NATIONAL BUREAU OF STANDARDS REPORT

3229

PRELIMINARY EXPERIMENTS

WITH A

RADIANT PANEL FLAME -SPREAD TEST METHOD

by

A. F. Robertson, R. Kreisler and D, Gross

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary NATIONAL BUREAU OF STANDARDS

A. V. Aslin, Director

THE NATIONAL BUREAU OF STANDARDS

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^Office of Basic Instrumentation ^Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT

1002-10-1029 April 13, 195^

NBS REPORT

3229

PRELIMINARY EXPERIMENTS

WITH A

RADIANT PANEL FLAME-SPREAD TEST METHOD

by

A. F. Robertson, R. Kreisler and D. Gross

NBS

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Approved for public release by the

Director of the National Institute of

part, Is prohibited

dardi, Washington

The publication, repri

unless permission isot 25.D.C, Such perml ““ rt has been specifically prepared If thal Standards and Technology (NIST) ,f,forit8 own use. on October 9, 201 5.

Preliminary Experiments

with a

Radiant Panel Flame -Spread Test Method

by

Ao Fo Robertson^ R* Kreisler and Do Gross

Abstract

A flame spread test method for wall surfacing

materials involving the use of a radiant panel heat

source is describedo Measurements have been made

of the energy flux field in front of the panel and

at the surface of the specimen as mounted for test*

A method for standardization of radiant output is

describedo Flame spread data are presented for a

number of both organic and mineral base materials,,

Limitations and significance of the test method

together with recommendations for further study are

included

o

lo INTRODUCTION

I!

' For many years a simple quantitative test for determining

the flame spread characteristics of small samples of buildiqg

I materials has been needed by both industry and government,

‘ Federal Specification SS-A-ll8a describes a method for evaluating

! the fire hazard of prefabricated acoustical tile j however, the

I method used is not one suitable for evaluation and expression

of the flame spread characteristics in quantitative terms.

: The ASTM currently is considering a tunnel test method developed

I' by Underwriters Laboratories, This test method involves use

; of large specimens and requires installation and operation cf

jj expensive and bulky equipment,

i!

j The present report describes a preliminary study of a new

j' method suggested as a possible means for evaluation of flame

I spread of finish materials. The apparatus is somewhat similar

Ij to one used by the British Fire Research Board, reference /!_/

j Factors considered in selection of the apparatus Involved the

j following; (a) The method should if possible be suitable for

I quality control, and product development tests by the manufacturer

1 as well as for fire spread evaluation by a test laboratory,

j (b) The method used should yield quantitative reproducible

I results, (c) The method should require the use of relatively

compact and inexpensive test equipment, (d) Specimen size

and the time required for performance of the test should not be

large

,

_2 »

Work accomplished to date on study of the method includes

the following; (a) Assembly of the radiant panel and auxiliary

equipment, (b) Calibration of the heat output of the panel,

(c) Preliminary study of flame spread rate of finish materials.

2o TEST APPARATUS

The apparatus used for the tests described consisted of a

radiant panel with its associated gas-air supply and controls,

a frame for support of the test specimen, a calorimeter for

measuring and permitting standardization of the heat output.

Figo 1 presents a photograph of the assembly as used for some

of the work. Here the radiant panel is near the upper center

of the picture just below the hood. The specimen support frame

is to the left, and the water cooled shutter is at the right of

the panel. A filter, lower right cleans air entering the blower.

This blower supplies the gas -air mixture to the panel by means

of the vertical pipe and a venturi gas-air mixer , the latter

not shown in the photograph.

It will be recognized that the apparatus described is in

many ways similar to that described in British Standard BS 476,

1953s reference (1). The use of the particular radiant panel

and general method of test was suggested by reports made

available by the Fire Research Station of Department of Scientific

and Industrial Research, England. However, most of the work

reported here was done before BS476, 1953, which describes an

"Apparatus for Preliminary Surface Spread of Flame Test", was

released for distribution.

2.1 Radiation Panel

The radiant panel Fig. 1 & 2 consists of a cast iron frame

enclosing a porous refractory material which was imported from

England* The panel is mounted in a vertical plane. It is

fed from the rear with a pre-mixed gas-air supply, combustion

of which is nearly completed within its pores. After warm-up,

the only visible flames from the panel surface are less than 1/8

in. in length. The panel approaches its equilibrium temperature

of approximately 800° C in about 25 minutes after ignition. Three

different types of calorimeters are investigated for monitoring

the output of this panel. These included the water cooled

shutter, the copper disc calorimeter, and the radiation pyrometer.

2.2 Water-Cooled Shutter

A water-cooled shutter was constructed as a possible method

of measuring and monitoring the heat output of the panel. This

shutter shown at the side of the panel, Fig. 1, consisted of a

*The panel used for the tests reported was manufactured by

Radiant Heating Ltd,, Barnsbury Park, London, N. 1, It is 12'in.

square, their Type 1 surface combustor.

