A research project has investigated the time it takes for a common type of household fire to become established.
Incipient fire development is the process that occurs from the initial exposure of a combustible object to an ignition source, through various growth stages, to a point when sustained flaming is established.
Incipient fire development can take from a few seconds to several tens of minutes, although, in many instances, a fire does not develop at all. The outcome depends on the materials the item is made of and the type and magnitude of the ignition source.
A recent BRANZ project exposed 12 purpose-built upholstered polyurethane foam chairs to a variety of small ignition sources, to measure the incipient fire development time. The aim was to determine if a time allowance for incipient fire development should be included in a proposed new Department of Building and Housing framework for performance-based fire engineering.
Slow burn hard to find
First, extensive preliminary trials were conducted to determine a suitable ignition source that would reliably start a slow-developing fire with a relatively long incipient fire development period, but these trials failed to produce any ignitions. Although the results will be useful for research into fire scenarios where a serious fire does not result, they were of no value in this project. Cigarette ignition fell into this non-fire category, because although cigarettes are responsible for many fires, there are many more instances where the cigarette extinguishes and no fire results.
In the end, the ignition sources chosen for the research were single matches and fire starters.
Chairs replicate those on sale
Twelve upholstered polyurethane foam chairs were specially prepared to replicate those sold into the lower end of the New Zealand market and those considered most likely to readily ignite.
The typical sofa chair construction used for this study comprised non-fire retardant treated polyurethane foam with a polyester wadding overlay and polyester covering fabric. The wadding gives the seat a plusher feel and prevents friction damage that would occur if the outer covering fabric was in direct contact with the foam. The seat and back cushions were placed on a steel frame in the form of a lounge chair (see Figure 1).
Fire tests carried out
Tests were conducted under the extraction hood of the ISO9705 room apparatus, permitting the heat release rate (HRR) to be measured by oxygen consumption calorimetry. From the application of the ignition source, the time of the incipient fire development was taken as the time until a HRR of 30 kW, at the establishment of a t2 fire curve, was reached (see Figure 2). The data for the entire burn was collected for each trial as this yielded valuable information for further analysis.
Statistical analysis of the incipient fire development times indicates a relatively narrow band for the ignition scenarios trialled (see Figure 3). The lower 5th percentile was about 88 seconds for the likely time for the incipient fire development and also represents the uncertainties due to the ignition source type and its location on the furniture.
Based on the HRR results for the 12 chairs tested, it was found that:
- the end of the incipient phase was satisfactorily characterised by the HRR exceeding 30 kW
- the location of the ignition source influenced the timeline of incipient fire development
- the closer the ignition source was to the centre of mass of the combustibles, the shorter the incipient fire development timeline
- the type or size of fire source influenced the incipient fire development timeline, with larger fire sources, such as fire starters, resulting in more rapid fire spread compared with matches
- the lower 5th percentile incipient fire development time assessed in the trials is considered to be significantly less than the 5% applicable to a more representative sample of furniture found in buildings. Consequently, these results are expected to be conservative estimates of the incipient fire development period.
No allowance recommended
On the basis of the relatively short incipient fire development times observed in the experiments representing low-end performing furniture, it is recommended that no allowance for incipient fire development be included in the timeline of fire development in the period up to the establishment of a t2 growth rate at 30 kW.
However, there is a need for a case study analysis to be undertaken for a range of buildings, with calculations of ASET (available safe egress time) and RSET (required safe egress time) including detection times and an incipient period in the design fire. Perhaps conclusions can then be drawn about whether the advantages (if any) of including an incipient period are justified from the point of view of recommending changes to current fire engineering design practice.
This work was funded by the Building Research Levy. More information is available in the BRANZ study report SR194, downloadable from www.branz.co.nz.
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