NATIONAL INSTITUTE OF STANDARDS &

TECHNOLOGY

Research Information Center

Gaithersburg, MD 20890

A11100 nOBbQ PUBLICATION

NBSIR 80-2170

Fire Protection Systems for Rail

Transportation of Class A

Explosives: Interim Report

Richard W. Bukowski

Center for Fire Research

National Engineering Laboratory

National Bureau of Standards

U S. Department of Commerce

Washington, DC 20234

November 1980

'’repared for: .S. Department of Transportation

. U56 Vashington, DC 20590

80-2170

1980

100 : ederal Railroad Administration

NBSIR 80-2170

FIRE PROTECTION SYSTEMS FOR RAIL

TRANSPORTATION OF CLASS A

EXPLOSIVES: INTERIM REPORT

National Bureau of Standards

Library, EOl Admin. Bldg.

FEB 2 7 1981

06 CLCC- -Ci'rc . &Ctoo

no. 26 io

Mfo

Richard W. Bukowski

Center for Fire Research

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Washington, DC 20234

November 1980

Prepared for: U.S. Department of Transportation

Federal Railroad Administration

Washington, DC 20590

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology, and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director *
e, ;t" • b .. *.• J ii . ! a

i .nlfT.'i 10 j .V'SIdij

<>1 fS £31
TABLE OF CONTENTS

Page

LIST OF FIGURES iv

Abstract 1

1. INTRODUCTION 2

2 . BACKGROUND 2

3. SCOPE 4

4. HAZARD SCENARIOS 4

4.1 Derailments 5 4.2 Boxcar Fires 5 4.2.1 Brake Fires 5 4.2.2 Yard Fires 6

5. FIRE PROTECTION SYSTEMS 6

5.1 Active Systems 7 5.2 Passive Systems 9

6. EXPERIMENTAL PROGRAM 9

6.1 Reduced-Scale Steel Floor Test 10

6.2 Reduced-Scale Wood Floor Test 11

6.3 Full-Scale Steel Floor Tests 12

7. HAZARD CRITERION 12

8. DISCUSSION OF RESULTS 13

8.1 Reduced-Scale Steel Floor Tests 13

8.2 Reduced-Scale Wood Floor Tests 13

8.3 Full-Scale Steel Floor Tests 14

9. DEVELOPMENT OF COMPUTER MODEL 14

9.1 Model Assumptions 15

9.2 Model Predictions 16

9.3 Model Limitations 17

10. CONCLUSIONS 17

11. RECOMMENDED FURTHER WORK 18

12 . REFERENCES 18

iii
LIST OF FIGURES

Page

Figure 1. Photograph of reduced-scale steel floor test

assembly and burner 19

Figure 2. Photograph of reduced-scale wood floor test

assembly and burner . 20

Figure 3. Photograph showing boxcar wall involvement

several minutes after flame penetration of

floor. . . 21

Figure 4. Photograph of full-scale steel floor test

assembly and burner 22

Figure 5. Summary of data from reduced-scale steel floor

tests 23

Figure 6. Test results for reduced-scale steel floor

test no. 6 24

Figure 7. Test results for reduced-scale wood floor

test no. 10 25

Figure 8. Predicted vs measured temperatures at 30

minutes - full-scale tests 26

Figure 9. Predicted vs measured temperatures - 500’ KW

fire without insulating barrier 27

Figure 10. Predicted vs measured temperatures - 500 KW

fire with 1 inch calcium silicate board as a thermal barrier 28

Figure 11. Predicted top surface temperature of 1 inch

calcium silicate board at 30, 60, and 120

minutes as a function of fire size 29
FIRE PROTECTION SYSTEMS FOR RAIL TRANSPORTATION OF

CLASS A EXPLOSIVES: INTERIM REPORT

Richard W. Bukowski

Abstract

As a result of several accidents involving fire

induced detonation of military explosives during rail

shipment, a research project, funded by the Federal

Railroad Administration (FRA), was initiated at the

Center for Fire Research (CFR) at the National Bureau

of Standards (NBS) . This project was initiated to

evaluate various methods of protection of Class A

explosives from fire, and to identify one or more

cost-effective approaches which could be explored

in greater detail in later studies.

Active systems (detection, notification, and

extinguishment) and passive systems (thermal insulating

barriers) were evaluated regarding cost, feasibility

and level of protection provided for the major hazard

scenarios involved in rail shipment of explosives.

The passive, thermal barrier approach was selected

as the most reliable and less costly of the options

studied while providing an acceptable level of

protection.

Small-scale and full-scale tests were conducted

to obtain performance data on one specific thermal

barrier material. Based on this data, a computer

model was developed which can predict temperatures

of the boxcar floor, top surface temperature of a thermal barrier, and casing/explosive interface

temperature of a wood-pallet mounted bomb for a range of fire sizes. The model predictions compare

favorably with measured results from a limited

number of experiments. Further experimental data

are needed to refine the model and establish an acceptable confidence level in the predicted values.

The proposed work necessary to provide this refinement and verification is described.

Key words: Bombs (ordnance); boxcars; computer

models; fire detection systems; fire suppression;

full-scale fire tests; heat transfer; railroad

accidents; small-scale fire tests; thermal protection.

