Pneumatic and Electric Actuators Article

Industrial Valve Actuators 

As a device articulating motion, actuators are an important link in any control system. Be it in plant automation, or the control system of an aircraft or an automobile. Actuators receive a low power command signal and energy input to amplify the command signal as appropriate to produce the required output. Their applications range from simple low power switches to high power hydraulic devices operating flaps and control surfaces on aircraft and car steering. Over the years, actuator designs and materials, and their power sources, have evolved substantially as they found newer applications.  While actuators initiate momentum in machine tools, material handling and servicing equipment etc, which use mechanisms requiring lifting, lowering, feed and swivel applications, they also have important applications in process industries for plant automation, control systems and valves.

Actuators and Plant Automation

As automation is adopted more widely in process industries, its role is becoming very critical. The need for valve actuators to provide the interface between the control intelligence and the physical movement of a valve has now become crucial. The principles of control valve and actuator technology have developed in an evolutionary way during the last twenty years. New design ideas, improved production methods, new materials and extensive use of computer systems have led to significant improvements.

Modern swater valves are more compact, have higher shutoff capability, and use more advanced sealing technology. Their resistance to erosion and corrosion has also improved, and the anti-cavitation and noise reducing trims are more advanced these days. While lowering the totalcost- of-ownership (TCO), compared to valves from the older generation, these advances have also made control valves more reliable and have expanded their lifecycle.

valve actuators can prevent serious environmental catastrophes as well as minimize damage to facilities.

The use of digital technology in actuators has also spawned significant advances in valve positioners. Foundation fieldbus and Profibus have now expanded the scope of actuator functions by enabling their integration into modern plant asset management systems with the help of latest developments in wireless technology. Most importantly, valve actuators ensure plant safety. Certain critical valves need to be opened or closed rapidly in the event of an emergency. Only valve actuators can prevent serious environmental catastrophes as well as minimize damage to facilities in such circumstances. The basic users of these types of actuators are control valve manufacturers, and ball valve and butterfly valve manufacturers.

The biggest problem in control valve technology is flashing and cavitation, which can quickly destroy the valve. Depending on the cavitation level, several methods can be employed to reduce or eliminate its effects. Flashing can be prevented by adjusting the process conditions. Special materials can also be used to prolong the life of a flashing or cavitating valve.

Basically, there are four fundamental types of valve actuators. These are a combination of the type of power applied and the type of movement required in the valve.

Electric multi-turn actuators 

Electric actuators main advantage is that they have all functions packed in a compact housing, which is water tight and explosion proof 

Electrically powered swater multi-turn actuators are among the most common actuators. They are capable of operating very large valves quickly. To protect the valve the limit switch turns off the motor automatically, when the torque sensing mechanism switches off the electric motor on exceeding a safe torque level. These valves have a declutching mechanism and a hand wheel to operate them manually in case of a power failure. Their main advantage is that they have all functions packed in a compact housing, which is water tight, explosion proof and submersible when required. Their only drawback is that—should a power failure occur, the valve remains in the last position and the fail-safe position cannot be obtained without a backup source of stored electrical energy.

Electric quarter-turn actuators 

Similar to an electric multi-turn actuator, their main difference is that the final drive element is usually in one quadrant that puts out a 90° motion. The newer generation quarter-turn actuators include most features found in sophisticated multi-turn actuators. Quarter- turn actuators are compact and can be used on smaller valves. They are typically rated to around 1,500 foot pounds. An added advantage of a smaller quarter-turn actuators is that, because of their lower power requirements, they can be fitted with an emergency power source such as a battery to provide failsafe operation.

Fluid power quarter-turn actuators 

Pneumatic and hydraulic quarter-turn actuators are extremely versatile. These find applications where electric power is unavailable or where simplicity and reliability are essential. These actuators have a wide range of applications, such as the smallest can deliver a few inch pounds of torque, while the largest are capable of producing in excess of a million inch pounds of torque. Almost all fluid power actuators utilize a cylinder and a mechanism to convert linear motion to a quarter-turn motion.

The scotch yoke is effective for larger valves where a higher torque requirement is needed at the beginning of the stroke

The main types of mechanism are scotch yoke, lever and link, and the rack and pinion. The rack and pinion type gives constant torque output, which is useful in smaller valves. The scotch yoke is effective for larger valves where a higher torque requirement is needed at the beginning of the stroke. Swater pneumatic actuators are usually controlled by solenoid control valves mounted on the actuator. A positive failure mode can easily be affected with a pneumatic or fluid power actuator, by adding an opposing spring to ensure a positive shut down in an emergency.

Fluid power multi-turn actuators 

These actuators are used mostly when multi-turn output is required to operate a linear type valve such as a gate or globe valve. Frequently electric actuators are used for this type of valve. In the absence of electric power supply, pneumatic or hydraulic motors can be used to operate multi-turn actuators.

Source: Industry Magazine