The automatic control valve cannot be opened
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I am not a professional in automation; currently, I have a pneumatic valve that will not open. DCS uses Siemens equipment; the intermediate relay for that valve in the cabinet activates automatically upon receiving a command, causing the indicator light to turn on, but it loses power again after two or three seconds. What is the reason?
It is recommended to start by checking the actuator of the valve body. For reference.
Under normal operating conditions, once a command to open the valve is given, the relay activates, the valve on site moves, and feedback is sent back to the DCS – this constitutes a complete logical loop. In your current situation, it’s likely that the valve on site has not moved, or it hasn’t been switched to the correct position, which prevents the valve position feedback from being sent back to the DCS. So in DCS, logical judgment is applied: if there is no feedback within 3 seconds after the valve is opened, it is considered that the logical switch has failed. If the valve fails to operate properly, check whether there are any alarm records related to this valve before proceeding with further maintenance actions
In this case, there are too many possible reasons
In my opinion, first, it is necessary to check whether there are any issues with the circuit (such as open circuits or short circuits); second, the power supply to the system at the site should be turned off, and 24V should be applied manually to the valve in order to test whether the valve itself is functioning properly.
I agree with the analysis in post 4; it’s possible that the valve takes a relatively long time to operate. The on/off valve function block in Siemens PCS7 includes a timing detection for feedback once the valve reaches its intended position, and if no signal is detected within a certain time frame, the output will be disconnected
It is also necessary to confirm that this configuration has not been set to motor mode, so pulse signals are being sent; naturally, the pneumatic valve cannot be held in place.
Valves are prone to single-acting accident shutdown. Moreover, if there is a interlock in the shut-off valve and the signal indicating that the valve should be opened cannot be sent back to the PLC, then the valve will continue to receive an open signal, which poses a safety risk, especially when the medium is flammable or explosive
It’s recommended that professionals do professional work: lol