Actuators are machines used to control the position of a valve remotely and automatically. They are attached to the control mechanism of a valve to replace the manual lever or handle. Valve actuators are essential components in flow-control systems. The primary function of an actuator is to control the position of a valve. The actuator can close a valve, open a valve, move a valve to a specific position, hold the valve in place, prevent valve leakage by creating a tight shut-off, operate in failure mode, modulate flow through a valve and so on.
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In industrial applications, numerous types of actuators are available, and they can be connected to a variety of valve types. Selecting the proper actuator for a given application is a task that must be considered seriously. Several factors influence the kind of actuator that is suitable for a process. This article is an all-in-one actuator guide that covers the essential function of actuators, outlines actuator types, and describes important considerations for valve-actuator selection for the particulars of an application.
Actuators control valves through three standard valve operations: quarter-turn operation, multi-turn operation and linear operation.
The quarter-turn operation involves valves that rotate 90 deg from the closed position to the open position (for example, ball valves and butterfly valves). The actuator control on quarter-turn valves can be on/off or modulated. Actuators for these valve operations are usually easy to install and maintain.
On the other hand, multi-turn valve operation requires the actuator to turn the valve mechanism several rotations before moving the valve from the opened position to the closed position. The mechanism can be a rising non-rotating stem or a non-rising rotating stem. Examples of valve types with this operation include globe valves, gate valves and needle valves.
Some valves are opened and closed through the linear operation of the valve mechanism. Linear operation involves motion in a straight line (in contrast to the circular motion of quarter-turn and multi-turn valve operation, although some multi-turn valve operations have the same mechanism as a linear valve operation — for example, certain types of globe or sliding gate valves). Linear valve operation can be driven in a number of ways other than by an electric motor. Linear motion can also be achieved by mechanical, hydraulic, pneumatic or piezoelectric power sources.
Two main types of actuators, based on their motion, are linear and rotary. However, actuators are generally defined by the source of power that drives the actuator. They can be sorted as the following:
1. Electric actuators (powered by electricity; Figure 1)
FIGURE 1. The photo shows an electric actuator mounted to a carbon-steel three-way ball valve
2. Pneumatic actuators (powered by pressurized air; Figure 2)
FIGURE 2. The pneumatic actuator shown here is attached to a two-way ball valve made of brass
3. Hydraulic actuators (powered by fluid; either water or oil)
Electric actuator. Electric actuators use electrical energy — usually 24V, 110V, 230V, 400V, single- or three-phase, to drive an electric motor whose rotor is connected to the shaft/stem mechanism of the valve. The electric motor can be powered by alternating current or direct current. They are an energy-efficient, clean and quiet method of valve control. The electric motor may be connected to the valve mechanism through gears to increase torque or regulate speed. Electric actuators are available for both very small-sized applications, as well as for actuators on large valves in industrial applications.
Electric actuators are capable of relatively high speeds if needed, but they tend to be slow-reacting if standard-specification actuators are used. There is an option to install a positioner that converts an on/off actuator to a modulating actuator capable of precise flow control. However, if electric actuators are modulating constantly, the motors can burn out. Electric actuators often have a declutching mechanism to allow rotation of the drive during a power failure or installation. Emergency power can be provided through a battery to ensure a fail-safe operation.
A specific category of electric actuators is called linear actuators. They also convert electric energy from an electric motor to mechanical motion. However, the motion is not rotational to turn a valve, but to move a stem attached to a load or a valve in a linear direction. They are used in globe and gate valves and many other functions requiring linear motion of a load.
Pneumatic actuator. Pneumatic actuators are highly reliable actuators that are popular in industrial applications. They convert compressed-air energy into mechanical motion and can be used in locations with no electricity. Pneumatic actuators are of two types: single-acting or double-acting. Single-acting pneumatic actuators use a single compressed-air source to turn the valve with a spring to return the valve to the normal position. A double-acting pneumatic actuator has two compressed-air sources that turn the valve and return it to the original position, otherwise known as a fail position.
Pneumatically controlled valves are relatively simple when compared to electric actuators — they are easy to install and maintain, and have a very fast operating speed. There is a cost benefit of using pneumatic actuators, but it only applies in valves up to a certain size.
Pneumatic actuators often use a cylinder with a mechanism that converts the linear motion from the compressed air into rotational motion. The most common mechanism is the rack and pinion, but it can also be a diaphragm, piston or scotch yoke. Most pneumatic actuators are used for quarter-turn valves. The mechanism can be spring-loaded to return to a normal shut-down position in emergencies.
Solenoid valves are used to regulate airflow into the actuator. Electrical signals from a controller energize the solenoid valve position to either open or closed, allowing compressed air to flow through to the pneumatic actuator’s sides. It is important to note that, in order to actuate a valve with a pneumatic actuator, there must be a supply of clean, instrument-quality air, normally at 60 or 80 psi.
