Fire spread tests: a critique

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Fire spread tests: a critique

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NATIONAL RESEARCH COUNCIL OF CANADA

CONSEIL NATIONAL DE RECHERCHES DU CANADA

Fire Spread Tests

-

A

Critique

b y F. R. S. CLARK

A N A L Y Z E D

104Sii

Price: $1.00 Reprinted from FIRE TECHNOLOGY Vol. 17, No. 2, May 1981

p. 131-138

DBR Paper No. 1001 Division of Building Research

OTTAWA NRCC 19707

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la lumihe de rkcentes dkcouvertes, on examine certains param2tres jusque-18 oublib, qui rkgissent la propagation du feu lors des essais courants. Parmi ceux-ci, citons l'exposition au feu, I'orientation de l'kchantillon, la direction des flammes, l'enceinte expkrimentale, la rktroaction, le site de combustion et l'kchelle.

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FIRE TECHNOLOGY Vol. 17, No. 2, May 1981

Fire S ~ r e a d

A

Tests

-

A

Critique

F. R. S. CLARK Fire Research Section

Division of Building Research

National Research Council o f Canada

Propagation is perhaps the most alanning characteristic of a fire. Rate of advance is an extremely important factor in determining the likelihood of escape from a burning building and the ability of fire pro- tection systems to protect property.

N

ATIONAL FLAME spread standards address the problem of flame propagation in a variety of ways. In the United States and Canada the Steiner tunnel test is often cited.'-' The flame spread index derived from this test is an estimate of the rate of flame propagation along a horizontal 20-ft (6-m) specimen. Various formulae are used to calculate the flame spread index, the choice depending on the behavior of the specimen. Thus in certain cases the calculation is designed to give proper emphasis to the spread behavior early in the burning sequence. Assessment of flame propagation in the United Kingdom is based on the results of the procedure listed in the British Standard BS 476.7;3 the distance a flame propagates along a specimen 900 mm long during 1.5 and 10 min is measured. Similar procedures are used in the U.S.S.R.' For cellular plastics, the Australian standard AS 2122.1 (1978)= defines procedures that call for the measure- ment of the total flaming combustion time and mass or volume changes of a small specimen.

Propagation is also studied by researchers probing the more funda- mental aspects of this phenomenon by means of a wide variety of techniques. For example, Tewarson and Pion6 measured the mass burning rate and compared it with the results of full-scale propagation tests for several synthetic polymers.

Criticism of the propagation tests cited in building codes and regula- tions in the early 1970s led to the development of generally larger scale test

Copyright O 1981 NATIONAL FIRE PROTECTION ASSOCIATION All Rlghts Reserved

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132 Fire Technology rigs more closely resembling the "real fire situation" and avoiding the scal- ing problems necessitated by small-scale tests. Corner assemblies ranging from 4 to 50 f t (1.2 to 16 m) in their largest dimension were studied and many room-burn facilities were constructed. Although in particular circumstances these assemblies are capable of satisfactorily reproducing real fires and allow the assessment of the flame propagation characteristics of materials, application of the results of such tests to other fire scenarios is a t least as difficult as for small-scale, less representational tests.

The recent use of room and corner wall test configurations has led to the development of new measurements of various stages of flame propagation. Of these the most important is undoubtedly f l a s h ~ v e r . ~ , ~ In Canada, in cases where flame propagation neither proceeds steadily down the Steiner tunnel nor undergoes well defined advance and recession, the time to flashover in a 4-ft (1.2-m) corner wall assembly lined with the test material is used to derive a flame spread index.2 The time to flashover represents an optimistic upper limit of the time of survival within a compartment on fire. Unfor- tunately, however, the behavior of a sample material in the 4-ft (1.2-m) corner wall rarely reflects the survival time possible within an actual room lined with the same material. Furthermore, the low density thermoplastic foams for which the comer assembly is used to derive flame spread indices are usually required to be covered by noncombustible materials.

