Journal of Building Physics 1977 Hilado 116 28

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DOI: 10.1177/1097196377001002021977 1: 116Journal of Building Physics

Carlos J. Hilado and Heather J. CummingFire Safety Aspects of Thermal Insulation

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FIRE SAFETY ASPECTS OF THERMAL INSULATION

Manuscript received June 23, 1977

CARLOS J. HILADOAND HEATHER J. CUMMING

Fire Safety Center

University of San Francisco

San Francisco, California 94117

ABSTRACT: Thermal insulation materials and systems have an importantrole in the conservation of energy, in the economical maintenance of com-

fortable environments, and in the profitable operation of industries. Properuse of thermal insulation should take into account fire safety aspects. In

industry, one important fire safety aspect is prevention of self-heating and

spontaneous combustion; ethylene oxide, heat transfer fluid, polyalkylene

oxides, and polyether polyols are given as examples. Fire protection tomaintain structural integrity and prevent decomposition of reactive chemi-cals is highly desirable. In any structures occupied by people, fire toxicityis an important factor in ability to escape from fires and in extent of fire

injuries.

INTRODUCTION

THERMAL INSULATION HAS an important role in the conservation of energy, in

the economic maintenance of comfortable environments, and in the profitableoperation of industries. With the steady increase in the cost of energy, energyconservation becomes an increasingly important factor in the economics of every

application which involves heat. One of the principal energy conservation measures

in any environment is the judicious use of thermal insulation materials and systemsto minimize heat losses. i

Because safety should not be sacrificed in the search for energy conservation, the

proper use of thermal insulation should take into account the possible hazards in

exposure to fire. The purpose of this paper is to discuss some fire safety aspects of

thermal insulation.

FIRE PREVENTION CONSIDERATIONS

Many thermal insulation systems consist of essentially noncombustible materials.Those systems which utilize combustible materials are generally restricted to tem-

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perature ranges which provide a margin of safety below the temperatures at which

such materials would ignite.As a fire prevention measure, improper use of combus-

tible materials should be avoided.

One important fire prevention aspect of thermal insulation systems is prevention

of self-heating and spontaneous combustion.Although an insulation material may

be noncombustible, it is likely to be near or in contact with combustible materials.

At ambient conditions, a combustible material may undergo exothermic oxidation,

with a reaction rate so slow that the heat generated is easily dissipated to the

surroundings. If the reaction rate is increased for any reason, and heat dissipation is

restricted by the presence of thermal insulation, the combustible material may

exhibit self-heating, in that its temperature is observed to increase without any

external application of heat. If this process continues, it may lead to ignition of the

combustible material.

SELF-HEATINGAND SPONTANEOUS COMBUSTION

Self-heating or spontaneous heating, and its related, more spectacular phenom-

enon, spontaneous combustion, have been observed in thermal insulation contain-ing combustible liquids and vapors which have leaked or spilled into the insulation.

In England, the temperatures of the insulation material or insulated material at

which fires have been reported to occur range from 80 to 150C. for essential oils,160 to 250&dquo;C. for coal tar distillates, and 200 to 300C. for mineral oils [ 1 ] .

The reaction causing self-heating may be the result of air oxidation, attack by

microorganisms, decomposition, action by water or water vapor, or catalysis byimpurities. Self-heating can occur if the volume of material exceeds the safe size,much as an atomic pile goes critical.Any material capable of self-heating will

generate heat which is dissipated to the surroundings by normal mechanisms of heat

transfer, i.e., conduction, convection, and radiation. Under stable conditions, heat

generation balances heat loss. When heat is generated more rapidly than it can be

dissipated, there is a continuing temperature rise in the self-heating material, and a

hazardous situation can be created.When a combustible material exposed to air is progressively heated, an increase

of internal sample temperature above the temperature of the heat source is a true

indication of self-heating. Since the observed self-heating temperature in any appa-ratus is affected by sample size, surroundings, and heating rate, the deviation be-tween the self-heating temperatures thus obtained and a &dquo;true&dquo; self-heating tem-

perature (assuming a true value exists) is not known. However, it can be neglected ifmaterials are compared under the same test conditions and the test results are usedfor comparison only and not as absolute values.

