Actuated Valves

Valve actuation may be manual or automatic. There maybe hundreds of valves on a facility, and there maybe a requirement to have a mix of actuation types. Some valves may be located in remote, extremely hostile or toxic environments that make manual operation not practical and/or safe. Also as a safety feature, certain types of automatic actuators may be required to operate quickly, shutting down a valve in case of emergency

An automatic actuator has an external power source to provide the force and motion to operate a valve remotely or automatically. There are typically three types of power sources for actuation, pneumatic, hydraulic and electric. The choice of power source may depend on a number of factors, availability of the power source, control requirements of the valve, torque required to move the valve.

The different types of actuators used on valves discussed in the following are Manual Actuators, Pnuematic Actuators and Hydraulic Actuators:

Manual Actuators

A manual actuator utilises levers, gears, or wheels to facilitate movement of the valve manually.

Advantages of Manual Actuators

• Simple to operate
• Can be locked off and/or removed to avoid inadvertent operation
• Inexpensive compared with remote operated valves
• Used in many applications where infrequent operation is requried

Dis- Advantages of Manual Actuators

• Cannot open or close quickly, relys on manual intervention
• Not suitable for applications where precise control is required and valve has to be moved
• Depending on application and torque required to move valve, may require gear box to enable manual operation

Pnuematic Actuators

Pneumatic actuators use pressurized air to operate a valve. They do this by applying the force of the air to a piston or a diaphragm attached to the valve stem. Pneumatic actuators are used to provide automatic or semi-automatic valve operation, and are the most popular type in use due to their dependability and simplicity of design.

Advantages of Pneumatic Actuators

• Simplicity of realisation relatively to small back and forth motions;
• Sophisticated transfer mechanisms are not required;
• Low cost;
• High speed of moving;
• Ease at reversion movements;
• Tolerance to overloads, up to a full stop;
• High reliability of work;
• Explosion and fire safety;
• Ecological purity;
• Ability to accumulation and transportation

Dis-Advantages of Pneumatic Actuators

• Compressibility of the air;
• Impossibility to receive uniform and constant speed of the working bodies movement ;
• Difficulties in performance at slow speed;
• Limited conditions - use of compressed air is beneficial up to the definite values of pressure;
• Compressed air requires good preparation

Hydraulic Actuators

Hydraulic actuators use a pressurized fluid to control valve movement. The hydraulic fluid used is either water or oil and is fed to either one or both sides of a piston to cause movement. Hydraulic valves provide for automatic and semi-automatic valve operation.

Advantages of Hydraulic Actuators

• Infinitely variable control of gear-ratio in a wide range and an opportunity to create the big reduction ratio;
• Small specific weight, i.e. the weight of a hydro actuator is in ratio to transmitted capacity (2-3 kg / kWt);
• Opportunity of simple and reliable protection from overloads;
• Small sluggishness of the rotating parts, providing fast change of operating modes (startup, dispersal, a reverse, and stop);
• Simplicity of transformation of rotary movement into reciprocating one;

Dis-Advantages of Hydraulic Actuator

• Efficiency of a volumetric hydraulic actuator is a little bit lower, than efficiency of mechanical and electric transfers;
• Conditions of operation of a hydraulic actuator (temperature) influence its characteristics;
• Efficiency of a hydraulic actuator is a little reduced in the process of exhaustion of its resource owing to the increase in backlashes and the increase of outflow of liquid (falling of volumetric efficiency);
• Sensitivity to pollution of working liquid and necessity of high stringent practices to ensure fluid is not contaminated.

Classification of Pneumatic and Hydraulic Cylinder Actuators

• Cylinders are typically classified by operating principle or by construction type
• Single-acting or double-acting
• Single-acting cylinders exert force either on extension or retraction.
• They require an outside force to complete the second motion
• Double-acting cylinders generate force during both extension and retraction
• Directional control valve alternately directs fluid to opposite sides of the piston
• Force output varies between extension and retraction

Electric Actuators

Electric actuators include electric motors and solenoid-actuated valves. Electric motors can be used to open, close, and position a valve manually, automatically, or semi-automatically. The motor operates in both directions and drives the valve stem by means of gear couplings. Electric actuators are frequently used on multi-turn valves such as gate or globe valves. With the addition of a quarter-turn gearbox, they can be utilised on ball, plug, or other quarter-turn valves.

Solenoid valves use electric power to attract a magnetic slug attached to the valve stem and are used in automatic open-close applications. Solenoid valves are often used as pilot valves.

Advantages of Electric Actuators

• faster transmission signal
• the signal transmission distance (within reason) doesn’t effect performance like pneumatic and hydraulic transmission signal for long distances.
• sensitivity is relatively high
• good anti-deviation from the ability of electric actuators
• good anti-deviation from the ability of electric actuators
• failure state can be programmed, including fail/stay-put in last position something which pneumatic and hydraulic actuators can’t do in certain failure modes
• unlike pneumatic and hydraulic which are susceptible to air or hydraulic power source becoming contaminated and damaging actuator

Dis-Advantages of Electric Actuators

• location susceptible, i.e. need power source, and if located in hazardous area must be certified for use.
• relatively complex in design and more difficult to fault find and repair on site
• a small thrust,
• the average failure rate is higher than the pneumatic and hydraulic actuator due to complexity.
• the technical requirements of the site maintenance personnel is relatively high
• also increases the wear of reduction gear

Common Terms and Defintions Used for Electric, Pnuematic and Hydraulic Valve Actuators

Term	Definition
ambient temperature	environmental temperature of the location where the actuator is working
blistering	formation of bubbles or pimples on a coated surface, caused by the local loss of adhesion and lifting of the film from the underlying substrate
emergency closing/opening	overriding operation allowing the actuator to be closed or opened under emergency conditions
emergency shut down	specific function of an actuator designed to perform a pre-determined operation (open/close/stayput) in an emergency situation
end of travel	predefined position related to a fully open or a fully closed condition
end stop	mechanical part, designed to stop the actuator drive train at an end position
end torque/thrust	actuator maximum output torque/thrust available at the end of the stroke
fail-safe actuator	multi-turn, part-turn or linear actuator which is able to operate in a defined pre-determined way on loss of external power
fail-safe position	defined pre-determined position in which the actuator operates on loss of external power
indicating arrangement	device, externally visible, showing the position of the actuator/valve obturator
limit switch	contact that changes status when the stroking position of the actuator reaches a preset position
linear actuator	actuator which provides thrust for a defined linear stroke
manual override	device designed to operate manually the valve when required
motive energy	energy used to operate the actuator which can be electric, pneumatic or hydraulic
operating cycle	one complete opening and one complete closing stroke of the valve, including the stopping phases
operating/stroking time	duration of a complete stroke of the actuator

output torque/thrust	torque/thrust delivered by the actuator
position transmitter	device transmitting a signal proportional to the actuator position
rated torque/thrust	characterising figure indicated by the actuator manufacturer used to define the maximum actuator operating torque/thrust capability
test room temperature	ambient air temperature where the actuator is tested
run torque/thrust	actuator output torque/thrust developed between the seating/unseating positions
seating/unseating torque	actuator output torque required to seat/unseat the valve
start torque/thrust	actuator output torque/thrust at the beginning of the stroke, in the direction of the motive force defined
stroke	single and complete movement from one end of travel to the other
torque/thrust characteristics	values which may vary through the actuator stroke
travel limitation	any device integrated in the actuator and designed to limit the travel/stroke
travel	value of actuator output turns, angular or linear movement between ends of travel