Control Valve Disc Position-Holding Scheme I After Air Failure

For pneumatic control valves, the situation is more complicated, so we mainly discuss the three-break method of pneumatic control valves. Generally speaking, when we choose a pneumatic control valve, we must first determine whether to choose air 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 the two position-holding schemes of single-acting pneumatic diaphragm control valve and double-acting pneumatic control valve.

1. Single Acting Pneumatic control valve scheme (the control valve is equipped with electro-pneumatic positioner)

This plan is mainly composed of pneumatic control valves, electro-pneumatic valve positioners, power loss (signal) comparators, single electric control solenoid directional valves, pneumatic retaining valves, valve position signal returners, 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 and locks the output signal pressure of the positioner in the diaphragm chamber of the pneumatic control valve, and the output signal pressure is opposed to 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 slightly below the minimum value of the air supply.

2. Cut off the Power: When the power supply of the control system fails (power loss), 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 of 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. Cut off the Signal: When the control system signal is faulty (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 directional valve slides under the action of the return spring. The solenoid valve reverses to empty the pressure of 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.

The position feedback signal is given by the valve position signal returner.

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

The disadvantage 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.