A valve can be operated through a series of drive mechanisms classified according to the mode of operation: manually or automatically. Manual type operators can be steering wheel, chain, gearbox and lever. Automatic type consists of an actuator which is activated by a pneumatic or electrical medium.
What is an actuator?
It is a device used to control and facilitate the operation of the valves; can be electric or pneumatic. This type of equipment can be operated remote or local mode. For proper operation, in the industry in general, the designer specifies a number of parameters, such as type of service, installation site (weather requirements), opening and closing speed, line pressure, maximum differential pressure, temperature, pressure and operating fluid, voltage type – phase – frequency, air or gas pressure for pneumatic pressure actuators. For an electric actuator it must be specified: AC or DC voltage, number of phases, motor type. In addition, the position indicator must be specified. The gate, ball, plug, balloon and butterfly valves may be equipped with actuator, according to certain criteria; instead the check valves for their operation do not require it.
In the chemical, petrochemical, oil and gas, pharmaceutical industries, among others; it is generally recommended to use actuator in the following cases:
- When the valve location makes it difficult to operate or remote valve operation is required.
- When the frequency of use of the valve is high.
- When the operating torque of the valve does not allow its operation manually.
- Where the valve’s operating conditions present a risk to operators.
For each case, COVNA valve has a wide variety of actuators specially designed for your process needs.
An actuator is a device whose function is to provide force to move or “act” another mechanical device. The force caused by the actuator comes from three possible sources: pneumatic pressure and electric driving force (electric motor). Depending on the origin of the force, the actuator is called “pneumatic” or “electric”.
To operate the pneumatic actuator, compressed air is connected to one side of the plunger generating a force in the direction of the expansion of the space between the plunger and the wall of the cylinder or body.
Using a mechanical device that can be the pinion and zipper, Scotch yoke, or a simple vane, the movement is transformed into rotary. The torque generated by the actuator is directly proportional to the compressed air pressure, but depending on the design can be variable according to the current position of the actuator.
The AT Series pneumatic actuators of COVNA Valve is characterized in the use of high precision and integrative pinions, designed according to yours requirements and with high quality materials, made from nickelled – alloy steel, stainless steel, full conform to the latest standards of ISO5211, DIN3337, NAMUR.
The Quarter Turn Electric Actuators incorporate electric motors specially developed and designed for the automation of valves precisely, reliably and easy to install. To operate the electric actuator, the corresponding terminals must be energized for the motor to operate in the proper direction. The torque generated by the electric motor is increased by a reducer to output the final torque at the selected time. The motors are fully enclosed cage type, small in size and inertia and large in torque. For the remote actuator automation, the wiring diagram that comes with the actuator is considered. Connections should consider force, stroke and torque limit signals, analog or digital position and torque signals, etc.
Uses and applications of actuators in the industry
The use of actuators is highly related to the operation and service of valves within a certain industrial process. Below is shown a classification according to the type of service and key features of COVNA Valve products, specifically for pneumatic and electric actuators
A normal process service consists of routine operations carried out in order to produce a specific product. In this case, the operation of any type of actuator (pneumatic or electrical) can work properly according to the needs of the process. The pneumatic and electric actuators of COVNA Valves are characterized by having actuator bodies designed and manufactured according to different requirements. Materials such as aluminum alloy are used for production under international standards. Additionally, treatments based on hard anodic oxidation and coated by polyester powder are applied for corrosion protection. In the case of pneumatic actuators, COVNA valves actuators have a series of mechanisms capable of operating with high reliability; an example of this are the high-pressure and integrative pinions, twin rack pistons configured under a symmetrical assembly of long cycle life and fast operation. For electrical actuators – likewise has high reliability and precision in operation – the COVNA Valves electrical actuators incorporates safety factors such as the fully enclosed of motors in cage type, high sealing performance and its self-locking with a accurate turbo-worm structure can output large torques with high efficiency and little noise.
Valves required to be operable for a specified number of operations in order to ensure the safety of the process in the event of loss of the main actuator fluid medium. In many cases, pneumatic actuators are used. Its operating condition in failure mode allows it to operate more safely.
