Discussion on a Case of Troubleshooting a Rare Fault in a On/Off Valve
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Today, a switch valve in one of the processes within the company’s production unit failed to operate as per the process feedback. Upon arriving at the site, it was confirmed that there was an air supply, but the switch still did not function and showed no response. The voltage measured was 0.6 volts; when the coil was disconnected, the voltage was 21.16 volts. The resistance of the coil was around 50 ohms, which is within the normal range. But it’s not for sale. Now I understand why the voltage measured while it’s powered is close to 0. With no other choice, I replaced the coil, but the problem persisted. The cable running from the junction box to the valve works fine in terms of continuity. . . I’m asking my brothers, why is this? What are the possibilities? What’s wrong with this valve exactly? :Lol, it’s not to be mysterious – I just want to discuss this together and see how everyone determines what to do when they encounter problems.
The biggest problem with on-off valves is when they fail to operate for extended periods of time; this can be caused by rust or blockages inside the valve. If the solenoid valve is working properly, then this is likely the cause
It’s similar to a resistor being connected in parallel with a coil – is water entering the solenoid valve? Coil to ground?
Check whether there is instrument air at the outlet of the solenoid valve when it is powered on; if there is, it may indicate that the valve spool is stuck； If not, it might be that the solenoid valve wires are connected incorrectly
One should check the changes in the gauge air pressure output for this. In the special cases I’ve seen, it’s common for the valve disc to get stuck in butterfly valves, and I’ve also encountered situations where the exhaust port of the cylinder gets blocked.
I just want to know if you use those terminals with diodes in your control room – when the fuse blows, the LED lights up! If so, it’s definitely the fuse that has blown! When this type of terminal fuse burns out, voltage is still output; it’s the same as the supply voltage when there is no load, but no power is available when a load is connected! Moreover, burning doesn’t necessarily mean it will emit light. I encountered the same problem and it took me two days to figure it out! If it’s 220V and it gets damaged, there will still be a 90V output; there’s no way around that when it comes to hitting people……
I agree with what was said on the sixth floor: on the terminal block, resistors with high resistance values are connected in series with the light-emitting diodes, which are in parallel with fuses. When the fuse is not blown, the fuse resistance is much smaller than the parallel resistance, and the voltage is supplied to the instrument by the fuse ； After the fuse blows, the voltage is supplied to the instrument through a resistor; due to the high value of this resistor, significant voltage drop occurs there, leaving very little voltage to reach the instrument. This is why the voltage appears low when measuring it without disconnecting the wires ； Remove the wire to measure the voltage; since the circuit is floating, the voltage remains at 24 volts ； The current passing through the resistor is low, which is just sufficient for the LED to function; this is why the diode emits light after the fuse blows. Experiments can be conducted: when the signal cable is disconnected on-site, the light of the indoor fuse will not turn on.
Shouldn’t the original poster reveal the answer now?
It is mainly considered from two aspects: 1. The actuator. A. First, check whether it is a problem with the feedback switch; then examine the air supply, check whether the solenoid valve is receiving power, and check the output end of the solenoid valve’s air supply to see if the solenoid valve is switching. B. Everything is fine above; check whether there is any air leakage in the actuator. 2. Valve body: After confirming that the actuator is functioning properly, check whether the valve is stuck. If possible, disconnect the connections and operate the valve separately to determine which side is not functioning.
The actuator was ruled out, as in my experience, the usual procedure when a on/off valve does not function is to first check the air supply, then verify whether there is any output from the air supply after the solenoid valve is activated; in this case, the air supply was present, but no output occurred after the DCS was turned on, which is why I disconnected the wires to measure the voltage. Then. . . ========== The problem lay in the cable running from the junction box to the valve; it seemed fine when testing for continuity, but there was definitely an issue. The fault was resolved after replacing it with a brand new cable, and during the replacement process, joints were found in the cable. . That’s probably where the problem lies. . . .
Judging from the issues reflected by the poster. The voltage across the solenoid valve at the site is 0.6V, while it is 21.6V when it is disconnected; I think there must be a short circuit somewhere in the wiring