ASET and other fire models shortcomings when modelling a rack fire. Models show ESFR sprinkler activation after 600 seconds or so whereas all tests carried out by FM Global, Tyco, Viking etc. show activation time after less than 120 seconds, usually around 60-90 seconds. Can these problems be overcome?
- The model assumes a single source at floor level whereas in fact as the fire grows the distance between the source and the sensing elements decreases as the fire moves vertically upwards and is not stationary at ground floor level just releasing more energy.
- The T2 heat release rate formula is not correct as the heat release rate from each tier is additive whereas the model assumes that there is a single heat release source
- Once activated the sprinkler starts as a control mode device i.e. it stops any further growth, once that is achieved it changes to a suppression device i.e. causes the heat release rate to reduce. The model assumes that the fire stops growing but that the heat release rate stays constant i.e. the system has reached steady state and the sprinklers keep it in that state
- Once smoke is cooled the density changes as does its capacity to absorb water vapour, most models do not account for this at all i.e. even if the mass flow of smoke is unchanged the volume is reduced as the density of colder smoke is substantially higher. Once the density of smoke is greater than that of air the smoke will start descending/dropping