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Control
Valve "Fail-Safe" Positions
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| Cause
of Fail-Safe Condition: Loss of Air Pressure |
A.
LINEAR SPRING/DIAPHRAGM ACTUATORS. Used with sliding
stem control valves: i.e. globe-style valves. Can be accomplished two ways:
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1. Fixed seat ring/plug
orientation. Springs are interchanged to either above or below actuator
diaphragm.
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2. Fixed spring orientation.
Plug and seat ring positions are reversed relative to each other. In the
Fail Open design, plug travel is above the valve seat. In the Fail Closed
design, plug travel is below the seat.
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Rotary
Spring/Diaphragm Actuators
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| Used with rotary control valves; i.e. butterfly,
eccentric plug. Reversing the fail mode for this type of valve is normally
accomplished by reversing the location of lever arm and plug. In order
to maintain consistency, ATO-FC action will be considered as "Reverse"
action for rotary or sliding-stem control valves. |
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Actuators
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Feature Comparison
| Spring
and Diaphragm |
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| Advantages |
Disadvantages |
| Lowest Cost |
Limited Output Capability |
| Can Throttle Without
a Positioner |
Large Size and Weight |
| Simplicity |
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| Inherent Fail-Safe Action |
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| Low Supply Pressure Required |
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| Adjustability |
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| Easily Maintained |
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| Pneumatic
Piston |
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| Advantages |
Disadvantages |
| High Torque Capability |
Fail-Safe Requires Accessories
or Addition of Spring |
| Compact |
Positioner Required for
Throttling |
| Lightweight |
Higher Cost |
| Adaptable to High Ambient
Temperature |
High Supply Pressure
Required |
| Fast Stroking Speed |
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| Relatively High Actuator
Stiffness |
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| Electric
Motor |
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| Advantages |
Disadvantages |
| Compactness |
High Cost |
| Very High Stiffness |
Lack of Fail-Safe Action |
| High Output Capability |
Limited Duty Cycle |
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Slow Stroking Speed |
| Electrohydraulic |
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| Advantages |
Disadvantages |
| High Output Capability |
High Cost |
| High Actuator Stiffness |
Complexity and Maintenance
Difficulty |
| Excellent Throttling
Ability |
Large Size and Weight |
| Fast Stroking Speed |
Fail-Safe Action Only
With Accessories |
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| Control
Valve ATO-FC Action |
| Control
Valve ATC-FO Action |
| Benchset
Range - ATO-FC |
| Benchset
Range - ATC-FO |
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Control Valve Concepts
/ Positioners / Positioner
and Controller Operating Modes / Flow
Characteristics / Seat Leakage
/ Packing / Helpful
Hints / Cashco Terminology /
Advanced Topics
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