The wide range of self-heating temperatures exhibited by the various thermalinsulation materials shows widely varying susceptibility to self-heating. This changeswith different combustible materials. Because one thermal insulation which is less

susceptible than another in one type of service may be more susceptible in a



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different type of service, each insulation should be evaluated for susceptibility with

the specific combustible material involved in the application. The variability of

composition, and therefore behavior, of a given brand of thermal insulation over a

period of time should be stressed.

Developing the ability to predict which thermal insulation materials would be

more susceptible to self-heating requires a thorough chemical and physical charac-terization of the materials already tested and of the materials to be considered in

the future. Such characterization is not feasible at the present time, because com-

mercial thermal insulation materials are proprietary in composition, difficult to

characterize, and variable in manufacture. Until such information is obtained and

the predictive ability developed, experimental tests continue to be necessary to

minimize fire hazard.

Because the surface temperatures reported in insulation fires are often signifi-

cantly lower than latoratory-determined autoignition temperatures of the combus-

tible materials involved [2] , an apparent lowering of autoignition temperature is

sometimes offered as an explanation. This viewpoint has some practical value in

that effective autoignition temperatures can be related to safety guidelines for

upper limits of service temperatures: a lowering of autoignition temperature means

that service temperature must be reduced accordingly to maintain the same marginof safety.

The presence of ferric oxide has been found to lower autoignition temperaturevalues for several organic compounds [3] , and the contribution of thermal insula-

tion to self-heating and lowering of autoignition temperature may be due to the

presence of certain impurities in the particular material. In the specific case of

ethylene oxide, a major factor in lowering of autoignition temperature may be the

conversion of ethylene oxide to acetaldehyde which has a much lower autoignition

temperature value [4].

Self-heating phenomena involving combustible vapors in contact with thermal

insulation can be illustrated by the example of ethylene oxide. The susceptibility of

various thermal insulation materials to induce self-heating with ethylene oxide hasbeen determined at Union Carbide Corporation by means of a self-heating test

which, in effect, determined the self-heating temperature observed when ethyleneoxide/air mixtures are in contact with a particular material.

The apparatus in which the tests are conducted consists of a vertical stainless

steel cylinder, 127 mm (5 in) in diameter and 305 mm (12 in) high, the sides of

which are heated by electrical heating tapes.A horizontal divider plate, 152 mm (6in) from the top, divides the interior of the cylinder into two equal compartments,the upper compartment serving as the test chamber. Only a portion of the cylindri-cal cross section, defined by a vertical baffle 51 mm (2 in) from one side, is filled

with thermal insulation material, in the form of roughly 6.4 mm (/a in) cubes. The

ethylene oxide and air are admitted through separate inlets of 25.4 mm (1 in) fromthe bottom of the sample chamber, directly opposite each other and 25.4 mm (1



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in) apart. The top plate is a loose fit which allows the gas mixture to vent. Thermo-

couples in the inlet tubes measure the temperatures of the entering gas streams, and

thermocouples in the sample measure the temperature at two levels. Depending on

the particular sample, either the upper or the lower sample thermocouple provides

the first indication of exotherms. The self-heating temperature is considered to bethe temperature at which an exotherm becomes noticeable on temperature-time

ch a rts.

A comparison of initial self-heating temperatures observed when various thermal

insulation materials were exposed to a 50/50 (by volume) ethylene oxide/air mix-

ture of steadily increasing temperature is provided by Table I, the data for which

were derived from a more detailed paper [5] .

