Gavelli_edit.qxp 28/10/08 1:52 pm Page 76
Modelling Pool Fire Hazards from Large-scale Liquefied Natural Gas Spills
a report by
Filippo Gavelli, Melissa K Chernovsky and Harri K Kytomaa
HSE – Fire Protection
Exponent Inc., Bowie, Maryland
Liquefied Natural Gas Pool Fires The regulations also specify a single tool, LNGFIRE3,
3
as acceptable
In recent years, hazards associated with the transport, storage or for performing thermal heat flux calculations, although other models
regasification of liquefied natural gas (LNG) have been the subject of may be considered if allowed by the authority having jurisdiction. In
considerable interest among regulatory agencies, emergency the case of offshore terminals, which are not subject to the same
responders and the general public as a result of the large number of regulations, thermal heat flux calculations are performed following
applications to site, build and operate LNG receiving terminals. The the approach outlined by Sandia.
4
Both LNGFIRE3 and Sandia’s
public’s concerns are focused primarily on the potential consequences thermal heat flux calculations are based on the ‘solid flame’ theory,
5
from large LNG spills as a result of accidents or intentional (i.e. terrorist) where the pool fire is modelled as a cylinder whose surface emits
acts. In the event of a large LNG release on land (e.g. within an onshore thermal radiation at a specified constant rate. This model determines
receiving terminal) or over water (e.g. from a breached LNG carrier the dimensions of the solid flame cylinder (base and height)
tank), the sequence of events would be approximately as follows: according to the diameter of the burning liquid pool and other
parameters. The cylinder is also assumed to be tilted in the direction
• the LNG spill forms a liquid pool, which will spread (if of the wind, and the angle of tilt is dependent on the wind speed.
unconfined); The heat flux from the fire is assumed to radiate uniformly from the
• heat transfer from the pool substrate (e.g. earth or water) causes surface of the cylinder. The thermal heat flux from the solid flame
LNG to evaporate, forming a cold, dense vapour cloud; cylinder to a target is determined from the following factors, which
• if a viable ignition source in the proximity of the pool comes into are also shown in Figure 1:
contact with the flammable vapour cloud, the cloud will ignite
and form a pool fire; or • the heat flux emitted by the fire surface, which is assumed to be
• if ignition does not occur in the proximity of the pool, the LNG uniform over the entire surface and is referred to as the ‘surface
vapour cloud will be dispersed by the wind until: emissive power’ (SEP) of the fire;
• it dissipates to below flammable concentrations; or • the fraction of the heat flux emitted by the fire that is not
• while still in the flammable range it encounters a viable absorbed or scattered by atmospheric gases and particulate,
ignition source remote from the pool, causing a flash fire with which is expressed as the ‘transmissivity’ (τ) of the atmosphere;
the potential to burn back to the pool. and
• the position (distance, direction and orientation) of the target
The focus of this analysis is on large LNG pool fires, and the goal is to relative to the fire, also known as the ‘view factor’ (VF).
review the current understanding and state of the art in estimating
the consequences to the public from a pool fire. Of particular interest Thus, the thermal heat flux from a solid flame cylinder to a target at
are LNG spill scenarios of 100m diameter or greater and their distance x is given by:
associated pool fires. Predicted hazard distances vary significantly
.
depending on the parameters selected for the calculation. This Q
”
(x) = SEP
.
τ(x)
.
VF(x, L) (1)
variation and its implications are the focus of this paper.
.
where Q
”
(x) is the thermal heat flux per unit area to a target at
Liquefied Natural Gas Pool Fire Models distance x from the centre of the pool fire, SEP is the surface
In the US, both federal regulations for onshore LNG terminals (49 emissivity of the fire, τ(x) is the atmospheric transmissivity to thermal
Code of Federal Regulations [CFR] 193) and the industry consensus radiation over a path length x, VF is the view factor and L is the pool
standard (National Fire Protection Association [NFPA] 59A) are fire flame height.
prescriptive in their requirements and specify both the LNG spill
scenario to be considered and the threshold thermal heat fluxes The thermal heat flux calculated according to Equation 1 decreases
from those scenarios to the public or public property. For example, as the distance of the target increases, as shown in Figure 2. For the
the maximum thermal heat flux allowed at a property line that can purpose of the hazard analysis, the target orientation is taken into
be built upon is equal to 5kW/m
2
.
1
The same heat flux level is also account by combining the VFs to a vertical and a horizontal target.
defined as the permissible energy level for personnel performing
emergency operations lasting for several minutes with appropriate As stated above, the focus of this article is on the prediction of
clothing.
2
The appropriateness of this threshold has been the subject thermal heat fluxes from large LNG pool fires, in the order of 100m
of intense debate over the last few years. However, 5kW/m
2
remains diameter or greater. LNG pools of this size are less likely to form on
the regulatory prescribed threshold for the above-mentioned land than over water, where the LNG pool may spread mostly
configuration and, as such, is utilised here. unconfined (or, at most, confined on one side by the vessel’s hull).
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© TOUCH BRIEFINGS 2008
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