Component Failures
Component Failures
A component (a pump, valve, cabinet, turbine or other piece of equipment) fails when the temperature it is exposed to rises above its failure temperature. FRI3D checks every component that is a target in the scenario against the simulation results once the run has finished, and records the first time the temperature is exceeded.
Components assigned directly to a fire source are the exception. They fail at time zero, whatever the simulation shows; see Assigning Equipment and Raceways to a Fire Source.
Failure temperature
| Where it comes from | When it is used |
|---|---|
| The component's Failure Criteria temperature | When the component has a failure criteria with a temperature set |
| The scenario's default component failure temperature | Otherwise. It is 205 °C unless changed. |
205 °C is the NUREG/CR-6850 (Appendix H) generic damage temperature for thermoplastic cable, which also governs most equipment, since equipment fails when its cables or controls do. Use a failure criteria to set a different value for a component. For example, thermoset cable is 330 °C in NUREG/CR-6850, and the NRC's screening value for sensitive electronics (Inspection Manual Chapter 0609, Appendix F) is about 65 °C.
The component fails at the first output time at which the temperature is strictly above the failure temperature. The failure method is reported as Max Temp.
Which temperature is compared
NUREG/CR-6850's damage temperatures are exposure temperatures: the temperature of the hot gas the item sits in, not the temperature of its own surface. FRI3D follows that, and adds the surface temperature only where the component's thermal behaviour has actually been modelled.
CFAST scenarios
Each component is a CFAST target. FRI3D compares the gas temperature around the target
(CFAST's TRGGAST column) with the failure temperature. CFAST's target surface temperature is not
used: CFAST models the target as a plate, and the plate's surface can lag the surrounding gas by
100 °C or more.
FDS scenarios
FRI3D places three FDS devices on each component, on the face of the component closest to the fire:
| Device | Measures | Placement |
|---|---|---|
<name>_TEMP | Gas temperature | Off the face, along its outward normal, by the larger of 0.3 m and 1.5 times the grid cell size |
<name>_WALLTEMP | Wall (surface) temperature of the face | A box around the face, sized from the FDS grid at the component so that it always contains at least one wall cell |
<name>_HEATFLUX | Gauge heat flux onto the face | The centre of the first gas cell off the face, looking outward |
Which of these decides failure depends on whether the component has a material:
| Component's material | Temperature compared with the failure temperature |
|---|---|
| A material of its own | The higher of the gas temperature and the wall temperature |
| None, or the Default material | The gas temperature only |
The wall temperature is only as realistic as the material behind it. In FDS, a component with no material is built from the Default material, which conducts heat like insulation board. Its surface heats far faster than steel equipment would. In one turbine-hall test, a turbine casing with no material would have "failed" after 36 s on that surface, while the gas around it stayed below 205 °C. So FRI3D ignores the wall temperature unless you have given the component a real material.
To use the wall temperature for a component, select it and set Material in its Object Properties to a material that describes it, for example steel for a pump casing or a cabinet. No other setting is needed.
The wall temperature is also ignored, and the gas temperature used, when FDS could not measure it. This happens when the component's face is buried inside another object or is thinner than one grid cell. FRI3D lists each such component in the model check after the run, so you can refine the grid or move the component.
The heat flux is recorded for every FDS component, but it is not yet used to fail components. NUREG/CR-6850 gives flux thresholds alongside the temperatures (6 kW/m² for thermoplastic and 11 kW/m² for thermoset cable). Using them for components is planned.
What's next?
- Raceway/Tray Failures: how raceways and the cables in them fail.
- FDS and CFAST: the simulations that produce the temperatures used here.