An Experimental Determination of Combustibility E. S M I T H , Ph.D. The Ohio State University

EDWIN

T h e author describes a laboratory method for determining ignitability, rate of heat release, and total heat release as a measure of the combustibility of materials. T h e results may be applied to building design and designation of contents with an eye towards reducing the possibility of a major fire. COMBUSTIBILITY of building materials, furnishings, a n d o c c u p a n c y m u s t be k n o w n before a rational e v a l u a t i o n of a s t r u c t u r e s resistance to the d e v e l o p m e n t of a c a t a s t r o p h i c fire can be made. T h e u l t i m a t e goal of c o m b u s t i b i l i t y studies is to enable t h e prediction o f fire involvem e n t w i t h i n a s t r u c t u r e for a given ignition source. T h a t is, if we k n o w t h e c o m b u s t i b i l i t y of e v e r y t h i n g within a room, the spatial o r i e n t a t i o n of e a c h item, and t h e s t r u c t u r a l features of t h e r o o m itself, we s h o u l d be able t o p r e d i c t the course a n d u l t i m a t e s e v e r i t y of the fire t h a t would occur a f t e r a given level of initiation.

COMBUSTIBILITY

DEFINED

The term combustibility as used above must be defined. Even the layman has a qualitative perception of the word. To him, the more it burns, the more combustible it is. Our problem is to quantitatively describe what is meant by "more" and determine the factors needed to specify " i t . " I n relation to t h e u l t i m a t e goal - - t h a t of predicting t h e kinetics of a fire s y s t e m - - we m u s t be able to describe, in a q u a n t i t a t i v e m a n n e r , h o w a n article of furnishing, wall covering, or ceiling tile will c o n t r i b u t e to t h e inception and s u p p o r t of a fire. F o r all p r a c t i c a l purposes, this c o n t r i b u t i o n (i.e., the more of " t h e m o r e it burns . . .") can be described b y t h r e e v a l u e s - ignitability or ease of ignition, rate of h e a t release ( R H R ) , and the total q u a n t i t y of h e a t t h a t NOTE: Professor Smith is Director of Chemical Engineering Research, Engineering Experiment Station, The Ohio State University, Columbus, Ohio.

109

110

Fire T e c h n o l o g y

can be released (THR). Combustibility is then defined b y t h e s e three values. One important aspect of this definition must be emphasized. Combustibility is no single value or set of values. It is a function of the enviromment to which the material is exposed, or time-heat flux h i s t o r y of exposure, as well as the physical shape and surface characteristics of the material. At first glance, this m a y appear to be hopelessly complicated, or at best an unwieldy definition; however, it is readily applicable both to fire hazard ratings and to the design of fire-resistant structures. The factors that comprise the definition for combustibility should be examined in relation to variables that affect their values. Configuration, exposed surface area, and surface characteristics (i.e., variables of the material itself rather than the environment to which it is exposed) are important to evaluating ignitability and R H R ; but these factors h a v e no effect on total heat release (THR). Take wood as an example. O n e pound of wood can release the same total quantity of heat whether it is in the form of a solid cube or shavings; b u t the R H R and ignitability w o u l d be vastly different. For m a n y materials, ignitability and the initial or m a x i m u m RI-IR will depend on the environment to which the material is exposed. Again using wood as an example, the maximum R H R and time to ignition will b e quite different ff the sample is exposed to a temperature of 600 ° F or 1200 ° F. The effect of exposure temperature varies with the t y p e of material. If the material forms a stable ash as it burns, the R H R will vary w i t h time, or in other words, the R H R is dependent on prior combustion history. If no ash is fo~zned, R H R will be a function of exposed surface area and essentially independent of prior burning.

