Communication compatibility: The vast majority of control valve actuators are pneumatically operated. Most control systems, however, either use a 4-20 mA electrical signal (alone or with HART communications) or are fully digital (fieldbus) systems. Bridging this communication gap requires a positioner that is compatible with the control system’s output signal and that can provide the required pneumatic output to the valve actuator.
Control accuracy: In a process control loop, the system is designed around the final control elements (valves) consistently responding to a change in output signal. If the response from the valve is different than expected or inconsistent, the control system will make corrections until the valve is within the desired parameters.
This takes time, however, during which the process is off set point. The larger and more complex the process, the longer it will take to bring that process back to setpoint. Meanwhile, energy, process fluid or end products can be wasted, and safety could be compromised. Quick response by the control valve, therefore, is important for accurate control. A positioner receives continuous feedback on actual valve position and adjusts its output automatically to ensure the valve maintains the desired position as defined by the control system, improving the overall accuracy and consistency of the process.
Actuators on some control valves can be very large to provide the required output thrust. The larger the actuator, the more air is required to be loaded or exhausted as the desired setpoint changes. A positioner typically includes an output amplification device (relay, spool valve, etc.) that increases its output and venting flow capacity. This ensures rapid valve positioning response and reduces offset errors.
A positioner is usually mounted to the actuator yoke; its measurement device is connected to the valve or actuator stem with a feedback lever system. Some rotary control valve designs incorporate a more direct, end-of-shaft mounting system that prevents lost motion in the interconnecting linkage.
Conventional analog positioners usually send a proportional output signal to the actuator. The further the valve is away from setpoint, the greater the output change the positioner will provide.
Newer digital positioners may include full Proportional-Integral-Derivative (PID) control systems. Most allow the control system or operator to retrieve information on key measured parameters and then set various tuning parameters used in the control algorithm within the positioner. Some digital positioners offer advanced features, such as automated calibration and tuning. High-end units offer advanced diagnostic capabilities to test and report on key valve performance criteria.
As electrical devices, electro-pneumatic and digital positioners must be matched to the environment and the system in which they will be used. National and international agencies test and certify electrical devices for the areas where the devices will be installed, and these standards must be followed. For example, an explosion-proof enclosure may be required for a device destined for a hazardous environment, and units installed outdoors would require watertight enclosures, at a minimum, to prevent damage caused by moisture.
I/P (or I to P) converters change an electrical current signal to a proportional pneumatic output. In control applications, they are most often used to convert the 4-20 mA signal from a control system to a 3 to 15 psig pneumatic signal that a pneumatic positioner can interpret and send to the control valve.
Control valves frequently include I/P converters and many positioners include built-in electro-pneumatic transducers that will convert the signal. Stand-alone I/P converters are called for, however, when safety or functional requirements dictate using another pneumatic device in the signal line or when maintenance considerations make separation advantageous.
I/P converters are electrical devices, so plant personnel must choose one that has the appropriate electrical characteristics and certifications for the zone in which it will be installed.
LIMIT SWITCH (POSITION SWITCH)
Limit switches are used to indicate when a control valve has reached a specific position. They are used most often to indicate the fully open or fully closed position, but can be set at intermediate points if desired.
A limit switch may be used for something as simple as turning a control panels’ indicator light on when the valve has fully opened or closed. Or, the switch can be wired into a sophisticated safety system. At either end of the spectrum, the limit switch simply provides a contact closure (or opening) at the position indicated.
Limit switches almost always are electrical devices. Most use a mechanical connection to the valve or actuator stem (shaft) to ensure accurate position triggering. Some use non-contacting electromagnetic triggering systems to activate internal contacts, but lever- and cam-actuating systems are more common.
In addition to choosing an environment-appropriate enclosure for a limit switch, specifiers must match the electrical contacts used within the switch assembly to the current that will be carried. The electrical contact rating information is typically expressed as a certain current-carrying capacity at a specified voltage. Exceeding the rating can lead to early failure of the switch contacts caused by arcing across those contacts. In a hazardous environment, that arcing can cause fires and explosions.