Evaluating the Fire Threat from Upholstered Furniture

Issue 39: Evaluating the Fire Threat from Upholstered Furniture

By Charles Fleischmann, Ph.D., P.E.

INTRODUCTION
When an unwanted fire occurs within non-storage buildings, there are few objects that have the potential to bring about untenable conditions as swiftly as upholstered furniture and mattresses. In the worst case scenario, the heat release rate (HRR) of an upholstered furniture item can reach values greater than 3 MW in a very short period of time (3-5 minutes) following ignition.1 Furthermore, it is not only the heat given off during the growth period, but also the toxic combustion products (primarily CO) produced that can overwhelm the occupants.

Over the last three decades, there has been a great deal of research and regulation on upholstered furniture and mattresses, which makes upholstered furniture and mattresses some of the most widely studied fuel packages available to fire protection engineers. In Europe, the Combustion Behaviour of Upholstered Furniture2 (CBUF) study incorporated both experimental results as well as enhanced fire modeling.

Yet, much of this modeling effort is nearly obsolete with the advances made in numerical modeling. Outside of the research environment, the Consumer Product Safety Commission (CPSC) in the USA has been investigating the hazard posed by upholstered furniture and mattresses. As part of this work, CPSC has carried out a number of experiments on both upholstered furniture and mattresses.

In 2007, a new open flame mattress standard (16 CFR Part 1633 Standard for the Flammability (Open Flame) of Mattress Set3) went into effect requiring all manufactured, imported or renovated mattresses to meet this standard. In 2008, the CPSC released a Notice of Proposed Rulemaking: CPSC NPR 16 CFR Part 1634 Standard for the Flammability of Residential Upholstered Furniture.4 Currently, it is not clear if this proposed standard for upholstered furniture will be adopted by the CPSC.

There has been significant opposition to the standard from the California Bureau of Home Furnishings Insulation and the National Association of State Fire Marshals and several other interested parties. Most of the comments, from outside the furniture industry, are critical of the standard, believing that it is not stringent enough to be effective at reducing the death toll from furniture fires. The public comments on the proposed standard can be found on the CPSC website: http://www.cpsc.gov/LIBRARY/FOIA/FOIA08/pubcom/flamm1.pdf.

Complex Behavior of Upholstered Furniture5
The complex nature of upholstered furniture fires is well known, and the melting behavior of polyurethane foam and synthetic fabrics is obvious in furniture fires, yet the detailed modeling of such phenomena is still very much in its infancy. After flaming ignition has occurred, the general burning behavior of an upholstered furniture item, ignited on the seat, can be broken down into four main phases: spread; burn through; pool fire; and burn out. Figure 1 shows the mass and heat release rate histories for a chair with polyurethane foam covered by wool fabric. The four phases of burning have been highlighted on the plot. The spread phase covers from ignition until the entire seat, back, and arms are burning. This can be seen in the early stages of the fire where the heat release rate is steadily growing.

Once the fire has spread over the surface, the fire enters a burn through phase marked by a quasi-steady heat release rate as the fire burns through the seat cushion. Once the seat burns through, the melted fuel will spill onto the floor marking the pool fire phase which is seen as a rapid increase in the heat release rate. This is typically the time of maximum heat release rate.

 

After most of the melted fuel is consumed, the fire enters the burn out phase where the heat release rate declines and the frame of the item continues to burn at a reduced rate. When a thermo plastic fabric is used, the burn through phase can be almost nonexistent, resulting in an extremely rapid growth to the maximum heat release rate.

 

Figure 1. Mass loss and heat release rate histories for a single seat upholstered chair with
wool fabric covering furniture foam.


Modeling the Combustion of Upholstered Furniture
The fabrics used, liners, foam chemistry and fire retardants combine to create a complex burning behavior. All of this makes the modeling of upholstered furniture extremely challenging.

The importance of the migration of the melted fuel to the floor cannot be overstated. Figure 2 shows the computer predictions from the Fire Dynamics Simulator version 5 (FDS5) for an upholstered chair compared with the experimental results. The computer animation of the burning chair is shown in the upper corner of Figure 2.

The results show good agreement during the quasi-steady burning phase, but the model over-predicts the growth rate during the first 100 seconds. The model is not able to capture the rapid fire growth that occurs around 200 seconds that results from the spilling of the melted fuel onto the floor from the seat area. The migration of fuel is not included within FDS5, and current efforts to model upholstered furniture in FDS5 have had only limited success due to the limitations of the physics of the model. The result is that the predictive capability for the heat release rate is not sufficiently accurate for many engineering designs.

 

Figure 2 – Fire Dynamic Simulator (FDS5) prediction compared with experimental
results for an upholstered chair.

More complex models that predict the melting behavior of thermo plastic materials are being developed using specific finite element codes in two-dimensions that require significant computer resources to model the melting behavior.6

As of yet, these models do not include the combustion the flammable gases that are released, and they only model simple geometries. Unfortunately it will be a number of years before accurate predictive models that incorporate melting fuel transport are available for engineering design applications. In the mean time, fire protection engineers must rely on experimental results and correlations, rather than models when evaluating upholstered furniture.

References

  1. Babrauskas, V., "Upholstered Furniture and Mattresses", Fire Protection Handbook, 18th Ed, National Fire Protection Association, Quincy, Massachusetts, 2002.
  2. Sundstrom, B. (Ed.) CBUF: Fire safety of upholstered furniture - the final report on the CBUF research program. Director-General Science, Research and Development (Measurements and Testing). European Commission. Report EUR 16477 EN, 1995.
  3. 16 CFR Part 1633 Standard for the Flammability (Open Flame) of Mattress Sets; Final Rule, Consumer Product Safety Commission, 15 March 2006.
  4. 16 CFR Part 1634 Standard for the Flammability of Residential Upholstered Furniture, Consumer Product Safety Commission, 4 March 2008.
  5. Fleischmann, C. and Hill G., "Burning Behaviour of Upholstered Furniture," Interflam '04, Interscience Communications Co., London, 2004.
  6. Butler, K. "A Model of Melting and Dripping Thermoplastic Objects in Fire", Proceedings of the Fire and Materials - 11th International Conference, Interscience Communications Co.,
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