Normally Open
Reference data and engineering information about normally open for miscellaneous applications.
normallyopen
Overview
Engineering reference data for Normally Open in miscellaneous.
Key Formulas
Unit Conversion
Multiply by conversion factor.
Linear Interpolation
Estimate between two known points.
Percentage
Part as fraction of whole.
Variables
| Symbol | Description | Unit |
|---|---|---|
| Input value | — | |
| Output value | — | |
| 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.