Selection of valve position switch?

Generally, control valves are installed in pairs as valve switches; here is an introduction to one type of such installation (based on what I have seen).

In engineering, valve position switches are generally handled in this way: 1. The control valve is equipped with a solenoid valve interlock; the “interlock position” provides a valve position signal (which is not necessarily the air-loss position). The purpose is to confirm that the interlock is in place. 2. For on-off valves, settings are generally provided for both the fully open and fully closed positions, offering two signals: “interlock in place” and “valve in service”. 3. Both linear and rotary motion can be “changed”; rotary motion is more convenient. 4. All of them are useful; microswitches are also good, as they have moving parts and, in theory, a limited number of operations. When using microswitches in programmable control valves, this factor must be taken into account. Magnet-coupled proximity switches are more expensive; NANUR proximity switches require a safety barrier (amplifier), which also incurs costs. @jiaguoyun

Your requirements for this control valve are quite strange. Since it’s an FC type, safety shutdown of the valve in case of air supply failures or other problems should be taken into account – so why does the solenoid valve need to be interlocked to open as well? The proximity switch can be installed in either the open or closed position; it depends on the process requirements to determine where the safe position of the valve is.

1. Is it required by the SIS to add a solenoid valve to the air supply of the control valve? If not, some systems have an interlock off pin on their PID module, and the signal can be connected directly to it. If valve sticking and failure to close fully are a concern, then choose two valve position switches. 2. Control valve FC: it shuts down in case of a gas supply failure, unless the valve is stuck. 3. All proximity switches are the same; you can change their installation location. 4. Under normal use of these two forms, no difference can be perceived.

Your idea is correct. But sometimes it is necessary to consider whether there are enough DCS points. The difference between electronic and mechanical types is that mechanical types are prone to being disturbed by vibrations, which can lead to damage or false alarms. Electronic proximity switches are prone to damage due to interference from humid environments. You can choose the appropriate one based on your own working conditions. The prices are actually pretty similar.

Interlock opening does not mean it is only activated in the event of a fault; some valves have both interlock opening and interlock stopping functions, just like pumps that start and stop. Fault shutdown refers to the shutdown required to ensure safety by cutting off power and gas; it is something different from the switching requirements imposed by interlocks.

How can the FC be interlocked to open? Problem contradiction

The FC is in interlocked off mode; if one wants to choose electromagnetic valve interlocked on mode, it is equivalent to switching to fault-on mode. To do this, simply swap the electromagnetic valve or change the outlet of the directional valve. It is possible to install either one or two valve position detectors, but for accuracy’s sake, it is best to have one detector for the closed position and another for the open position. If only one detector is used, then a point that represents a state other than either closed or open must be set in the system; in other words, when the sensor detects a value of “1”, it should not detect a value of “0”. Progressive and contact types are similar in function; one operates within a range of 0-24V while the other operates within a range of 4-8V, but the contact type offers better stability.