In order to successfully automate a process it is essential to make sure that the valve itself is appropriate to handle the special demands of the process and the product in the pipeline. It is the process or product that should dictate the type of the valve, the closure element of the valve, trim requirements and material of construction.
       Once a valve has been selected the next step is to consider the application’s automation requirements. These can be simplified by examining the two basic types of valve operation from an actuator’s perspective.

1.       Rotary or quarter-turn operation. This group would include plug valves, ball valves, butterfly valves as well as quarter-turn dampers. A simple 90° of movement at the prescribed torque demand is required.

2.       The other group of valves can be classified as multi-turn. These are valves that have rising non-rotating stems or non-rising rotating stems. In other words they require multiple turns to move the valve closure element from open to close. This group would include globe valves, gate valves, knife gates, sluice gates, etc. Alternatively, linear, pneumatic or hydraulic piston operators or diaphragm pneumatic actuators could be used.

Figure 2: Multi turn & quarter turn actuators

There are four fundamental types of actuators. These are a combination of the type of power applied and the type of movement required at the valve.

1.       Electric multi-turn actuators –The electrically powered multi-turn actuators are one of the most common and dependable configurations of actuators. A single or three-phased electric motor drives a combination of spurs and/or level gears, which in turn drive a stem nut. The stem nut engages the stem of the valve to open or close it, frequently via an Acme threaded shaft. Electric multi-turn actuators are capable of quickly operating very large valves. To protect the valve, the limit switch turns off the motor at the ends of travel. The torque sensing mechanism of the actuator switches off the electric motor when a safe torque level is exceeded. Position indicating switches are utilized to indicate the open and closed position of the valve. Typically a declutching mechanism and hand wheel are included so that the valve can be operated manually should a power failure occur. The main advantage of this type of actuator is that all of the accessories are incorporated in the package and are physically and environmentally protected. It has all the basic and advance functions incorporated in a compact housing which can be water tight, explosion proof and in some circumstances, submersible. The primary disadvantage of an electric multi-turn actuator is that, should a power failure occur, the valve remains in the last position and the fail-safe position cannot be obtained easily unless there is a convenient source of stored electrical energy.

2.       Electric quarter-turn actuators –These units are very similar to an electric multi-turn actuator. The main difference is that the final drive element is usually in one quadrant that puts out a 90° motion. The newer generation of quarter-turn actuators incorporates many of the features found in most sophisticated multi-turn actuators. For example, a non-intrusive, infrared, human machine interface for set up, diagnostics, etc. Quarter-turn electric 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.

3.       Fluid power quarter-turn actuators –Pneumatic and hydraulic quarter-turn actuators are extremely versatile. They can be used where there is no readily available electric power or where simplicity and reliability are essential. They can also withstand considerable mechanical abuse. Their range of applications is enormous. For example, the smallest can deliver a few inch pounds of torque where 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 covert the linear motion generated in the cylinder to a quarter-turn motion. 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 throughout the stroke so, they are useful for smaller valves. The scotch yoke is effective for larger valves where a higher torque requirement is needed at the beginning of the stroke. Pneumatic actuators are usually controlled by solenoid control valves that are mounted on the actuator, sometimes in combination with position indicating switches. A positive failure mode can easily be affected with a pneumatic or fluid power actuator by the addition of an opposing spring to provide a positive shut down in an emergency.
Figure 3: Gas driven actuator


Figure 4: Hydraulic spring return fluid power actuator  

4.       Fluid Power multi-turn actuators –These types of actuators are frequently used 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. However, if there is no electric power supply readily available, then pneumatic or hydraulic motors can be used to operate multi-turn actuators.