"3~

12 X 12 X 7/8 ino copper tank, one wall of which was formed of

a copper sheet l8 in<> square and l/l6 inc thicko Water was

circulated through this tank which was mounted on a set of

rollers to facilitate movement to and from a position in front

of the radiant panel

o

2o3 Disc Calorimeter

This calorimeter consisted of a copper disc with a copperconstantan thermocouple swaged into its rear surface, Figo 3 & 5°

The front face was coated with acetylene black the remainder

being polished o It was supported by wires within a cylindrical

stainless steel shield which in turn was supported by fine wires

within a frame used for positioning it before the panelo The

shield was used in an attempt to reduce heat transfer by

convection between the back of the disc and ambient gases,

2 A Test Specimen Holder

A holder made of 1” angle iron which could be set at various

distances from and angles to the radiant panel was used to hold

the 6” X 12” samples for the test. This frame, Fig. 1 & 2, was

hinged to permit loading new specimens outside the radiation field,

2,5 Timer

A record of the time of start of a test as well as the

Initiation of flaming and progress of the flame front past

various stations along the length of the specimen was secured

by means of an electrically operated paper tape chronographo

3, TEST METHODS AND RESULTS

The work reported here involved the study of a new test

method. Because of this no strictly uniform test procedure was

used through all the tests performed. The test methods mentioned

below are those used for the greater part of the work,

3<>1 Performance of Radiant Panel

(a) Measurement of Radiant Output

A test method such as the one proposed requires a simple

positive method of standardization of the energy output of the

radiant panel. Since it is energy output rather than panel

temperature that determines the heat transfer rate, it is desirable

that the method used for standardization of this rate measure

radiant energy rather than panel temperature. The emissivity cf

the panel surface is likely to change with use and thus measurement

and control of panel temperature does not provide an effective

means of control of radiant energy output. As mentioned earlier

three different methods were used for measuring output of the

panel, these included the water cooled shutter, the disc

calorimeter, and a wide angle radiation pyrometer. The latter

device proved the most practical but was used for only the

latter part of the experimental study. In use this pyrometer

was sighted on the 10 in. diameter circular central area of the

panel.

Measurements with both an optical and radiation pyrometer

permitted an estimate to be made of the surface temperature

and emissivity. These were found to be 790°C and O. 7O

respectively. Temperature variations across the heated face

of the panel were estimated to be on the order of 48°C.

During a series of 4-0 tests performed over a period of 13

days , it was found easy to maintain the energy output by means

of the radiation pyrometer within ±5^»

The total rate of heat output of the radiant panel was

estimated to be 63OO cal/sec or about 2^ kilowatt. This

estimate is based on the known gas supply rate and the

heating value of this gas. From the measured panel temperature

and emissivity it is estimated that only 17^ of this heat

output is released as radiant energy.

(b) Energy Field

The energy field produced by the panel was surveyed by

means of the disk calorimeter and a high speed recording

potentiometer. The calorimeter was, as mentioned previously,

surrounded by a shield in an effort to reduce convection heat

transfer between the back of it and ambient gases. Fig. 5

shov/s a sketch of this calorimeter as used for these tests.

In surveying the energy field before the plate a grid

system was selected and defined as shown in Fig. 6 . Measurements were made at various distances perpendicular to the panel

face at many of the coordinates marked here. Figs. 7 5 8 and 9

present the result of these measurements for four of the

vertical planes defined in fig. 6 , Fig. 8 presents a plot of

the panel, as measured for four of the coordinates lying in a

horizontal plane passing thru, the center of the panel.

The measured energy intensities just mentioned were

considerably different from those effective in heating

specimens when exposed during test. Fig. 9 shows the results

of measurements made when the energy field was changed by

introduction of a dummy specimen of incombustible material

within which the calorimeter could be mounted. This was

accomplished by boring a number of holes through the specimen

within which the disc calorimeter could be mounted with its

surface flush with the specimen. When making these measurements

the unused holes were plugged to present a continuous unbroken

surface. The considerable increase in measured intensity near
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these measurements were made prior to adoption of the radiation

pyrometer as a standardization procedure,

3o2 Flame Spread Tests

(a) Specimen Orientation

During initial tests it was found that orientation of the

specimen in a vertical plane, similar to that mentioned in

BS 476 , resulted in a curved flame front the position of which

was hard to define at a given time. In other tests of specimens

having surfaces finished with a fire retardant paint it was

found that ignition and burning took place on the edges and

back of the specimen rather than on the surface exposed to

radiation.

