Actuator selection and sizing for valves
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An actuator is a machine or component installed on the top of an industrial valve for automatically moving and controlling the valve. The performance of a valve is largely dependent on its actuator. Three factors are important for engineers to consider when selecting an actuator: frequency of operation, ease of access, and critical functions. Valve actuators should perform several functions including moving the valve closure member to an appropriate position, holding the valve closure member in the desired position, providing enough force or torque for seating the closure member and meeting the required shut down leakage class, providing fully open or fully close or failure mode as is, or providing a certain amount of closure member rotation with the right speed. In general, actuator can be hydraulic, pneumatic or electrical. This paper discusses the mechanism plus advantages and disadvantages of these three types of actuators. Affected parameters for actuator selection include the availability of a power source, torque and size of the valve, failure mode, speed of operation, frequency and ease of operation, control accessories, hazardous area, and cost. This paper presents a case study of breakaway torque (break to open) calculation and actuator sizing for a full-bore ball valve in pressure Class 300 equal to 50 barg nominal pressure and 22Cr duplex body material. The valve is fail close with an emergency shut down function, and a pneumatic actuator was selected for the valve. Fail close means that the valve will close in case of losing the power used as a source of actuator operation. Other four torque values were provided from the valve supplier. The calculated breakaway valve force and torque were used as a basis for actuator air cylinder sizing assuming air pressure of 7 barg and system efficiency of 90%. Force and torque for closing the valve were used to calculate the spring movement as well as spring piston length through Hook’s law.
KeywordsValve automation Actuator Quarter turn or linear movement Electrical Pneumatic Hydraulic
1 Introduction to valve actuation
An actuator is a machine or component installed on the top of an industrial valve for automatically moving and controlling the valve. Actuators are used in the plants more than before to provide better and more precise control on the valve operation . According to statistics published by European Industrial Forecasting in 2015, approximately 75% of the valves in the oil and gas sector are automated by actuators .
A valve actuator can be defined simply as a black box with a receiving signal or power supply through air or oil pressure that produces torque for valve movement as an output. The quality of a valve depends on many parameters such as metallurgy, mechanical strength, machining, etc., whereas the performance of the valve is largely dependent on the actuator . When selecting an actuator, engineers have three primary factors to consider: frequency of operation, ease of access, and critical functions . Valves with high numbers of operation are proper choices and possibilities for actuation. Valves in remote or hazardous areas where the presence of an operator is dangerous should be also considered for automation. Other reasons for valve actuation include ensuring safe, reliable, or fast operation ; excessive valve torque; and emergency operation of a valve to the open or closed position . Valve actuators should perform several functions including moving the valve closure member to the suitable position such as open or closed, holding the valve closure member in the desired position, providing enough force or torque necessary to seat the closure member and meet the required shut down leakage class, providing fully open or fully closed or failure mode as is, and providing a certain amount of closure member rotation with the correct speed .
2 Actuator types
2.1 Hydraulic actuators
Hydraulic actuators are smaller than pneumatic actuators since oil pressure is higher than air, so less oil is required for moving the spring compared to air. However, the hydraulic actuator is thicker than pneumatic due to higher oil pressure. For example, the air pressure in pneumatic actuators is 5.5–9 barg, which is much lower than oil pressure. It should be noted that barg is used for indicating the gauge pressure.
Hydraulic actuators are good choice for large size and high-pressure class valves having high force or torque requirements for operation at high speed.
Hydraulic actuators are more precise than pneumatic actuators since hydraulic oil is not compressible.
Hydraulic actuators have a higher degree of corrosion protection compared to air or gas, which is important in corrosive environments such as offshore.
Hydraulic actuators can reach the safe predefined position (open/closed) after losing the source of oil power through spring force.
The high pressure of hydraulic fluid is complex to manage, requiring many safety and environmental precautions.
The control system of hydraulic actuators is larger and more expensive compared to pneumatic.
2.2 Penumatic actuators
Control systems of pneumatic actuators are relatively inexpensive and more compact compared to hydraulic actuators.
This actuator provides high speed operation and returning to a safe position in case the air power source is lost, unlike electrical actuators.
Pneumatic actuators are more costly compared to electrical actuators.
Air is a compressible fluid, which can jeopardize the accuracy of the pneumatic actuators.
2.3 Electrical actuators
Electrical power is relatively inexpensive.
Electrical actuators provide good accuracy on valve movement and functioning and it is possible to monitor the opening percentage of the valve.