Table 1 Self-Heating of Insulation Materials with 50/50 Ethylene Oxidel

Air Mix tures

The susceptibility of various thermal insulation materials to induce self-heating

with combustible liquids has been determined at Union Carbide Corporation byexposing liquid-soaked samples of thermal insulation to air of steadily increasingtemperature. The apparatus in which the tests are conducted consists of a vertical

heated furnace tube 220 mm (8.75 in) long with a 102 mm (4 in) bor-e, heated byelectrical current passing through nichrome wire in an asbestos sleeve wound

around the tube, and an inner refractory tube 220 mm (8.75 in) long with a 76 mm

(3 in) bore, inside which the specimen is placed.Air is admitted at a controlled rate,and its temperature is steadily increased by controlled increase in electrical energy

input. The temperatures of the air near the specimen and of the interior of the

specimen are measured by means of thermocouples. The self-heating temperature isconsidered to be the temperature at which an exotherm becomes noticeable on

temperature-time charts, usually the temperature at which the sample internal tem-

perature exceeds that of the heating air.



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A comparison of initial self-heating temperatures observed when various thermal

insulation materials were soaked with DOWTHERM heat transfer fluid is provided

by Table 2, the data for which were derived from a more detailed paper [5] . The

dependence of self-heating temperature on the particular combination of thermalinsulation and combustible liquid is emphasized by more recent data on polyalky-lene oxides (Table 3) and polyether polyols (Table 4).

Table 2. Self-Heating of Insulation Materials with DOWTHFRM Heat Transfer

Fluid.

Table 3. Self-Heating of Insulation Materials with Polyalkylene Oxide

I

Table 4. SelfHeating of Insulation Materials with PolyetherPolyols



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In general, differences between the values of initial self-heating temperatures

given in Tables 1 through 4 should not be considered significant if the differences

do not exceed 20C. Reproducibility varies with the sample of material tested. For

example, the 268C. value given for calcium silicateA in Table 1 is the average of 4

individual test results ranging from 245 to 295C.;the

290C.value

givenfor

calcium silicate B in the same table is the average of 4 individual test results ranging

from 280 to 300C. The variation observed with supposedly the same materials

over a period of years can be considerably greater. For example, the 320C. value

given for perlite B in Table 1 is the average of 6 test results, ranging from 265 to

380C., obtained from 1970 to 1973; the 251C. value given for calcium silicate Cin the same table is the average of 4 test results, ranging from 229 to 290C.,obtained from 1968 to 1973.

This degree of variation should not be surprising. Commercial thermal insulation

materials are manufactured to specifications which include properties such as

density, thermal conductivity, and durability, but not susceptibility to self-heating.

Chemical composition could vary significantly within existing specifications.

FIRE PROTECTION CONSIDERATIONS

Thermal insulation systems play an important part in the efficient operation of

industrial plants because of the appreciable value of energy employed in heatingand refrigeration. Because of their widespread use in manufacturing facilities, theytend to be among the first materials subjected to flame impingement in an acci-

dental fire. Their performance under fire conditions is therefore of significant con-cern.

At the very least, a thermal insulation system should present no significant fire

hazard, that is, it should not contribute significantly either as fuel for the fire or as

a means of propagating the fire. It would be more desirable if it provided some

measure of protection for the insulated pipes or vessels, preferably for at least one

hour before pipe or vessel wall temperatures reach a critical level.

A thermal insulation system is judged as providing fire protection if it retards the

temperature rise of the insulated pipe or vessel from reaching a failure point for at

least 60 minutes.A failure point of 538C. ( 1000 F.) was selected for two reasons:first, steel rapidly loses its strength at temperatures above 538C., and, second, ator near 53$C., many reactive chemicals decompose with explosive violence.A reviewer has pointed out that it is not widely agreed that a criterion of 538C.

maximum allowable temperature after one hour of exposure is appropriate for

chemical plants, and that the results of investigations into plant disasters such as

that at Flixborough, when made public, will have an important bearing on this

question. This particular criterion was selected in the 1960s as the criterion for

giving fire protection credit to thermal insulation systems in certain plants of Union

Carbide Corporation, and to date there has been insufficient evidence for changingthis criterion. The 538C. temperature level is cited as a critical level inASTM E



8/14

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119 [6] in tests for structural steel columns, beams, and girders. Since theASTM E

119 procedure is firmly established among the fire protection and insurance organi-zations in the United States, selecting a different temperature level without consid-

erable technical evidence would be unwise.

