Control Valve: Pneumatic, Hydraulic, Electrical, and Manual Actuators

Valve actuator is a mechanical component \ device used to operate the valve using power source. It can be Manual, Electric, Pneumatic, or Hydraulic . There are two main types of actuators Rotary and Linear.

Rotary Valve Actuators: It produces the rotational motion needed to operate rotary valves such as ball, plug, and butterfly valves.

Linear Valve Actuators: It produces linear motion to operate such type of valves: globe, gate, and pinch valve.

a) Manual Actuators

• Basic Lever Actuator: The most common manual operator on smaller quarter-turn valves
• Hand Wheels: used for larger valves that require high torque to operate often incorporate geared hand wheels, (have a large hand wheel with a gearbox that further increases the mechanical advantage). These wheels will require multiple revolutions to turn the valve only 90 degrees.

Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. 

• Manual Valves with Limit Switches: Many times manual valves perform a function that does not require automated actuation, but the system still needs to know what position they are in. 

Limit Switches: (position indicators)  switches are used to communicate the valve's current position to the control system. It can either be a mechanical device or a proximity switch, referred to as a “prox switch, can send a confirmation signal to a PLC (Programmable Logic Controller) or a DCS (Distributed Control System) to let the computers know if the valve is open or closed.

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Pneumatic VS Hydraulic VS Electric Actuators

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.

Hydraulic Actuators: operate similarly to pneumatic actuators, but an in-compressible liquid (Hydraulic Fluid) from a pump rather than pressurized air moves the cylinder.

Electric Actuator: converts electrical energy into torque using electrical motor.

Pneumatic Actuators pros and cons

Pros of Pneumatic Actuators:

Benefits of pneumatic actuators come from their simplicity, they generate accurate linear motion, its applications involve areas of extreme temperatures (-40°F to 250°F ).

1. In terms of safety and inspection, cause of using air, pneumatic actuators avoid using hazardous materials
2. From explosion protection point of view and machine safety requirements because they create no magnetic interference due to their lack of motors.
3. Cost of pneumatic actuators is low compared to other actuators ($50 to $150), according to Bimba Manufacturing, also lightweight and require minimal maintenance.

Cons of Pneumatic Actuators:

1. Pressure losses and air’s compressibility make pneumatic less efficient than other linear motion methods.
2. Compressor limitations mean that operations at lower pressures will have lower forces and slower speeds and also compressor must run continually operating pressure even if nothing is moving.
3. It must be sized for a specific job what means they cannot be used for other applications.

Hydraulic Actuators pros and cons

Pros of Hydraulic Actuators:

1. They are suitable for high pressure applications where they can produce forces 25 times greater than pneumatic cylinders of equal size. They also operate in pressures of up to 4,000 psi.
2. Hydraulic actuator can hold force and torque constant without the pump supplying more fluid or pressure due to the incompressibility of fluids
3. They can have their pumps and motors located a considerable distance away with minimal loss of power.

Cons of Hydraulic Actuators:

1. Leak of fluid ,loss of fluid leads to less efficiency, leads to cleanliness problems and potential damage to surrounding components and areas.
2. Hydraulic actuators require many companion parts, including a fluid reservoir, motors, pumps, release valves, and heat exchangers, along with noise reduction equipment.

Electric Actuators pros and cons

Pros of Electrical Actuators:

1. Electrical actuators offer the highest accurate control positioning an example of the range of accuracy is (+/0.000315 in)
2. Electric actuators can be networked and reprogrammed quickly. They offer immediate feedback for diagnostics and maintenance.
3. They provide complete control of motion profiles and can include encoders to control velocity, position, torque, and applied force.
4. Environmental hazards are eliminated because there are no fluids leaks.

Cons of Electrical Actuators:

1. Initial unit cost of an electrical actuator is higher than other types of actuators (range from $150 to greater than $2,000 depending on its design and electronics)
2. Electrical actuators are not suited for all environments, unlike pneumatic actuators, which are safe in hazardous and flammable areas.
3. The chosen motor just suits actuator’s force, thrust, and speed limits to a fixed setting. If a different set of values for force, thrust, and speed are desired, the motor must be changed.

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Double Acting VS Spring Return Actuators

• Double Acting Actuators : are air/liquid supplied to both sides of the piston, with one side being higher pressure which achieves the movement required to actuate the valve (fluid both open and close the valve)

• Spring Return Actuators: 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 (fluid only open and spring close the valve or vice versa)

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b) Pneumatic Valve Actuators

Pneumatic Linear to Rotary Actuators (Piston Style):

• Scotch Yoke

Check this for more understand of mechanism

Rack and Pinion: Unlike traditional actuators, which produce a 90-degree turn of the pinion, rack, and pinion, actuators output a 180-degree turn.It's suitable for actuating plug valves.

Check here to recognize more about system components

Pneumatic Linear Actuators:

linear valves such as gate and globe valves, the linear motion is achieved directly. Air is used to push the "rising stem" up and down. There are 2 main styles of linear actuators, they are diaphragm and cylinder.

c) Hydraulic Valve Actuators

The same mechanism and types of pneumatic but it uses hydraulic fluid instead of air.

Piston Style – Piston-style hydraulic actuators function the same way at pneumatic piston-style actuators:

• Scotch Yoke 
• Rack and Pinion 

d) Electric Valve Actuators

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.

1. Motor: AC motors are mostly used as the driving force, for some applications also single-phase AC or DC motors are used.
2. Limit and torque sensors: The limit switches signal when an end position has been reached. The torque switching measures the torque present in the valve.
3.  Gearing: used to reduce the high output speed of the electric motor.
4. Valve attachment: consists of two elements. First: The flange used to firmly connect the actuator to the counterpart on the valve side, Second: The output drive type used to transmit the torque or the thrust from the actuator to the valve shaft.
5. Manual operation: In their basic version most electric actuators are equipped with a handwheel for operating the actuators during commissioning or power failure. The handwheel does not move during motor operation.
6. Actuator controls: Both actuator signals and operation commands of the DCS are processed within the actuator controls. This task can in principle be assumed by external controls such as a PLC.
7. Electrical connection: The supply cables of the motor and the signal cables for transmitting the commands to the actuator and sending feedback signals on the actuator status are connected to the electrical connection.
8. Fieldbus connection: Fieldbus technology is increasingly used for data transmission in process automation applications.

e) Electraulic Valve Actuators

It combines the simplicity of electric operation, the power of hydraulics,

This is all about different control techniques for the most important component of control system.

Resources:

• Research Gate Website
• Schlumberger Website
• Visual Encyclopedia Of Chemical Engineering Equipment - University Of Michigan
• Machine Design Website
• Assured Automation Website
• Youtube
• Wikipedia