The problems of design of flammability tests, the interpretation of results, and the rash of new test procedures has stimulated several institu- tions to prepare guidelines on test development; I.S.0.9 and the National Materials Advisory Board of the U.S.A.1° have already issued such guidelines; and those of the British Standards Institution" and the Stan- dards Association of Australia" are near completion.

Discrepancies in the ranking of products by propagation propensity according to the results of various tests have been demonstrated.I3 This is un- fortunate; despite caveats inserted before the test procedures listed by, for example, I.S.O. and A.S.T.M., these tests are often used for the approval of materials or composites (such as sandwich panels). The supply industry is organized primarily along lines of chemical composition. The identification of physical and chemical characteristics that have some bearing on propagation behavior would aid in the development of acceptable products. Char formation,14 thermal inertia, and surface emissi~ity,'~-'~ for example, have been identified as having a significant role in propagation.

In this paper some of the factors affecting fire propagation are discussed to show why different test procedures give different results.

F I R E E X P O S U R E

Many energy sources have been utilized to stimulate propagation. Exposure-to-flame methods use fuels such as natural gas, methane, pro- pane, butane, hydrogen, wooden cribs, and garbage pails filled with paper.

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Fire Spread Tests 133

Radiant heating methods use electrically- or gas-powered refractory panels, focused light from arcs and electrically-heated filaments, and flames suitably shielded to prevent convection and conduction to the specimen. Ig- nition by conduction from hot objects and by convective heat transfer from hot air streams have also been popular. Finally, since ignitability may be related to propagation rate,18 the use of CO, lasers for ignition studies should also be recorded." Aside from the characteristic monochromatic radiation they provide, lasers have allowed the identification of attenuation of energy by pyrolysis gases.

In addition to the method of energy transfer, consideration is necessary of thermal flux, spectral distribution of the radiation, free radical (and ion) concentrations at or near the sample surface, duration of exposure, and spatial relation between the energy source and the sample.

By far the greatest attention has been paid to the flux level used in fire propagation tests; small pilot flames and sparks impart very little energy to the sample (even if the local flux level is high), whilst the energy imparted by large cribs is very significant. Lasers and the focused light from hot filaments provide high flux. The influence of radiation on propagation has been st~died,~O-~l but the appropriate value of the energy to be com- municated to the sample tests remains unresolved.

Rarely is the applied flux homogeneous over the entire specimen area. In the Australian flame spread test AS 1530.3,,, where this condition is met, propagation is evaluated at several flux levels. More typically, a flux gra- dient is established along the length of the specimen, for example, BS 476.7 (1971). ASTM E648-78 (the flooring radiant panel test), the proposed I S 0 rate of flame spread test (developed by TC 92, WG-4) and the NRCC carpet flammability test.23 Still more commonly, flux inhomogeneity occurs in a less controlled, less predictable way (see References 1, 2, and 5, and most corner wall and room bum tests). Search for either a single flux level or universally-applicable flux-distance or flux-time curves is pointless because they are not found in real fires. Better would be an understanding of the relation between the flux and the propagation rate.

The influence of the spectral distribution of the radiant emission of the energy source on fire propagation along a sample has long been recognized. Near blackbody radiation, as provided by heated refractory panels, and hot filaments of known temperature are one answer to standardization of the source, although the selection of the most appropriate blackbody temperature is difficult. The use of monochromatic laser light is another solution, but in common with all radiant source exposures the absorptivity of the sample still determines the energy received.

In an attempt to match the spectral distribution of flux present in real fires and the mode of reaction initiation, many workers use flames as the source. Flame emission spectra vary considerably, however, both in laboratory and "real fire" flames. Besides, convective energy transfer and chemical reaction initiation by different mechanisms may be present. Koohyar et for example, minimized convection and radical impinge

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ment by flame sources in their experiments, but the emission spectrum was arbitrarily chosen.