1
1. INTRODUCTION

Accidents involving rail transport of hazardous materials are receiving increasing public attention. While most of these accidents

involve private sector shipments of industrial chemicals, the Federal

Government, primarily the Department of Defense (DOD) , ships significant

quantities of explosives by rail. Although there have been only eight

rail accidents since 1973 involving military shipments of munitions

(with no fatalities) both DOD and the Federal Railroad Administration

(FRA), which regulates the railroad industry, recognize the need to ship

hazardous cargoes safely.

In response to this need, in 1979 the FRA initiated a contract with

the Center for Fire Research (CFR) at the National Bureau of Standards

(NBS) to do a preliminary study of possible methods of protecting shipments of military explosives from fires occurring during rail transport.

Since DOD also has an ongoing project in this area titled "Safe Transport

of Munitions (STROM)", every attempt was made to coordinate the two

projects to minimize unnecessary duplication of effort.

2 . BACKGROUND

In 1973 two separate railroad accidents occurred in the western

U.S. involving trains carrying military shipments of bombs. Although

neither accident resulted in fatalities, both resulted in extensive

damage to railroad equipment and one resulted in considerable civilian

property damage and several injuries. Due to the extent of damage, and

the likelihood that under slightly different circumstances injuries and

deaths could have been extensive, these two accidents were investigated

in greater depth than most incidents. The National Transportation

Safety Board (NTSB) investigation report of the Benson accident resulted

in several safety related rules changes which impacted on the problem.

And in addition, the information from both of these detailed investigations were beneficial in developing the necessary research plan for this

study.

The first incident occurred on April 28, 1973, in a railroad yard

at Roseville, California [1]^. On the day of the accident, the yard

contained 21 boxcars loaded with 250 lb (114 kg) bombs, 10 car loads of

LPG gas, and two propane tank cars. A fire occurred from some undetermined cause which exposed these cars and resulted in the complete or partial detonation of 18 of the 21 bomb cars. The ensuing fire and

explosions resulted in 63 injuries and required evacuation of 4700

persons from the area. Shrapnel and debris was spread over a one mile

area with one house set afire almost one mile from the center of the

explosions; windows were broken in downtown Roseville - three miles

away, as well as in Rocklin almost ten miles away. The town of Antelope

Numbers in brackets indicate the literature references at the end of

this paper.

2
(less than one mile from the explosions) was almost totally destroyed,

with several buildings including the fire station leveled and burned.

The estimated direct cost to the U.S. Government as a result of the

accident was on the order of $5 million.

The second accident occurred on May 24, 1973 (36 days later) in the

area of Benson, Arizona [2], In this accident, a 106 car freight train

carrying 12 boxcars loaded with 500 lb (228 kg) bombs was proceeding at

approximately 45 miles per hour (73 kmph) when an explosion occurred in

one of the cars. As the train was brought to a stop, a second and third

explosion occurred resulting in a crater 115 feet (35 m) long by 93 feet

(28 m) wide by 25 feet (8 m) deep. The explosions scorched the desert

for about 1/4 mile (400 m) in all directions, destroying 460 feet (140

m) of track and road bed. All but approximately 500 of the 2,600 bombs

either exploded or were destroyed. Unexploded bombs were thrown as far

as one mile (1.6 km) from the main crater area and windows were broken

in a home 5 miles (8 km) from the accident site. Fortunately, there

were only two minor injuries of train crew members which occurred when

they jumped from the moving caboose.

Availability of information from investigation of the Roseville

accident has been somewhat limited due to litigation which was finally

completed in mid 1980. However, a litigation report released in June

1980 [3] indicated that the most probable causes were either: 1) a misapplied brake shoe on one explosives car which initiated a subcar

fire as the train entered the rail yard, or 2) a yard fire of unknown

origin occurring just after the train entered the rail yard.

Investigations indicated that the probable cause of the Benson

accident was overheated or sparking brakes on one of the bomb cars which

resulted in ignition of the underside of the wood boxcar floor. The

subfloor fire subsequently penetrated the floor and caused the initial

explosion.

Attempts were made to replicate the scenario through full-scale

testing conducted at the Naval Weapons Center, China Lake, California in

conjunction with the U.S. Government defense of law suits extending from

the two accidents. These tests revealed that a fire of moderate size,

burning under a wood floored boxcar, can penetrate the floor and expose

the bombs in as little as six to ten minutes. Additional tests demonstrated that cast iron brake shoes can, under conditions of jamming or heavily applied braking, produce sufficient sparks or can overheat

wheels to the point of ignition of the underside of the car.

As a result of the Benson accident, several rule changes were

promulgated by the FRA in an attempt to reduce the likelihood of this

type of incident [4]. These rule changes required that only the lower

sparking type of composition brake shoes be used on boxcars carrying

Class A explosives and that the cars be equipped with either a continuous

steel subfloor or metal spark shields located over each of the truck

assemblies. Also, explosive cars were required to use only roller type

bearings. These employ a sealed lubricant system that does not cause

3
excess lubricating oil to be sprayed on the underside of the car during

transit which is typical with the older "friction" type bearings.

Finally, the new rules required increased pre-and post-loading inspection

and increased surveillance during transport.

In addition to these rule changes, the National Transportation

Safety Board recommended that the FRA. and DOD conduct studies of other

methods of protecting Class A explosives from fire while being transported by rail.

