The thermal response of a pressurised storage vessel and its contents to simulated jet fire impingement.
The storage of pressure liquefied gas in vessels is subject to various regulations and
codes of practice. For example, Liquefied Petroleum Gas (LPG), a commercially relevant
product, is subject to Health and Safety Executive Guidelines regarding cylinder/tank
arrangements and spacing. In the event of an incident involving fire, the internal
pressure and shell temperature of an LPG vessel will rise, and the weakening of steel
at elevated temperatures can result in the structural failure of the shell. This can be
avoided by the fitting of pressure relief valves, which vent material at a pre-set pressure.
However, an ignited release can create a high velocity jet flame which, because of
significant radiative and convective components, can generate intense, localised heat
loads on neighbouring vessels or pipe-work. However, existing codes of practice have
no special provision for the possibility of jet fire incidents.
Owing to a lack of detailed information on the thermal response of a LPG vessel
exposed to jet flame impingement, a series of laboratory scale tests with simulated,
localised jet fire impingement on the exterior shell of a pressure vessel was required.
The thermal response and the effects of key parameters, Le. fill level, magnitude of
heated zone (Le. size and intensity) and position of simulated impingement, could then
be examined for the part-validation of a suitable computer model. In addition, these
studies could be used to interpret the results from concurrent full scale jet fire
An appropriate pressure vessel was constructed to standard design codes, which
incorporated a vent line and dump tank. A suitable LPG substitute was selected. Results
from the studies indicated that mixing, and therefore thermal stratification, was highly
dependent on the size of the heated zone and its position in relation to the liquid/vapour
interface. High Speed Micro-Cinematography was successfully employed to film
individual bubble streams within the vessel and to measure individual bubble sizes and
velocities for various experimental configurations. Studies were also made on the
venting characteristics. Sudden pressure relief caused severe agitation of the liquid
phase and the breakdown of thermal stratification. In addition, swelling and aerosol
generation through homogenous boiling within the liquid phase was observed.
Comparisons with the nodal computer model revealed that the use of only single vapour
and liquid nodes was a poor approximation to the detail observed in the small scale
studies, where the incident heat flux was relatively low and the simulated region of
impingement was highly localised. However, the bulk liquid and vapour temperatures
and the pressure response up to the time of venting was generally well predicted. As
the degree of engulfment increased the model became a better approximation.
Although the full scale trials employed an almost fully engulfing jet flame rather than
point source impingement, comparisons have allowed understanding of the liquid and
vapour thermal gradients, and the subsequent breakdown of these during venting.