As users design processes that eliminate manual valve operation, automation valves have become a larger feature of today’s process control systems. With the wide variety of control devices, protocols, and process area regulations and limits, having a thorough guide to explore alternatives available for practically any quarter-turn valve is beneficial.
There are many more methods and possibilities for precise control of both on/off and modulating control valves as more valve manufacturers produce their designs to support valve automation.
How to select automation valves accessories?
Process controls today range from full computer systems to the staff-monitored electromechanical variety (push buttons, heavy-duty relays, and so on). There may also be pressure switches, temperature controls, or other process-monitoring equipment in the process area that must be linked to the control valve and hence the actuator.
The pneumatic actuator is the workhorse of quarter-turn automation valve. When choosing a pneumatic rotary actuator for valve control in process applications, it is critical to ensure compatibility with other control system components (power medium, control signals, etc.), the environment (corrosion, temperature), the system (speed, cycle frequency, fail mode), and, of course, the valve.
The pneumatic actuator must be available with a few basic control accessories in order to work properly with an existing control network.
- Solenoid valve: A pilot device that is available in a variety of voltages and constructions for area classification.
- Limit switches are used to indicate valve position, sequence cycling, alarms, and so on.
- Positioner: A device used to throttle the valve in response to a changing control signal.
The actuator should be offered with corrosion-resistant (anodized, stainless-steel) trim, various coatings (polyurethane, epoxy, etc.), and weatherproof, hazardous-area, or intrinsically safe control accessories for environmental compatibility.
A pneumatic actuator pilot valve is a control device that accepts a manual or power signal and then distributes air pressure to the actuator’s air inlet ports to drive it to the desired position. The solenoid-operated valve is the most popular type of pilot device. It easily connects with widely used control systems as an electric device and may also be equipped with low-wattage coils for compliance with computer control signals.
The number of port holes or ways air can flow through pneumatic actuator pilot valves is classified.
A three-port (three-way) valve, for example, has a pressure port, an output port, and an exhaust port. Because only one air chamber is alternately pressured or emptied in normal operation, the three-way valve is an obvious choice for spring-return pneumatic actuators.
A pressure port, two output ports, and an exhaust function are all part of a four-way valve. It is utilized with these types of pneumatic actuators because the two output ports will pressurize one or both chambers of a double-acting cylinder.
The term limit switch may be misleading when applied to a pneumatic actuator. The name is more appropriately applied to electric rotary actuators equipped with limit switches that cut power to the motor when the actuator reaches its desired limit of rotation. When limit switches are used with pneumatic actuators, the name “position-indicating switch” (or feedback switch) is more appropriately applied.
Indeed, a switch installed on a pneumatic actuator does not limit its travel but rather informs (through switches) whether the actuator has reached or has not reached a particular point of rotation.
The position-indicating switch box, also known as a switch box, encloses the switch elements, cams, and terminal strip and features a rotating input shaft that is attached to the actuator’s auxiliary shaft to take up rotary motion. The switch housing is made up of an input shaft that is externally coupled to the actuator’s auxiliary drive shaft and is internally outfitted with adjustable cams, snap-acting switches placed to align with the cams, and a terminal strip for incoming cabling.
The input shaft of the switch box turns as the actuator cycles, and the cams actuate the switches. When the switches are used to signal the cycle’s limits, the cams are set to operate the switch when the desired position is reached.
Position-indicating switches are used for a number of purposes, including light indication (powering control panel indicator lamps), system sequence cycling, alarms, electrical interlocking, and so on. Other devices, such as a potentiometer or position transmitter, may be installed in some switch enclosures to provide constant feedback on the position of the valve.
When the switches are connected to signal lights, they should be configured so that both lights are illuminated in the middle of journey, with one or the other turned off at the ends of transit. This prevents the operator from being duped by a burned-out lamp.
The type and quantity of switches required for pneumatic actuator switch boxes are frequently specified. Mechanical (snap action) and proximity switches are two examples of accessible switches.
These switches are sometimes known as “snap acting” switches because the contacts within the switch shift with a unique sound (snap).
Mechanical switches are often specified by the number of poles and throws they have. A pole is a switch component that is moved by the switch action to make or break an electric contact. Throws are the various electric connections that a particular pole can make.
Electric limit switches are available in four configurations: single-pole-single-throw, single-pole-double-throw, double-pole-single-throw, and double-pole-double-throw.
When a metallic or magnetic object comes into contact with the switch sensing area, these switches/sensors activate. These switches are inherently dust and moisture resistant, and some require a power circuit. The proximity sensor and reed switch are two types of proximity switches.
Inductive sensors are switches that are activated when a metallic object comes into contact with the sensing face. The majority of inductive sensors meet multiple NEMA ratings. The sensors are dust, moisture, and oil resistant. Internal solid-state circuitry protects sensor operation from stress and vibration. Because the sensing region is an electro-magnetism field, they require power to work.
The reed switch is another low-current proximity switch (250 to 500 mA). When a magnet is positioned near the detecting region, action is triggered. Reed switches do not need to be powered.
The following are the primary benefits of reed switches:
Metal contact that is completely hermetically sealed.
