual Valve's Effect on Pressure

  • #1
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Main Question or Discussion Point

Think that there is 6 bar air (or water) feeding the system and there is a manuel valve before air comes to regulator.

My question is that if I open the manuel valve half position do I change pressure (reduce it) or just reduce the flow ?

For example, I have pneumatic valve whose working pressure is 4 bar and the pressure of coming water is 6 bar. In normal case , valve can’t work since it is not designed for such high pressure but what happens if I make manuel valve half open which is located before pneumatic valve ? In this way can I reduce the pressure of water and use these pneumatic valves whose operating pressure is 4 bar ?

Thanks in advance
 

Answers and Replies

  • #2
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Temporarily closed for moderation.
 
  • #3
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Your thinking is correct. Opening the upstream valve will reduce the pressure. How much depends on friction losses. If it drops enough, the pneumatic valve might be able to open.

What happens then? If you close the manual valve and the pressure stays at 6 bars, the pneumatic valve might be destroyed when it tries to close. Then you would be unable to shut off the flow any more. You didn't give enough info for us to tell if that is hazardous.

6 bars is about the pressure of a fire hose. Watch out for those powerful streams. Hit someone in the eye and it would cause injury.
 
  • #4
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Your thinking is correct. Opening the upstream valve will reduce the pressure. How much depends on friction losses. If it drops enough, the pneumatic valve might be able to open.

What happens then? If you close the manual valve and the pressure stays at 6 bars, the pneumatic valve might be destroyed when it tries to close. Then you would be unable to shut off the flow any more. You didn't give enough info for us to tell if that is hazardous.

6 bars is about the pressure of a fire hose. Watch out for those powerful streams. Hit someone in the eye and it would cause injury.
Thanks for respond. I actually noticed that I didnt explain situation well.
Now I have just started to work as a training engineer and I just saw they solved this situation using pressure regulator which decreases the pressure of water for 6 bar to 4 bar.
Then I thougt What would happened If we not fully open the manuel valve which let 6 bar water come to our pneumatic valve.
Now I’d like to ask why pressure stays 6 bar after I closed the maneul valve even I open it half way.
The question is valid for manuel upstream valve using for air when I turn the valve down a bit the power of air I feel on my hand decreases but I can’t understand why it is like that(as physical explanation) If that is such easy why we use regulator to reduce pressure ?
Thanks and regards.
 
  • #5
JBA
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What is the function of the "pneumatic" valve? Pressure or Flow regulation or what?
 
  • #6
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What is the function of the "pneumatic" valve? Pressure or Flow regulation or what?
It opens the way of water in case plc send commands.
 
  • #7
JBA
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The biggest risks to damaging the pneumatic valve while being protected by partially closing the upstream manual valve are:
1. The plc closes the pneumatic valve; because the pneumatic valve's inlet pressure will instantly rise to the 6 bar system inlet pressure regardless of the setting of the upstream manual valve.
2. The throttling of a valve downstream of the pneumatic valve; which, can create a backpressure sufficient to slow the flow thru the manual valve to a point that it no longer reduces the inlet pressure to the pneumatic valve to the 4 bar limit.
3. Someone passing by sees that the manual valve is partially closed and restricting the system throughput and opens the manual valve to full open to restore the system to its full capacity.
 
  • #8
LURCH
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The question is valid for manuel upstream valve using for air when I turn the valve down a bit the power of air I feel on my hand decreases but I can’t understand why it is like that(as physical explanation) If that is such easy why we use regulator to reduce pressure ?
Thanks and regards.
1)I’m pretty sure that what you are feeling there is a decrease in volume, and not pressure.

2) I hope someone else will check my reasoning on this, but I think the manual valve you are talking about would cause the pneumatic valve to chatter. The important thing to remember is that pressure is resistance to flow. When you partially close the manual valve, the pressure down stream from that valve may drop enough to allow the pneumatic valve to close. However, once that valve is closed, the manual valve will no longer be resisting flow, because there will be no flow. The pneumatic valve will then be taking the full pressure of the system, because it will be providing all of the resistance. When the pressure gets too great for the pneumatic valve, it will most likely pop open, allowing pressure to release. When that pressure drops below the threshold, the pneumatic valve will once again be able to close, which will cause pressure to rise again, and so on.

I am a bit rusty on this sort of thing, so somebody else please chime in…
 
  • #9
jrmichler
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I am a bit rusty on this sort of thing, so somebody else please chime in…
I have extensive experience with pumps, control valves, and control systems, and would like to help out. But, I'm not clear on exactly what the OP is talking about. I need a diagram. The diagram needs to show where this air is going - to open atmosphere, or to do something.
 
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