Why didn’t the valve maintain its position in this situation?
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Regarding the air-loss opening, air-loss closing, and air-loss holding functions of valves, how is such air loss determined? Based on the gas source pressure? A few days ago, the air line connected to the gauge of one of the device’s pressure relief valves leaked, which caused the valve that was originally closed to open. I’ve been wondering why, with air leaking from the gauge, the valve failed to maintain its original closed state
A loss of air pressure means that there is no more air supply to the control system. Depending on the design of the valve, some valves will close automatically while others will open automatically (this is usually determined based on process safety considerations). In your case, it seems that the position-control valve is not performing its function properly; it should maintain the current valve position in the event of an air supply interruption. It’s possible that there is a problem with the position-control valve.
Maintaining the position without pressure can only be done for a limited time; such valves should be double-cylinder types, relying on the residual air pressure inside the cylinders to keep the position in place. However, it’s impossible for the cylinders to be completely airtight, so they may gradually move towards either an open or closed position. Check the data sheet for this valve; normally, for hold-valves, it will indicate FHC (fault condition leads to a closed position) or FHO (open position).
What was said upstairs makes sense, but is it possible that the valve, while in operation, causes the air pressure supplied to the control instrument to drop to the value set by the instrument, which leads to the opening mentioned earlier? You can also use specialized instruments for control: there are instruments with two-stage protection as well as those with three-stage protection available on the market; of course, there are also ordinary instruments. In cases where the air supply is cut off but the signal source remains intact, the valve can still remain in its original position. QQ:120519927
I would like to ask which manufacturers produce mature three-break protection devices?
Valves like the one mentioned on the 3rd floor should be double-cylinder. I think you should check whether your air pipeline connection is correct. Such a position maintenance valve should have at least two pneumatic relays; in other words, the air circuit will only be conductive when the air supply pressure reaches a certain value (which can be set), and of course this happens simultaneously for the opening and closing actions. When the pressure drops, the relay activates to cut off the flow; the air supplied to the switch becomes trapped, and the valve will not move – it simply maintains its position! Depending on the gas piping configuration, it can also be set to off on gas loss or on on gas loss. On-site, valves of this type are usually equipped with an air source buffer tank as well.
Reply to 1# timwangwt: When this situation occurs, the check valve should first be disassembled for inspection. One possibility is that the sealing diaphragm is deformed and thus fails to provide a seal ； Another possibility is that the balance spring has failed, and its elasticity is not sufficient to press the sealing diaphragm tightly.
Ningchuan Yingao Te Automatic Control Company manufactures three-break protection devices