Hydraulic actuator. Hydraulic actuators convert hydraulic power to achieve mechanical work. They can be used for quarter-turn valves, such as ball valves, or multi-turn valves, such as globe valves. Hydraulic actuators consist of a cylinder and a mechanism for converting linear motion to rotational motion, such as a scotch yoke mechanism. Hydraulic actuators use high-pressure oil from a hydraulic pump to drive the valve.
Like pneumatic actuators, hydraulic actuators can be single-acting, with a spring as a fail-safe, or double-acting. They are relatively small compared to pneumatic actuators, but with thicker parts due to the high-pressure operation. They are also more precise than the pneumatic actuators because oil is incompressible. Hydraulic actuators are commonly used in large valve sizes that require a large turning force.
Before purchasing an actuator, several basic parameters should be considered. These parameters are based on the function for which you will use the actuator, as well as the environment. However, some actuators have unique features in addition to the basic parameters, which makes them unique. Always read the manufacturer’s documentation for recommendations and features of each actuator.
Presented below is a list of important parameters that should be considered when deciding on which actuator would be most suitable for an application.
Operating conditions. The actuator’s operating conditions and environment go a long way toward determining what type will fit best for your application. Operating conditions can include the following:
Temperature: Electric actuators can overheat if the operating temperature is too high. Pneumatic actuators are more commonly used and best suited for high-temperature operations.
Pressure: While all actuator types can operate at high pressure, consider the pressure differential across the valve, which will determine the amount of required torque for the actuator to turn the valve.
Hazardous environment. Make sure to select the actuator and actuator accessories with the correct IP code (ingress protection code against intrusions) if your environment has dust or moisture. Electric actuators with IP 67 and below are vulnerable to damage from moisture and condensation. Pneumatic actuators are preferred in wet environments. If the actuator operates in an explosive environment, one must consider whether the subject actuator meets a specific explosion protection standard, such as those that carry an explosion-proof NEMA rating or a flameproof ATEX classification. Another thing that must be considered is the duty cycle. Establishing an accurate duty cycle can help to decide which actuation mechanism should be used for a specific application. An electrically actuated valve can provide reliable service for a piping system that operates a few times a day. However, as the frequency of operations increases, and with it, the duty cycle, the electric actuator may suffer from burnout due to motor coils heating up. Pneumatic actuators are the most suitable choice for applications that require frequent valve operations as they can handle high-frequency duty cycles without failure.
Connection type. Consider that actuators have different connection types based on various standards. In order for the actuator to be connected to the valve, specific adapters must be used so that it may be mounted on the valve stem properly and perform its function as expected.
Consistency. While sometimes not obvious, the simplest way to decide which actuator is suitable for your application is by checking to see what type of actuators are already in use in the process.
Control functions. The type of control the process requires, either on/off or modulating, will determine whether the actuator requires a positioner or end switches. Consider the type of signal that will be sent to achieve this control. There are digital signals for on/off controls and various types of analog signals for modulating flow control.
Sizing. The actuator should be sized according to the torque requirement. It is quite common that manufacturers supply both the actuator and valve as one unit. When you already have a valve and want to select an actuator, check for the size range to fit the mounting flange and the valve’s minimum torque requirement. This information is available in the valve documentation.
Torque. When deciding the valve’s torque, consider the minimum torque required to start motion (otherwise known as breakaway torque) when the valve is at rest on the valve seat and the torque needed to close the valve into the valve seat completely. The actuator torque should be some percentage — usually 10–30% — higher than the minimum torque required from the valve.
Frequency of use. Some actuators are designed for on/off positions with limited use frequency, while others are designed for continuous modulation throughout their working life. Consider the control function in your process before deciding what type of actuator will be the best fit. The frequency of use directly affects an actuator’s durability as the actuator is a mechanical device that wears with use. This is particularly crucial for modulating valves that may be constantly operated and could overheat or fail.
Operating speed. Actuators can be fast-acting, closing a valve from a fully open position in a few seconds, or slow-acting, which takes several minutes. The particular process application will determine what kind of actuator speed you need. For example, a ventilation actuator in a building will be slow-acting because its thermal mass will prevent quick temperature changes. On the other hand, a hot-water supply line in a brewery will be fast-acting to ensure precise flowrate and volume is distributed for the brewing process.
The speed of an actuator is also directly related to the power used by the actuator during operation. The more the speed that is required, the higher the power rating. For electric actuators, this defines the electric motor power and gear system, while for pneumatic and hydraulic actuators, it defines the actuator’s operating pressure and size. When choosing the operating speed of an actuator, it is crucial to make sure that the valve is not closing too fast. Closing the valve too quickly may lead to a hydraulic shock to the system, which may result in damage to the connected pipes and equipment.
