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Pneumatic Valve Actuators in Sub-Arctic Climates what can happen
- Jul 09, 2018 -

There is never a good time for an actuator to fail, but a failure when the temperature is at 60 below or when a blizzard limits capacity for transporting repair parts could be especially troublesome. Careful selection of valve actuation is critical.

vmsum12_actuators_fig1.png

When considering safety integrity, one assesses what might cause failure in a product. To illustrate, let’s begin with an off-the-shelf pneumatic actuator intended for normal service and then consider what may fail if applied at –60° F (–51° C). Our basic parameters are as follows:

  • The actuator housing is steel.

  • The drive shaft is a precipitation-hardened stainless steel.

  • The seals are buna.

  • The shaft bushings are Nylon 6.

With these parameters in mind, at –60° F (–51° C):

The steel housing has become brittle. It is not necessarily weaker, but a sudden impact or an imperfection that would have no effect on the metal when ductile, could result in a sudden fracture at these sub-zero temperatures because the temperature is below the brittle transition temperature of steel. The question “will it fail at –60° F?” cannot be answered without knowing if there will be sudden impact loading, but possibility of failure has increased there.

vmsum12_actuators_fig2.png

A normally ductile metal will yield before fracture while a brittle metal will fracture without yield (Figure 2).

The precipitation-hardened shaft material has also become brittle and may fracture given an impact load. If, for example, the driven valve resists opening and then breaks free, the resulting sudden impact may cause the actuator shaft to fail.

The shaft material will have contracted at a rate of 9.4 by 10–6 per inch of diameter per degree F, while the Nylon 6 bushings will contract at a rate of 44.2 x 10–6 per inch per degree F. If the original gap between the bushing bore and the shaft surface was .002 inches and if the shaft diameter was 3 inches, then 130° F (54° C) temperature change from 70° F (21° C) to –60° F (–51° C) would contract the shaft diameter to 2.996 inches and the 3.002 inches diameter bushing bore to 2.985 inches, causing binding and ­failure.

The buna seals have turned to stone. Their normal resiliency, which allows flowing into and sealing leak paths, is gone. The seals cannot flex and leakage will occur at all seal points resulting again in actuator failure.