Shortly after the study began at NBS, it was determined that the

DOD was conducting a concurrent study of rail transport of Class A

explosives. Contact was made with the DOD project manager and meetings

were arranged to coordinate the studies to minimize unnecessary duplication of effort. From that time on, NBS project personnel have attended

the regular DOD project meetings in order to enhance coordination between

the two programs. Furthermore, since the DOD project was well established, the scope and plan of the NBS project was adjusted to complement

similar research areas in the DOD project and to focus on research areas not generally pursued under the DOD project.

3. SCOPE

The scope of the project at CFR was essentially to evaluate current

methods of fire protection technology and to determine which would best

apply to the hazards of rail transport of Class A explosives.

One of the subtasks included in the DOD project (conducted by the

Naval Weapons Center, China Lake, California) involved the development

of an "on board" detection and suppression system. However, because a large portion of the testing in this subtask was being conducted as part

of the U.S. defense in the law suits extending from the Benson and

Roseville accidents, it focused exclusively on wood floored railcars.

Discussions with DOD personnel revealed that since the 1973 accidents,

the character of the boxcar fleet has changed. Wood floored boxcars are becoming obsolete and are being replaced by all steel cars. DOD records

indicate that by 1979, 85 percent of all Class A explosive shipments

were being made in all steel cars [5]. And, it is expected that this

percentage will be continually increasing as the wood floored cars reach

their end of service life and are replaced by steel cars. Based on this

information it was determined that NBS could provide a significant

contribution by focusing its study on the all steel boxcar.

4. HAZARD SCENARIOS

The first step was to identify the major hazard scenarios involved

in the transportation of munitions. Once these scenarios had been

defined, various intervention strategies could be evaluated in order to

determine which will best prevent fire penetration of the railcars, and

subsequent detonation of the bombs. A study of railroad and DOD accident

4
reports resulted in the identification of three principal fire hazard

scenarios. These included derailments, brake fires, and rail yard

fifes

.

4.1 Derailments

By far, the largest single accident type involving rail shipment is

derailment. However, since munitions are unfused during shipment, their

sensitivity to detonation from impact is relatively low. Thus, the

derailment of munition cars would not ordinarily result directly in

explosion unless the derailment led to a fire or explosion of some other

hazardous train cargo.

Another difficulty encountered in derailment is that the resultant

damage to the boxcars carrying the munitions is unpredictable. The cars may remain upright or may be thrown into any conceivable orientation.

In addition, the cars could be torn open and the load spilled. This

makes the derailment situation complex, and a difficult one to handle

due primarily to its unpredictable nature. This complexity suggests

that the best approach is to try to prevent the derailment in the first

place

.

One of the sub tasks in the DOD project involved the study and

prevention of derailments. The primary causes of derailments identified

by the DOD study were 1) bad track, and 2) instability of the car when

rounding curves, a result of improper train make-up (short/long or heavy/light car combinations) . Since the causes and solutions to the

derailment problem do not directly involve fire hazards it was felt that

this problem was not reasonably within the scope of the NBS study and

was not addressed.

4.2 Boxcar Fires

As exemplified by the Roseville and Benson accidents, there are two

primary scenarios for boxcar fires. The first, similar to the Benson

accident, involves sparking brake shoes or overheated wheels or bearings

igniting the underside of a wood floored car. Fire subsequently burns

through the floor and exposes the load. The second is the rail yard

fire (Roseville) where a boxcar is exposed to an external fire while the

car is stationary in a yard awaiting a train make-up. These two fire

related scenarios involve different effects as a function of whether the

boxcar is a wood floored or a steel floored car.

4.2.1 Brake Fires

Sparking brake shoes or overheated brakes or wheels are significantly

more hazardous with wood floored cars than with steel floored cars. This is because of the combustibility of the flooring material itself

and the fact that the underside of the wood floor typically becomes

coated with lubricating oil sprayed from the friction type wheel bearings

common on older wood floor cars. Once the underside of the flooring is

5
soaked, the hot sparks associated with a heavy application of brakes, or an overheated wheel or bearing resulting from a stuck brake, can ignite

the underside of the wood floor. Steel cars typically employ roller

bearings which do not produce this oil spray.

As was discussed earlier, rule changes promulgated following the

Benson accident required the use of lower sparking types of brake shoes

and spark shields to protect the underside of wood floored cars from

this exposure. A review of accident records since the promulgation of

these rules indicates that no further subfloor fires have occurred in cars so equipped [5].

In the case of steel floored cars the noncombustible nature of the

flooring minimizes the hazard from sparking brakes or overheated wheels.

For example, data taken by the Southern Railroad shows that the typical

radiant flux from a locked brake with cast iron brake shoes is as high

as 7 to 10 BTU per square foot second (24 to 34 kw/m2

) (with heavy

sparking) , and from a composition brake shoe is only 2 BTU per square

foot-second (7 kw/m2

) (no-sparking) for a maximum period of 10 minutes

before the brake burns out [1]. In the case of steel floored cars this

level of radiant flux would be insufficient to cause a fire problem in

the load. Thus, the lack of fires subsequent to the rule change in wood

floored cars, and the apparent lack of hazard potential due to locked

brakes and brakes sparking in steel floored cars, coupled with the

current 85 percent steel car shipping rate all indicate that this

scenario is no longer a significant problem.

