High-temperature and high-pressure valves, also known as high-temperature and high-pressure control valves, play a vital role in industrial automation process control. They act as the final control elements by receiving control signals from the regulating control unit and using dynamic operation to adjust process parameters such as fluid flow, pressure, temperature, and liquid level. Generally, they consist of an actuator and a valve. Depending on the stroke characteristics, high-temperature and high-pressure valves can be divided into straight stroke and angular stroke types. Based on the power used by their actuators, they can be categorized as pneumatic, electric, or hydraulic high-temperature and high-pressure valves. Additionally, they can be classified based on their function and characteristics into three types: linear, equal percentage, and parabolic. High-temperature and high-pressure valves are suitable for air, water, steam, various corrosive media, slurry, oil, and other substances.
In case of faults, several position-holding solutions are available for high-temperature and high-pressure valves. The following are some approaches:
The role of high-temperature and high-pressure valves in process control is well-known. In many control processes, it is necessary for high-temperature and high-pressure valves to be in a specific position in case of a fault to protect the process from accidents. This requires the design of high-temperature and high-pressure valves to implement a fault-safe "three-disconnection" (air disconnection, power disconnection, signal disconnection) protection mechanism. For electric high-temperature and high-pressure valves, it is relatively simple. When the signal is disconnected, the valve can stay in any position preset by the control module, such as fully open, fully closed, or hold. When the power is disconnected, the valve naturally remains in the fault position, or an electric actuator with a reset device can move the valve to fully open or fully closed.
For pneumatic high-temperature and high-pressure valves, the situation is more complex. Therefore, we mainly discuss three-disconnection position-holding methods for pneumatic high-temperature and high-pressure valves. Generally, when selecting pneumatic diaphragm high-temperature and high-pressure valves, we first determine whether to choose normally open or normally closed, which defines the protective position of the valve when the air is disconnected. If the process requires the valve to be open when the air is disconnected, we choose a normally open (normally closed) high-temperature and high-pressure valve; otherwise, we select a normally closed (normally open) high-temperature and high-pressure valve. This is just a basic solution. If the process requires the "three-disconnection" protection of air disconnection, power disconnection, and signal disconnection, the high-temperature and high-pressure valve needs to be equipped with accessories to form a protection system to meet control requirements. These accessories mainly include position-holding valves, high-temperature and high-pressure control valves, and air reservoirs.
The following are two position-holding solutions for single-acting pneumatic diaphragm high-temperature and high-pressure valves and double-acting pneumatic high-temperature and high-pressure valves:
Solution for single-acting pneumatic diaphragm high-temperature and high-pressure valves (high-temperature and high-pressure valve equipped with an electric-pneumatic valve positioner): This solution mainly consists of a pneumatic high-temperature and high-pressure valve, an electric-pneumatic valve positioner, a power (signal) comparator, a single-control electromagnetic directional valve, a pneumatic position-holding valve, and a valve position signal returner. The working principle is as follows:
1. Air disconnection:
When the control system air source fails (air loss), the pneumatic position-holding valve automatically closes, locking the output signal pressure of the positioner in the diaphragm chamber of the pneumatic high-temperature and high-pressure control valve. The output signal pressure balances with the force produced by the spring of the high-temperature and high-pressure control valve, maintaining the valve position in the fault state. The position-holding valve should be activated when the air source is slightly below the minimum value.
2. Power disconnection:
When the control system power supply fails (power loss), the power (signal) comparator controls the single-control electromagnetic directional valve to lose the output voltage. With the loss of power, the single-control electromagnetic directional valve loses power, causing the spool in the single-control electromagnetic directional valve to slide under the action of the reset spring, changing the direction of the high-temperature and high-pressure control valve, exhausting the pressure in the diaphragm chamber of the pneumatic position-holding valve, closing the pneumatic position-holding valve. This locks the output signal pressure of the positioner in the diaphragm chamber of the pneumatic high-temperature and high-pressure control valve, balancing the output signal pressure with the force produced by the spring of the high-temperature and high-pressure control valve, maintaining the valve position in the fault state.
3. Signal disconnection:
When the control system signal fails (signal loss), upon detection by the power (signal) comparator, the voltage signal of the single-electric control solenoid reversing valve is cut off, causing the single-electric control solenoid reversing valve to lose power. The spool inside the single-electric control solenoid reversing valve slides under the action of the reset spring. The high-temperature and high-pressure regulating valve switches, emptying the pneumatic position maintenance valve chamber pressure, and closing the pneumatic position maintenance valve. The output signal pressure of the positioner is locked in the diaphragm chamber of the pneumatic high-temperature and high-pressure regulating valve, and the output signal pressure balances with the force generated by the spring of the high-temperature and high-pressure regulating valve. The valve position of the pneumatic high-temperature and high-pressure regulating valve remains in the fault position. The position feedback signal is provided by the valve position signal returner. The advantage of this scheme: when the "three-fail-safe" protection is activated, the system responds quickly and operates swiftly. The overall cost is relatively low. The disadvantage of this scheme: the high-temperature and high-pressure regulating valve remains energized for a long time, affecting its service life. The scheme requires additional accessories, making installation and debugging somewhat complex. Valve position feedback requires an additional valve position signal returner, which is more complicated when a handwheel is used in conjunction.
