Friday, April 27, 2018

Valve Actuation 101: The Three Basic Types of Pneumatic Valve Actuator

Pneumatic valve actuators come in three basic design varieties:
  1. Scotch-yoke
  2. Rack & pinion
  3. Rotary vane
All three types provide the same function - converting air pressure to rotational movement intended to open, close, or position a quarter-turn valve (ball valves, plug valves, butterfly valves, or other 90 degree rotational valves).

All three styles are available in either direct acting or spring return versions. Direct acting actuators use the air supply to move the actuator in both directions (open and close). Spring return actuators, as the name implies, uses springs to move the actuator back to its "resting" state. Converting from direct acting to spring return is done through simple modifications, typically just adding an external spring module, or removing the end caps from rack and pinion actuators and installing several coil springs.

Scotch yoke
Scotch yoke (Limitorque)
Scotch-yoke actuators use a pneumatic piston mechanism to transfer movement to a linear push rod, that in turn engages a pivoting lever arm to provide rotation. They come in a wide variety of sizes, but are very often used on larger valves because they are capable of producing very high torque output. Spring return units have a large return spring module mounted on the opposite end of the piston mechanism working directly against the pressurized cylinder.

Rack and pinion
Rack and pinion (Delval)
A rack & pinion pneumatic actuator uses opposing pistons with integral gears to engage a pinion gear shaft to produce rotation. Rack & pinion actuators (sometimes referred to as a lunch box because of their shape) tend to be more compact than scotch yoke, have standardized mounting patterns, and produce output torques suitable for small to medium sized valves. They almost always include standard bolting and coupling patterns to directly attach a valve, solenoid, limit switch or positioner. Rack and pinion actuators use several smaller coil springs mounted internally and provide the torque to return the valve to its starting position.

Vane actuators generally provide the most space savings when comparing size-to-torque with rack and pinion and scotch yoke. They have a reputation for long life because then contain fewer moving parts than rack and pinion and scotch yoke actuators. Vane actuators use externally mounted, helically wound "clock springs" for their spring return mechanism.

The practical difference between these three types of pneumatic actuators comes down to size, power, torque curve and ease of adding peripherals. For the best selection of valve actuator for any quarter turn valve application, you should seek the advice of a qualified valve automation specialist. By doing so your valve actuation package will be optimized for safety, longevity, and performance.

Monday, April 16, 2018

Swanson Flo Markets - New Video

Here's a new short video highlighting the markets that Swanson Flo serves in the upper midwest United States.

Wednesday, April 4, 2018

Valve Automation Basics: Electric Actuators

Electric Actuator
Electric Actuator Assembly (Limitorque)
Electric actuators bring automation to industrial valve operation, allowing complex processes to be managed and controlled by remotely located control systems.

There are other motive forces used for valve actuators, including hydraulic and pneumatic, but electric actuators carry their own particular set of operating characteristics that make them an advantageous choice for many applications.

Valve actuators are available in uncountable variants to suit every application scenario. There are three basic valve actuation motions.

  • Multi-turn, with repeated rotations of the valve shaft needed to move the valve trim from fully open to fully closed. A gate valve is a multi-turn valve. These are also called linear, with respect to the motion of the closure element. The term "linear", in this case, refers only to the movement of the valve trim and not the flow characteristics of the valve.
  • Part Turn, where a 90 degree rotation of the valve shaft produces a change from opened to closed. Ball valves are in this category.
  • Lever, generally associated with damper control.

Electric Actuator
Completed Electrically Automated Valve
An electric actuator is a combination of motor and gearbox with sufficient torque to change valve trim position. A local self-contained control commands the motor and provides feedback to the process master controller regarding position, travel, torque, and diagnostics. Several interface options are available to facilitate communication between actuator and master controller.

There are numerous considerations to take into account when selecting an electric actuator.

  • Torque needed to effectively operate the subject valve.
  • Actuator enclosure type - wash down, hazardous area, dust, etc.
  • Service area for the assembly - corrosive environment, temperature extremes, and more
  • Valve movement - linear, multi-turn, part turn, lever
  • Operation mode - open and close only, positioning, modulating
  • Frequency or duty cycle - infrequent, frequent, or almost continuous positioning
  • Communication - How will the local controller communicate with the central control system?
  • Electrical - What electric power characteristics are available for operation?
  • Protections - Motor overload, torque limit, others
  • Process Safety - Among other things, what happens if power fails? 

There are certainly other elements to consider when applying an electric actuator for industrial use. Share your valve and actuator requirements and challenges with product specialists, combining your process knowledge with their product application expertise to forge the most effective solutions.

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800-288-7926