4.2.2 Yard Fires

Railroad and DOD munition transport data indicate that as much as 90 percent of the total transit time of a munitions laden car is spent

stationary in rail yards awaiting train make-up [5]. Also, yard fires are relatively common, particularly in certain yards. For example, it was reported that the Roseville, California yard averaged one fire every

three days for seven years prior to the accident [1],

Most major U.S. cities contain rail yards, often near high density

population areas. Since trains carrying explosives are not specially

routed to avoid these yards (such routing would be impractical) , the

consequence of an accident like Roseville occurring in one of these

yards could be catastrophic. The possibility of a large population

exposure, coupled with an apparent high incidence of yard fires and

long standing time in the yards, make the yard fire the primary scenario

of interest.

5. FIRE PROTECTION SYSTEMS

Having identified the exterior exposure fire in the rail yards as the most frequent scenario, the focus was directed at identifying various

alternative fire protection systems which would be effective intervention

strategies for this scenario, and assessing their feasibility in terms

of implementation. Fire protection systems can be divided into two

6
general categories: active systems (including automatic detection,

notification, and extinguishment) and passive systems (structural

elements which will resist the fire exposure and prevent the development

of a hazardous condition in the load for a given period of time)

5.1 Active Systems

An active fire protection system designed for use in a railcar

would consist of a number of hardware components. Automatic fire

detectors would be required on the interior and/or exterior of the car

to provide early detection of an impending hazardous condition. These

detectors would need to be connected to a control/notification system by

which the train crew, or in the case of a car in a rail yard the yard

master, would be notified that a fire condition existed and the system

had activated. Also, an "on-board" extinguishing system, actuated by

the detection system, would be necessary to prevent fire spread to the

interior of the car.

A study was conducted in 1977 by the Naval Civil Engineering

Laboratory at Port Hueneme, California in which a heat-actuated alarm

system for railroad boxcars carrying explosives was developed [6]o The

system consisted of bimetal snap-disk type heat detectors located on the

underside of the car above each of the truck assemblies, connected to a control and radio transmitter unit which transmitted an alarm signal to a receiver unit located in the caboose.

Tests conducted as part of this study indicated that the design was feasible in that it would not activate from a heavy application of

brakes but would detect an overheated wheel, and could transmit signals

reliably to the caboose in the case of long trains or trains running

through tunnels. Problems of shock and vibration associated with rail

transport could be overcome by proper shock mounting and design of

system components. However, a major problem with this approach was that

dirt and grime accumulation on the underside of boxcars would necessitate

a regular maintenance program of cleaning the sensors.

As part of the DOD project, the Naval Weapons Center at China Lake,

California built upon the Port Hueneme system. Smoke detectors were

added in the interior of the car to provide a "second line of defense"

should the subcar heat detection system fail to operate due to this

grime accumulation. Their tests indicated that (for wood floored cars)

a subfloor fire created sufficient smoke inside the car to actuate

interior smoke detectors before the floor was breached by the fire. In

tests run with both ionization and photoelectric type smoke detectors in

the car, their test data showed a stronger signal from the photoelectric

type due to the character of the smoke produced.

Furthermore, advancements in electronics during the time period

between the Port Hueneme tests and the Naval Weapons Center tests resulted

in the availability of an improved transmitter unit capable of identifying

the specific car and the specific sensor activated.

7
The Naval Weapons Center work also included the development a prototype suppression system. This system consisted of a 300 gallon

(1135 £) pressure tank using several nitrogen cylinders as a pressurizing

source. When activated by the detection system, the tank was pressurized

and, after pressure stabilization, water was provided to a series of fog

nozzles located around the interior of the car and connected to the

pressure tank with high pressure flexible hose. In order to maximize

the amount of operating time, the system was designed to apply water for

30 seconds and then shut off for 30 seconds and repeat. This would

result in an approximate 1-1/2 hour water supply with 200 gallons (757 £)

of water in the tank.

Several tests conducted on this system indicated that a significant

amount of interior wetting on the floor and walls was achieved. However,

a major problem as yet unresolved is that the fire could develop in the

open stud space between the exterior metal wall and the interior plywood

sheathing. In this location, the fire would be shielded from the water

fog and could spread up through the stud space and involve the combustible insulation on the interior of the roof. Currently, it is unknown

whether the cooling effect of the water fog would prevent the achievement

of critical temperatures and explosions in the load if this roof insulation became involved.

While this active system appears to provide an improvement in fire

protection, there are a number of disadvantages, including cost and

complexity. Naval Weapons Center estimates for the cost of such a complete active system are over $6,000 per car [1], This cost is some- what variable as a function of the number of cars per train since only

one receiving system is necessary for each train. Not included in these

cost figures would be the cost of installing a receiver system in each

yard master’s tower in every rail yard through which these trains pass.

This would obviously be necessary because of the 90 percent of transit

time spent in yards and the frequency of yard fires. The system would

also need coordination among the car identification codes so that, if

more than one train is in the yard at a given time, the specific car could be identified quickly.