Ease of operation and maintainability. Pneumatic actuators are the easiest to install and maintain because they have very few parts and a simple operation mechanism. Electric actuators can be sophisticated and may be the most difficult to troubleshoot when they do not work. Pneumatic and hydraulic actuators are also very durable when compared with electric actuators.
Available power source. Sometimes, the choice of an actuator type is strictly determined by the available power source within the valve environment. Electric actuators can be powered by 24-V, 110-V, 230-V, 400-V, single- or three-phase power, and by direct or alternating current. Pneumatic actuators require compressed air between 60 to 150 psi (4–10 bars), while hydraulic actuators could have the oil operating pressure at 2,900 psi (200 bars).
Failure mode. If the valve is required to be at a particular position when there is a sudden loss of power source, a fail-safe mechanism is needed. Pneumatic and hydraulic valves can have a spring-loaded actuator that returns the valve to a default position, while electric actuators have a battery. They may also have springs for fail-safe operation. This adds to weight, size and cost. The fail-safe functions can be one of the following: close at no power, close at no control, open at no power and open at no control. When adding a fail position in a pneumatic or hydraulic actuator, the actuator must be upsized. Now, the actuator is overcoming not just the torque of the valve, but the spring’s pressure as well.
Available space. A pneumatic actuator can be significantly bigger than the valve it controls, especially for low compressed-air pressure applications. Check that available space can accommodate a particular actuator type before final decisions are made.
Actuator cost. Regardless of an actuator’s suitability to your application, the cost is often a determinant of the type of actuator selected. The higher the torque, power, size, extra features like ATEX, positioner, and so on, the higher the actuator’s cost will be. While certain features can be avoided to reduce cost, others, such as torque and power requirements, cannot be compromised.
The starting point for the actuator selection is always the question, “What function will the actuator perform in your process?”. After this is clearly determined, take care to go through the selection criteria one after the other to ensure that all have been considered before selecting the type of actuator, as well as its size, and the features applicable within your budget.
The selection process for the right valve actuator can be complicated. Sometimes, existing infrastructure, such as power sources, communication modules, distance to the control room, as well as the experience of the engineer, can play a significant role in the decision-making process. In other situations, valve-actuator selection is done first with few desired specifications and other parameters have to be created to support the operation of the actuator.
The ultimate purpose of a valve actuator is to perform a certain operation, or series of operations, in the most cost-effective way possible, while automating the process. There are many features to consider when selecting a valve actuator, but remember that these features will affect the quality and cost of the actuator. Any additional torque capacity beyond the specification, or an ATEX approval or a positioner, may not be needed for your application. Leaving them out can save you quite a bit of money. If needed, consult a valve engineer to assure the sizing and selection has been done correctly for your specific application.
Electric valve actuators are ubiquitous in today’s industrial space. They can be found in various industries and applications, including water treatment and wastewater plants, hydroelectric power generation, oil refineries, shipbuilding and numerous processing industries such as chemicals, food and beverage, pulp and paper, and pharmaceuticals. For anyone charged with the selection and operation of these products, it is important to understand the power source, controls, feedback, commissioning, security, backup power and failsafe required for the application.
This article will review different types of control systems and configurations available to electrically operate valves, which some in the industry refer to as motor-operated valves (MOVs), and how to make an informed decision on which type of actuator to use.
An electric actuator must have a power source available. Prior to selecting an actuator, it is important to determine the type of electricity that is available on site.
This will decide the type of electric actuator supplied for the application. The power source will be used to operate the motor through a set of reversing contactors, either located within the actuator, or in some cases located in a remote cabinet.
The power source must be capable of supplying the proper voltage and current required by the electric actuator to operate the valve. An electric actuator may be required to operate the valve as an open/close or modulating actuator; make sure the actuator is manufactured and supplied according to the required specifications.
Actuator-mounted local controls. All photos courtesy of AUMA
Controls are what operate the electric actuator so the next step is to determine how the actuator will be controlled.
There are two main types of controls:
Remote control system.
Discreet open and close signals from a PLC or other system are most commonly used with open/close valves and can be used for modulating service as well. This is done with open/stop/close signals sent from pushbuttons or switches (pilot devices) from the PLC or remote-control station to the actuator. Control power is typically a 24VDC or 110VAC signal. Remote-control power can be provided on-site, or in some cases, actuators come equipped with a control power transformer (CPT) that provides this voltage for the customer’s control system to use.
Analog control signals are common with modulating valves. The analog control signal can also be referred to as a positioning signal or set point. Most analog controls utilize a 4-20mA signal, with 4mA being fully closed and 20mA being fully open. A 0-20mA is typically not used because 0mA could be misinterpreted as a loss of signal. The valve’s position in between close and open can be reached by setting the mA control signal to a signal position in between 4mA and 20mA, respectively. This signal could be sent from the control room, or in some cases, it could be the output of a flowmeter upstream of the valve used to regulate flow.