DETERMINING

COMBUSTIBILITY

FACTORS

A quantitative expression for ignitability, RHR, and THR must be available to define combustibility. Equipment and procedures are being developed at the Ohio State University to determine ignitability and RHR as a function of exposure. THR may be found by the procedures described by Gross and Natrella.* The original equipment used at OSU to experimentally measure ignitability and RHR is schematically represented in Figure I. The principle of operation is simple. A controlled flow of air heated to some selected temperature passes through an environmental chamber, and the differential temperature between inlet and outlet is recorded. If there is no sample or no heat released in the chamber, the differential temperature AT will be zero. If heat is released in the chamber, the air leaving will be hotter than that entering, and the AT recorded will be proportional to the rate at which heat is being released. The area under the AT vs. time curve would *Gross, D. and Natrella, M. G., "Interlaboratory Comparison of the Potential Heat Test Method," NBS Report No. 9865, August 26, 1968.

Combustibility

111 TO SCALES

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Figure 1. Ignitability and rate of heat release test chamber.

be proportional to the heat released over the time period considered. Initial runs were made with the configuration shown in Figure 1. Simultaneous R H R and weight loss measurements were made. Ignition was either by self ignition or b y a small pilot flame located approximately 2 in. above the top edge of the sample suspended vertically in the chamber. In recent studies, the method of sample support and exposure has been changed. The sample is now mounted 60 ° from the horizontal and subjected to direct flame impingement over approximately 10 percent of its exposed underside surface. This revised procedure allows testing of all materials, including those not thermally stable; permits evaluation of ease of ignition; and extends the range of temperature over which significant data can be obtained. Figure 2 illustrates a typical AT vs. time curve obtained for a cellulosic material. One of the problems is how to interpret the wealth of data obtained from a series of such runs at different temperatures. A few of the

I12

Fire Technology

significant points and features of the AT vs. time, or R H R , c u r v e s are shown. The time from initial exposure until a significant change in R H R (slope of curve) is reached is an important characteristic denoted by A. I t is a measure of how readily the material can be "fired" or ignited u n d e r the exposure conditions of the run. How rapidly R H R will build up is given by the m a x i m u m slope of the curve (Slope B). The m a x i m u m rate of heat release is denoted by C. The period of major heat release is indicated by values D-1 and D-2, which are used to differentiate between m a t e r i a l s t h a t burn rapidly and completely in a few seconds and those t h a t m a y have a lower but sustained rate of heat release. These points or slopes are the numerical values that, taken together, measure the sample's combustion characteristics under the exposure conditions of a particular r u n . Note t h a t the slope of the ~ T vs. time plot is equivalent to d2H/dt2; t h e AT is equivalent to d H / d t ; and the area under the curve is equivalent to H or the a m o u n t of heat released. .6

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I Figure 2. Rate of heat release curve for a cellulosic material.

The features of Figure 2 t h a t express the characteristics of ignitability are A, the time from initial exposure until significant R H R develops, and Slope B, the rate at which R H R increases. Slope E, or origin slope, drawn from the origin tangent to the AT curve, is a composite value of A and Slope B. Since the flow rate of air through the combustion chamber is held constant during a run, the change in temperature (AT) of the air as it passes

Combustibility

113

through the combustion chamber is proportional to the rate at which the sample releases heat. H e a t released during any time period is proportional to the area under the AT vs. time curves over this period of time. To convert chart readings of AT to B t u / m i n values, calibration charts were used. These calibration charts, a sample of which is shown in Figure 3, were determined b y burning natural gas at a known rate in place of a sample. From knowledge of the heating value of the gas, the rate of heat released b y the burning gas was calculated. The AT caused b y this R H R was recorded.

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The calibration charts were derived from a series of these runs made at different air flow rates, absolute temperature of entering air, and R H R ' s (i.e., flow rate of natural gas). For each run, a value of K, as defined by the equation K~ = (RHR) ,/ (Chart reading)~, where RHR~ is the rate of heat released b y the natural gas for run i and Chart reading~ is the chart reading of AT for run i, was determined. The values of K were then plotted as a function of inlet air temperature with air flow rates as a parameter to give the calibration chart shown in Figure 3. Air flow rates were determined as a function AP, the pressure drop across the orifice meter in the air line to the combustion furnace. Using Figure 3, the R H R in B t u / m i n can be determined for any point on the AT vs. time curve. For example, the run shown b y Figure 2 was