The complexity problem is even more difficult to overcome. This

active system involves a large number of individual components, both

electrical and mechanical, which must all function reliably. In addition, the system must be portable in nature and capable of rapid and

reliable installation and removal by untrained personnel. There are any

number of things that can go wrong with such a system including loss of

primary power (batteries), improper installation, component failure, and even the failure to properly activate the system, such as a switch or valve being left closed when installed. Also, the portable nature of

the system could result in difficulties in securing the equipment

adequately within the car. For example, in the case of a derailment, it

is possible that the impact would tear loose the heavy suppression

equipment and damage both the load and the protection system.

8
All of these factors have a detrimental effect on the feasibility

of such an active system.

5.2 Passive Systems

In contrast to the active systems approach which serves the

multiple-purposes of detection, notification, and extinguishment or control; passive fire protection measures serve a single purpose - containment. That is, passive fire protection measures involve structural elements which are designed to withstand the effects of a given

fire exposure for a period of time.

The primary advantage of passive systems is simplicity. If properly

designed and installed, a passive fire protection system will function

until its design limits (fire severity and time) are exceeded. In

addition, passive systems require little or no maintenance and have an almost unlimited useful life unless physically damaged.

For the specific case of fire protection of explosives in rail

transport, and particularly in the case of the all-steel car, the

passive protection approach is an appealing alternative to the more

complex active systems. The steel car flooring is noncombustible and

will maintain its structural strength when exposed to temperatures

exceeding 1000°C. Therefore, the more immediate problem in a fire

situation is not structural collapse, but rather that the steel floor is a good conductor of heat. When the floor temperature reaches approximately 400°C, the wooden pallets on which the explosives rest will begin

to burn. Once this combustion takes place, the explosives will be

heated to their critical temperature quickly and detonation will occur.

In addition, even if the ignition temperature of the wood is not reached,

heat transfer through the steel by radiation and convection can even- tually heat the bombs to their critical temperature anyway. Therefore,

the problem associated with exposure fires becomes one of simply maintaining an interior floor temperature at a low enough level so as to

both prevent ignition of the wood pallet and overheating of the load.

Since the passive approach appeared to be feasible, less costly,

and potentially more reliable than the active approach, and since

considerable work on the active system had already been done as part of

the DOD project, it was decided that the major effort of this project

should be concentrated in the area of the passive protection. Therefore,

an experimental program was developed to evaluate the feasibility, and

estimate the performance of a passive protection system for the yard

fire scenario.

6. EXPERIMENTAL PROGRAM

Three groups of experiments were conducted as a part of the program.

They included simulations of (1) reduced scale tests of a wood palletmounted bomb in a steel floored car, (2) reduced scale tests of a wood

pallet-mounted bomb in a wood floored car, and (3) full-scale tests of a wood pallet-mounted bomb in a steel floored car with and without an

9
insulating, thermal barrier above the floor. Since the DOD project

included full-scale tests with wood floored cars, it was decided that it

would not be necessary to reproduce these tests. The purpose of these

tests was to obtain the necessary data to evaluate the feasibility and

performance of a passive thermal barrier in preventing the attainment of

critical load temperatures when the railcar is exposed to an external

fire and to develop a first approximation computer model to estimate the

temperature rise in a typical explosive load as a function of fire size

and characteristics of a thermal barrier.

6.1 Reduced-Scale Steel Floor Test

Considerable experience has been attained at NBS and other fire

research organizations in recent years in reduced-scale physical testing

in order to reduce the cost of collecting performance information by

full-scale replication testing. Reduced-scale tests permit the researcher

to gather approximate, qualitative information concerning the performance

of a structure or configuration so that the number of full-scale tests

necessary to fully define the situation can be minimized.

The initial experiments were conducted in a 1/3 scale configuration.

For these experiments, the boxcar floor was represented by a 2 foot (61

cm) square, 10 gauge flat steel plate. The bomb was represented by a 12

inch (30 cm) long, 2 inch (5 cm) diameter steel pipe threaded at both

ends and fitted with steel end caps. The pipe was rested on two 1-inch

(2.5 cm) high pine strips representing the wood pallet. The pipe was filled with sand to represent the explosive (as is normally done in

thermal testing of inert munitions) and thermocouples were mounted on

the inside and outside of the pipe as well as on the upper surface of

the steel floor-plate directly above the fire.

The test assembly was suspended 12 inches (30 cm) above a burner

formed of 3/16 inch (0.5 cm) inside diameter stainless steel tubing.

This burner was fed from a methane tank through a calibrated flow meter

and pressure regulator. Figure 1 shows the test assembly and burner.

The tests were conducted in a laboratory fume hood, which caused

a small air flow across the assembly. Air flow measurements with a hot

wire anemometer indicated a peak flow of 50 feet per minute (0.25 m/sec).

It was felt that this air flow had a minimal affect on the temperatures

achieved.

Four tests were run (tests #5-8) at constant heat release rates

ranging from 1.8 to 11 kilowatts for the 30-minute test period. During

each test, three temperature points were recorded at 30 second intervals

by an automatic data acquisition system. The system had an overall

accuracy (including conversion to temperature) of plus or minus 3°C.

10
6.2 Reduced-Scale Wood Floor Test

Discussion of the results of full-scale wood floor car tests

conducted at China Lake with Naval Weapons Center personnel revealed

several interesting phenomena. Shortly after ignition, dense smoke was

observed in the interior of the car, coming from the seams between the

wooden floor boards. This smoke production appeared to increase con- tinually until, between 6 and 10 minutes after ignition, small flames

broke through at these seams. It was then observed that these flames

spread along the seams, eventually reaching the floor /wall junction.