The frequency distribution of radiation and the flux delivered to a sample from a source also depends on the absorption characteristics of the intervening space. In studies of the ignition of poly(methy1 methacrylate) and

red oak by a CO, laser, KashiwagiZ5 demonstrated that there is strong at-

tenuation of incident radiation by decomposition products in the gas phase. The effect of these absorbing and scattering species will clearly vary according to the containment of the pyrolytic and combustion products, a factor that varies greatly among flame spread tests. In addition, absorption will vary according to the temperature of the intervening material.

Propagation involves a complex set of free radical and ionic chain reac- tions sensitive to the concentrations and activity of radicals and ions present. Thus the interference of a second source of radicals and ions, such as a flame, would be expected to be considerable. Certainly the nature of propagation in the presence of an initiating flame will be different from that of propagation initiated and sustained by thermal energy transfer alone.

Flame propagation tests differ in the spatial relation between the sample and the energy source. In the Steiner tunnel, the gas flame source is located at the entrance and is directed to one end of the sample. In contrast, in the ASTM El62 test the whole specimen receives heat from the radiant panel, which is placed 30 degrees from parallel to the major axis of the specimen. In Butler chimney-derived tests, such as AS 2122.L5 a bunsen-like flame licks over most of the small, vertical specimen. In each of these examples, the energy source is separated from the sample in a different way. Further- more, the spatial relation varies during the course of burning in a fashion particular to the test apparatus.

S A M P L E C H A R A C T E R I S T I C S

The buoyancy of hot gases forces the flame front to have a pronounced vertical velocity component. This fact makes the orientation of the major axis of the sample very important in the study of propagation. Propagation along a horizontal sample has a character quite different from that of propagation up a vertical sample. Essentially very conceivable orientation of the sample has been utilized, each exerting a different influence on the feedback

from the flame to the sample. Samples need not be fixed in space and there

are advantages to having the flame front stationary while the specimen is moved.

The method of sample support may be critical, especially in small-scale

tests. The extent to which the support method affects the test result

depends on the thickness of the sample, the thermal inertia of the material, the duration of the test, and the flux. Free vertical suspension, use of "semirestrained" holders and metal spikes, use of composite panels of the specimen with noncombustible plates, and wrapping of the sample in metal

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Fire Spread Tests 135

foil are all techniques that have been used. They rarely bear any resemblance to the actual mounting methods used in buildings or in common practice.

I t is often convenient to consider flame propagation as a one dimensional phenomenon for ease of modeling and for more direct transfer of knowledge gleaned from the study of flame propagation in premixed, laminar gas flames. Test methods used for material appraisal, however, more typically involve two- or three-dimensional flame propagation. This renders difficult the determination of flame front position by simple visual observation, but it does not affect other methods such as mass burning rate or total flaming time.

A I R F L O W

Airflows, when present, may be in the direction of propagation (e.g., burning wood in the Steiner tunnel) or against it (e.g., the limiting oxygen index test). Sirnms" found that, in order to ignite cellulose samples in an airflow, turbulence is required, presumably to ensure adequate mixing of ox- idant and fuel. Under some circumstances, turbulence caused by adjacent ignition may also influence the rate of flame spread. This suggests another coupling mechanism between the fire response of a material and the en- vironment of burning.

T E S T E N C L O S U R E

Major research effort is currently focused on the modeling of fires within compartments, subject to physical and thermal boundary conditions. The variables that influence the course of a fire thus confined are well established. The effect of the enclosures used in flame spread tests has been less

I studied; dimensions, volume, and thermal properties are most often chosen

' _{to meet practical requirements such as operator safety, airflow control and}

I convenience. Of the many enclosure effects, feedback and oxygen depletion

i

will be discussed here.

Oxygen depletion occurs when a specimen does not receive sufficient air for complete combustion. The effect of oxygen depletion was studied during the evaluation of the surface flammability of specimens by ASTM E84.l' In tests where samples exhibited rapid flame spread, the oxygen level was

below that sufficient for combustion of volatiles. Castino et a1 concluded

that the flame front development was "not critically limited by the oxygen depletion over the ten-minute test period." This conclusion, however, is rather contentious. Tests that do not call for the use of an apparatus that restricts air access to the specimen are less sensitive to oxygen depletion.

