Fail Last

Overview

Engineering reference data for Fail Last 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

Fail-Last vs Other Failure Modes

When Fail-Last is Used

Fail-Last (FL) designation is typically applied in scenarios where:

• Process continuity is critical and sudden valve movement could cause damage
• Thermal shock must be avoided in heat exchange systems
• Pressure stability needs to be maintained during signal interruption
• Gradual shutdown procedures are preferred over emergency closure

Signal Types and Fail-Last Behavior

For pneumatic actuators: FL typically requires a locking relay or volume tank to trap air pressure in the actuator.

For electric actuators: FL is achieved by maintaining the last position command without requiring continuous power.

For hydraulic actuators: Check valves or pilot-operated locks maintain position when hydraulic pressure is lost.

Safety Considerations

⚠️ Fail-Last valves may not meet safety requirements for processes where continued flow during signal failure presents a hazard. Always conduct a Failure Mode and Effects Analysis (FMEA) before specifying FL valves in safety-critical applications.

References

• Original Source