How a Typical Control Valve Loop Works

In every control system of the process industry, the control valves are the most essential element to control or regulate the fluid flow in a line for a particular process operation. In general, it is referred to as a final control element. In every control loop, the control valve provides the adjustment or actuation of the control strategy.

In the case of an air compressor system in a process plant, the control valve behaves as a means for flow adjustment.

Below are the basic elements in the control loop of the valve to understand the concept of working the control valve loop.

What are the basic elements of a control valve loop?

The basic elements of the control valve loop are

1. A sensor or measurement device is used to measure the process parameters such as temperature, level, and pressure.
2. The transducers are used to receive the detected signal from the sensor and convert the signal to proportional electrical quantity.
3. An electronic controller receives signals from the transducers or transmitters and compares the received signal (process variable) with the set point and then initiates the required action to control the process.
4. If the process variable like temperature, level, flow, or pressure is above or below the set point then a controller initiates the control action to the final control element.
5. The current to pneumatic transducers convert the electrical quantity received from the electronic controller into the proportional pneumatic signal for the control valve action.
6. The current to pneumatic (I-P) transducers may not be required when pneumatic controllers are used for controlling the process. Because this current to pneumatic transducers are used only with electrical signals.
7. The control valve will manipulate the process to control the process parameters as required.
8. The positioner is attached to the control valve to overcome the valve friction and ensures that the valve closure member will act for the closing and opening of the valve as directed by the pneumatic controller by increasing its actuating pressure.

What are the types of control loops?

There are four types of control loops

1. Flow control loop
2. Pressure control loop
3. Temperature control loop.

Flow control loop system:

1. A flow sensor may be a magnetic flow meter or mass flow meter that measures flow and sends this signal as input to the flow transmitter.
2. The flow transmitter (FT) receives the signal from the sensor and transmits it to the flow indicator cum controller.
3. The flow indicator cum controller (FIC) compares the process variable with the given set point and sends the corresponding signal to the I-P converter. This FIC has a dual action that indicates the process variable and along with controlling the process by comparing it with a set point.
4. The current to pneumatic (I-P) converter will convert the electrical signal 4-20mA received from the controller into a proportional pneumatic signal 3 to 15 PSI to actuate the control valve.
5. Finally, the flow control valve (FCV) will open or close the valve stem based on the signal received from the controller through the current to a pneumatic (I-P) converter to either start the flow, restrict the flow, or control the flow.

The pressure and the temperature control loops are explained by considering PRDS as an example.

Pressure loop control system:

Components of the pressure loop control system are:

1. Pressure-reducing valve (PCV).
2. Pressure transmitter (PT),
3. PID controller,
4. Current to the pneumatic converter,
5. Pressure gauges (PG), etc.

• In the pressure loop control system, the pressure of the superheated steam is reduced by opening the pressure control valve (PCV).
• The required output pressure value is given to the controller as a set point.
• The pressure sensor detects the outlet steam pressure and sends the proportional signal to the PID controller through a pressure transmitter (PT).
• The PID controller will compare the given set point (SP) with the process variable or the measured value received from the pressure transmitter.
• The controller will generate an error signal and send this error signal to the positioner of the pressure control valve (PCV) through the current to the pneumatic converter.
• The positioner commands the control valve to open and releases excess pressure until the pressure reaches the desired value.
• When the steam pressure reaches the desired set point the positioner commands the control valve to close and prevents the further release of pressure after normal conditions have been restored,
• The generated error signal is represented mathematically by the equation e = SP – PV.
• For the stable operation of the process, the process variable (PV) must be equal to the set point (SP).

PV = SP

• In the case of PRDS, the steam inlet isolation valve and steam outlet isolation valve of the pressure control valve (PCV) are controlled by using two remotely operated motorized valves for isolating the pressure control valve during maintenance.

Temperature loop control system:

Components of temperature loop control system:

1. De-super heater or temperature control valve (TCV)
2. Temperature element (TE)
3. Temperature transmitter (TT)
4. PID controller
5. I/P converter
6. Temperature gauges (TG).

• The water strainer is provided at the inlet side of PRDS to prevent the entry of dust particles into the temperature control valve & spray nozzles.
• The pressure gauge at the inlet side indicates the pressure of the atomized spray water.
• The temperature control valve (TCV) controls the quantity of the spray water.
• Isolation valves are used to isolate the temperature control valve, and the bypass valve will allow the flow of spray water during the maintenance period of the temperature control valve when becomes inoperable
• The steam backflow is prevented by a non-return valve.
• In the desuperheating station, the temperature of superheated steam is reduced by spraying attemperation water at a high velocity through spray nozzles.
• The Temperature sensor RTD or Thermocouple detects the temperature of the steam and sends a proportional 4-20 mA current signal to the PID controller through the temperature transmitter.
• The PID controller now compares the set point with the process variable or measured value received through the temperature transmitter and generates an error signal (e).
• This error signal is now given to the temperature control valve through the positioner.
• The valve opens accordingly and sprays atomized water through the spray nozzle until the steam temperature reaches the set value, and re-closes to prevent the further spray of water after normal conditions have been restored.
• The generated error signal is represented mathematically by the equation e = SP – PV.

For the stable operation of the process, the process variable (PV) must be equal to the set point (SP).

PV = SP

What is a control loop?

A control loop is a system designed to maintain the stability of the process. By controlling the process variable to the desired set point. Every step in the control loop must work in conjunction with each other to manage the system.

What are the basic parts of a control loop?

The basic parts of a control loop in the case of a feedback control system consist of five basic components.

The input signal, sensing elements, the process being controlled, controller and actuating devices, and output signal.

What are the 3 basic elements of the control system?

The basic elements are the error detector, the controller, and the output element.

What are the steps involved in a control loop?

The steps involved in a control loop are:

• Sense or detect:

In this step, the current conditions of various process parameters such as flow, level, pressure, and temperature are measured using appropriate sensors or detectors.

• Compare :

In this step, the process variables measured by sensors are compared with the given set point to maintain the stability of the process.

This step is carried by electric or pneumatic controllers.  Note that the electric controllers are used for only electric signals. And pneumatic controllers are used only for pneumatic signals.

• Respond :

In this step, the final control element reacts to any error that exists between the measured value and the set point by generating a corrective pneumatic signal.

Here an error signal generated from the controller is sent to the final control element through the current to the pneumatic converter to carry the required operation within that loop.

• Feedback :

This step only occurs in the closed-loop control system. The feedback element sends the process value to the controller to compare with the set point to maintain the stability of the process loop.

The loop repeats continuously in cycles following steps 1 to 4, affecting the process variable to maintain at the desired set point.

We know that e = SP – PV, if SP = PV then e= 0. For the stable system, the error function is maintained at zero (e=0).