I Pyrolysis of a solid sample produces gaseous products that oxidize exo-

;

thermically and thus feed the endothermic pyrolysis. In addition, where

; char forms, oxidation of char may further enhance pyrolysis.'* Three

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136 Fire Technology

conduction. There is present, as well, a complex mass transfer process in- volving initiating and chain carrying chemical species. Existing flame spread tests differ widely in the manner in which the various transfer

mechanisms are handled. In a flame propagating vertically down a sample,

feedback processes are different from those operating when the flame propagates vertically upwards on the same sample. In cases where sample con- finement influences heat and mass transfer processes, the energy feedback will also be affected and will typically vary during a fire.

I t is convenient to distinguish between microscopic feedback, with short-range energy and mass transfer back to the fuel, and macroscopic feedback in which long-range transfers occur partly from gross features of the test apparatus. In samples for which microscopic feedback dominates, the influence of the test box on the propagation rate should be less than in samples for which macroscopic feedback is more important. I t is predicted that char-forming polymers should tend to exhibit performance ratings that are at least qualitatively similar in a variety of flame spread tests. The behavior of non-char-forming polymers is more likely to depend on the test method.

S I T E O F C O M B U S T I O N

If oxygen is withheld, a polymer heated above its surface ignition temperature may not ignite until the hot gases released from it are well removed from their source. If combustion occurs far from the fuel, feedback

will be weak and propagation may cease. A graphic demonstration of this ef-

fect occurs when some low density thermoplastic foams are tested in the Steiner tunnel; rapid propagation over part of the specimen is often followed by an arrest, and sometimes an apparent recession, of the flame front. The propagation outstrips the ability of the feedback mechanisms to supply heat for pyrolysis and the production of gaseous fuel, or the ability of the airflow to supply oxygen.

Combustion may occur above the fuel surface in the gas phasez8 or at the

surface. The latter mechanism has until recently been believed to be of secondary importance owing to the difficulty of oxygen access to the sur-

face of burning polymers once pyrolysis is well established. Stuetz et a129-31

have assembled evidence to support a theory of combustion of solids that involves chemisorption of oxygen on the polymer surface. Oxidation at the surface releases heat for pyrolysis of fuel beneath, but the relative importance of the two mechanisms of combustion is not yet apparent.

C O R R E L A T I O N S

A common method of evaluating a new test procedure is to seek correlation between the results of the new test and those of well-established procedures. This paper has sought to indicate that a high degree of correlation

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Fire Spread Tests 137

between widely differing test methods would be surprising, especially for samples for which microscopic feedback mechanisms are less important. Further, it is clear that the existence of such correlations does not indicate validity for either the new or the old methods.

C O N C L U S I O N

The results of flame spread tests are, in general, characteristic of the test procedure and do not reflect the complex influence on the performance of materials of factors extrinsic to the specimen. Without a much more complete knowledge of the peculiarities of existing test methods and of the factors that determine the behavior of a burning sample, advice given on the basis of test results as to the performance to be expected in a fire cannot be accurate. The search for methods of assessing the intrinsic material properties that influence flame propagation is important, but even if successful the predictions of real fire performance of materials in complex assemblies will probably remain difficult.

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'

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bustion and Flame, Vol. 26 (1976), p. 85.

'

Babrauskas, V., "Estimating Room Flashover Potential," Fire Technology, Vol. 16

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Academy of Sciences. Washin on. D.C. (1979), p. 164.

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'*

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'@

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ldd.

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Igniting Condensed-Phase Materials," Twelfth Symposim (International) on Combustion, Combustion Institute, Poitiers, France, 14-20 July 1968, p. 215.

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'*

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"

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ACKNOWLEDGMENT: This pa er is a contribution from the Division of Building Research,

National Research Council of tanada, and is published with the approval of the Director of the Division.

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