Reed switches can work in damp and dusty environments.
Temperature range: -60° C to +155° C (-76° F to 311° F).
The operation requires no power.
A completely integrated solution
Because both a feedback limit switch and a pilot valve are commonly found in an automated valve package, there are completely integrated devices that combine all of these parts into a single enclosure. SVF Flow Controls’ Nexus-LP is a nice example.
One advantage of this device is that the field wiring connects through a single conduit entrance, connecting both the pilot valve power wiring and the limit switch feedback terminals.
Other methods of position indication
A potentiometer can be installed in a switch box to provide continuous monitoring of an actuator’s position, as in modulating or “jogging” applications. As the shaft of the switch box spins, so does the potentiometer’s input shaft. At the control panel, the continuously decreasing or increasing resistive signal can be transformed into a valve position. Because of the inherent resistance of the lengthy wire, when the actuator is positioned distant from the control system, the outcome may be an unreliable resistive signal.
A resistance-to-current transducer circuit may be preferable in this scenario. The circuit board is often mounted in the switch box alongside the potentiometer and generates a 4-to-20 mA signal to show valve position constantly.
Switch boxes used in explosive situations must be capable of withstanding an internal explosion without igniting the explosive mixture surrounding the switch enclosure. Thus, the enclosure is designed to withstand the maximum expected internal explosion pressure without damage or excessive distortion, as well as to provide pressure venting through channels of such dimensions that gases are cooled below the ignition temperature before reaching the surrounding atmosphere. Thus, the design of a hazardous area switch enclosure must take into account housing thickness, cover fit, and tolerances.
Many switch enclosures include various National Electrical Manufacturers Association construction requirements to meet a wide range of applications. ATEX also covers hazardous area device selection and area definitions.
Selecting controls for modulating valve applications
A rotary control valve is one that is utilized for modulating/throttling rather than basic on-off service. A control valve is a process control element that changes the flow of fluid in response to a system control signal. An actuator must be equipped with a pneumatic positioner to enable fast, sensitive, and accurate positioning in response to a control input. A pneumatic positioner is essentially a relay that detects and compares an instrument signal and the position of the valve stem. It senses valve position via the actuator shaft since it is often located on the top of a rotary actuator.
The majority of fundamental positioners are linearly characterized. This means that the rotation of the input signal to the output is directly proportional, allowing the process engineer to select a valve that will give system characteristics. For example, standard ball valves, like many other quarter-turn valves, provide equal % flow.
Terms associated with positioners
- Direct acting — Increasing input signal opens the valve (increases flow).
- Reverse acting — Increasing input signal closes the valve (decreases flow).
- Resolution — The smallest possible change in valve position.
- Deadband — The maximum range through which the input signal can be varied without initiating change in valve position.
- Hysteresis — The maximum difference in valve position for a given input signal during a full range traverse in each direction.
Transducers are devices that translate one sort of signal to another. A current-to-pneumatic transducer accepts an analog milliamp control signal from a field instrument and converts it to a proportional pneumatic signal for the positioner in the case of control instrumentation.
The most typical control valve conversions are for systems controlled and monitored by electronic equipment but using pneumatically actuated control valves. The most practical approach for linking the two types of equipment is to use a transducer. A transducer, as an electromechanical device, must be carefully selected for environmental compatibility, hazardous locations, sensitivity, vibrations, and so on.
One disadvantage of transducers is that they might be difficult to detect near the positioner, necessitating extensive lines of wire or pneumatic tubing. Some manufacturers have integrated the transducer inside the positioner to meet this requirement. Electro-pneumatic positioners are the name given to these hybrids.
Standard instrument signals
Instrument signals are used to connect various elements in the control process. Information can be sent from a sensor to a controller, or from a controller to an actuator, and so on. Standard instrument signals enable different manufacturers’ products to communicate with one another. The following are typical standard instrument signal ranges.
The value at the high end of a normal instrument signal range is typically five times that of the low end. For example, 20 mA is 5 4 mA, 15 psi is 5 3 psi, and so on. Normally, the low end does not have a value of zero. This provides a good way of distinguishing between a device that signals the low end of a range and a device that is not working. This is known as live zero.
The main exceptions to these rules are resistance-type inputs, which typically have a low end of zero and a range of high end values.
Typically, split ranges are fractions of regular instrument signals. 3 to 15 psi, for example, is frequently divided into 3 to 9 psi and 9 to 15 psi, each of which is half of the usual range. Split ranging is the procedure of piloting two control valves using the input signal range [3 to 15 psi (0.2 to 1 bar). In practice, the first control valve cycles through its full stroke at 3 to 9 psi (0.2 to 0.6 bar), while the second valve cycles through its full stroke at 9 to 15 psi (0.6 to 1 bar).
Pressure [psi (bar)] is the most common variable for instrument signals in pneumatic equipment. The variable in electric devices can be current (mA), DC voltage (VDC), or resistance [ohms (O)]. The instrument signal ranges for pneumatic and electric devices are shown in Table 1.
The following are some applications for a pneumatic positioner.