All control components are integrated into the actuator, unlike pneumatic and hydraulic actuators.
Electrical actuators are cheaper, more compact, and lighter than pneumatic and hydraulic actuators.
Electrical actuators cannot maintain fail safe positions unless they are combined with hydraulic power (electrical-hydraulic actuators).
Electrical actuators contain more complex and sensitive components compared to pneumatic and hydraulic actuators.
Electrical actuators are not as economical as pneumatic or hydraulic actuators above certain sizes.
Electrical actuators require more certifications in hazard areas with explosive fluids such as ATEX. ATEX stands for atmospheres explosive and it is a European Union Directive covers equipment and protective systems for use in potentially explosive atmosphere.
3 Parameters that affect actuator selection
Availability of power source: A hydraulic actuator cannot be used on a plant if no source of high-pressure oil is available.
Torque and size of the valve: Large size and high pressure class valves (such as a 30″ Class 1500 ball valve) require high torque for operation. Selection of a very large pneumatic actuator for such a large valve is not economical. A hydraulic actuator is recommended in this case.
Failure mode: Pneumatic and hydraulic actuators stand in open or closed positions during a power loss, unlike electrical actuators. These types of valves are spring return, which means that upon power or signal failure, the spring returns the valve to a predefined safe position . Therefore, for example, electrical actuators are not suitable for emergency shut down valves that should be fully closed if power is lost.
Speed of operation: Electrical actuators operate valves more slowly than pneumatic and hydraulic actuators, so an electrical actuator may not be an appropriate choice if an operations speed of 1 in./s or faster is expected from the valve.
Frequency and ease of operation: It is common to use electrical actuators for certain large size valves that are operated frequently instead of manual operation, for ease of operation. For example, a 20″ class 300 manual ball valve with frequent operation is proposed to be equipped with an electrical actuator just for ease of operation.
Control accessories: Control accessories in electrical actuators are integrated into the actuator, unlike pneumatic and hydraulic actuators. In fact, electrical actuators do not require any space for control accessories, which is an advantage. Hydraulic actuators have larger control panels compared to pneumatic actuators.
Hazardous areas: The use of electrical actuators in a hazardous environment may be limited in some cases. Different hazard zones and classes are defined based on present of flammable gases or vapours that may put restriction in usage of the electrical actuators.
Cost: Electrical actuators are the cheapest type of actuators, hydraulic actuators are the most expensive and pneumatic lie between.
4 Case study of torque calculations on a valve
Typical packing friction forces 
Stem diameter (mm)
ASME pressure class
Packing type and related friction forces
Break to open (BTO): This torque is measured when the valve is closed and the ball opens against just one seat under pressure. This torque, also called breakaway torque, was calculated earlier.
Running torque (RT): The torque of the valve when the ball opens at approximately 35°–45°.
End to open (ETO): The torque of the valve when the ball opens at 80° position closed to fully open the valve.
Break to close (BTC): When the valve is in fully open position, the torque required to break the open position of the valve to close the valve.
End to close (ETC): The torque required to fully close the valve when the valve is about to close.
BTO with double block: This torque is measured when the valve is close and the ball opens against both seats under pressure.
The other torque values for the ball valve had been measured by the ball valve supplier:
5 Case study of actuator sizing
Refer to Eq. 1, τ = F × r, which can be used for the BTO torque created by the actuator. r is the movement distance of the rod due to air pressure.
Assuming 16 mm for thickness of the rod end, the length of the air cylinder is 450 mm.
Assuming the preliminary length of the spring (compressed spring) to be 100 mm, then the spring piston is 513 + 100 = 613 mm.
The piston diameter is assumed to be equal to the cylinder diameter, which is 160 mm.
This article examined a case study of breakaway torque (Break to Open) calculation and actuator sizing for a bare stem full bore ball valve in pressure Class 300 and 22Cr duplex body material. The valve is fail close with an emergency shut down function, and a pneumatic actuator was selected for the valve. The force for opening the valve should overcome the packing friction as well as unbalanced force due to the pressure differential. Four other torque values were provided from the valve supplier. The safety factor for actuator sizing is assumed to be equal to 2. The calculated breakaway valve force and torque were used as a basis for actuator air cylinder sizing assuming an air pressure of 7 barg and system efficiency of 90%. Additionally, force and torque for closing the valve were used to calculate the spring movement as well as spring piston length through Hook’s law. The diameter of the spring housing was assumed to be equal to the diameter of the air cylinder.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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