In order to successfully automate a process, it is important to ensure that the valve itself meets the special requirements of the process and the medium in the pipeline. Usually the production process and process medium determine the type of valve, the type of spool, and the structure and material of the trim and valve. After the valve is selected, the next step is to consider the requirements of automation, that is, the choice of actuator. Actuators can be simply considered in terms of two basic types of valve operation.
Rotary valves
These valves include: plug valves, ball valves, butterfly valves, and dampers or baffles. This type of valve requires an actuator that rotates 90 degrees of torque to the required torque.
Multi-turn valves
These valves can be either non-rotating poppet stems or rotating non-poppet stems, or they require multi-turn operation to actuate the valve to the open or closed position. Such valves include: straight-through valves (globe valves), gate valves, knife gate valves, etc. As an option, pneumatic or hydraulic cylinders or diaphragm actuators with straight output are also available to actuate the valves described above.
At present, there are two types of electric actuators, which are generally divided into part-turn electric actuators (Part-Turn Electric Valve Actuator) and multi-turn electric actuators (Multi-Turn Electric Valve Actuator), the former mainly controls valves that require partial rotation such as ball valves, butterfly valves, etc., and the latter requires multi-turn rotating valves, such as gate valves, etc.
Electrically driven multi-turn actuators are one of the most commonly used and reliable types of actuators. Use a single-phase or three-phase motor to drive a gear or worm gear*** rear drive stem nut, which causes the stem to create a motion that causes the valve to open or close the valve.
The multi-turn electric actuator can quickly actuate large valves. In order to protect the valve from damage, the limit switch installed at the end of the valve stroke will cut off the power supply to the motor, and when the safety torque is exceeded, the torque sensing device will also cut off the motor power supply, the position switch is used to indicate the switching status of the valve, and the handwheel mechanism installed with the clutch device can manually operate the valve when the power supply fails.
The main advantage of this type of actuator is that all components are housed in a single housing, which integrates all basic and advanced functions in a waterproof, dustproof and explosion-proof housing. The main disadvantage is that the valve can only remain in place when the power supply fails, and the valve can only achieve a fail-safe position (fault on or fault off) with a backup power system
This actuator is similar to an electric multi-turn actuator, the main difference is that the final output of the actuator is 1/4 of the 90-degree movement. The new generation of electric single-turn actuators combines the complex functions of most multi-turn actuators, such as parameterization and diagnostics using a non-intrusive, user-friendly operator interface.
Single-turn actuators are compact enough to be mounted on small valves, typically with an output torque of up to 800 kgm, and with the small power required, they can be battery-mounted for fail-safe operation.
The following factors must be considered when selecting the right valve actuator type and size:
Drive energy
Commonly used driving energy is power supply or fluid source, if the power supply is selected as the driving energy, the three-phase power supply is generally selected for large-size valves, and the single-phase power supply can be selected for small-size valves. Generally, electric actuators can be selected from a variety of power supply types. In some cases, DC power supply is also an option, in which case fail-safe operation of the power supply can be achieved by installing a battery. There are many types of fluid sources, first of all, they can be different media such as: compressed air, nitrogen, natural gas, hydraulic fluids, etc., secondly, they can have various pressures, and thirdly, actuators have various sizes to provide output force and torque.
Valve type
When choosing an actuator for valves, it is important to know the type of valve so that you can choose the correct type of actuator. Some valves require multi-turn drive, some require single-turn drive, and some require reciprocating drive, which affects the choice of actuator type. Generally, multi-turn pneumatic actuators are more expensive than electric multi-turn actuators, but reciprocating linear output pneumatic actuators are cheaper than electric multi-turn actuators.
Moment magnitude
For 90-degree rotary valves such as: ball valves, disc valves, plug valves, it is good to obtain the corresponding valve torque through the valve manufacturer, most of the valve manufacturers are required to test the operating torque required by the valve at the rated pressure, and they will provide this torque to the customer. For multi-turn valves, these valves can be divided into: reciprocating (lifting) motion - stem does not rotate, reciprocating motion - stem rotation, non-reciprocating - stem rotation, the diameter of the stem must be measured, and the size of the stem connection thread has determined the actuator specification.
Actuator selection
Once the actuator type and the required actuating torque of the valve have been determined, the actuator manufacturer can be selected using the datasheet or selection software provided by the actuator manufacturer. Sometimes the speed and frequency of valve operation also need to be considered.
Fluid-driven actuators regulate the travel speed, but electric actuators for three-phase power only have a fixed travel time.
Some small specifications of DC electric single-turn actuators can adjust the stroke speed.
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