Valve actuators are a type of control valve, and there are many options available to meet plant-wide and individual automation requirements. While simple in concept, consisting of a box with an input, output, and a mechanism to operate a valve, there are actually quite a few considerations in choosing the right valve actuator. And because the valve actuator plays a more significant role in a valve’s overall performance in the control loop, it’s a decision that engineers shouldn’t take lightly.
Image via Wikimedia Commons
In this guide, we’ll discuss the two primary types of valve operation, types of valve actuators, important valve actuator functions, valve sizing information, and the key considerations in selecting the right valve actuator. Use the links below to navigate to a specific section:
- Types of Valve Operation
- Types of Valve Actuators
- Important Valve Actuator Functions
- Considerations in Choosing a Valve Actuator
Types of Valve Operation
There are dozens of types of control valves. From the perspective of a valve actuator, however, there are two primary types of valve operation. Understanding how a valve operates is the first step in choosing the appropriate actuator.
- Rotary (Quarter-Turn) Operation – This includes plug valves, ball valves, and butterfly valves. Quarter-turn dampers would also fall into this category. These types of valves are generally easier to fit with the appropriate actuator as the operation is relatively simple, requiring a 90-degree rotation at the appropriate torque.
- Multi-Turn Operation – This group of valves have either rising non-rotating stems or non-rising rotating stems and require multiple turns to move the valve closure element from the open to the closed position. Some examples of the types of valves found in this group include globe valves, knife gates, gate valves, sluice gates, and others.
Types of Valve Actuators
Just as there are many types of valves, there are several specific types of valve actuators. Most, however, fall into a few common categories, based on the type of power applied and the type of movement required:
- Pneumatic and hydraulic actuators (fluid power) – quarter-turn – These types of actuators are versatile and can be used where electric power isn’t readily available or in applications in which simplicity and reliability are prioritized. They also have a wide range of capabilities, from smaller actuators delivering a few inch pounds of torque to the largest actuators, which can deliver a million inch pounds of torque or more. Most pneumatic and hydraulic actuators use a cylinder coupled with some mechanism that turns the linear motion generated in the cylinder to the quarter-turn motion required for valve operation. Adding an opposing spring allows for positive shut-down in emergencies.
- Pneumatic and hydraulic actuators (fluid power) – multi-turn – When multi-turn output is required for operating a linear-type valve, such as a gate valve or a globe valve, fluid power actuators are a common solution. While electric actuators are often used for these types of valves, pneumatic and hydraulic actuators are viable options for applications in which electricity isn’t readily available.
- Electric actuators – multi-turn – These types of valves are among the most common and the most dependable. They’re capable of quickly operating some of the largest valves, and they’re powered by a single or three-phased electric motor that drives a combination of level gears and spurs. Subsequently, these gears and spurs drive a stem nut, which engages the stem of the valve to open or close it. They often include a declutching mechanism and a hand wheel that allows for manual operation in the event of a power failure.
- Electric actuators – quarter-turn – Similar in design to multi-turn electric actuators, the primary difference is that the final element is positioned in one quadrant offering 90 degrees of rotation. These types of actuators are compact and often used in smaller valves, and because they have lower power requirements, they can be configured with an emergency power supply (such as a battery) for fail-safe operation.
- Manual actuators – Manual actuators use levers, wheels, and/or gears to facilitate movement. Manual actuators differ from automatic actuators, as automatic actuators have an external power source providing the force and motion needed to operate a valve automatically or remotely. For many valves, manual operation isn’t an option, either because the application includes valves in remote pipelines or due to the sheer force required for operation. Additionally, manual actuators aren’t a practical solution for valves located in toxic or hostile environments, and they’re not as useful in applications requiring the safety precaution of allowing for immediate shutdown.
Screenshot via Valve-World.net
Pneumatic and hydraulic actuators are described together and operate similarly; however, they differ slightly in how the cylinder is moved. Hydraulic actuators move the cylinder with an incompressible liquid from a pump, while pneumatic actuators move the cylinder using pressurized air. Pneumatic actuators aren’t as practical for large equipment that requires large bore cylinders because of compressed air consumption.
Hydraulic actuators, on the other hand, can have a higher cost per unit compared to pneumatic and electric actuators. They also tend to leak fluid, and they require a variety of companion parts including motors, pumps, release valves, fluid reservoirs, heat exchangers, and noise-reduction equipment.
Valve actuators can also be categorized as diaphragm actuators, direct-acting and reverse-acting actuators, direct-acting diaphragm actuators, field-reversible multi-spring actuators, and piston actuators, although there are other types as well.