Smoke issuing from the seams near the wall appeared to be drawn into the

wall "stud space" which acted as a chimney. When the flames at the

floor board seams reached the floor/wall junction, the gases contained

in the smoke in the stud space were ignited, resulting in flame spread

up through the stud space and out the top, impinging on the insulation

on the underside of the roof. Since this insulation is combustible,

fire involvement of the entire car interior was observed shortly afterward. Examination of the damage after the test indicated that the floor

had burned through above the fire and that the plywood sheathing on the

interior side of the wall had burned completely through between the two

wall studs. The metal exterior car wall showed a clearly defined burn

pattern at the affected stud space.

It was desirable to be able to reproduce the above described

phenomenon to obtain a relative comparison of the performance of the

wood and steel floored cars in the 1/3 scale test arrangement. Two

floor/wall sections were constructed of 3 foot (0.9 m) long tongue and

grooved oak flooring bolted at both ends to steel angles representing

the side rails of the car. Wall studs were formed of 3/4 by 1-1/2 inch

(1.9 by 3.8 cm) pine strips with a light gage steel plate forming the

exterior wall and 1/4 inch (0.6 cm) plywood forming the interior wall

surface. The bottom edge of the plywood interior wall surface was 1/2

inch (1.3 cm) above the floor as is typical in wood floored boxcars.

The 2 inch (3 cm) steel pipe and 3/4 by 1 inch (1.9 by 2.5 cm) wood

strips upon which the pipe rested were the same as used in the 1/3 scale

steel floor test. The same gas burner was used, and the spacing between

the gas burner and the underside of the floor was maintained at 12

inches (30 cm). The rate of heat release used for both of the wood

floor tests was 11 kilowatts. Figure 2 shows the reduced scale wood

floor test assembly.

In the first wood floor test (test #9) the simulated bomb and

burner were located in the center of the floor. For the second wood

floor test (test #10) the bomb was located approximately 4 inches (10

cm) from the floor/wall junction. In test #9 the entire burner flame

was contained under the floor and in test #10 (due to the proximity to

the wall) a portion of the flame curled over the edge and up the outside

wall

.

11
The phenomenon observed in the full-scale wood floor tests conducted

by Naval Weapons Center was also observed in the reduced scale tests

conducted at NBS. That is, dense smoke issued from the floor seams,

flame penetration at the seams was observed in about 8 minutes, and

(test #10) one of the wall stud spaces became involved shortly after

flame appeared through the floor (see figure 3)

.

6.3 Full-Scale Steel Floor Tests

Eight experiments were conducted in the full-scale steel floor

tests, five without an insulating barrier and three with a barrier. Two

64 by 40 inch (163 by 102 cm) test specimens of nailable steel boxcar

flooring were obtained from a manufacturer. These floor sections were rested on concrete blocks 3 feet (1 m) above the test burner. No attempt

was made to simulate walls or the enclosed volume of a railcar. The

burner used for the full-scale tests was a 12 inch (0.3 m) square diffusion type burner connected to bottled methane through calibrated flow

meters and pressure regulators. A MK-82 500 pound (227 Kg) practice

bomb was obtained from the U.S. Ammunition Plant at Rock Island, IL.

The bomb was mounted on two 4 by 4 inch (10 by 10 cm) wood blocks and

was filled with sand and instrumented in the same manner as in the 1/3

scale test. Heat release rates ranging from 16.5 to 500 kilowatts were used for the full-scale uninsulated test and heat release rates of 66,

99, and 500 kilowatts were used for the insulated tests. Figure 4 shows

the full-scale steel floor test assembly.

One instrumentation change was made for the insulated tests. Since

the temperature differential between the outside and inside surface of

the bomb was negligible during all of the uninsulated tests, and since

it was necessary to know the upper surface temperature of the insulating

barrier, the thermocouple which had been located on the exterior of the

bomb was moved to the upper surface of the insulator. It should also be

noted that, in the full-scale test, all tests except for the 500 kilowatt

heat release rates were run for 30 minutes. In the case of the 500

kilowatt heat release rate, two full methane tanks were sufficient only

for about 17 minutes. Therefore, the total test time for the 500 kilowatt

experiments was 15 minutes.

7. HAZARD CRITERION

In order to evaluate the success or failure of the protection

scheme, some indicator of a hazardous condition must be established; in

this case a thermal initiation temperature for the explosive. Reference

[7] discusses, both through theoretical calculations and experimental

results, the chemical reactions which occur for various explosives when

heated under known conditions of confinement. Table 1 of this paper

gives experimental values for temperatures at which runaway reactions

occurred in five common explosive materials. These temperatures range

from 189°C (462°K) for PETN to 288°C (561°K) for TNT. On this basis, it was decided to use a value of 180°C (453°K) as the critical temperature

beyond which an explosion may result.

12
8. DISCUSSION OF RESULTS

8.1 Reduced-Scale Steel Floor Tests

The purpose of the reduced-scale steel floor tests was to obtain a qualitative, preliminary estimate of the range of temperatures which

would likely be encountered under full-scale conditions as a function of

fire size, so that the number of more-expensive full-scale tests could

be minimized. It was also hoped that a scaling relationship could be

developed which would allow a direct comparison of small to full-scale

results

.

