I have a question regarding faults in control valve circuits. . .
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RT. . . . I would like to ask for your advice. I’ve encountered such a problem in my work related to instrument maintenance: a certain control valve often appears to be in a disconnected state during operation, which causes it to switch from automatic mode to a faulted, disconnected state. (Here, I wonder if the circuit of the control valve is in an open state?) ) Interestingly, the on-site inspection showed that there were no issues with the wiring or the safety barriers (tests were conducted to check continuity, including the resistance of the positioners, as well as using a megohmmeter to test the circuits). On site, it was sufficient to simply remove and reattach the terminal connections (in other words, to manually disconnect the circuit and then reconnect it to supply power), after which the control valve returned to its manually controllable state, allowing it to switch back to automatic control mode normally. . . Colleagues suspect that there might be a problem with the locator, or an internal soft fault. But I think it might be a fault or defect in the AO channel where the control circuit is located, which causes the output to exhibit periodic step-like interruptions; by manually disconnecting the circuit and then reconnecting it to power, the circuit can be reactivated. Or could it be that the configuration settings selected are abnormal or incompatible, causing such a problem?
This is a bit confusing, but based on the testing process, a fault in the circuit is more likely; at least I’ve never seen similar problems with the positioner.
Is it an intelligent locator? Is the communication on?
Check whether the normal locator is functioning correctly, and examine the settings of the intelligent locator to see if there are any issues.
It’s probably a problem caused by the configuration.
I think you can connect an ammeter online to check whether the AO value remains stable (provided that the valve mode is set to manual to avoid current fluctuations), in order to determine if it’s a problem with the AO card. If the above checks show no issues, try using a different controller. Or try giving the locator self-tuning
Since the wiring and safety barrier are functioning properly, proceed with elimination testing: first check whether the configuration is correct, then try a different AO channel; if that doesn’t work, test the locator.
I encountered a similar problem years ago: a valve would open fully on its own without anyone operating it. We checked the circuit and tried everything that could be done with a new actuator; we even brought in the manufacturer’s technicians who spent two or three days trying to fix it. Hehe, the solution I found later was that, in my carelessness while checking the circuit and unplugging wires, I broke the clip behind the safety barrier; I simply used a new safety barrier instead, and then the problem was resolved. . . . . . The safety barrier that was replaced was later installed by me in a wiring box attached to a matching small device, and it has been working well ever since. . . . . . To this day, I still haven’t figured out what exactly caused the malfunction; I’m mentioning it here just for the original poster’s reference.
Personally, I think your analysis makes sense; it seems that the locator is receiving incorrect signals – for example, excessive current can cause the locator to enter fault mode. You should check the locator’s manual to determine the type of fault.
I’ve encountered configuration issues; the connection would break and need to be reconnected, and the signals worked fine when set manually, but after a while the connection would break again. I’m sure it wasn’t a problem on the field side, so I asked the DCS manufacturer to modify the configuration, and everything worked properly after the new configuration was installed!