In any control system, the option potentially exists to select either a control valve or a regulator, so it is useful to compare the respective performance and economics of these approaches to arrive at some general selection guidelines.
Unlike regulators, control valves are not standalone products. A control valve is the final control element in a control system and needs to be evaluated in that context. A control valve is most frequently used in the control of the following parameters: temperature, pressure, flow and level.
However, in principle, any continuously varying system parameter that can be measured and compared to a set point can be controlled. Also, keep in mind that regardless of the parameter being controlled (the controlled variable), the control valve itself can only change the flow rate.
All control is to a set point, and the control system can be diagrammatically illustrated as shown in Figure 1.
The essential feature of the controller (whether acting as a single-loop controller or as a component within a PLC or within a DCS or fieldbus device) is the incorporation of proportional and integral control modes capable of returning the measured variable to the set point following load changes or system disturbances. Control is generally within 5%, often within 1 to 2%.
Rising stem control valves are typically globe valves commonly used to the 2-inch size. However, globe valves can extend to at least 24 inches in size with special trims and cages for severe service and high-noise applications.
For economic reasons, rotary control valves are generally applied as line sizes increase above 2 inches and a variety of ball valves, eccentric plug and segmented ball valves, and butterfly valves exist in this segment of the market. The segmented ball valve is used in many applications and the largest size rotary control valves, such as butterfly valves, can extend to 72 inches in diameter.
Globe-style valves offer advantages in that the plug and/or cage can be more readily characterized to optimize the installed flow characteristic. Most rotary valves have an inherent flow coefficient characteristic that increases approximately exponentially with the increase in travel.
Although a range of actuators can be used, pneumatic actuation predominates. The diaphragm actuator is generally the style of choice due to minimized resistance and hysteresis for small changes in travel associated with control to within 1 to 2%. For improved control and minimized deadband, positioners are generally specified. It is also common practice for the current-to-pneumatic conversion to occur at the positioner. There is also a continuing increase in the use of intelligent or smart positioners, thereby providing automatic commissioning, higher accuracy, tight shut off, customizable characteristics and diagnostic capabilities.
There are obvious economic considerations associated with the choice of a control valve, which, at a minimum, require the incorporation of sensors and transmitters, controllers, positioners, instrument valves, tubing, wiring, calibration, tuning, etc. Generalized cost estimates can only be considered representative, at best, and can vary significantly depending upon application, size, material, accuracy and commercial circumstances.
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.
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.
The principle characteristics of a regulator are proportional control and a rapid response in the order of milliseconds. Regulators are highly adaptable to a range of functional control modes including downstream pressure, backpressure, differential pressure, flow and temperature.
Regulator accuracy is expressed in terms of the offset (commonly referred to as “droop”) as a function of flow. Flow rates for regulators are typically published at 10, 20 and 30% droop or offset for various media such as steam, air and water (Figure 3).
An effective method of increasing regulator accuracy involves using a pilot regulator to maintain a near constant pressure on the regulator diaphragm to improve the droop or offset performance to within about 5% over the operating range.
Typical regulator applications can include any application requiring control within 5 to 30%. These applications include a myriad of “set and forget” functions:
- Pump bypass
- Steam heating and/or pressure control
- Gas cylinder pressure control
- Air sets
- Tank blanketing, etc.
- Various backpressure relief functions, etc.
However, regulators also incorporate a unique capability to react within milliseconds and may offer superior control in fast systems such as liquid pressure control and in systems with positive feedback to disturbances such that rapid corrective action is required.
In addition, regulators may offer advantages in economic control in heating, drying and evaporator applications using saturated steam.
Regulators: Advantages and Limitations
The basic advantages and features of a regulator can be summarized as follows:
- Low cost
- High reliability/ease of maintenance
- No requirement for auxiliary systems
- No stem sealing/low friction
- Fast acting with unique advantages in certain applications
- Concerns with potentially explosive environments are eliminated with a hermetically sealed, self actuated regulator.
The limitations associated with regulators are:
- Best accuracy about 5%
- Maximum available size is typically 6 inches
- Failure modes are fixed
- Regulators are generally not applicable for severe service
Making a Decision
When you need to decide between using a control valve and a regulator, be sure to address the following:
- What accuracy of control is required?
- What are the installation cost parameters?
- Is power available for actuation?
- Is rapid response a consideration?
- What line sizes and materials are required?
- Are noise and cavitation considerations?
- Is a predetermined failure mode required?
- Is the environment potentially hazardous or explosive?
The answer to these questions will help you determine the best choice for your application.