Alarm gauge signal issue
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May I ask, for a pressure alarm instrument that has set high and low alarm values and is interlocked with a pneumatic valve, does it send a signal to cut off the air supply to the valve or a signal to supply air when the alarm values are reached? Thank you!
Under normal circumstances, it should be cut off at the high limit and supplied at the low limit.
There are two issues that need to be clarified: 1. Is the purpose of installing this pneumatic valve to release air and reduce pressure, or to increase pressure?； 2. Is this pneumatic valve a normally open valve or a normally closed valve? Once these are clarified, the problem is no longer a problem. :lol
What was said on the 4th floor is correct. First, one should consider process safety when deciding whether to use a gas-actuated or air-actuated control valve; in other words, it’s necessary to determine whether the control valve should be in the open or closed position under safe conditions. Only after that can it be decided whether the solenoid valve should be powered on or off. I’m not sure if the original poster understands this.
I understand that you are looking at the issue from a design perspective, while I am asking questions based on the PID diagram, which is quite complex. The PA instrument controls the N2 pneumatic valve (which opens when pressured) and the vent pneumatic valve (which closes when pressured). The PA instrument also provides high-pressure and low-pressure alarms; its function seems to be to close the vent valve by sending nitrogen under high pressure, and to open the vent valve under low pressure. The diagram does not show the solenoid valves clearly
This depends on the process requirements; your description is not clear.