Spring return actuators are not inherently cost effective, as actuators must be sized to overcome the spring tension and to the required torque of the valve and system. This requires the actuator to be over sized which increases the cost of the actuator. Add the spring assembly to these costs and you now have a price that can be 30 to 80 percent higher than the standard double-acting actuator assembly.
Springs are also subject to corrosion from atmospheric conditions and degradation from constant compression. Industry experts and end users are aware of an alternative to the use of springs to accomplish the fail-safe mode of operation. This is accomplished with the use of external air reservoirs to store compressed air needed to supply the required force to fail the valve to the fail-safe position. Applications for such fail-safe setup can be observed in applications for actuating large emergency shutdown valves.
New generation actuators
New generation actuators need a cost effective and space conservative design to supply enough pressurized operating medium to perform the fail-safe action, without the use of springs or external reservoirs. The ultimate design would be to integrate the reservoir into the pneumatic actuator housing to provide the necessary stored energy (Figure 1).
With the proper pilot assembly, the reservoir would be constantly pressurized and available to
perform the fail-safe operation during a power failure or a catastrophic air failure.These features have been designed, built,and field tested as seen in (Figure 2)
This reservoir is internal to the actuator and is sized to allow for the fail-safe operation. With the reservoir internal to the actuator, springs are not required, therefore reducing the size, weight and footprint of the actuator (Figure 3)
The fail-safe operation is accomplished by utilizing a properly ported solenoid valve or pilot assembly. Both of these will pressurize the reservoir during normal operation with full operating pressure. At the loss of power or catastrophic air loss, enough air is maintained in the reservoir to fail the valve in the pre-selected fail position.
The introduction of a pneumatic fail-safe actuator without the use of springs opens up new possibilities for pneumatic
actuator designs. By unshackling from the mechanical restriction and space requirements necessary for springs, actuator designers can now trade the space once reserved for springs for more user driven designs such as friendlier valve adaptation, simplified inventory, tighter repeatability on control valves, and air consumption efficiency designs.
For example, actuator designers can utilize the additional real estate from the internal air reservoir to create more real estate space for valve mounting.
Traditionally, actuators can only accommodate two ISO patterns on their housing bodies, but by manipulating the space created by the air reservoir, some manufacturers have been able to add three or more flange patterns per model size for mounting to valves. This addition leads to a better valve adaptation experience, reduced valve adaptation hardware cost, and reduced model size requirements.
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