While control valves are an essential part of most flow processes today, they cannot accomplish their key tasks without help from the many accessories that aid in smooth operation.
Control valves are a ubiquitous essential in today’s industrial facilities. Whether they are managing flow or controlling process conditions such as temperature or pressure, they are key to maximizing production, maintaining product quality and ensuring safety.
However, they cannot accomplish these tasks on their own. Equally important to the processes are the accessories that work with a control valve—helping ensure accuracy, boosting output, making various control system components compatible with one another and providing added layers of safety.
In this article, we will review some of these devices, the roles they play in a control system and factors that should be considered when specifying these devices.
Positioners are required in almost all control valve applications. Many positioner types and communication protocols are available, but a positioner’s primary function is always the same: to accurately position the throttling element within a control valve as directed by a signal from the process control system.
The positioner receives a sign from an external process controller, compares the valve’s actual position to the desired position, and sends a command change to the actuator to correct the valve position in a controlled feedback loop. The process controller measures the change in the process and sends further changes to the position signal as necessary.
Without a positioner, a control valve position is an open loop; the valve moves in response to the signal but nothing is providing feedback to ensure the valve is in the desired position.
Four factors make positioners vital components in today’s process control systems:
Higher-pressure actuators: Positioners make it possible to use today’s higher-pressure actuators in traditional pneumatic control systems, which can reduce costs and make the valve more responsive.
Most pneumatic control systems use an output signal range of 3 to 15 psig or 6 to 30 psig. Traditional spring-diaphragm actuators are widely available in these ranges, but the output force from a given actuator size is significantly limited. And, piston actuators almost always require higher air pressures to perform optimally.
As a result, most actuators are designed and sized to operate at air pressure ranges well above the 3 to 15 or 6 to 30 psig delivered by the control system. Therefore, a positioner or other amplifying device is required to make them compatible with the control system’s lower output pressure. The higher air pressure allows selection of smaller actuators reducing overall equipment cost.
Forces within the control valve: Friction from valve stem sealing systems (packing) can be significant, particularly in higher-temperature applications in which graphite-based materials must be used. Unbalanced forces or flow-induced forces within the valve also will affect the actuator’s ability to consistently position the throttling element or plug within the valve.
A positioner continually monitors the effects of these forces and sends the appropriate feedback to the controller, ensuring that necessary adjustments are made quickly to keep the control valve performing as expected.
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.
A position transmitter is similar to a limit switch in that it is connected to the valve or actuator stem (shaft) to detect valve position and is often housed in a similar enclosure. The transmitter’s function, however, is quite different.
While a limit switch triggers at one defined position, a position transmitter provides a continuous output signal proportional to the actual valve position. That data can then be fed into a control system to compare the valve’s actual position to the desired one, or the data may be used to only provide a visual indication on a display panel.
Like a limit switch, a position transmitter is an electrical device. There are, however, no contact switches. Instead, a position sensor is used to follow the valve position and send a signal to an amplification circuit, which then provides the transmitter output. That output is usually a 4-20 mA signal, though other ranges are available.
Position transmitters are often loop-powered devices (two-wire) in which the electrical supply is derived from the control loop power. Externally powered (four-wire) versions also are available for use when there is insufficient power available within the 4-20 mA loop. The sensors can range from simple potentiometers to high-tech optoelectric and other non-contacting technologies.
COMBINED POSITIONER ASSEMBLIES
A number of positioners on the market can be supplied as combined assemblies, incorporating the function of the positioner, limit switch and position transmitter in the same housing and using the same feedback connection.
Solenoid valves are often found in applications in which a control valve under certain conditions must be quickly driven to the fail position.
Solenoid valves are usually electrically actuated on-off or three-way valves installed into the air system and designed to take a specific action when tripped. They range from small, single-coil units to large, high-volume piloted designs, depending on the desired function and required capacity.
The solenoid can be thought of as an on-off switch for a pneumatic system. The signal to the solenoid controls the action of the internal valve assembly, allowing flow through the valve in one position and either isolating or venting the pressure in the other.
