Should the control valve be selected as air-open or air-close?

First and foremost, safety considerations must be taken into account: in the event of a failure in the control system, the control valve should be in either the open or closed position； Then, from the perspective of process quality, if there are no requirements, it makes no difference whether to choose an air-open or air-close type.

First and foremost, safety considerations must be taken into account: in the event of a failure in the control system, the control valve should be in either the open or closed position； Then, from the perspective of process quality, if there are no requirements, it makes no difference whether to choose an air-open or air-close type.

First and foremost, safety considerations must be taken into account: in the event of a failure in the control system, the control valve should be in either the open or closed position； Then, from the perspective of process quality, if there are no requirements, it makes no difference whether to choose an air-open or air-close type.

Fail-open type

The selection of the control valve (air-operated open or air-operated closed) depends to a large extent on the system’s safety requirements and process needs. 1. **Safety considerations**: In the use of pressure vessels, safety is usually the primary consideration. If the system loses its control air supply or instrument air, it is necessary to ensure that the faulty position of the valve is safe. In many cases, for pressurized vessels, if the control system fails, it is usually necessary to close the valves for safety reasons, to prevent excessive leakage from causing excessive pressure in the vessel or other safety issues. If this is the desired location for a safety failure, a fail-close valve might be more suitable. 2. **Process control requirements**: If, during the normal operation of a pressurized vessel, the control logic requires that the gas discharge rate be maintained or increased in the event of a failure of the control signal, in order to keep the pressure inside the vessel, a fail-open valve may be more suitable. In your case, if the control strategy is to release more gas when the control signal is lost in order to reduce the pressure inside the container or prevent it from rising further, then a gas-operated valve should be selected. In summary, if safety requirements dictate that the gas be kept enclosed within the container in the event of a loss of control, a gas-shut-off valve is a suitable choice. If safety requirements dictate the release of more gas in case of loss of control to prevent excessive pressure, a gas-operated valve should be selected. When making decisions, please carefully consider the overall security requirements of the system as well as the specific details of the operation process. .

From a process safety perspective, in the event of a failure in the control system, the control valve should be in either the open or closed position, or at its original position； In the pressurized vessels you mentioned as examples, if only the safety of the vessel is considered, it is possible that such situations may arise due to insufficient conditions. ,

This equipment should be equipped with a safety valve to prevent overpressure; it should not rely solely on the downstream control valve for regulation, and other measures should be in place. For this control valve, whether it operates on air opening or air closing basis, the impact of the fault location on the downstream system also needs to be considered.

As you requested, select the gas shut-off valve, but also take into account whether there are any risks associated with emissions from subsequent processes, and whether it might affect those processes. Generally, pressure vessels are equipped with safety valves to release pressure. If it is toxic, harmful, and poses a high risk, the pressure from the safety valve is released into other containers for treatment or combustion.

Based on your description, the pressure inside the tank should not be high under accident conditions. In that case, a pneumatic shut-off valve should be used.