March 12, 2025
The role of the control valve in the process control is well known, in many control processes require the control valve at fault in a certain position to protect the process from accidents, which requires the control valve in the design of the realization of the fault - safety of the three-break protection measures. For electric control valves, it is relatively simple, broken signal, according to the control module can be set and stay in the full open, full closed, hold in any position, and power failure, naturally stay in the fault position, or with a reset device electric actuator can also be operated to the valve position of the full open or full closed.
For pneumatic control valves, the situation is more complicated, so we mainly discuss the pneumatic control valve three breaks to keep the position method. Generally speaking, we are in the selection of pneumatic thin-film control valves, are to first determine the selection of gas open or gas closed, which is the choice of regulating valves to protect the position of the gas break, if the process requires a gas break when the valve is open, the choice of normally open regulating valves, and vice versa, the choice of normally closed regulating valves. This is only a rough program, if the process requires gas, power, signal cut off the three-break protection, the control valve needs to be configured with some accessories to form a protection system in order to achieve the control requirements, these accessories are mainly hold the valve, solenoid valve, gas tanks and so on. The following is a single-acting pneumatic diaphragm control valve and double-acting pneumatic control valve of two kinds of position preservation program.
First, the pneumatic membrane control valve program
This program is mainly composed of pneumatic control valve, electric - gas valve positioner, power loss comparator, single electric control solenoid valve, pneumatic holding valve, valve position signal return device and so on. Its working principle is as follows:
1, cut off the gas source: when the control system gas source failure, the pneumatic holding valve automatically closes the positioner's output signal pressure will be locked 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 is maintained in the fault position. The position-holding valve should be set at slightly lower than the minimum value of the gas source to start.
2, power failure: when the control system power failure, power loss comparator control single electric control solenoid valve output voltage disappears, single electric control solenoid valve loss of power, single electric control solenoid valve within the slide valve in the reset spring under the action of the slide valve, the solenoid valve commutation, the pneumatic holding valve membrane chamber pressure emptying, the pneumatic holding valve closes, the positioner's output signal pressure is locked in the pneumatic control valve membrane chamber, the output signal pressure and the control valve spring generated counterforce, the output signal pressure and the control valve spring generated counterforce. The output signal pressure of the positioner is locked in the diaphragm chamber of the pneumatic control valve, and the output signal pressure is balanced with the counterforce generated by the spring of the control valve.
3, break the signal: when the control system signal failure, power loss comparator detects, cut off the voltage signal of the single electric control solenoid valve, single electric control solenoid valve is de-energized, single electric control solenoid valve slides under the action of the reset spring, the solenoid valve is reversed, and the pressure of the membrane chamber of the pneumatic holding valve is emptied, and the pneumatic holding valve is closed, and the output signal pressure of the positioner is locked in the membrane chamber of the pneumatic control valve, and the output signal pressure is balanced with the counterforce generated by the control valve spring, and the valve position is maintained in the faulty position. The output signal pressure is balanced with the counterforce generated by the control valve spring, and the valve position of the pneumatic control valve is maintained in the fault position.
The position feedback signal is given by the valve position signal return.
Advantages of this program: when the “three breaks” protection is activated, the system reacts faster and acts quickly. The overall cost is cheaper.
Disadvantages of this program: the solenoid valve is charged for a long time, affecting the service life. With more accessories, installation, commissioning complexity, valve position feedback needs to be equipped with another valve position signal return device, in the case of handwheel, more complex.
Second, double-acting pneumatic control valve program
This program is mainly composed of control valves, pneumatic directional valves, positioners, self-locking valves, check valves, pressure reducing valves, air tanks and so on. Its working principle is as follows:
When the control system gas source failure, the self-locking valve automatically opens, the air-controlled reversing valve control gas source withdrawal, air-controlled reversing valve slide valve reset under the action of the spring, two air-controlled reversing valve in one of the exhaust, the other into the air, the check valve closes, the gas source stored in the tank by the gas supply to the valve to realize the valve fully closed or fully open. The conversion of fully closed or fully open can be realized by adjusting the connection of the pneumatic control reversing valve.
If you want to realize the valve hold position, add pneumatic hold position valve and change the pipeline connection, use the self-locking valve to directly control the hold position valve, cancel the air-controlled directional valve, check valve, gas storage tank can be.
If you want to realize that when you cut off the air source, you can ensure that the valve has a number of actions, you can use the following program.
