Control Valve Failure Response Plan

The role of the familiar control valve in control is unquestionable. In many places where control is required, the control valve is required to be at which point when a failure occurs to ensure that there will be no or fewer accidents in the process, so The control valve is designed to realize the protection measures of failure-safe air cut, power cut, and signal cut off. This is very simple for electric control valves. When the signal is cut off, it can be fully closed according to the setting of a certain control module. Any place of open and hold, and when the power is cut off, it will naturally stop at the fault position, or the electric actuator with reset device can also run the valve position to fully open or fully closed.

For the pneumatic control valve, the situation is more complicated, so we mainly discuss the three-break method of the pneumatic control valve. Generally speaking, when we choose a pneumatic membrane control valve, we must first determine whether the air is open or air closed. This is the protection position when the air is cut off. If the process requires the valve to be opened when the air is cut off, select normally open (air closed). ) Type control valve, otherwise, select normally closed (air-open) type control valve. This is just a rough solution. If the process requires three-break protection for gas, power, and signal, the regulating valve needs to be equipped with some accessories to form a protection system to achieve the control requirements. These accessories mainly include retaining valves and solenoid valves. , Gas tank, etc. The following are two position-holding schemes of single-acting pneumatic diaphragm control valve and double-acting pneumatic control valve.

1. Pneumatic membrane control valve scheme (the control valve is equipped with electric-pneumatic valve positioner)

This program is mainly composed of pneumatic control valve, electro-pneumatic valve positioner, power loss (signal) comparator, single electric control solenoid directional valve, pneumatic retaining valve, valve position signal returner, etc. Its working principle is as follows:

1. Cut off the air source: When the air source of the control system fails (lost air), the pneumatic retaining valve automatically closes to lock the output signal pressure of the positioner in the diaphragm chamber of the pneumatic control valve, the output signal pressure and the reaction force generated by the control valve spring Phase balance, the valve position of the pneumatic control valve remains at the fault position. The retaining valve should be set to start when it is slightly lower than the minimum value of the air supply.

2. Power off: when the power supply of the control system fails (loss of power), the output voltage of the power loss (signal) comparator controlling the single electronically controlled solenoid directional valve disappears, the single electronically controlled solenoid directional valve loses power, and the single electronically controlled solenoid The spool valve in the reversing valve slides under the action of the return spring, the solenoid valve is reversed, the pressure in the diaphragm chamber of the pneumatic retaining valve is emptied, the pneumatic retaining valve is closed, and the output signal pressure of the positioner is locked in the pneumatic control valve In the diaphragm chamber, the output signal pressure is balanced with the reaction force generated by the control valve spring, and the valve position of the pneumatic control valve remains at the fault position.

3. Signal disconnection: When the control system signal fails (signal loss), after the power loss (signal) comparator detects it, it cuts off the voltage signal of the single electronically controlled electromagnetic directional valve, and the single electronically controlled electromagnetic directional valve loses power. The spool valve in the single electronically controlled electromagnetic reversing valve slides under the action of the return spring. The solenoid valve reverses to empty the pressure in the diaphragm chamber of the pneumatic retaining valve, and the pneumatic retaining valve closes to lock the output signal pressure of the positioner. In the diaphragm chamber of the pneumatic control valve, the output signal pressure is balanced with the reaction force generated by the control valve spring, and the valve position of the pneumatic control valve remains at the fault position.

Position feedback signal is given by valve position signal returner.

Advantages of this scheme: When the "three breaks" protection is activated, the system reacts quickly and acts quickly. The overall cost is relatively cheap.

Disadvantages of this scheme: The solenoid valve is charged for a long time, which affects its service life. There are many accessories, and installation and debugging are more complicated. Valve position feedback needs to be equipped with a valve position signal returner, which is more complicated when it is equipped with a handwheel.