114

Fire T e c h n o l o g y

made at a P = 6 in. and 400 ° F. The R H R corresponding to P o i n t C is equal to 0.50 (the chart reading for aT) times 480 (the K value f r o m Figure 3) or 240 Btu/min. Quantitative values for ignitability and R H R can be determined from an experimental run as shown by Figure 2, but this is for one exposure level. Combustibility data must be available for a broad range of environmental conditions. However, acquisition of these data does n o t require an infinite number of experimental runs. By combining a knowledge of the combustion mechanism for the sample in question with three or f o u r experimental runs covering a wide range of exposures, it is possible to interpolate R H R data for the entire range of exposures with a relatively high level of precision. Figure 3 can be used to illustrate the point just made. Note t h a t increasing exposure temperature decreases time of ignition and increases the rate at which the material starts to burn. However, once the entire exposed surface has been charred, the R H R ' s are nearly the same. These curves are for a cellulosic material t h a t forms a structurally stable ash. Evidently rate of heat penetration after the initial burning period is det e r m i n e d by the radiant heat from its own flame, which is absorbed and transferred through the ash layer. For certain rigid plastic foam material t h a t leaves no appreciable ash on burning, the time to ignition (with flame impingement) is essentially zero, even at room temperature, and the R H R is practically the s a m e regardless of exposure temperature. The major variable is the exposed surface area. With these combustibility data available, it is possible to specify an enclosure and its fuel loading* to prevent a catastrophic fire for different initiation source levels. As an example, a room will be taken as t h e system to be analyzed. To determine if the fuel loading within the room is large enough to allow a catastrophic fire, the rate processes occurring within the system must be analyzed. If the rate at which heat is being released is greater than t h a t being absorbed and transferred from the s y s t e m to the surroundings, the temperature within the enclosure will increase. Increasing temperature increases R H R , which further increases t e m p e r a t u r e and radiant heat flux. I f the fire loading is such that an accelerating increase continues, a catastrophic fire results. Conversely, if the R H R becomes less t h a n t h a t dissipated to the surroundings, temperature within the enclosure will decrease. The fuel loading within the enclosure will determine when this condition will be reached. With sufficient data it is possible to design the fuel loading of an enclosure to elLminate a catastrophic fire at any assumed level of ignition. Mathematically, a heat balance can be made to calculate t h e change *Fuel loading in this context is not only the weight of combustible material, but also features t h a t dete~maine the material's combustibility, such as exposed surface area and the spatial relationship of combustible surfaces.

Combustibility

115

in temperature within the system as a function of R H R , losses to surroundings, and accumulation in the system's mass. That. is, the change in sensible heat content of the air within the enclosure over time period LX0would be equal to the sum of the R H R ' s minus the rate of heat loss b y radiation, conduction, and convection, and the rate of heat accumulation in the system all multiplied b y ~0. This analysis of the fire system is conceptually different than t h a t generally made. In this case, the emphasis is on the design of fuel loading to eliminate a catastrophic fire. The usual analysis is made on the assumption that a catastrophic fire will occur and that all the combustible material in the system (enclosure) will burn under exposures developed by a catastrophic fire. The concept of a design to prevent a catastrophic fire emphasizes the need for combustibility data of the type described here. To design for a given level of fire involvement~ combustibility must be known for low heat flux conditions as well as for catastrophic fire environments. COMPARISON

OF

TESTING

METHODS

These same data that describe combustibility can be used to characterize or classify materials in respect to their fire hazard. Values for ignitability and maximum rate of heat release at one or more specific temperatures would serve as a basis for a fire hazard classification. These data also can be related to conventional classification tests. For example, one would intuitively expect to find a relationship between the factors comprising ignitability and flame spread rating such as that given by NFPA No. 255 (also published as ASTM E-84 and UL 723). Combustibility tests on treated and untreated cellulosic materials indicate that the time of exposure at 600 ° F -until the slope of RHR vs. time curve exceeds 50 Btu/min 2 is inversely related to the flame spread rating. Table 1 lists these numerical values and potential heat values for five cellulosic materials. Figures 4 through 7 are computer printout charts of RHR vs. TABLE 1.