This program consists of gas storage tanks, check valves, blocking valves, shut-off valves and other components. Its working principle is as follows:
When the gas source failure, the check valve closes, the blocking valve lost gas, the slide valve in the blocking valve reset under the action of the spring, the gas line reversal, disconnect the system's gas source pipeline, connect the gas storage tank pipeline, the gas supply from the gas tank to the valve to ensure that the valve has a number of actions to achieve the purpose of continuous control. Due to the limited capacity of the gas storage tank, and the pressure of the gas source in the tank with the valve action continues to decline, can not be used for a long time to supply gas to the valve tank. The capacity of the gas storage tank used in this program should be larger than that of the gas storage tank used for general protection. In this program, the number of times the valve is disconnected from the gas source, the number of times the valve action is related to the capacity of the storage tank.
For pneumatic thin-film control valves to keep the program, there is a reference: between the positioner and the actuator in series to keep the valve and two three-way solenoid valve each one, in the break in the gas to keep the valve to keep the position, in the break in the signal, with the solenoid valve to keep the position, however, the solenoid valve must be interlocked with the locator, i.e., locator signals, solenoid valves must be electric, locator, once the signal is lost, solenoid valves must be powered off immediately.
Three-break protection program
What is the three-break protection: that is, cut off the power supply, cut off the signal source, cut off the gas source of the valve to maintain the original position, to protect the system from damage! Next I will ABB TZID-C positioner, Siemens PS2 intelligent positioner is how to do three off protection to do a comparative analysis! If there is something wrong, please include more corrections!
A: Siemens three-break protection program
This program mainly consists of pneumatic control valves, two-wire Siemens valve positioner (6DR50xx-, 6DR51xx-), electronic switches (loss of signal comparator), a single electronically controlled solenoid valve, pneumatic hold position valve components
This program mainly consists of pneumatic control valves, two-wire Siemens valve positioners (6DR50xx-, 6DR51xx-), electronic switches (loss of signal comparator), single electrically controlled solenoid reversing valves, and pneumatic holding valves, see Figure 1.
1, cut off the gas source: single-acting positioner can choose single-channel pneumatic holding valve, double-acting positioner can choose double-channel pneumatic holding valve. When the gas source system failure (loss of gas), the pneumatic holding valve automatically cuts off the channel between the positioner and the pneumatic control valve, the output signal pressure of the positioner is locked 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 or the pressure balance between the two chambers, and the valve position of the pneumatic control valve is maintained in the position of the last control signal before the failure. The hold position valve should be set at slightly lower than the minimum value of the gas source to start, because Siemens positioner normal operating pressure range is 0.14 ~ 0.7MPa, the switching point can be set to 0.14MPa.
2, cut off the power supply: solenoid valve can choose two two-way 24VDC single electric control normally closed type. When the control system power failure (power failure), the electromagnetic directional valve power loss, electromagnetic directional valve slide valve under the action of the reset spring, the solenoid valve reversal, the pneumatic holding valve membrane chamber pressure emptying, pneumatic holding valve to perform the equivalent of the “gas” when the action, so that the valve position of the pneumatic control valve to maintain in the last control signal position before the failure. Signal position.
Signal break: The electronic switch can be installed in the Siemens positioner with special module for Siemens positioner. When the control system signal failure (loss of signal), the electronic switch detects it and cuts off the voltage signal of the electromagnetic directional valve, and the solenoid valve performs the action equivalent to that of “power failure”, so that the valve position of the pneumatic control valve is maintained at the last control signal position before the failure.
Advantages of this program: “three breaks” protection start, the system reacts faster, rapid action. Disadvantages of this program: the solenoid valve is electrified for a long time, the requirements are higher. With more accessories, installation, commissioning complexity.
Second: ABB three-break protection program
TZID-C intelligent positioner itself is able to realize the power cut, cut signal protection, so that the valve in the system failure locked in the current position (when the gas supply is normal), in order to realize the valve in the three breaks in the protection of the air protection, it is necessary to increase the air lock valve in the air circuit.
III: Siemens positioner with triple-break protection function
Siemens PS2 positioner series intelligent positioner with three-break protection, itself has three-break protection function, no need to add any air-control accessories in the gas circuit, only one positioner can realize three-break protection function. Simplify the structure of air circuit, save installation space, and convenient for inspection and maintenance!
