FOCUS
Modelling applications
important to bear in mind different cooking styles and cultures, as well as methods of cooking, which vary substantially. A worst case example might be a pan full of cooking oil used for deep frying.
A study published in the NIST Special Publication 1066 (2007)3
considers the
UL 300A ‘standard fire’ consisting of a 400kW heat release rate in a 0.25m diameter, 0.175m deep pan containing 0.1m of corn oil, in which the heating of the deep pan from below continues.
As the influence of a suppression system
such as sprinklers on all hob arrangements is relatively unknown, the impact of such a system on fire size has been disregarded here for conservatism. However, further research is needed here.
Human tenability
When looking at the acceptable limits of received heat radiation, typical fire engineering standards, such as BS 7974-6: 20044 Guide E5
, provide reference to 2.5 kW/m2
and CIBSE as a
Travel speed The travel speed should represent the prospective range of potential occupants of the apartment. This might vary from single occupant adults (with/without mobility impairments) of various ages, or groups of occupants (eg families) which might move at the speed of the slowest occupant, if they cannot be carried.
Final exit door Negotiation of the final exit door should be considered if in direct line of sight with the hob. Considering that the final exits will require the person to ‘pull’ the door towards them, research data indicates that an eight second door opening time2
would be sufficient to
accommodate the majority of persons with disabilities, while potentially allowing a little extra time to accommodate the stress factor associated with the fire event (see Figure 1 on page 23).
Design fire assumptions
The size and fuel of the fire will have a direct impact on the amount of radiation experienced by the occupants. It is therefore important to use a design fire that is accurate to what would be likely to occur in the event of a hob fire. It is also
recommended upper limit for short term exposure, as this value can cause severe pain. This is widely accepted in the consulting industry as a recommended design value in fire engineering building design. The Society of Fire Protection Engineers (SFPE) handbook6
indicates that 1.7 kW/m2 is the
critical heat flux required to begin causing pain with sufficiently long exposure. The difference here is important to note, as the desire to pass the fire location and successfully evacuate decreases as more pain is received by the occupant for a longer duration. Other industries have operating guidance7
for staff which report that an incident heat flux of 6kW/m2
is the heat flux required to cause
pain within 10 seconds, and for which rapid egress is required. It recommends that egress routes should be considered impassable when undertaking quantified risk assessments in the offshore oil and gas sector for an incident thermal radiation heat flux above 6kW/m2
.
Based on the above, it can be considered that pain is received between 1.7 kW/m2 6kW/m2
and , and that within these boundaries,
occupants can continue to pass a fire incident, depending on the duration of egress and the thermal dose they receive.
Methodology/appliance location
BS 9991 does not provide clear guidance with regards to the ‘acceptable’ proximity of a hob cooking appliance (deemed to be the primary
24 DECEMBER 2018/JANUARY 2019
www.frmjournal.com
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