Direct acting and reverse acting are two different configurations of control valves based on the relationship between the valve stem movement and the process variable (e.g., flow rate, pressure, temperature) they control. Here's an explanation of each:
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Direct Acting Control Valve:
- In a direct acting control valve, the stem movement in the valve is in the same direction as the change in the process variable that it controls.
- For example, if the control valve is controlling flow rate and is configured as direct acting, opening the valve (moving the stem upward) will increase the flow rate, while closing the valve (moving the stem downward) will decrease the flow rate.
- Direct acting control valves are commonly used for processes where increasing the valve opening results in an increase in the controlled process variable.
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Reverse Acting Control Valve:
- In a reverse acting control valve, the stem movement in the valve is in the opposite direction to the change in the process variable that it controls.
- Using the same example of flow rate control, in a reverse acting control valve, opening the valve (moving the stem upward) will decrease the flow rate, while closing the valve (moving the stem downward) will increase the flow rate.
- Reverse acting control valves are used in processes where the control system requires a decrease in the valve opening to increase the controlled process variable.
The selection of direct acting or reverse acting control valves depends on the specific requirements of the process and the control system. Factors such as the control loop configuration, desired response of the system, and characteristics of the process variable being controlled influence the choice between direct and reverse acting configurations.
It's important to note that the terms "direct acting" and "reverse acting" refer to the relationship between valve stem movement and the process variable, not necessarily the physical orientation of the valve itself. Control valves can be installed in various orientations depending on the specific application and piping layout.