By applying a scaling relationship equal to the dimensional scale

factor squared (an approximation usually used when radiation predominates

as in this case) , the reduced-scale tests indicated that heat release

rates below ^ 15 kw posed little hazard and that steady-state floor

temperatures increased the most rapidly between 15 kw and 70 kw (see

figure 5). On this basis, test values for heat release rate of 16.5,

33, 66, 99, and 500 kw were selected for the full-scale tests.

When the full-scale data had been obtained and the results compared

to the reduced-scale using the same scaling relationship, satisfactory

agreement was not found. The temperatures in the full-scale tests were

lower than would be expected from the reduced-scale data. The most

probable reason for this is that the area and mass of the 2-foot square

floor plate was too small, resulting in much less conduction loss to the

edges than in the full-scale. In addition, differences in flame height

and the complex geometrical nature of the nailable steel flooring as compared to a flat plate were also probable factors in the lack of

correlation.

8.2 Reduced-Scale Wood Floor Tests

The two reduced-scale wood floor tests were conducted to attempt to

reproduce the fire spread phenomena observed in the Naval Weapons Center

full-scale wood floor tests and to demonstrate the differences in fire

performance between rail cars with wood floors and with steel floors.

Figures 6 and 7 show the floor and simulated bomb temperatures for

1/3 scale tests at 11 kw for steel and wood floors respectively. With a steel floor, the floor temperature increases rapidly, reaching a high

steady-state value in about 6 minutes. With a wood floor, however, the

upper surface temperature increased at a slow, almost linear rate until

flame penetration occurred at about 15 minutes. At this point the

temperature increased rapidly to a value nearly double that of the steel

floor steady-state value.

The simulated bomb temperatures for the two flooring types were

also quite different. With the steel floor the bomb temperature increased

steadily, reaching 160°C at 30 minutes. In the wood floored car the

bomb temperature remained almost at ambient until flame penetration of

13
the floor assembly; then rapidly rose rising to the critical temperature

(180°C) [7] in less than 3 minutes.

The results of these wood floor tests demonstrate that wood is a good thermal insulator in a fire; but that its combustibility leads to

early failure, particularly at the floor board seams where the fire

penetrates first. This leads to the observation that a floor made of a thermally insulating noncombustible material which prevents fire penetration might be effective in delaying achievement of a hazardous temperature long enough for the fire to be discovered or extinguished.

8.3 Full-Scale Steel Floor Tests

As was discussed in section 4.2, when a steel floored railcar is

exposed to a subcar fire, it is principally the heat transferred through

the floor that raises the temperature to a critical value. If a thermal

barrier is placed over the floor to inhibit temperature rise of the top

surface, this critical temperature could be either prevented or delayed

for a long enough time to allow discovery and extinguishment of the

fire

.

Figure 8 illustrates a comparison of predicted (by the model

discussed in section 9) and measured floor temperatures at 30 minutes as

a function of fire size (filled- in symbols indicate measured values)

.

For the 500 kw fire the bare steel floor reached a temperature of 625°C

(circle) . Placing a 1 inch (25 mm) thick sheet of calcium silicate

board over the steel floor resulted in an increase in the steel temperature to 725°C (square) , but the top surface of the insulating board was much cooler, measuring only 94°C at 17 minutes (end of the test). The

extrapolated temperature at 30 minutes is 137 °C (open triangle)

.

Calcium silicate board was selected because it has excellent

insulating properties. The material comes in 4’ x 8' (1.2 x 2.5 m)

sheets and appears similar to gypsum wallboard without a paper cover.

It is available in various thicknesses from 1/2" (1.2 cm) to 2" (5 cm).

In addition, at 900 psi, its compressive strength should be sufficient

to withstand railcar loading and unloading operations. It may be necessary however to provide metal cladding to protect the surfaces from

impact damage. The same process used to produce metal clad gypsum board

might be used in this case.

9. DEVELOPMENT OF COMPUTER MOD CL

Since the cost of full-scale tests to evaluate various thermal

barrier materials of different thicknesses would be prohibitively high,

a computer model of a steel floor railcar exposed to a subfloor fire was developed to estimate the necessary parameters. The model calculations

can be run with or without a thermal barrier of any desired thermal

conductivity or thickness, and will compute steel floor temperature,

thermal barrier top surface temperature (if included) , and bomb temperature vs time for any fire size (heat release rate)

.

14
9.1 Model Assumptions

The model employs a one-dimensional (lumped) thermal network

analysis, which is the simplest and most conservative approach. The

bare steel flooring was treated as a mass of metal at uniform temperature,

losing heat from the bottom by radiation and from the top by radiation

and convection (assumed values: heat capacity (Cp) = 0.46 j/g-s; mass

(m) = 173.8 Kg; heat transfer coefficient (h

c

) = 0.0016 J/s-cm 2 -°C). No

attempt was made to model the increase in ambient temperature within the

enclosed volume of the boxcar.

A fire burning below a finite horizontal surface will transfer part

of its energy to the surface and part will flow around the surface

edges, and be lost to the atmosphere. The fraction absorbed by the

surface will also change with time as a function of the difference

between the flame temperature and the surface temperature, the emissivities of the flame and surface, convective flow patterns under the

surface, and others. Without direct flux measurements under the surface,

estimates of the fraction of heat absorbed by the surface must be made

in order to calculate the temperature rise in the system. Since sufficient data was not available to estimate the changes in this fraction

with time, only a single-point estimate could be made at steady-state

conditions

.