Actuators on valves identified as “critical service” shall have a reliable supply of the actuator fluid as well as a correct selection of manufacturing materials. The pneumatic actuators of COVNA Valve show high performance in each of its parts, as is the case of preloaded coated springs, which are made of a high quality material to resist corrosion and a longer life.
- Emergency process service
Valves installed at strategic locations to isolate / shutdown equipment or depressurize, remove or displace flammable / toxic / environment contaminating material inventory of process lines / equipment.
On items classified as emergency process service the actuator shall have an additional set of actuating pushbuttons and position indicating lights located at least 10 meters from the valve and operable at grade level. The actuators and power/instrument leads on Emergency Classified valves shall be fireproofed. For these types of services, the actuators generally used are the pneumatic types. The pneumatic actuators of COVNA Valve have been designed, manufactured, and tested using the latest technology, materials, and innovation, providing excellent service in various environments. For emergency services, the actuators must be properly specified according to the operational requirements and the corresponding risk studies.
Criteria for the selection and sizing of pneumatic and electric actuators
Performance characteristics are often the key factors when deciding between pneumatic and electric actuators.
- Speed: pneumatic actuators can operate at variable and very high speeds; Electric actuators operate at fixed and moderate speeds.
- Dangerous environment: pneumatic linear actuators are explosion, shock and spark proof.
- Overheating: Once again, the advantage applies to pneumatic actuators, which are not subject to overheating and run smoothly in humid environments. Electric actuators, on the other hand, can overheat and must be sealed against moisture.
- Duty cycle: pneumatic actuators work with a duty cycle of 100%; the electrical units have a standard 25% duty cycle that can be increased.
- Blockage: electric actuators, which cannot be stopped, have a high probability of damage. Pneumatic actuators can be stopped indefinitely.
- Torque to weight ratio: There is a high torque to weight ratio for pneumatic actuators, while electric actuators have a low torque to weight ratio.
- Spring return: pneumatic actuators have the advantage here. (ie fail-safe) Pneumatic varieties have a spring return option that is practical and economical. This spring return option is generally not available for electric actuators.
There is much to consider when deciding whether to use a pneumatic or electric actuator. Each has inherent advantages and disadvantages, but the best option will depend on the specific needs of the application, budget constraints and performance requirements. Taking the time to thoroughly evaluate the options will help ensure optimum quality and longevity of the actuator, allowing operations to run smoothly and efficiently.
Regarding the general and particular criteria for sizing pneumatic and electric actuators, the following technical aspects are taken:
- First, the torque needed to generate the rotational movement must be determined.
- Consider the operational pressure, which most likely fights against the actuator.
- With the torques already determined, and using the torque tables of the different models, it is possible to choose a suitable model for a given application.
- It is important to determine the final over sizing factor that is calculated by dividing the actuator torque by the original torque required by the valve. It is important to keep this in mind so as not to lose sight of how much torque we are actually delivering to the valve, especially when the customer or the engineer supplies the maximum allowable torque for the valve stem.
- Check the maximum allowable torque for the valve stem.
- Establish the controls that will govern the actuator: Positioner, solenoid valves, career switches, position transmitters, etc.
- Determine the minimum air pressure available at the point. It is in this situation that the actuator is in its worst condition. The valve must be operated even when the air pressure drops to a minimum. The maximum expected pressure must also be achieved, and compared with the maximum pressure supported by the actuator and with the maximum torque supported by the valve shaft.
- If the maximum torque at maximum air pressure exceeds the maximum allowable torque of the valve stem, it is important to consider installing a pressure regulator to limit the maximum air pressure.
- Obtain power availability at the installation point. Voltage, frequency, number of phases.
- When choosing the actuator with its motor, take the consumption and power factor data.
- Electric actuators have longer operating response times than pneumatic actuators, so it is a fact to consider. Depending on the size of the valve, these times typically range from 20 seconds to 90 seconds or more.