Additional equipment is required for good control valve performance, and a few of the more important items are described in this section.
Actuator: The actuator provides the power that is used to move the valve stem and plug. The power source used in the process industries for the vast majority of the actuators is air because it is safe and reliable. Many actuators are described as diaphragm because the pneumatic signal pressure is transmitted to the actuator volume that is sealed with a flexible diaphragm. As shown in Figure 16, the valve stem is connected to the diaphragm, as is a spring that forces the valve to be either fully opened or fully closed when the opposing air pressure in the diaphragm is atmospheric. The diaphragm pressure is equal to the pneumatic control signal, usually 3-15 psig representing 0-100% of the signal, which forces the diaphragm to distort and moves the valve stem to the position specified by the control signal.
|For a picture of a valve actuator and much more information on actuators, select this button to be directed to a site on the WWW. See Section 6.5 in the reference.|
|For further details on actuators, including discussion of failure positions and explanatory sketches, select this button to be directed to a site on the WWW.|
Booster: The flow rate of air in the pneumatic line is not large and significant time may be required to transfer sufficient air into the actuator so that the actuator pressure equals the line pressure. This time slows the dynamic response of the closed-loop system and can degrade control performance. When the delay is significant in comparison with the other elements in the control loop, a booster can be located in the pneumatic line near the valve which increases the volumetric flow rate of air and greatly speeds the dynamic response of the actuator.
Failure position: Major failures of control equipment, such as the break of a pneumatic line or air compressor, lead to a low (atmospheric) pressure for the signal to the actuator. In such situations, where control has been lost, the valve should be designed to attain the safest possible position, which is usually fully opened or closed. The proper failure position must be determined through a careful analysis of the specific process; usually, the pressure and temperature near atmospheric are the safest. The failure position is achieved by selecting the design in which the actuator valve places the valve stem in its safest position. The design is usually described as fail open or fail closed. Other failure modes can be achieved in response to unusual circumstances, for example, fail to a fixed position and fail slowly to the safe position.
Positioner: The valve is a mechanical device that must overcome friction and inertia to move the stem and plug to the desired position. Typically, the valve does not achieve exactly the position specified by the control signal. This imperfection may not be significant because feedback controllers have an integral mode to reduce offset to zero at steady state. However, the difference might degrade control performance, especially in a slow control loop. A positioner is a simple, proportional-only controller that regulates the measured stem position close to the value specified by the control signal to the valve. For further discussion on positioners, see Hutchison (1976).
|For additional discussion of valve positioners, including a few sketches, select this button to be directed to a site on the WWW.|
Hand wheel: Some control valves must occasionally be set to specified positions by personnel located at the process equipment. A manual hand wheel provides the ability for local personnel to override the control signal to the valve.