Digital bus communication can be used on open/close valves or modulating valves. Digital bus communication is also referred to as fieldbus or two-wire control, as the control signal communicates along wires connected in between a group of actuators and a digital bus controller.
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To use a fieldbus communication, first determine which protocol will be used. Some of the available protocols are:
If the site is communicating via fieldbus, make sure the actuator can use the same protocol.
Some actuators can provide the option to operate with discreet inputs and fieldbus communication.
Feedback are the signals coming from the actuator to the control system, also referred to as output signals.
Analog feedback. Actuators can supply a 4-20mA signal to give the control system the existing valve position, like the setpoint analog control signal. 4mA or 20mA may be set to indicate the open or close position based on the application. Some actuators can also offer torque feedback in the form of a 4-20mA signal indicating actuator output torque, if the application requires.
Discreet feedback signals. Many actuators have status relays available to the customer signaling actuator position, torque-fault, general fault, run indication or intermediate position. In the case of non-intrusive (defined below in Commissioning) actuators, they will have relays that can be programmed to the customer’s desired feedback signal requirements.
Bus communication feedback. Fieldbus monitoring can be done across the fieldbus signal to include many more signals than are capable using the discreet feedback. For example, this can be the actuator’s position or status, and fieldbus system status, in addition to many others depending on the fieldbus protocol.
Visual position feedback screen.
Limit/torque switches. These are available as voltage-free dry contacts to give position/torque feedback status.
Visual position feedback. Most actuators come with a physical position indicator that shows the position of the valve and actuator; this may be in the form of a mechanical dial or on-screen indication. If the valve actuator is mounted to an additional gearbox, it often is supplied with a pointer cover to offer visual position indication.
Visual mechanical position feedback indicator.
Commissioning of an actuator is required to ensure proper operation of the valve and integration to the control system. Commissioning requirements may include verification or setting of end positions, setting of torque and the feedback signals. Commissioning is performed based on the type of actuator provided.
Hands-on and/or wireless commissioning.
There are different levels of security available for the customer when it comes to an electric actuator.
Physical security: Actuators can be supplied with lockable pilot devices, such as selector switches. Additionally, lockable covers may be supplied to cover the local controls. There is also the option to have padlocks on the handwheel/declutch devices, which are used to operate the valve manually.
Actuator and hand wheel with locks and an alternative option for a faceplate cover. Photo credit: courtesy of AUMA
Backup or emergency power may be important in an instance where the main power on-site is interrupted. A decision must be made as to the type of actuator operation required upon the loss of main power. Does the actuator still need to operate the valve normally or just place the valve in a safe position?
These options can operate the actuator and/or place the actuator in a safe position in a loss of main power:
UPS (Uninterrupted Power Supply): If a solution for actuator normal operation during a power outage is needed, a UPS backup power system could be a good option for the actuator. This will allow a customer to open or close the valve in the event of an emergency power outage.
DC (Direct Current Power): Actuators are also available in a DC voltage version. These can be used if a site has backup DC voltage available to operate an actuator during an emergency power outage.
An actuator may be supplied with a device to place it in a safe position. This device often uses a spring or other technology to place the actuator in the defined safe position.
Power Source
Type of Controls
Feedback
Commissioning
Security
Backup Power
Make sure the actuator and backup power sources are compatible (voltage, current, etc.)
Failsafe
Electric actuators have an important role in safe and efficient operation valve operation. Understanding the various elements, such as those discussed here, allow the user to successfully define, set up and operate a basic control system.
In the realm of industrial automation, electric actuators are a standout solution renowned for their superior precision, adaptability, and energy efficiency. Offering precise control over speed, position, and force, they excel in dynamic operating conditions, making them ideal for applications that demand fine control. The surge in their popularity is attributed to their on-demand energy consumption and eco-friendly design, aligning with new global regulations seeking sustainable solutions across industries, especially in the industrial sector. The use of electricity opens doors to leveraging renewable energy sources like solar and wind. Their streamlined design and reduced maintenance requirements further increase their appeal, positioning electric actuators as one of the main choices when seeking efficient, environmentally friendly, and low-maintenance automation solutions.
Landscape Source: Courtesy of AUMA
The move toward net zero emissions by 2050 was just released as part of the COP28 meeting in Dubai.
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Coker inlet feed valves at a bitumen upgrader in Canada’s Oil Sands were experiencing run-time and reliability issues due to coking up. The severe internal damage to the valve body and actuation reduced the plant’s efficiency and significantly affect daily operation output and performance for the client.
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