What types of solutions should be used to address valve faults, and which solution is best suited for maintaining the valve position?
The regulating valve plays a critical role in process control systems. In many industrial applications, it is essential that the valve remains in a specific position during a fault to prevent accidents or damage to the system. This requires the valve to be designed with fail-safe mechanisms—commonly known as "three-break" protection: gas loss, power failure, and signal loss. For electric control valves, this is relatively straightforward. When the signal is lost, the valve can remain fully open, fully closed, or in a holding position based on the settings of the control module. If the power is cut, the valve will naturally stay at its last position, or if equipped with a reset mechanism, it can be forced to close or open.
Pneumatic control valves, however, present more complex challenges. Therefore, we focus primarily on the three-break protection method for pneumatic valves. When selecting a pneumatic diaphragm control valve, the first step is to determine whether it should be normally open or normally closed, depending on the required safety position when the system fails. If the process demands that the valve closes when air supply is lost, a normally closed (air-to-open) valve should be chosen; otherwise, a normally open (air-to-close) valve is appropriate. However, this is just a basic approach. If the system requires full three-break protection, additional accessories must be integrated into the system to ensure proper operation. These may include retaining valves, solenoid valves, and gas storage tanks. Below are two common retention schemes for single-acting and double-acting pneumatic regulators.
1. **Single-Acting Pneumatic Diaphragm Valve Solution** This system typically includes a pneumatic control valve, an electro-pneumatic positioner, a power/signal comparator, a single-electrically controlled solenoid valve, a pneumatic retaining valve, and a valve position feedback device. Here's how it works: - **Gas Loss**: When the air supply fails, the pneumatic retaining valve automatically closes, locking the positioner’s output pressure inside the actuator’s diaphragm chamber. This pressure balances with the spring force, keeping the valve in its last position. - **Power Failure**: When the power is lost, the comparator disables the solenoid valve, causing it to reverse and release the pressure from the retaining valve. This locks the positioner’s output, ensuring the valve stays in place. - **Signal Loss**: When the control signal is lost, the comparator detects the issue and de-energizes the solenoid valve, triggering the same sequence as a power failure. The valve maintains its position due to the balanced forces. The position feedback is provided by the return signal. **Advantages**: Quick response during failures, cost-effective. **Disadvantages**: Long-term energization of the solenoid valve reduces its lifespan, and the system requires multiple components, making installation and maintenance more complex. 2. **Double-Acting Pneumatic Regulating Valve Solution** This setup usually consists of a control valve, a pneumatic reversing valve, a positioner, a self-locking valve, a check valve, a pressure-reducing valve, and a gas storage tank. Its operation is as follows: - During gas loss, the self-locking valve opens, cutting off the control air supply to the reversing valve. The spool returns to its default position, allowing the gas storage tank to provide pressure to the valve, enabling it to either close or open. - To achieve valve retention, a pneumatic retaining valve can be added, and the piping adjusted accordingly. - For limited number of valve operations, a gas storage tank, check valve, and lock valve can be used. When the air source fails, the check valve closes, and the lock valve reverses the airflow, supplying gas from the storage tank to the valve. However, since the storage capacity is limited, this method is only suitable for short-term use. Another alternative involves using a series of retaining valves and a two-position three-way solenoid valve between the positioner and the actuator. The solenoid valve helps maintain the position during gas loss, but it must be synchronized with the positioner to ensure safe operation. In conclusion, choosing the right protection system depends on the specific requirements of the process, the type of valve, and the available resources. Proper selection ensures both safety and reliability in critical control scenarios. Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
The regulating valve plays a critical role in process control systems. In many industrial applications, it is essential that the valve remains in a specific position during a fault to prevent accidents or damage to the system. This requires the valve to be designed with fail-safe mechanisms—commonly known as "three-break" protection: gas loss, power failure, and signal loss. For electric control valves, this is relatively straightforward. When the signal is lost, the valve can remain fully open, fully closed, or in a holding position based on the settings of the control module. If the power is cut, the valve will naturally stay at its last position, or if equipped with a reset mechanism, it can be forced to close or open.
Pneumatic control valves, however, present more complex challenges. Therefore, we focus primarily on the three-break protection method for pneumatic valves. When selecting a pneumatic diaphragm control valve, the first step is to determine whether it should be normally open or normally closed, depending on the required safety position when the system fails. If the process demands that the valve closes when air supply is lost, a normally closed (air-to-open) valve should be chosen; otherwise, a normally open (air-to-close) valve is appropriate. However, this is just a basic approach. If the system requires full three-break protection, additional accessories must be integrated into the system to ensure proper operation. These may include retaining valves, solenoid valves, and gas storage tanks. Below are two common retention schemes for single-acting and double-acting pneumatic regulators.
1. **Single-Acting Pneumatic Diaphragm Valve Solution** This system typically includes a pneumatic control valve, an electro-pneumatic positioner, a power/signal comparator, a single-electrically controlled solenoid valve, a pneumatic retaining valve, and a valve position feedback device. Here's how it works: - **Gas Loss**: When the air supply fails, the pneumatic retaining valve automatically closes, locking the positioner’s output pressure inside the actuator’s diaphragm chamber. This pressure balances with the spring force, keeping the valve in its last position. - **Power Failure**: When the power is lost, the comparator disables the solenoid valve, causing it to reverse and release the pressure from the retaining valve. This locks the positioner’s output, ensuring the valve stays in place. - **Signal Loss**: When the control signal is lost, the comparator detects the issue and de-energizes the solenoid valve, triggering the same sequence as a power failure. The valve maintains its position due to the balanced forces. The position feedback is provided by the return signal. **Advantages**: Quick response during failures, cost-effective. **Disadvantages**: Long-term energization of the solenoid valve reduces its lifespan, and the system requires multiple components, making installation and maintenance more complex. 2. **Double-Acting Pneumatic Regulating Valve Solution** This setup usually consists of a control valve, a pneumatic reversing valve, a positioner, a self-locking valve, a check valve, a pressure-reducing valve, and a gas storage tank. Its operation is as follows: - During gas loss, the self-locking valve opens, cutting off the control air supply to the reversing valve. The spool returns to its default position, allowing the gas storage tank to provide pressure to the valve, enabling it to either close or open. - To achieve valve retention, a pneumatic retaining valve can be added, and the piping adjusted accordingly. - For limited number of valve operations, a gas storage tank, check valve, and lock valve can be used. When the air source fails, the check valve closes, and the lock valve reverses the airflow, supplying gas from the storage tank to the valve. However, since the storage capacity is limited, this method is only suitable for short-term use. Another alternative involves using a series of retaining valves and a two-position three-way solenoid valve between the positioner and the actuator. The solenoid valve helps maintain the position during gas loss, but it must be synchronized with the positioner to ensure safe operation. In conclusion, choosing the right protection system depends on the specific requirements of the process, the type of valve, and the available resources. Proper selection ensures both safety and reliability in critical control scenarios. Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
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