Answer / carl_ellis

Similar to Valtek's fail-safe "air spring":

Where service conditions exceed the capabilities of the
standard fail-safe spring to drive the valve to its failure
position, and where specially designed, extra-strong failure
springs may be both mechanically and economically
unfeasible, air spring fail-safe systems on Valtek control
valves provide the thrust necessary to drive the plug to its
failure position. An air spring provides a pressurized
volume of air to drive the actuator piston in the failure
direction.

The volume of air is sometimes provided within the actuator
itself, or where the cylinder volume is insufficient, a
separate external volume tank is provided. Air spring
systems are used primarily to close valves upon air failure.
And sometimes they must open valves upon air failure. A
fail-closed Valtek valve is customarily operated with the
flow direction over the plug. Thus, with the plug on the
seat, the upstream pressure acts to hold the valve closed.

Fail-open Valtek valves customarily operate with the flow
direction under the plug. Thus, when a general system
failure occurs, the upstream pressure will keep the plug off
the seat and the valve open.

Air springs on Valtek valves are practical because the
locked-up air is used only at the instant of air failure to
drive the valve to the fail position. Line pressure will
insure that the valve stays either closed or open.

Occasionally, service conditions require that the valve
remain in the last operating position upon loss of air
supply. For such applications, Valtek valves can be
equipped with a fail-in-place lock-up system. If air
failure occurs, the system activates two pilot-operated
lock-up valves that trip and lock the existing cylinder
pressures on both sides of the piston, thus maintaining the
last throttling position.

Carl Ellis
Measure First

Answer / ayaz

Sdv or bdv work on demand of emergency, so we use the fail safe position
1-when any electrical or pneumatic supply fail the valve will be go on safe position
2- when use double action means two side supply produce not a spring to close.

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