Important Valve Actuator Functions
All valve actuators must perform several functions including:
- Moving the valve closure member to the appropriate location. A closure member is typically a plug, disc, or ball, and an actuator must have enough force to move it even in difficult or undesirable conditions. Additionally, it has to be fitted with the necessary controls to direct it.
- Holding the valve closure member in position. Once in the desired position, a valve actuator must be able to hold it in place. In some applications, such as throttling applications, this requires a robust spring or fluid power or mechanical stiffness to keep the closure member securely in place.
- Seating the valve with enough torque to meet desired shutoff specifications. Some types of valves may require special accessories for actuator sizing to sustain enough torque to maintain closed positions.
- Having a failure mode. A failure mode needs to occur in the event of a system failure. Depending on the application, failure mode may be as-is, fully closed, or fully open.
- Having the proper rotational value. Some valves require a certain amount of rotation, often 90 or 180 degrees. Multi-ported valves often require more than 90 degrees of rotation, and electric actuators are usually preferred for applications requiring rotation of greater than 180 degrees, as they’re not limited in rotation mechanically.
- Providing the right operating speed. The cycle speed of an actuator can be regulated with control circuit elements, but cycle speeds less than half of the typical actuator cycle time require careful valve selection. Specially prepared pneumatic actuators may be required for high cycle speeds without risking damage to valve parts.
Considerations in Choosing a Valve Actuator
Now that we’ve covered the basic types of valves and valve actuators, as well as the main functions valve actuators must perform, there are a few important considerations to weigh when selecting the appropriate actuator for your application. There are a variety of factors to consider, ranging from usage factors, to sizing, supply pressure requirements, safety and reliability, cost considerations, and more.
Usage factors are a significant considerations in selecting the right valve actuator. These factors include:
- Compatibility – What power source is available? As discussed above, electric actuators, by nature, require electricity to operate. If there is no readily available source of electricity, a pneumatic or hydraulic valve actuator is the logical choice. Pneumatic actuators require an air supply between 40 and 120 psi. Higher pressures can be difficult to come by, and lower pressures will require a diaphragm or piston with a larger diameter in order to achieve the necessary torque. Electric actuators require a power supply of 110 VAC, but valve actuators can be purchased with DC and AC motors in other sizes.
- Temperature Range – Pneumatic actuators can operate in temperatures ranging from -4 to 150° F (-20 to 70°C), or in some cases in the range of -40 to 250°F (-40 to 121°C), provided the right seals, grease, and bearings are in place. Electric actuators can operate in temperatures ranging from -40 to 150°F (-40 to 65°C).
- Hazardous Areas – Pneumatic actuators are often preferred in hazardous or toxic environments due to their explosion-proof nature, but if there’s a lack of compressed air or a pneumatic actuator is unable to meet other operating specifications, electric actuators may be used. Electric actuators used in hazardous environments must have an NEMA VII enclosure to protect it from explosions. NEMA guidelines are discussed in more detail below.
The National Electric Manufacturers Association (NEMA) has established guidelines on the construction and installation of electric valve actuators in hazardous areas. This guideline states:
VII Hazardous Location Class I (Explosive Gas or Vapor)
Meets application requirements of National Electric Code; conforms with specifications of Underwriters’ Laboratories, Inc. used for atmosphere containing gasoline, hexane, naphtha, benzene, butane, propane, acetone, benzol, lacquer-solvent vapors, natural gas.
Most manufacturers of electric valve actuators offer NEMA VII-compliant versions of their standard products to meet the requirements for hazardous applications. In some cases, a more cost-effective option is to use electric controls with pneumatic valve actuators. However, when feasible, pneumatic actuators are generally the safer and more practical choice.
Sizing and Force
Force refers to the torque requirement of the valve, meaning the amount of force required to move the valve from the open to the closed position. In a sliding stem valve, the force required would be a linear push-and-pull force; multi-turn or part-turn valves require a rotary force. Determining the amount of force required depends on a number of factors such as:
- Minimum and maximum supply pressure
- Actuator type
- Fail mode
- Valve torque
The size and design of the valve and the differential pressure across the valve are also considerations, as well as the stem packing friction, media temperature, and the mechanical characteristics of both the valve and the valve stem. Note that for quarter-turn valves, it’s not possible to calculate the valve’s torque demand accurately. Instead, physical measurement of each valve size under various differential pressure conditions is required.