Control of the level
dividing the range
Reverse action with relation to the actuator
Split ranging is the procedure of piloting two control valves using the input signal range [3 to 15 psi (0.2 to 1 bar). In practice, the first control valve cycles through its full stroke at 3 to 9 psi (0.2 to 0.6 bar), while the second valve cycles through its full stroke at 9 to 15 psi (0.6 to 1 bar).
Positioners come in a variety of construction materials, accessories, characterized cams, position transmitters, and incorporated transducers.
Pneumatic valve actuators with manual override mechanisms
It is now possible to have greater control over a process system than ever before in this era of automation. Indeed, one key rationale for automating a valve for a given function or functions in a system is to have even more full control over the process by giving feedback, sequencing, and rapid reaction, as well as by eliminating human mistake.
Interfacing an automated valve with a control system may necessitate that an actuator be outfitted with a solenoid-actuated pilot valve, positioner, limit switches, a mechanical position indicator, transducer, and so many other control accessories that if power to the actuator is lost, or the actuator fails to operate for any reason, it may be rendered inoperable and thus useless, becoming a potential hazard or causing an unnecessary shutdown of the producti Control of the valve, and possibly the entire operation, is thus lost.
The employment of a manual override mechanism is the simplest and most reliable technique of ensuring the continuing operability of an automated valve in the case of a system failure. As more quarter-turn valves are introduced into larger process control systems, there is growing worry about their capacity to operate in the case of an actuator or power failure. Actuator makers have acknowledged and solved this challenge. For pneumatic quarter-turn actuators, there are currently a number of manual override provisions available.
A wrench override is essentially a handle with an engagement provision that fits onto the actuator’s auxiliary drive shaft. When the actuator fails, the wrench can be put to the flats of the shaft to manually override it. Because it is difficult to override and hold spring-return actuators in position, this method should only be utilized with double-acting actuators. Torque should be kept under 1500 lbf-in (170 Nm).
To prevent loss, the wrench is frequently linked to the actuator or mounting bracket by a cable or chain. It may also have a locking feature to keep smaller spring-return actuators in place until the situation is remedied. A wrench override should never be permanently linked to the actuator’s drive shaft since it may injure personnel working near the equipment if it activates automatically.
Disengageable gear manual override
The disengageable gear override is a modular component that fits between the valve and the actuator to provide simple and dependable manual positioning. Even with spring-return actuators, the self-locking worm gear design allows for safe and easy operation as well as positive manual placement. The clutch lever, placed at the base of the handwheel, engages the worm gear with the output drive sleeve immediately, allowing operation. With a minor adjustment to the actuator, manual override modules can be retrofitted to existing control valves.
Manual overrides have proven to be an accessory that requires more thought in many cases. Consider modular construction, quick operation, and compatibility to standard actuators.
Two-wire control (As-I)
An increasingly common technique for controlling and communicating with automated valves in process areas is two-wire control.
There are a variety of systems available that range from simple on/off valve control to full system integration, diagnostics, and control and are based on various bus protocols (DeviceNet, As-I, and so on). The decision becomes a plant/platform-wide one.
AS-Interface (Actuator Sensor Interface, AS-I) is a two-conductor cable developed for connecting simple field I/O devices (such as actuators and valve position sensors) in discrete process applications.
AS-Interface is a “open” technology that is supported by numerous automation equipment makers. It is a networking alternative to field device hard wiring that can be used as a partner network for higher level fieldbus networks such as Profibus, DeviceNet, Interbus, and Industrial Ethernet. It provides an inexpensive remote I/O solution.
Applications: Bus technology can help systems with eight or more valve actuators. These systems typically have automated valves that are controlled by a programmable logic controller (PLC).
AS-Interface vs. traditional system
AS-Interface is a low-cost, adaptable alternative to standard hard-wired I/O. It has the potential to replace traditional point-to-point wiring with a more flexible solution that is easier to install, operate, and maintain, as well as re-configure.
Typical batching valve wiring networks connect each I/O to a central location, resulting in several wire lines for each field device. Large sums of money are required for cabling conduit, installation, and I/O points. In order to connect merely a few field devices, space for I/O racks and cabling must be made available.
A straightforward gateway connects the network to the field communication bus. Data and power are sent to each AS-Interface compatible field equipment through the two-wire network.
Each valve communication module includes an AS-Interface ASIC and other electronics for gathering open or closed position status and turning on or off solenoids or other ancillary devices. Other AS-Interface modules for gathering inputs and switching power outputs are available.
On/off batch process valves and other discrete applications are ideal.
Each network master has 62 field devices.
Simple electronics for cost-effective and long-lasting performance.
For both data and power supply, the transfer media is an unshielded two-wire cable.
Signal transmission is very resistant to EMI.
Simple to install, offering the biggest cost reductions with the least amount of complexity.
The freedom to choose network architecture provides for optimal network wiring.
A variety of gateways are available to connect to high-level bus networks.
It is expected that a process control scheme will be designed for highly specific outputs, rates, and materials. It may also have to deal with pressure and temperature issues, as well as hazardous region locations. With so many options, classifications, and control methods available today, working with a highly experienced automation supplier is always a good decision.