F ire protection is desirable, but in many cases it is more economically provided

by water spray and other systems. The fire protection provided in a structure oftenaffords sufficient protection for vessels and piping. In chemical process plants, units

handling reactive chemicals require the maximum fire protection and thus call for

fire-protective insulation, but such units usually comprise less than 10 per cent of

insulation applications in any particular plant.

FIRE ENDURANCE OF INSULATED PIPES

A fire exposure test has its principal value in simulating the conditions that

would probably exist in an accidental fire. In industrial plants, particularly in the

chemical process industries, a substantial proportion of accidental fires involve

burning liquids, so that vessels and piping tend to be exposed to direct flame

impingement. Perhaps the most vivid description of a typically severe fire would be

&dquo;a sea of fire&dquo; at the base of a still column or beneath a pipe rack.The requirements of a fire exposure test should therefore include provisions for

direct flame impingement, for a fire source of substantial area, for direct impinge-ment of hot combustion gases, for a rapid rate of temperature rise at the start of

the fire, and for generous access of air. These conditions were provided by burninggasoline in a large open pan. The exposure fire should provide a maximum heat

rate, which is achieved by a considerable thickness of flame. Thicknesses approach-

ing 1 meter are desirable to reduce radiation losses to the surroundings.The test apparatus originally developed by Union Carbide Corporation [7,8]

consisted of the following: a steel fuel pan measuring 2.25 m (7.5 ft) long, 1.2 m (4ft) wide, and 0.45 m (1.5 ft) deep; a steel pipe support rack to hold five 6 m (20 ft)

lengths of 3 in. IPS pipe in a horizontal plane spaced on 41 cm (16 in) centers; five6 m (20 ft) long, 3 in. IPS, schedule 40 carbon steel pipes per test; chromel-alumel

thermocouple wire with porcelain insulators and connections; a multi-point record-ing potentiometer (two points minimum for each pipe); fuel supply and control

equipment; and water supply and equipment for water stream and water sprayapplication.

Each 6 m. test pipe was cut at the center on a 30 angle to longitud inal center-line to form two sections.A chromel-alumel thermocouple was installed on theinside wall of each section at points that located the thermocouples 180 apart and3 m ( 10 ft) from either end when the pipe was reassembled. The two sections were

welded together to reform the original 6 m. long test pipe. The pipes were placedon the support rack so that the thermocouples were centered over the fuel pan. Theopen ends of each pipe were packed with glass wool insulation to minimize airmovement through the pipes.



9/14

123

The fuel pan was charged with water until the water level was 0.5 m (1.67 ft)

below the horizontal centerlines of the test pipes. Gasoline was fed into the fuel

pan until the water surface was covered, using a fuel sparger system to distribute

the gasoline evenly over the surface of the water in the pan. The fuel was ignited so

thatthe five test

pipeswere

fully exposed simultaneouslyto the fire. The fuel flow

into the pan was controlled so that the temperature in a bare pipe would coincide

with theASTM E 119 [6] standard fire temperature curve: 538C. (1000F.) in 5

minutes, 704C. (1300F.) in 10 minutes, 843C. (1550F.) in 30 minutes, 927C.

(1700F.) in 60 minutes.

Reviewers have questioned the desirability of following theASTM E 119 fire

temperature curve. The author agrees that theASTM E 119 fire temperature curve

does not necessarily apply for all the situations in which attempts are made to use

it, and is aware that other fire temperature curves have been proposed which mayhave equivalent and perhaps superior technical justification. However, developmentof a &dquo;standard&dquo; fire temperature curve appropriate for industrial plant fires does

not seem feasible at this time. Chemical plant fires are probably much more diffi-

cult to generalize than building fires, because of the wide variety of organic com-

poundswhich

couldserve as the fuel in

spill fires,and the multitude of

plantlayouts which could constitute the fire environment.Any proposed plant fire tem-

perature curve would be economically prohibitive to develop and validate, and

would probably never attain the same acceptance as theASTM E 119 fire tempera-ture curve among the fire protection and insurance organizations in the United

States.

