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Several kinds of maintenance schemes for regulating valve failure

2018-04-27

Control valve in the process of the role is well known, in many control process requires the control valve in a certain position at the time of failure, in order to protect the process does not have an accident, which requires the control valve in the design to achieve fault-safe triplex protection measures.
Control valve in the process of the role is well known, in many control process requires the control valve in a certain position at the time of failure, in order to protect the process does not have an accident, which requires the control valve in the design to achieve fault-safe triplex protection measures. For the electric regulating valve, it is relatively simple. When the signal is off, it can stay in any position of fully open, fully closed, and hold according to the setting of the control module. When the power is off, it will naturally stay in the fault position, or The electric actuator with a 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 pneumatic control valve of the three break retention method. Generally speaking, when we choose the pneumatic diaphragm regulating valve, we must first determine whether the gas is open or closed. This is the protection position when the regulating valve is cut off. If the process requires the valve to be opened when the gas is cut off, the normally open regulating valve is selected, otherwise the normally closed regulating valve is selected. This is only a superficial scheme. If the process requires three-break protection of gas cut-off, power cut-off and cut-off signals, the regulating valve needs to be configured with some accessories to form a protection system to achieve the control requirements. These accessories mainly include retaining valves, solenoid valves, gas tanks, etc. The following is a single-acting pneumatic diaphragm control valve and double-acting pneumatic control valve two kinds of retention program.
 
Scheme of 1. pneumatic diaphragm regulating valve
This scheme is mainly composed of pneumatic control valve, electric-gas valve positioner, loss of power comparator, single electric solenoid directional valve, pneumatic valve, valve position signal return device, etc. Its working principle is as follows:
1. Cut-off air source: When the air source of the control system fails, the pneumatic holding valve automatically closes the output signal pressure of the positioner and locks it 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. The valve position of the pneumatic control valve remains in the fault position. The valve shall be set to start slightly below the minimum value of the gas supply.
2. Power cut-off: when the power supply of the control system fails, the power loss comparator controls the output voltage of the single electronically controlled electromagnetic directional valve to disappear, the single electronically controlled electromagnetic directional valve loses power, the slide valve in the single electronically controlled electromagnetic directional valve slides under the action of the return spring, the solenoid valve reverses, empties the pressure of the diaphragm chamber of the pneumatic valve, closes the pneumatic valve, and locks 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 in the fault position.
3. Break signal: when the control system signal fails, the voltage signal of the single electronically controlled electromagnetic directional valve will be cut off after the power loss comparator detects it. The single electronically controlled electromagnetic directional valve loses power, the slide valve in the single electronically controlled electromagnetic directional valve slides under the action of the return spring, the solenoid valve reverses, empties the diaphragm pressure of the pneumatic valve, closes the pneumatic valve, and locks 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 in the fault position.
The position feedback signal is given by the valve position signal return.
The advantages of this scheme: when the "three-break" protection is started, the system responds quickly and acts quickly. The overall cost is cheaper.
The disadvantage of this scheme: the solenoid valve is charged for a long time, which affects the service life. There are many accessories, and the installation and debugging are more complicated. The valve position feedback needs to be equipped with a valve position signal return device, which is more complicated when equipped with a handwheel.
 
Scheme of 2. double-acting pneumatic control valve
This scheme is mainly composed of control valve, pneumatic directional valve, positioner, self-locking valve, check valve, pressure reducing valve, gas storage tank, etc. Its working principle is as follows:
When the air source of the control system fails, the self-locking valve automatically opens, and the control air source of the air-controlled reversing valve is canceled. The slide valve of the air-controlled reversing valve is reset under the action of the spring. One of the two air-controlled reversing valves is exhausted, the other is intake, and the one-way valve is closed. The air source is supplied to the valve from the air source stored in the air storage tank, thereby realizing full closing or full opening of the valve. The conversion of full close or full open can be realized by adjusting the connection mode of the pneumatic control reversing valve.
If you want to realize the valve retention, install the pneumatic valve and change the pipeline connection, use the self-locking valve to directly control the valve, and cancel the pneumatic directional valve, one-way valve and gas storage tank.
If you want to realize the gas source, can ensure that the valve has several times of action, can use the following scheme.
This scheme is composed of gas storage tank, one-way valve, locking valve, stop valve, etc. Its working principle is as follows:
When the air source fails, the one-way valve is closed, the locking valve loses air, the slide valve of the locking valve is reset under the action of the spring, the air path is reversed, the air source pipeline of the system is disconnected, and the air storage tank pipeline is connected. The gas tank supplies gas to the valve to ensure that the valve has several actions to achieve the purpose of continuous control. Since the capacity of the gas tank is limited and the pressure of the gas source in the gas tank decreases continuously with the action of the valve, the gas tank cannot be used for a long time to supply gas to the valve. The capacity of the gas storage tank used in this scheme shall be larger than that of the gas storage tank for general protection. In this scheme, when the gas source is cut off, the number of valve actions is related to the capacity of the gas storage tank.
For the position retention scheme of the pneumatic diaphragm regulating valve, there is another one for reference: the position retention valve and the two-position three-way solenoid valve are connected in series between the positioner and the actuator, the position retention valve is used to retain the position when the gas is cut off, and the solenoid valve is used to retain the position when the signal is cut off. However, the solenoid valve must be interlocked with the positioner, I .e. the positioner must have electricity.