In case of faults, several position-holding solutions are available for high-temperature and high-pressure valves. The following are some approaches:
The role of high-temperature and high-pressure valves in process control is well-known. In many control processes, it is necessary for high-temperature and high-pressure valves to be in a specific position in case of a fault to protect the process from accidents. This requires the design of high-temperature and high-pressure valves to implement a fault-safe "three-disconnection" (air disconnection, power disconnection, signal disconnection) protection mechanism. For electric high-temperature and high-pressure valves, it is relatively simple. When the signal is disconnected, the valve can stay in any position preset by the control module, such as fully open, fully closed, or hold. When the power is disconnected, the valve naturally remains in the fault position, or an electric actuator with a reset device can move the valve to fully open or fully closed.
For pneumatic high-temperature and high-pressure valves, the situation is more complex. Therefore, we mainly discuss three-disconnection position-holding methods for pneumatic high-temperature and high-pressure valves. Generally, when selecting pneumatic diaphragm high-temperature and high-pressure valves, we first determine whether to choose normally open or normally closed, which defines the protective position of the valve when the air is disconnected. If the process requires the valve to be open when the air is disconnected, we choose a normally open (normally closed) high-temperature and high-pressure valve; otherwise, we select a normally closed (normally open) high-temperature and high-pressure valve. This is just a basic solution. If the process requires the "three-disconnection" protection of air disconnection, power disconnection, and signal disconnection, the high-temperature and high-pressure valve needs to be equipped with accessories to form a protection system to meet control requirements. These accessories mainly include position-holding valves, high-temperature and high-pressure control valves, and air reservoirs.
The following are two position-holding solutions for single-acting pneumatic diaphragm high-temperature and high-pressure valves and double-acting pneumatic high-temperature and high-pressure valves:
Solution for single-acting pneumatic diaphragm high-temperature and high-pressure valves (high-temperature and high-pressure valve equipped with an electric-pneumatic valve positioner): This solution mainly consists of a pneumatic high-temperature and high-pressure valve, an electric-pneumatic valve positioner, a power (signal) comparator, a single-control electromagnetic directional valve, a pneumatic position-holding valve, and a valve position signal returner. The working principle is as follows:
1. Air disconnection:
When the control system air source fails (air loss), the pneumatic position-holding valve automatically closes, locking the output signal pressure of the positioner in the diaphragm chamber of the pneumatic high-temperature and high-pressure control valve. The output signal pressure balances with the force produced by the spring of the high-temperature and high-pressure control valve, maintaining the valve position in the fault state. The position-holding valve should be activated when the air source is slightly below the minimum value.
2. Power disconnection:
When the control system power supply fails (power loss), the power (signal) comparator controls the single-control electromagnetic directional valve to lose the output voltage. With the loss of power, the single-control electromagnetic directional valve loses power, causing the spool in the single-control electromagnetic directional valve to slide under the action of the reset spring, changing the direction of the high-temperature and high-pressure control valve, exhausting the pressure in the diaphragm chamber of the pneumatic position-holding valve, closing the pneumatic position-holding valve. This locks the output signal pressure of the positioner in the diaphragm chamber of the pneumatic high-temperature and high-pressure control valve, balancing the output signal pressure with the force produced by the spring of the high-temperature and high-pressure control valve, maintaining the valve position in the fault state.
3. Signal disconnection:
When the control system signal fails (signal loss), upon detection by the power (signal) comparator, the voltage signal of the single-electric control solenoid reversing valve is cut off, causing the single-electric control solenoid reversing valve to lose power. The spool inside the single-electric control solenoid reversing valve slides under the action of the reset spring. The high-temperature and high-pressure regulating valve switches, emptying the pneumatic position maintenance valve chamber pressure, and closing the pneumatic position maintenance valve. The output signal pressure of the positioner is locked in the diaphragm chamber of the pneumatic high-temperature and high-pressure regulating valve, and the output signal pressure balances with the force generated by the spring of the high-temperature and high-pressure regulating valve. The valve position of the pneumatic high-temperature and high-pressure regulating valve remains in the fault position. The position feedback signal is provided by the valve position signal returner. The advantage of this scheme: when the "three-fail-safe" protection is activated, the system responds quickly and operates swiftly. The overall cost is relatively low. The disadvantage of this scheme: the high-temperature and high-pressure regulating valve remains energized for a long time, affecting its service life. The scheme requires additional accessories, making installation and debugging somewhat complex. Valve position feedback requires an additional valve position signal returner, which is more complicated when a handwheel is used in conjunction.
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