Normally Open

Overview

Engineering reference data for Normally 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

Symbol	Description	Unit
$x$	Input value	—
$y$	Output value	—
$k$	Conversion factor	—

Comparison: NO vs. FO

Understanding the distinction between Normally Open (NO) and Fail Open (FO) is critical for safe system design.

Feature	Normally Open (NO)	Fail Open (FO)
Applies to	Manual valves or dampers	Automatic control valves
Normal State	Open	Depends on control signal
Failure Behavior	Not defined (assumed to stay in last position)	Moves to open position
Signal Types	N/A (manual)	Pneumatic, hydraulic, or electric
Primary Concern	Operator must ensure it's open	Safety on signal/power loss

Practical Considerations

• Labeling: Mislabeling a valve as NO when it should be FO is a common and dangerous error. Always verify the valve's intended failure state during design.
• System Impact: A control valve failing closed (FC) when it should fail open (FO) can lead to pressure buildup, overheating, or system blockage. Conversely, an FO valve in a critical isolation service could cause unintended flow during a failure.
• Redundancy: In high-safety systems, a normally open (NO) manual valve might be placed in series or parallel with an automatic FO control valve to provide a mechanical backup.

References

• Original Source