Issues related to pneumatic control valves with solenoids
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Under what circumstances is a solenoid valve used with a pneumatic control valve, and what is the purpose of the solenoid valve? What is the difference in function from a valve positioner?
Go for safety; relieve pressure in dangerous situations
Pneumatic control valves all come equipped with air supply solenoid valves. What you are referring to is probably a control valve with a quick shut-off function; such valves are used in processes where rapid opening or closing is required. The solenoid valve is used to quickly turn the air supply on and off in order to operate the valve, and its principle is the same as that of a quick shut-off valve. A valve positioner is a mechanism that controls the vectorial movement of control valves; it can be considered as a more intelligent version of a positioner
Looking at the control valves in the ESD system, several of them are equipped with solenoid valves
Solenoid valves are used to control the air circuit of pneumatic control valves; they typically switch on and off during emergency interlocks, thereby enabling rapid closing/opening of the pneumatic control valves.
For designs that require an emergency shut-off (or opening) function, solenoid valves are necessary; valve positioners are used to adjust the degree of valve opening
Their functions are different: the solenoid valve controls the air circuit, while the positioner controls the valve position.
It can be understood in this way: valve positioners are primarily used for process control, especially in PCS systems, while solenoid valves are installed to cut off the gas supply in emergency situations; they enable the valve to operate in a fault-open or fault-close mode depending on the valve’s characteristics, and they serve mainly ESD systems. After shutdown, it is necessary to reset the control valve on site, primarily by resetting the solenoid valve.
The signal for the solenoid valve generally comes from the SIS system (or other interlock systems); it goes beyond the control functions of the DCS (positioner) and directly operates the source of power for the valve – namely, instrument air. The solenoid valve directly controls the air inside the diaphragm head, ignoring the air pressure from the positioner, and pulls the valve to the desired safe position.
I think floors 6 and 7 express this quite correctly; when an urgent shutdown or activation is required from a technical perspective, a solenoid valve needs to be added. This solenoid valve is two-position, operating either in the on state or the off state, with fast response times. The main functions of a valve positioner are: 1. To improve the static characteristics of the control valve and enhance the linearity of the valve position. 2. Improve the dynamic characteristics of the control valve and reduce the transmission lag of the control signal. 3. Change the flow characteristic of the control valve. 4. Change the response range of the control valve to the signal pressure to achieve step control. 5. Reverse the valve operation. Wait, to put it more straightforwardly, it’s about determining the valve position.
Beyond DCS is the priority of interlock systems; the overriding mechanism used in some interlock systems is also a form of priority