Screenshot via Sizing and Selection of Actuators for Valves, SlideShare
Due to the complexity involved in properly sizing valve actuators, it’s imperative that actuators are sized by technically trained experts. Simply opting for an oversized actuator isn’t a safeguard, either; an oversized valve actuator can damage the stem of the valve (unless additional components such as relief valves are used), plus it’s wasteful spending. An undersized actuator simply won’t be able to operate the valve on demand, which is particularly important in Emergency Shutdown Valves (ESDVs).
Minimum and maximum supply pressure are among the most important sizing factors to consider. Normal pressure, a metric often provided, should never be used to size an actuator. An actuator must be able to develop enough torque to operate the valve at the minimum supply pressure, ensuring that it functions properly even when supply pressure is at its lowest point. Maximum pressure must also be considered, as it’s imperative that the actuator be able to handle the maximum potential pressure safely. Regulators and relief valves may be used in applications in which the actuator is oversized or unable to handle the maximum supply pressure safely.
Pneumatic actuators have an advantage in that it’s easier to control their speed. The easiest way to implement speed control in a pneumatic actuator is to fit it with a needle valve, or a variable orifice, at the air pilot exhaust port. Electric actuators have geared motors and therefore it’s difficult (impossible, really) to control the speed of these actuators without making gear adjustments. Pulsing circuits can be added in some cases to allow for slower operation.
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For electric actuators, the required operating speed determines the motor power required. For fluid power actuators (pneumatic and hydraulic actuators), the required operating speed determines the necessary size of supply and exhaust lines in addition to the size of the direction control valve.
Frequency of Movement
The frequency of operation (or duty) impacts mechanical drive durability and the robustness of the controller. Obviously, valves that operate infrequently (such as isolating or regulating duty valves) mean less wear and tear on mechanical components and controls. Other valves, however, such as modulating process valves, operate on a near-constant or continuous basis, requiring a more durable valve and actuator assembly.
Actuator capability is generally defined as a specified number of starts per hour. According to the ISA Guidelines for the Specification of Electric Valve Actuators, “The reversing contactor as standard should be rated for a minimum of 60 motor starts per hour. Optional solid state reversing contactors should be electrically interlocked and be available for high rate modulating service and rated for minimum of 1200 starts per hour.”
Screenshot via Indelac Controls, Inc.
Another general rule of thumb is based on the duty cycle:
- Isolation – the valve operates only a few times per day
- Regulating – 30 to 60 starts per hour
- Modulating – 600 to 1,800 starts per hour
In certain applications, such as hazardous environments, guidelines such as NEMA VII aim to improve safety by requiring certain precautions for different types of actuators. Safety should always be a consideration when selecting a valve actuator, and when possible, the safest and most reliable option based on the application specifications should be used. An actuator should be able to maintain the fail-safe position should a fire break out, for example.
However, in some situations, the ideal actuator can’t be used due to mitigating circumstances. When a different type of actuator must be used than the most ideal solution, there are often additional components that can be added to improve safety and meet regulatory specifications.
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For example, pneumatic actuators can be stalled indefinitely without issue, but electric actuators cannot be stalled without risking damage to the motor due to excessive current generating heat in the motor. To protect the device, torque switches or heat and current sensors can be used.
Spring returns, or fail-safe options, are another safety component often specified in process industries. Spring returns drive the valve to a pre-determined safe position in the event of a power or signal failure. With pneumatic actuators, this is both a practical and inexpensive option. Accumulator tanks may be installed to store air pressure when spring returns cannot be used (which is sometimes the case with large or heavy actuators).
Most electric actuators aren’t available with a spring-return option, but a battery backup is often a viable fail-safe option for these actuators. Or, an electro-hydraulic actuator may be used to achieve spring-return functionality, requiring only an electric power supply. This is done by energizing a hydraulic pump which pressurizes a spring-return cylinder, driving the actuator to the original position in the event of a power failure.
All of the factors discussed above impact the cost of a valve actuator. In general, the greater the torque required, the more power necessary for operation, the larger and more expensive the actuator. While selecting the proper valve actuator for the application, including any additional components required for safe and desirable operation, may mean higher upfront costs, the final configuration will benefit from greater reliability and durability. Writing clear and unambiguous technical specifications, being diligent about quality control, and ensuring adequate supplier qualifications are essential steps in procuring the right actuator for your application.
The wrong actuator won’t just fail to adequately operate the valve but can lead to damage to the valve stem and the valve itself, as well as introduce unnecessary safety risks. Choosing the right valve actuator means fewer unexpected shutdowns and less frequent actuator and valve replacements, and ultimately lower long-term costs.