Experience with the test showed that theASTM E 119 fire temperature curve

could be approximated by controlling the fuel at a rate of 3.8 1/min (1 gpm) forthe first 5 minutes exposure, 6.9 1/min (1.8 gpm) for the next 10 minutes expo-sure, and 9.5 1/min (2.5 gpm) for the remainder of the exposure.After one hour of fire exposure, during which period temperatures were re-

corded as a function of time, the fuel was shut off and the fire allowed to go out.

Each test pipe was inspected and the observations recorded.After 16 minutes, the

fuel was reignited and burned for 5 minutes to restablish full exposure conditions.The fuel was then shut off and water at a rate of 280 1/min (75 gpm) was appliedto the insulation in a straight stream through a 38 mm (1.5 in) nozzle from a

distance of 18 m (60 ft) for 1.5 minutes. The water application was then changedfrom a straight stream to a spray and continued until the residual fuel burned out.

A second inspection of each test pipe was made and observations recorded.

This test procedure was duplicated and modified by Dow Chemical Company[9] . Sensitivity to weather conditions was reduced but not entirely eliminated by

conducting the test inside a specially designed building.In general, thermal insulation materials which exhibit shrinkage or thermal shock

upon exposure to fire give their best performance when applied in double layerswith staggered joints to prevent direct impingement of flame on the pipe through



10/14

124

breaks in the insulation system. Fire resistance is also influenced by the thickness of

insulation applied, and by the coating or covering applied.The fire performance data made available by Union Carbide Corporation and

Dow Chemical Company on thermal insulation systems evaluated by this test pro-cedure are presented in Table 5. Because of economic and air pollution considera-

tions,it is doubtful that many more tests will be conducted on this scale; smaller-

scale, less polluting tests are being developed.

Table 5. Fire Endurance of Pipe Insulation Systems.



11/14

125

Table 5. Fire Endurance of Pipe Insulation Systems (continued)

Many of the thermal insulation materials listed in this table are no longer com-

mercially available in the compositions tested because of replacement of any asbes-tos used and other composition changes, but they are included for the followingreasons: first, the availability of as complete a compilation as possible provides auseful information source to workers in this field, and is particularly helpful to

those developing smaller-scale tests; second, information on insulation materialsused for years, though in many cases replaced with asbestos-free modifications,helps those experienced with insulation to relate this type of information to their

experience; and third, many materials which are no longer commercially availableare still in service and may continue to be in service for years.A smaller-scale fire exposure test for pipe insulation which offers the advantages

of lower cost and reduced smoke generation has been developed [ 10] . The fire

performance data made available by Union Carbide Corporation on materials evalu-ated by this test are presented in Table 6. On the basis of test results, it appears that

some of the asbestos-free insulation materials are comparable in fire behavior to the

older asbestos-containing types.



12/14

126

Table 6. Fire Performance of Pipe Insulation Materials in 2%-Inch Double Layer

Application.

FIRE TOXICITY CONSIDERATIONS

Toxic gases generated from the pyrolysis and combustion of thermal insulation

materials can be an important factor in the ability of occupants to escape from a

burning sturcture, and in extent of fire injuries. Carbon monoxide poisoning has

been found to be a major factor in fire deaths; the extent to which gases other than

carbon monoxide increase the toxic threat in fires depends on the chemical com-

positionof

the material andon

the manyvariables in an actual fire.

A screening test method for relative toxicity used at the University of San

Francisco involves the exposure of four Swiss albino male mice in a 4.2 liter

hemispherical chamber to pyrolysis effluents generated by pyrolyzing 1.00 g of

material in a quartz tube in a tube furnace. Most of the work has been performed

using a heating rate of 40C/min from 200 to 800C without forced air flow, tosimulate the conditions of a developing fire; 270 materials have been evaluated

using this test procedure [11 ] . Relative toxic test data on some construction and

thermal insulation materials are presented in Table 7.



13/14

127

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