For example, an application might require that the operator in an emergency situation be able to manually drive the valve to its fail position instantaneously. A switch is connected to a three-way solenoid valve. When the switch is in the normal “on” position, power is supplied to the solenoid, which in turn opens its internal valve, allowing the positioner output pressure to flow to the actuator. When the switch is turned to “off,” power to the solenoid is removed, which opens the solenoid, venting the control valve and moving the valve to the fail-safe position.
Because of the wide variety of solenoid valve actions, voltage ratings and current ratings available, an almost infinite number of possible system configurations exist. When specifying a solenoid valve, it is important to describe the desired action under loss of electrical power, as well as the required voltage and current ratings.
Airsets, also known as filter regulators, are small pressure-reducing regulators that manage the air supply to pneumatic instruments and valves. They perform two critical functions: providing a constant air supply pressure to the instrument or valve, and filtering the instrument air.
The pressure-reducing function is essential to a plant’s performance and safety. Most plant instrument air systems operate at pressures of 100 psi (6.9 bar) or higher, while most control valves and other instruments are designed to run at much lower air supply pressures—as low as 20 psi (1.4 bar) in some cases. Exceeding the rated supply pressure can lead to early failure, mechanical damage, system shutdowns and potentially unsafe conditions.
Air supply pressure requirements can vary significantly from device to device, and having separate instrument air systems to meet the demands of each control system component would be impractical. Therefore, the standard practice is to have a single air system and install airsets on individual devices to reduce the instrument air pressure to the appropriate level.
Control valve actuators are designed to be sealed devices with no air leakage. As a result, any internal leakage through the control devices could create pressure buildup. Airsets, therefore, typically have an internal relief that will vent any undesired pressure buildup within the system.
Finally, clean instrument air is critical for consistent instrument performance. Valve positioners are made of precisely machined and fitted components, so even a small debris particle or droplet of condensate can affect performance. An airset ensures a clean air supply to keep everything operating as designed. Periodic maintenance is required to ensure the filter is clean and condensate is drained from the dripwell.
A volume booster is a mechanical relay that amplifies the flow of air.
Valve positioners, controllers and transmitters have limited flow capacity. To ensure accuracy, the internal nozzles, relays and other mechanical devices must be kept small to maximize responsiveness. Without external amplification, the output capacity of these instruments can be quite low.
Valve actuators, on the other hand, often become quite large as capacities and/or pressures increase. These large actuators contain much greater volumes of air that must be loaded or exhausted as the valve moves through its travel. To handle the higher capacities, valve positioners often include an output relay or high-capacity spool valve to amplify flow capacity. Even with this built-in amplification, however, the response of large valves can be slower than required.
The solution is an external volume booster. It provides a 1:1 amplification of flow while keeping the output pressure as close as possible to signal pressure. Since the booster has a separate supply connection, it is not limited by the capacity of the devices ahead of it in the loop.
Volume boosters used on control valves are normally applied with a bypass or gain adjustment to provide stability. The volume booster normally has a built-in deadband where a certain amount of signal change is necessary to activate the volume booster. The bypass or adjustment works by mixing the positioner output with the pressure in the actuator, allowing small signal changes to flow through the bypass without opening the main plug within the booster. Controlling the amount of bleed adjusts the sensitivity of the booster, and prevents overshoot that can be caused by excess capacity, ensuring smooth, controlled responses to small changes.
MANUAL OVERRIDE (HANDWHEEL/HANDJACK)
A manual override is an emergency backup device that allows manual operation of a control valve in the event power and/or air supply is lost. This should not be confused with a manual operator. Unlike a manual operator, an override device is not intended for continual use. Instead, the override is a limited-use device, often designed to counter the built-in spring return within the actuator to push the valve to its fully open or closed position, depending on fail action.
Several designs are available to accommodate various valve and actuator types and force requirements. They range from a simple wheel and drive screw attached to the top of a spring diaphragm actuator to a mechanical wheel and lever system connected to the valve stem or hydraulic handjacks (pumps) that attach to or are integral within a piston operator.
Control valves are indeed essential to the operation of any process facility. Equally important, however, are the accessories that allow a control valve to perform optimally. This article has only scratched the surface, touching on the most commonly used accessories. Numerous other options are available, giving control system designers and plant operators a virtually bottomless toolbox to draw from in meeting the needs of any application.