Delay of the feedback signal
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The feedback signal of a programmable valve is delayed; when it is turned on, the feedback is disconnected, which is normal. When it is turned off, the feedback loop and on-site response are both very rapid, but it takes over 10 seconds for the signal to reach the DCS. The signal is transmitted via 2x2 shielded cable. What causes this! I have no idea. The cable has been replaced with a spare one, and the feedback switch has also been replaced with a new one.
The on-site switches are working properly, which indicates that the transmission of DO signals is normal. The slow feedback from the DCS suggests that the problem lies with the DI signals. There are several aspects that can be checked; first, verify whether there are any loose connections in the terminal blocks corresponding to the on-site instrument wiring boxes – trying a different terminal block might help. Second, check whether the DI signals pass through a safety barrier; the safety barrier terminals should be inspected as well. Third, determine whether the feedback signals are in the form of simple dry contact signals or Namur signals. Finally, does this phenomenon occur with a single valve or with several valves? If it is the latter, then it is necessary to consider whether the distance between the field instruments and the DCS control room is too great.
If possible, short-circuit the DI points at the DCS cabinet terminal block to see if it causes a delay
If you’re stuck, use a simple method: try using a limit switch next to the cabinet, then test it at the local terminal block, and finally connect the wires from the valve to the limit switch to test it. Just compare them and you’ll find out where the problem lies
Many problems with instruments can have their causes identified by using a comparison method. Like it.
I’ve found the cause of the fault. It was a lack of clear communication between the site and the control room that led to the repair efforts going in the wrong direction. The actual fault manifestation is that the DCS has sent a shutdown signal, but nothing happens on site. It was only about 10 seconds ago that the site received a signal and was quickly shut down. I always thought it was a problem with the feedback signal, but it turned out to be a problem with the command signal. Replacing the solenoid valve on site resolved the issue.