Sample PB A B F** Q**

Comparison of Test Methods

Potential heat F l a m e spread (B t u /lb ) rating 7620 7100 7000 300 540

150 45 20 15 15

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D-2

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*Max: M a x i m u m R H R i n B t u / m i n for 100 in. ~ of s a m p l e exposed. T i m e t o slope = 50: T i m e in m i n u t e s u n t i l t h e slope of t h e R H R vs. t i m e c u r v e r e a c h e s 50 B t u / m i n ~. * * N o n c o m b u s t i b l e b y A S T M E-136 test.

116

Fire Technology I

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Combustibility

117 I

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118

Fire Technology

Time, or combustibility data, for these same samples. There is obviously no correlation between potential heat, flame spread, or m a x i m u m R H R , nor should one be expected. One might think t h a t heat release (area under the R H R vs. Time curve) should be proportional to potential heat. Actually, even at 900 ° F, treated cellulosic material does not burn completely, or more accurately, all products of decomposition do not burn as compared to burning in an oxygen bomb calorimeter. The effect of exposure conditions on comparative fire hazard ratings can be illustrated b y comparing R H R data for two different t y p e s of material - - one that forms a structurally sound ash, such as wood, a n d a rigid plastic foam, which leaves no stable ash. Figure 8 shows a typical heat release curve for a thin wood panel and for rigid foam at 300 ° F and 1200 ° F. At 1200 ° F there appears t o be little Temp. = 1200 ° E CI

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difference in combustibility of the two materials. However, at a temperature of 300 ° F, the ignitability of the foam is much greater than the wood. In interpreting these curves, one would infer that the thin wood panel and rigid foam, when exposed to a catastrophic fire, would make a similar

Combustibility

119

c o n t r i b u t i o n to t h e fire in t e r m s of ignitability, R H R , a n d T H R . H o w e v e r , t h e c o n t r i b u t i o n e a c h would m a k e to t h e initiation a n d d e v e l o p m e n t of a fire a t low e x p o s u r e levels is v a s t l y different, as i n d i c a t e d b y t h e difference in i g n i t a b i l i t y a n d r a t e of h e a t release a t 300 ° F. T h e c o m p a r i s o n m a d e in F i g u r e 8 e m p h a s i z e s t h e n e c e s s i t y of e v a l u a t ing c o m b u s t i o n kinetics of m a t e r i a l s o v e r a r a n g e of e x p o s u r e c o n d i t i o n s before a realistic p i c t u r e of c o m b u s t i b i l i t y is available.

SUMMARY Combustibility has been defined in terms of three values -- ignitability, rate of heat release, and total heat release at a given exposure condition. In the context of this definition, combustibility cannot be denoted by one or even a group of numbers, but is a function of the physical characteristics of the sample and of the environment to which the sample is exposed. A complete description of combustibility requires that rate of heat release be known as a function of time for the range of exposure conditions of interest. A few experimental heat release curves at three or four exposures is believed to be adequate to establish the effect of temperature or heat flux on rate of heat release. Data of the type described in this report are required in order to rationally design a building and its contents, so that it will not be subject to a catastrophic fire under a given initiation level. BIBLIOGRAPHY Smith, E. Edwin, "Experimental Determination of Combustibility," Project No. EES 307X, The Ohio State University, 1970. Columbus, Ohio. Nixon, Walter R., II, "Development of Design for Environmental Chamber for Combustibility Studies," Thesis for Master of Science Degree, The Ohio State University, 1970, Columbus, Ohio. ACKNOWLEDGMENT: This work has been sponsored by the American Iron and Steel Institute. Their financial support and the technical assistance provided by the task / group monitoring the project is gratefully acknowledged. This is a summary of a paper ¢ presented at the Conference on Fire Protection of Steel in Building Construction, sponsored by the AISI at the University of Maryland, July 14-17, 1970.