27.7. VALVE FAILURE MODE |
2131 |
A hybrid of hand and pneumatic valve actuation is seen on this Valtek brand control valve, where the control valve assembly is actuated by a pneumatic piston actuator, but is also equipped with a manually-operated “handwheel”:
A handwheel mechanism may be used to override the pneumatic actuator simply by overpowering it in either direction (i.e. providing a greater force on the valve stem than the piston actuator provides) or it may be left in a neutral position to allow the pneumatic actuator full control over valve stem position. Handwheels may be used to override the control valve’s pneumatic actuator to either the full-open or full-closed positions, or it may simply be used to assert a highor low-limit “stop” to the valve stem to prohibit stem motion beyond a certain position.
Note the “lockout” tab flipped to the horizontal position near the handwheel, located between two of the handwheel’s spokes. This simple mechanism permits the handwheel to be locked out to prevent accidental turning.
27.7Valve failure mode
An important design parameter of a control valve is the position it will “fail” to if it loses motive power. For electrically actuated valves, this is typically the last position the valve was in before loss of electric power. For pneumatic and hydraulic actuated valves, the option exists of having a large spring provide a known “fail-safe” position (either open or closed) in the event of fluid pressure (pneumatic air pressure or hydraulic oil pressure) loss.
2132 |
CHAPTER 27. CONTROL VALVES |
27.7.1Direct/reverse actions
The fail-safe mode of a pneumatic/spring valve is a function of both the actuator’s action and the valve body’s action. For sliding-stem valves, a direct-acting actuator pushes down on the stem with increasing pressure while a reverse-acting actuator pulls up on the stem with increasing pressure. Sliding-stem valve bodies are classified as direct-acting if they open up when the stem is lifted, and classified as reverse-acting if they shut o (close) when the stem is lifted. Thus, a sliding-stem, pneumatically actuated control valve may be made air-to-open or air-to-close simply by matching the appropriate actuator and body types.
The most common combinations mix a direct-acting valve body with either a reverseor directacting valve actuator, as shown in this illustration:
Air in
(vent)
Air in |
open
(vent) |
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Reverse-acting |
Direct-acting |
actuator |
actuator |
Air-to-Open (ATO) |
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Fail-Closed (FC) |
open |
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Air-to-Close (ATC) |
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Fail-Open (FO) |
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Direct-acting gate valve body |
Direct-acting gate valve body |
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27.7. VALVE FAILURE MODE |
2133 |
Reverse-acting valve bodies may also be used, with opposite results:
Air in
(vent)
Air in |
close
(vent) |
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Reverse-acting |
Direct-acting |
actuator |
actuator |
Air-to-Close (ATC) |
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Fail-Open (FO) |
close |
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Air-to-Open (ATO) |
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Fail-Closed (FC) |
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Reverse-acting gate valve body |
Reverse-acting gate valve body |
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The reverse-acting gate valve body shown in the left-hand illustration is open, with fluid flowing around the stem while the wide plug sits well below the seat area. Reverse-acting valve bodies tend to be more complex in construction than direct-acting valve bodies, and so they are less common in control valve applications. An interesting exception to this trend – although not technically a control valve but rather a self-actuated device – is the Fisher model 1098EGR pilot-operated pressure regulator which uses a reverse-acting valve body to throttle the flow of gas through it.
2134 |
CHAPTER 27. CONTROL VALVES |
27.7.2Available failure modes
Valve fail mode may be shown in instrument diagrams by either an arrow pointing in the direction of failure (assuming a direct-acting valve body where stem motion toward the body closes and stem motion away from the body opens the valve trim) and/or the abbreviations “FC” (fail closed) and “FO” (fail open). Other failure modes are possible, as indicated by this set of valve symbols:
Fail open
(or)
FO
Fail locked
(or)
FL
Fail last/drift open
(or)
FL/DO
Fail closed
(or)
FC
Fail indeterminate
Fail last/drift closed
(or)
FL/DC
In order for a pneumatic or hydraulic valve to fail in the locked state, an external device must trap fluid pressure in the actuator’s diaphragm or piston chamber in the event of supply pressure loss.
Valves that fail in place and drift in a particular direction are usually actuated by double-acting pneumatic piston actuators. These actuators do not use a spring to provide a definite fail mode, but rather use air pressure both to open and to close the valve. In the event of an air pressure loss, the actuator will neither be able to open nor close the valve, and so it will tend to remain in position. If the valve is of the globe design with unbalanced trim, forces exerted on the valve plug will move it in one direction (causing drift).