Working of Direct Acting and Reverse Acting Control Valve Loop

What is the controller action?

• The controller action is defined as the function of the controller.
• The controller action determines whether
• The controller output increases when PV increases above the set point, or
• The controller output decreases when the PV is increased above the set point.
• The operator has to select the controller action based on their process requirements.

What are the factors that determine the controller action?

The factors that determine the controller actions are

• The controller action depends upon the location of the flow control element. In this case, it is important to know whether the flow control valve is located on the input side or at the output side of the process.
• The control action depends upon the need for fail-safe operation of the process.
• It depends upon the type of valve action may be ( ATC or ATO )

The direct acting and reverse acting valve loops can be determined,

• The control valve loop is said to be direct acting if the controller output and measured value are in direct proportion to each other. When the process value raises, the controller output increases and vice versa
• The control valve loop is said to be reverse acting if the controller output and measured value are in inverse proportion to each other. When the process value raises, the controller output decreases and vice versa.
• The desired fail-safe action is achieved for the minimum output of the controller.

For example, in a distillation column, there are two controllers, the reflux and the other one is net overhead.

• The reflux controller is reverse-acting and the reflux control valve is Air-to-Close, Fail Open (ATC – FO), and
• The net overhead controller will be direct-acting, and the net-overhead control valve is Air-to-Open, Fail Close (ATO – FC).

Generally, the control valve loop includes some basic elements

• The process parameters like flow or pressure are under control.
• The control action to the control valve is initiated by the process controller.
• A valve positioner is the most essential component and is always required in every application.
• The control valve action.

What are the modes of operation of control valve loops?

The control valve loops are operated in two basic modes

1. Direct acting loops.
2. Reverse acting loops

Direct Acting Control Valve Loops:

In a direct-acting control valve loop, an action of instruments such as the controller, valve positioner, and the control valve is shown below.

• Controller Action:  As the process variable or measured value of process parameters such as flow, level, temperature, or pressure increases numerically the controller output signal also increases in proportion to it and vice versa.

• Valve Positioner Action: Here an air pressure or output load from the valve positioner is increased when an output signal from the controller is increased and vice versa.

• Control Valve Action: The valve plug travels towards the close position for a valve that is Air-to-Close, Fail Open (ATC – FO) when the load or air pressure on the valve actuator increases.

Reverse Acting Control Valve Loops:

In a reverse-acting control valve loop, an action of instruments such as the controller, valve positioner, and the control valve is shown below.

• Controller Action:  As the process variable or measured value of process parameters such as flow, level, temperature, or pressure increases numerically the controller output signal decreases in proportion to it and vice versa.

• Valve Positioner Action: Here an air pressure or output load from the valve positioner decreases when an output signal from the controller is increased and vice versa.

• Control Valve Action: The valve plug travels towards the close position for a valve that is Air-to-Open, Fail Close (ATO – FC) when the load or air pressure on the valve actuator increases.

Depending upon the requirement the controller, valve positioner, and control valve can be configured for both direct acting and reverse acting in the same control loop.

Let us consider a basic example

• The flow control valve is installed at the input side of the process.
• An ATC control valve is used to control the flow.
• The tank level is considered a process variable.
• When the level of the tank rises above the set point, then the controller output must be increased to turn off the valve for error correction, hence direct action is selected from the controller.
• If the flow control valve used is Air to open type instead of Air to close, then the reverse action is selected from the controller.
• In the case of direct-acting valves, the diaphragm extends the valve stem when the pneumatic pressure applied is increased. This closes the valve for a normally seated valve.
• In the case of direct action sliding stem valves, the valve stem is pushed down with the increase in pressure. And the valve stem is pulled or lifted up in case of reverse acting.
• In the case of reverse-acting valves, the diaphragm lifts the valve stem when the pneumatic pressure applied is increased. This will open the valve for a normally seated valve.
• The fail-safe mode of a pneumatic valve or spring valve is a function of action of both the actuator and the valve body.