Is a gas open valve always in the fault closed state, and is a gas close valve always in the fault open state?
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Since I started working, I have known that an air-on valve must be in the fault-off position, and an air-off valve must be in the fault-on position. There are three fault conditions for pneumatic control valves: lack of air supply, lack of power supply, and loss of signal ； For the first two types of faults, namely gas supply interruption and power loss, it is possible to change the fault mode from off to on by altering the gas circuit; for the third type of fault, which is a signal interruption, it is also possible to switch the gas circuit and change the fault mode from off to on by adding a current comparator. In other words, a gas-on valve can also be set to fail-open by making changes, and a gas-off valve can similarly be set to fail-close. If that’s the case, why does the valve go into fault mode when we try to open it, and go into fault mode when we try to close it? In confusion. . . Let’s discuss it!
In its default state, a gas-open valve is closed due to the action of a spring; therefore, it remains closed when there is no gas supply. The gas-close valve is the opposite
I’m referring to pneumatic piston-type control valves without springs.
Because the so-called fault usually refers to a gas supply fault. Actually, as LZ said, there should be three types of faults, and they should also be considered as three separate fault scenarios.