Fail Open

Overview

Engineering reference data for Fail Open in miscellaneous.

Key Formulas

Unit Conversion

$y = x \cdot k$

Multiply by conversion factor.

Linear Interpolation

$y = y_1 + \frac{(x - x_1)(y_2 - y_1)}{x_2 - x_1}$

Estimate between two known points.

Percentage

$p = \frac{\text{part}}{\text{whole}} \times 100\%$

Part as fraction of whole.

Variables

Common Applications

Fail-open (FO) valves are commonly employed in processes where the safe state upon a loss of signal or power is an open flow path. Typical applications include:

• Cooling Water Systems: Ensures coolant continues to flow to critical equipment (e.g., engines, heat exchangers) during a control system failure to prevent overheating.
• Venting and Relief: Used on vessels or systems that require pressure relief or ventilation if control is lost, avoiding dangerous overpressure conditions.
• Bypass Lines: Allows process flow to maintain through a bypass circuit if the main control valve fails.
• Safety Interlock Systems: As part of a broader safety system, an FO valve can be part of a final element that moves to a pre-determined safe state upon an emergency shutdown signal.

Advantages and Considerations

Key Advantages:

• Process Safety: Directly aligns the valve's failure mode with a process safety requirement, often as a simple, passive safety measure.
• Simplicity: For pneumatic actuators, this is often the default, spring-return configuration, making it a straightforward and reliable design.
• Redundancy: Can be used in conjunction with other safety instrumented functions to achieve a higher overall safety integrity level (SIL).

Important Considerations:

• Process Impact: The open position upon failure may lead to uncontrolled flow, which could be wasteful or create secondary issues if not accounted for in the process design.
• Actuator Selection: The fail-safe state is primarily a function of the actuator and its springs, not the valve body itself. The same valve body can be configured for FO (Fail-Open), FC (Fail-Closed), or FL (Fail-Last) with different actuators.
• Signal Type: While the principle is the same, implementation details differ between pneumatic (spring-return), electric (spring-return or battery-backed), and hydraulic systems.

References

• Original Source