How does it work: Actuators

Every valve needs a means by which it can be operated (e.g., cycled or actuated). There are a variety of options to achieve this including: handwheels, levers, gears, and actuators.

Actuators are a means by which a valve can be automated so that no human interaction with the valve package is necessary to cycle the valve. Actuators can be remotely operated and can act as shutdown mechanisms in case of an emergency situation, wherein human interaction can be dangerous.

“At a basic level, an actuator is a control mechanism that is operated by an energy source. This energy can be hydraulic pressure, pneumatic pressure, or electric current which moves the internal mechanical parts of the actuator.” said Russ Robertson, actuation product manager, “They can be designed to fail-open (in the case of actuator failure, the valve will stay open) or fail-close (in the case of actuator failure, the valve will stay closed). They also are distinguished by whether they are for quarter-turn (e.g., ball valves, plug valves) or linear (e.g., gate valves) valve operation.”

Types

Double Acting – Actuators in a double acting configuration have air/liquid supplied to both sides of the piston, with one side being higher pressure which achieves the movement required to actuate the valve. This configuration uses the air/liquid as energy to both open and close the valve.

Spring Return – Actuators in a spring return configuration have air/liquid supplied to only one side of the piston, and the energy to move the mechanisms comes from a spring on the opposite side. This configuration uses the air/liquid as energy to open or close the valve, while the spring acts to affect the opposite motion.

Pneumatic (see Figure 1) – Pneumatic actuators utilize compressed air to generate the operating energy. These actuators are quick to respond, but are not ideal for environments under high pressures, as gas is compressible. Pneumatic actuators can be either spring return or double acting.

  • Piston Style – Piston style actuators generate linear force by the air acting on the piston. The conversion of this linear force to torque (for use in rotary valves) is achieved by specific actuator designs.
    • Scotch Yoke – A scotch-yoke actuator includes a piston, connecting shaft, yoke, and rotary pin. The yoke is offset 45 degrees from the axis of the piston at the two ends of travel and at 90 degrees to the piston shaft when in the mid travel position. The canted scotch-yoke design is ideal for offset butterfly valve actuation.
    • Rack and Pinion – Unlike traditional actuators, which produce a 90-degree turn of the pinion, rack, and pinion, actuators output a 180-degree turn. This style of actuator is particularly suitable for actuating plug valves.
  • Diaphragm Style – The diaphragm-style actuator includes a rubber diaphragm and stem in a circular steel housing. This style of actuators is ideal for valves requiring shorter travel, such as diaphragm valves and globe valves.

Hydraulic (see Figure 2) – Hydraulic actuators use liquid as a means to apply pressure to the actuators mechanical components. They generally can exert a large amount of force, because liquid is not compressible, but are generally limited in acceleration and speed. Hydraulic actuators can be either spring return or double acting.

  • Piston Style – Piston-style hydraulic actuators function the same way at pneumatic piston-style actuators, but utilize liquid instead of gas to generate the operating energy.
    • Scotch Yoke – See description above.
    • Rack and Pinion – See description above.

 

Direct Gas (see Figure 3) – Direct gas actuators utilize a high-pressure natural gas or nitrogen supply to achieve on/off control of a valve in any natural gas transmission application. Direct gas actuators only come in double acting configurations.

Gas-Over-Oil (see Figure 4) – Gas-over-oil actuators use high-pressure gas supplied from the pipeline, suspended above a hydraulic fluid to move the mechanics of the actuator. Gas-over-oil actuators only come in double acting configurations.

Electric – Electric actuators use a power source, such as a battery, to run the actuator. They usually include intricate electrical circuitry to program when the actuator operates. Because of their use of electricity as a power source, however, they may not be the best actuator for remote installations.

Applications

The Industrial Revolution brought about the use of water to hydraulically actuate valves and by the 1920s, pneumatic actuation was in use. With the invention of more advanced process plants with higher pressure requirements, more sophisticated electric designs as well as innovations like the gas-over-oil actuator developed. Around the 1950s, high-pressure gas actuators were created to meet the high pressure demands of the pipeline industry as well as electro-hydraulic actuators for critical failsafe applications.

Actuators are ideally suited for installations where human interaction is either not possible or is dangerous, such as where space or installation location inhibits access to the valve operator.

Subsea actuators (see Figure 5) are designed to withstand the low temperatures, extremely high pressures, and remote accessibility of underwater installations. Only specific actuators are used in subsea applications, because the gas in a pneumatic actuator, for example, would compress under the high pressures, rendering the valve package inoperable. Integrated interface panels allow the subsea actuator to be operated by an ROV and easily added to a subsea tree. The Cameron LEDEEN line of subsea actuators includes a shallow-water series (for installations to 500 ft [152 m]) and a deepwater series (for installations deeper than 500 ft [152 m]).

Compact actuators (see Figure 6) are utilized on FPSOs or other locations where space is limited and weight is of concern. These actuators are designed to provide the powerful torque/thrust output of their larger counterparts, but with a reduced installation footprint. 

Direct gas and gas-over-oil configurations are commonly utilized in pipeline applications and are ideally suited for sweet natural gas lines, wherein the actuators use the gas in the line. Direct gas actuators are known to have very smooth operation and are, due to inherent design characteristics, less likely to leak hydraulic fluid to the environment, which can sometimes be an issue when utilizing gas-over-oil actuators. Direct gas actuators also require less cleaning and routine maintenance than gas-over-oil actuators. In both configurations, a biodegradable hydraulic fluid is utilized in offshore applications for safer operation.

Images

Actuator Figure 1
Figure 1LEDEEN Quarter-Turn Pneumatic Spring Return Actuator
Actuator Figure 2
Figure 2
LEDEEN Quarter-Turn Hydraulic Spring Return Actuator
Actuator Figure 3
Figure 3
LEDEEN Direct Gas Actuator with Hydraulic Override 
Actuator Figure 4
Figure 4
LEDEEN Gas-Over-Oil Actuator 
Actuator Figure 5
Figure 5

LEDEEN Shallow-Water Subsea Actuator

Actuator Figure 6
Figure 6
CAMERON Compact Quarter-Turn Hydraulic Double Acting Actuator