【Weekly Question 20090511】On the selection between electric actuators and pneumatic actuators (generous prizes await you)
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This post was last edited by baodingzhai on 2009-5-15 at 17:01. Control valves can be found everywhere in factories, and the quality of their use directly affects the quality of the products. Let’s discuss the principles for choosing between electric and pneumatic actuators when selecting control valves, as well as the calculation methods involved. For experienced engineers, this is an opportunity to reinforce and deepen their knowledge; for beginners, it’s a chance to learn, make progress, and draw conclusions. I hope everyone will actively participate in the discussion to arrive at correct and comprehensive answers! 1. Main considerations for selecting an actuator: The main considerations for choosing an actuator are: ① Reliability ； ②Economy ； ③Smooth movement and sufficient output torque ； ④Simple structure and easy maintenance. 2. Comparison of selection between electric actuators and pneumatic actuators: (1) Pneumatic actuators are simple and reliable ； The poor reliability of old-fashioned electric actuators has been a persistent weakness of them in the past; however, the development of electronic actuators in the 1990s completely resolved this issue, allowing them to operate without maintenance for 5 to 10 years, and their reliability even exceeds that of pneumatic actuators. (2) Power source: The biggest drawback of pneumatic actuators is the need to install a separate air supply station, which increases costs ； The power source for electric valves is available everywhere. (3) In terms of price, pneumatic actuators require a valve positioner as well as a gas supply, and their cost is comparable to that of electric valves (imported electrical valve positioners are roughly as expensive as imported electronic actuators) ； Domestic positioners are on par with domestic electric actuators). (4) Thrust and stiffness: they are comparable. (5) Fire and explosion protection: “Pneumatic actuator + electrical valve positioner” is slightly better than electric actuators. 3. Recommendations: (1) Where possible, it is recommended to use imported electronic actuators in combination with domestic valves for applications involving localization or new projects. (2) Although film actuators have drawbacks such as insufficient thrust, low stiffness, and large size, their simple structure makes them the most widely used actuators at present. However, what we emphasize here is that it is preferable to use compact membrane actuators of types ZHA and ZHB to replace the older membrane actuators of types ZMA and ZMB, in order to achieve a lower weight, smaller size, and greater output force. (3) Considerations for selecting a piston actuator: ① When the thrust of a pneumatic diaphragm actuator is insufficient, a piston actuator should be used to increase the output force ； For control valves with large pressure differences (such as medium-pressure steam cut-off valves), when DN ≥ 200, a double-piston actuator should even be selected ； ②For ordinary control valves, a piston actuator can also be used in place of a diaphragm actuator, thereby reducing the size of the actuator. From this perspective, pneumatic piston control valves will be used more frequently ； ③For diagonal stroke control valves, their angular stroke actuators typically feature a dual-piston gear-rack rotary design. It is worth emphasizing the traditional approach of \"linear piston actuator + angle iron + crankshaft and connecting rods\".
In terms of reliability, cost-effectiveness (without considering the cost of gas supply), simple structure, and maintenance, pneumatic mechanisms have considerable advantages
In explosion-proof areas, pneumatic actuators combined with electrical valve positioners are generally used; for ordinary control valves, mechanical positioners are much cheaper! Electric actuators are not convenient to calibrate on-site! In terms of power supply, electrical valve positioners generally operate on 24V, which is a voltage commonly used in instrumentation; they can share the same cable tray with field transmitters and other devices! Electric actuators are generally powered by 220V, and it is necessary to run cables from a 24V power source!
In certain special situations, electric actuators cannot replace pneumatic control valves, in terms of response speed, the pace of operation, and stability. A simple electric bicycle with adjustable settings will also do.
A lot has been said above; as a supplementary point: Pneumatic actuators are generally used in petrochemical plants because the air supply is easy to obtain, and they are also cheaper in terms of price, among other advantages. However, there are also situations where electric actuators are used, mainly because an air supply cannot always be provided, such as in some tank farms and storage and transportation systems. In some applications, pneumatic actuators are essential because they can keep the valve in a safe state in the event of a gas supply failure, whereas it is more difficult for electric actuators to achieve this. Another advantage of electric actuators is that, thanks to the possibility of bus connection, it is easier to diagnose the actuators, and data collection is more comprehensive, which facilitates the acquisition and management of information.
Our rubber plant replaced more than 20 cut-off valves with Rotork electric actuators of the bus-type; they kept breaking down, and the operators didn’t know how to deal with those problems. In the end, cylinder-type actuators were used instead, which only led to further complications.
Electric actuators are generally used in places such as power plants where it is inconvenient to obtain a gas supply, and their reliability is not comparable to that of pneumatic actuators.
If it’s a single application, I recommend going with the electric version; if an air source needs to be added, it will result in additional costs； For centralized applications, it’s better to go with pneumatic systems. That’s what we use here – pneumatic systems are cost-effective and practical, easy to maintain, and less prone to problems!
With the development of electronic technology, the advantages of electric actuators have become increasingly evident. Excluding cost considerations, electric actuators are the better choice; once properly installed and calibrated, they operate stably. Moreover, they meet the requirements of modern bus systems better, offering simple calibration and powerful functions. If cost savings are a concern, domestic valves can be used in combination with imported actuators. Lottock’s electric actuators are also quite good.
Chemical plants have high explosion-proof requirements, so pneumatic systems must be used
I think pneumatic drive is slightly better than electric drive. 1. Pneumatic systems are cost-effective, as companies already have compressed air pipelines; compressed air can be used as the power source, eliminating the need for additional investment. 2. Pneumatic valves are safer than electric ones, and this advantage is even more evident in applications where explosion prevention is required. 3. Pneumatic valves are relatively stable and less affected by external factors