Measures to address the failure of pneumatic control valves
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Question: What do positive and reverse action mean in air-operated open and air-operated closed control valves? When the instrument air system fails, should the positive or negative action of the control valve be taken into consideration when the pneumatic control valve is controlled by a downstream valve or a bypass valve? For example, if the control valve is a air-actuated valve, it should be controlled by a valve located downstream. Is this approach effective when the regulator is set to positive action, and is it also effective when the regulator is set to reverse action?
The terms \"air-open\" and \"air-close\" for control valves refer to the fact that when there is a loss of air or power, an air-open valve will close automatically while an air-close valve will open automatically. This is done to ensure the safety of the process operation as well as to take into account the properties of the materials being handled. In the event of a failure in the instrument air supply, it is possible to use downstream valves for control; however, if an air-open valve is used, then the valve will be in a closed state at that time. It is certainly necessary to consider the status of such a valve. The positive and negative action of a valve simply refers to the increase or decrease in the relationship between control and the input signal; it does not function when there is a loss of air or power.
A control valve consists of an actuator and a regulating mechanism. Actuators come in two types: direct-acting and reverse-acting. In the case of direct-acting actuators, the valve stem extends when the signal increases; reverse-acting actuators behave in the opposite way； The adjustment mechanism comes in two forms: with the valve core installed in the normal orientation and in the reverse orientation. Different actuating and regulating mechanisms can be combined to form air-open valves or air-close valves. The choice between wind on and wind off is primarily based on the safety of the manufacturing process.
This post was last edited by zeqing on 2009-8-27 at 12:27. When there is a problem with the instrument air, the control valve ceases to function; whether it is a gas-operated or air-operated valve, it must be taken out of service and controlled through a bypass line. As for the on/off of gas and the positive/negative effects, you can look up some information.