Several other specimen positions were tried and it appeared

that a favorable arrangement involved placing the specimen face

down inclined at an acute angle to the surface of the radiant

panel. With the specimen in the position shown in figure k,

it was ignited across the top and the flame front, which for

homogeneous materials was nearly straight, was not obscured

by smoke and flames, therefore its progress down the specimen

was easy to follow. It was because of these advantages that

this position was chosen for the tests herein reported.

(b) Test Procedure

In performing the flame spread tests the radiant panel

was ignited and allowed to heat for about 25 minutes. The

radiant output was then measured and if necessary adjusted with

the aid of the radiation pyrometer. A small pilot flame was

located above but not in contact with the upper edge of the

specimen to ignite combustible gases leaving the specimen

during radiant exposure, A pre-conditioned test specimen was

then mounted in the holder and after starting the chronograph

tape the specimen was swung into position in front of the panel.

An electrical push button switch was used in the timing marker

circuit and was actuated at specimen exposure, ignition, and

arrival of the flame front at the 3? 6, 9, and 12 inch

stations along the length of the specimen.

The duration of the radiant exposure varied from less than

a minute to over three minutes depending on the behavior of the

specimen being tested. It was estimated that when testing a

number of specimens, the average time interval required per unit

would be about five minutes.

-6 -

(c) Typical Test Results

Table I presents a list of various organic-base materials

which have been tested. A plot of the position of the flame

front vs time for two specimens of each of these materials is

shown in figure 10. Table II and Fig. 11 presents similar data

on several composite or mineral base materials tested. Fig. 12

presents data taken over a period of 12 days during 6 of which

tests were performed on similar specimens of a factory finished

fiber board. This gives some indication of the day to day

reproducibility of data on a given specimen.

4. DISCUSSION

Study of the flame spread-time data of figures 10, 11, and

12 indicate an initial rapid flame spread over the first 3 to

6 in. of the specimen with a slower progress over the remaining

exposed face. Referring to figure 9 it will be observed

that performance such as this is to be expected since the energy

flux incident on the specimen decreases very rapidly in the

range from 0 to 4 in. It seems quite likely that more uniform

rate of flame spread over the specimen face could, if desirable,

be achieved by modification of the position of the specimen

with respect to the radiant panel.

Experience with the test method has indicated that the

tests are simple to perform and tests on 10 or 12 specimens

can be completed within an hour. Such repetitious testing

would however require modification of the specimen support

frame. It is believed that a number of light sheet metal or

wire frames which could be clipped onto the radiant panel would

be more desirable both from the standpoint of preventing

preheating of the specimen before exposure, and maintenance

of a standardized specimen location with respect to the radiant

source

.

No attempt has as yet been made to define the scale by

which flame spread hazard of the various materials would be

specified when a test method such as this is -used. This might

be done in a number of different ways;

1. Flame spread rate at a given position along the

specimen length.

2. Time after exposure required for flame to travel

a given length on the specimen.

3o Combination of 1 or 2 and maximum flame spread

observed on specimen.
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covering the range of 1 = 6 ino/min to l5->6 in. /min for the 9 in.

position and the materials shown in figure 10. Figure 12

indicates that the 6 or 9 in. positions would probably be the

most desirable ones at which this flame spread rate should be

measured. Selection of one of these stations would, however,

provide an indeterminate classification for materials such

as two of those shown in figure 11 where the slope of the time

distance curve is uncertain because the flame did not travel

the full lengLxi of the specimen.

A test method of the type described does not provide a

quantitative method for distinguishing between materials which

exhibit a large heat release on combustion and those which do

not. An example of this is the mineral base acoustic tile

No. 12 which exhibits initial flame spread characteristics

quite similar to some of the organic base materials but is

known to release only a small amount of heat on combustion,

A measurement of the heat release of the materials tested might

be made by determining the maximum exhaust gas temperature.

This difficulty is not unique to the test method as it is also

encountered with the tunnel test methods now under study by

ASTM.

5. RECOMMENDATIONS FOR FURTHER STUDY

It is suggested that as time and man power are available

study of this test method should continue. The following factors

seem to require further experimental work;

1. Revision of the specimen support frame.

2. Installation of some sort of short stack above the

upper end of the specimen to permit measurement of

exhaust gas temperature and smoke produced by the

specimen.

3. Study of ambient variables on test results,

h. Study of desirable conditioning and pre-test

storage methods to be used for specimens.

5. Perform a series of tests on a wide variety

of materials and if possible obtain a correlation

with other flame spread test methods,

6. REFERENCE

1. British Standard BS476-1953 "Fire Tests on Building Materials

and Structures" British Standards Institution, 2 Park Street,

London Wl.

This standard may be obtained from the British Information

Services, 30 Rockefeller Plaza, New York 20, New York.
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THE NATIONAL BUREAU OF STANDARDS

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3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

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