Urgent question: Why are control valves divided into gas-shut valves and gas-open valves?
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In our actual production, some of the control valves used are air-shut valves, while others are air-open valves. What is the basis for determining whether a valve is an air-shut valve or an air-open valve? What can be used in on-site production to determine whether a valve belongs to one category or the other? This is urgent; those who know the answer please share their insights!
:The handshake concept regarding open/closed operation applies to single-acting pneumatic actuators. Open with air supply – closed by spring: The valve opens when there is air supply, and it closes automatically when the air supply is cut off. Closed with air supply – open by spring: This is the opposite of the above scenario. Since the spring force generated by the spring cannot be exactly equal to the pressure provided by the air source, it is important to ensure that both the torque produced by the spring and the torque generated by the air source are greater than the torque required to open the valve.
The choice between a gas-operated valve and a air-operated valve is determined by the requirements of the production process and the installation location. For example, in the case of emergency vent valves for polymerization reactors, it is necessary for such valves to remain open in emergency situations. If a gas-operated valve is used, a failure in the gas supply will cause the valve to close automatically, which is very dangerous; therefore, an air-operated valve – one that opens due to a spring mechanism in case of failure – must be used. Similarly, in some cases, a gas-operated valve – one that closes due to a spring mechanism in case of failure – is required. As for how to determine whether to use a gas-operated or an air-operated valve, those with professional knowledge can make this decision based on the shape of the actuator. Those without such expertise can check whether the valve is open or closed when there is gas inside the cylinder; if it is open, then it is a gas-operated valve, and if it is closed, then it is an air-operated valve.
Put simply, when the air supply to a control valve is suddenly cut off, you need to know what position you want the valve to be in. If you want the valve to be closed – for example, in the case of a steam heating control valve – continuous steam supply could lead to a steady increase in temperature, which can be dangerous; in such situations it’s necessary for the valve to be in a closed state. This is what is referred to as a gas-open type valve. Conversely, there is the gas-close type valve
Determining whether a control valve should operate in air-open or air-close mode (or in emergency-close or emergency-open mode) is done from a safety perspective, with the aim of ensuring safety through the appropriate setting
The issue of air-open/air-close applies to single-acting pneumatic actuators. Air-open – spring-close: The valve opens when there is air pressure, and it closes automatically when the air pressure is lost. Air-close – spring-open: This is the exact opposite of the above. In many cases, the situation is reversed: lol :lol
This is entirely determined by the requirements of the production process; it is not determined by the control valve itself.