The following range of additional components is considered to be illustrative of the potential requirements of an installed control valve in a system compared to a standalone regulator:

• Transducers: differential pressure, pressure and low-cost (non HART)
• Controllers: 1 to 4 loops; single-loop PID; DCS – computer card in component; PLC – computer card
• Final Control Element: control valve with positioner vs. regulator; control valve with positioner vs. piloted regulator

Depending on the selection of sensors and the controller, the additional cost of a control valve in a system compared to a regulator can typically range from $1,000 to more than $5,000.

Further costs will be associated with limit switches, position indicator/feed­back, air sets, instrument valves, tubing, wiring, installation, calibration and tuning, and may or may not factor directly into the selection decision.

Control Valves: Advantages and Limitations

The advantages of selecting a control valve can be summarized as:

• Control within 5% and potentially within 1 to 2% for critical systems
• Availability of a wide range of sizes and valve types
• Severe service capabilities
• Selectable failure mode
• The controlled and measured variables can be in different loops

The following limitations are associated with control valves:

• Cost and complexity
• The requirement for auxiliary systems

In practice, the results—due to the difficulty in matching the control valve characteristic to the system, dynamic instabilities and incompatibilities, over-sizing and larger than anticipated deadbands due to friction or backlash within the control valve—have not always justified the expense of selecting a control valve.

More importantly, the issue of the required accuracy of control should be critically assessed as a regulator may offer acceptable accuracy and high reliability with considerable savings.

Regulator Characteristics

A regulator is a standalone, self-acting proportional controller. The essential characteristic of a proportional controller is that the controlling action is proportional to the deviation from the set point. Depending on the gain or sensitivity of the regulator, this controlling action minimizes the error or deviation on load change or system disturbance, but does not eliminate the “error” or offset.

A typical pressure regulator with its proportional control action is shown in Figure 2.