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.

 

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.

 

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.

 

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.

 

Recommend Valve Actuator Manufacturer

If you have any interest in more information about valve actuator, I recommend that you can try to visit the website of Sun Yeh Electrical Ind. Co., Ltd. – it is one of the leading valve actuator manufacturers in Taiwan. The company can provide high quality electric valve actuator, spring return valve actuator, heavy duty electric actuator, motorized valve actuator, and much more products for customers. Get further details, please do not hesitate to check out its product catalog and send inquiry to Sun Yeh.

 

 

Article Source: https://www.mpofcinci.com/blog/how-to-select-a-valve-actuator/

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