A valve actuator is a device that produces force to open or close the valve utilizing a power source. This source of power can be manual (hand, gear, chain-wheel, lever, etc.) or can be electric, hydraulic or pneumatic.
Basics of Actuators
Basic  actuators  turn  valves  to either  fully  opened  or  fully  closed  positions.  But  modern actuators have much more advanced capabilities. They not only act as devices for opening and closing valves, but also provide intermediate position with high degree of accuracy.
The valve actuator can be packaged together with logic control and digital communication  ability to allow remote operation as well as allowing predictive maintenance data.
Type of Actuators
Two types of actuators are common: pneumatic and electric actuators.
Pneumatic:
Pneumatic actuators utilize an air signal from an external control device to create a control action via a solenoid. These are commonly available in two main forms: piston actuators and diaphragm actuators.
- Piston actuators  –  Piston  actuators  are  generally  used  where  the  stroke  of  a diaphragm actuator would be too short or the thrust is too small. The compressed air is applied to a solid piston contained within a solid cylinder. Piston actuators can be single acting or double acting, can withstand higher input pressures,  and can offer smaller cylinder volumes which can act at high speed.
- Diaphragm actuators – Diaphragm actuators have compressed air applied to a flexible membrane called the diaphragm. These types of actuators are single acting, in that air is only supplied to one side of the diaphragm, and they can be either direct acting (spring-to-retract) or reverse acting (spring-to-extend).
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.
Electrical Actuators:
Electric actuators are motor driven devices that utilize an electrical input signal to generate a motor shaft rotation.  This rotation  is, in turn, translated  by the unit’s linkage  into a linear motion,which drives the valve stem and plug assembly for flow modulation.
In case of electric signal failure, these actuators can be specified to fail in the stem-out, stem-in, or last position. Commonly used motors for electric actuators include steppers and servos.
- A step motor uses gears with increments in the range of 5,000 to 10,000 at 90 degree rotation for accurate positioning at lower speeds. The disadvantage is that steppers may lose synchronization  with the controller when employed in an open loop without an encoder or if they are undersized for an application.
- Servos, by  definition,  are  closed  loop  and  provide  superior  performance  at  high speeds,  but at a higher  cost.  High  precision  screws  and anti-backlash  mechanics provide accuracies to ten-thousandths of an inch. Standard precisions with standard components range from a few hundredths to a few thousandths of an inch.
Brush DC motors and AC motors are sometimes used with limit switches when positioning accuracy is less critical. The motor is connected  to a gear or thread that creates thrust to move the valve.
To protect the valve the torque sensing mechanism of the actuator turns off the electric  motor when a safe torque level is exceeded.  Position  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.
Pneumatic v/s Electric Actuators
The major difference between pneumatic and electronic actuators is the speed of operation. The two technologies are so different that one cannot be a drop-in replacement for the other.
Each has inherent advantages and disadvantages.
Advantages of Pneumatic Actuators
- The biggest advantage of the pneumatic actuators is their failsafe action. By design of the compressed spring,  the engineer  can determine  if the valve will fail closed  or open, depending on the safety of the process.
- Provide high force and speed, which are easily adjustable and are independent  of each other
- Have a delayed response which makes them ideal for being resilient against small upsets in pressure changes of the source.
- Most economical  when  the  scale  of  deployment  matches  the  capacity  of  the compressor.
- Provide inherent safety and are ideal for hazardous and explosive environment.
- Low component  cost  and  smaller  footprint.
Limitations of Pneumatic Actuators
Maintenance and operating costs can be high, especially if a serious effort has not been  made  to  quantify  and  minimize  the  costs.
Maintenance  costs  include replacement  cylinder  costs  and  plugging  air-line  leakages  whereas  the operating costs include the cost of compressed air, i.e. electricity for the compressor.
Electric actuators:
Advantages
- Provide precise control and positioning in comparison to pneumatic actuators.
- Response time is essentially instantaneous.
- High degree of stability.
- Help adapt machines to flexible processes.
- Low operating cost. Controllers and drivers low voltage circuitry consume power to a far lesser degree.
Disadvantages
- The primary disadvantage of an electric 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.
- Higher cost than pneumatic actuators. High component costs often deter the use of electric actuators because savings in operating costs compared to pneumatics are often not adequately considered or are outright ignored.
- The actuator  needs  to be in an environment  that is rendered  safe.  Generally  not recommended for flammable atmospheres.
