HangZhou Hoyee trade Co.,Ltd

WELL DONE OR NOTHING

Home > Industry news > Content
Pneumatic Valve Actuators in Sub-Arctic Climates the solution
- Jul 10, 2018 -

Failure of the example standard actuator is a certainty in this case. While there is nothing in the above example that is not obvious to every designer and user, the solution to avoid failure is to apply what we know.

               fig3                                   fig4    




                  fig5                               fig6    

Metals

For example, metals that have a brittle transition temperature that falls within the range of possible application temperatures should not be used unless absolutely no impact loads can occur. Examples of suitable metals are 300 series stainless steel and aluminum—neither has brittle transition temperature. Because of its greater strength, stainless steel may be the best choice for larger actuators.

Figures 3 and 4 show a simplistic representative impact test performed on a notched steel bar. One end was locked in a vice, and a hammer blow served to provide an impact. At room temperature, the hammer blow bent the specimen, but there was no fracture.

Figures 5 and 6 show an identical specimen that was brought to a temperature of –60° F. (An interesting side note is that the air cans used to clean a keyboard, when turned over, emit a liquid that has a measured temperature of –60° F, which proved convenient for testing.)

vmsum12_actuators_fig7.png

             fig7   

Material Compatibilities

If at all possible, where there is close fit between moving parts, select materials having the same coefficients of thermal expansion/contraction (Figure 7).

Seals

vmsum12_actuators_fig8Select seal materials that retain adequate resiliency at the lowest temperatures to be encountered. Additionally, eliminate every possible seal via basic design. A seal that is removed by design cannot fail. Figure 8 shows a simple drop test where a bar is dropped against an O-ring at both room temperature and at –60° F. Basic buna, ethylene propylene diene monomer (EPDM), Viton and silicone O-rings were tested. All except Viton showed excellent “bounce” at room temperature but only silicone retained resiliency and bounce at –60° F. Suppliers show low-temperature options for each of these elastomers, but in-house testing is recommended.

Designers and users who read this may respond: “We’ve used essentially standard actuators and all we did was select a seal material that remained resilient at the low temperatures.” This article is not stating such a combination will fail, only that it may fail—the steel plates of the Titanic may not have fractured if it had not hit the iceberg, and not all World War II Liberty ships cracked in half. But by considering the above suggestions, the supplier can greatly reduce the risk of failure.


USERS

Users, as well as designers, have responsibilities regarding pneumatic actuators under extreme cold conditions.

First, and most obvious, users should shelter the actuator from weather extremes where possible. ­Second, users must assure a dry air supply, at least 15° F (–9° C) below the lowest temperature that may be ­experienced since ice plays havoc with air flow and mechanical motion. ­Finally, users should assess the recommended actuator and whether all possible precautions have been incorp­orated by the supplier.

Clearly, pneumatic actuators can perform their intended functions despite having to operate in extreme temperatures. However, they need to be designed and manufactured ­properly, and users need to take ­responsibility to keep them functioning correctly.