The unprotected floor temperature data obtained in the full-scale

experimental portion of the study were used to estimate the fraction of

heat released by the fire which was transferred to the floor at steadystate. Table 1 shows these estimates for the five fire sizes tested

along with the measured and predicted floor temperatures at the end of

the test using these estimates. It is expected that the fraction of

heat transferred to the floor would decrease with increasing fire size

(due to increasing steel floor temperatures) as was observed. The value

(0.60) for the 33 kw fire appears low, being bracketed by values of 0.79

and 0.75, but the reason for the low value is not apparent. For consistency (and to remain conservative) a fraction of 0.75 was used in the

model for the 33 kw case even though this results in a predicted temperature 30° C higher than measured.

Table 1

Estimated Heat

Heat Release Rate Transfer Rate to Temperatures

from Fire (kw) Flooring (kw) Fraction Measured (°C) Predicted

16.5 13.0 0.79 127 126*

33.0 19.7 0.60 166 167*

66 .

0

50.0 0.75 290 292*

99.0 75.0 0.75 360 359*

500.0 276.0 0.55 625 628**

* Temperature at 30 minutes

** Temperature at 15 minutes

15
For the insulated case, the same fractions of heat transferred to

the steel floor were used. This is conservative since the higher steel

temperatures which occur with the thermal barrier in place would result

in an overall smaller fraction of transferred heat.

For the insulated case, a one dimensional thermal network analysis was again used. It was assumed that there is heat loss from the bottom

of the steel by radiation, conduction from the steel to the thermal

barrier, and loss by convection and radiation from the top of the

barrier layer. The properties of the calcium silicate board were taken

from the manufacturer's data.

In both the uninsulated and insulated cases, the bomb was treated

as a 300 lb (136 kg) steel cylinder 11.2 in (28.4 cm) diameter by 61 in

(154.9 cm) long. Heat is gained in the bomb by convection and radiation

from the bottom and lost by radiation from the top.

9.2 Model Predictions

Figure 9 shows a comparison between predicted and measured temperatures for the 500 kw fire without a thermal barrier. The predicted

steel temperatures are slightly lower initially, then higher, and then

converge as steady-state values are reached. These differences are most

likely due to the single-point estimate of the fraction of heat transferred to the steel. Likewise the predicted bomb temperature is somewhat

low initially, becoming slightly higher than measured at the end of the

test

.

Figure 10 shows the comparison between predicted and measured

temperatures for the 500 kw fire with the 1 inch (2.5 cm) thick calcium

silicate board. As was discussed previously, the predicted steel temperature is significantly higher than measured. The predicted thermal

barrier temperature is much closer to measured (but still slightly

conservative at later times) and the predicted bomb temperature is

within 2°C of measured values. If the predictions are extended for

longer times for this 500 kw case, at 2 hours the predicted bomb temperature is approximately 84°C and in 10 hours it still remains below

100°C.

It would be useful to determine the time to reach steady-state

temperatures on the top surface of the thermal barrier. Figure 11 shows

predicted top surface temperature as a function of heat release rate at

30, 60, and 120 minutes. From this it can be seen that 60 minutes is

adequate time to reach steady-state for all but the lowest fire sizes,

and here it would appear to be a satisfactory approximation. Thus, if

steady-state conditions were desired in a full-scale test, it would have

to be run for at least 1 hour.

16
9.3 Model Limitations

As discussed above, this first approximation model provides

conservative results. While it shows that a thermal barrier approach is

feasible, more data are needed to allow the replacement of several

estimated parameters with measured values. Also, the model must be

tested against more than one barrier material and thickness before

acceptable confidence in the predictions can be obtained, providing a more generalized tool. However, the model is useful, not only because

it shows the feasibility of the approach, but also because it provides

an indication of the range of temperatures encountered and the time

necessary for the system components to reach steady-state (limiting)

values

.

10. CONCLUSIONS

The results of the experiments conducted indii ate that, for the

case of a steel-floored boxcar exposed to a subcar fire, a thermal

barrier such as 1 inch thick calcium silicate board will be effective in

preventing overheating and subsequent detonation of an explosive load

for more than 1 hour at subcar fire rates of heat release up to 1000

kilowatts. This passive thermal barrier approach has a number of advantages over an active system including simplicity, greater reliability,

and lower cost. In addition, since explosive laden boxcars are almost

always weight limited in the amount of load shipped, every pound of

material added for fire protection results in one less pound of load

which can be shipped. The weight of a complete 1 inch layer of calcium

silicate board in a 50 foot (15.2 m) boxcar is under 1,800 pounds (818

kg) , exclusive of any metal cladding or decking used to mechanically

protect the insulator. While a total weight of the detection/suppression

system designed by Naval Weapons Center is not available, the weight of

300 gallons (1136 i) of water alone would be 2400 pounds (1091 kg)

,

which is more than the weight of the insulating barrier. Thus, the

weight penalty for the passive approach would be less than for the

active approach.

The primary disadvantages of the passive system are that 1) it does

nothing about extinguishing or controlling the fire, and 2) it does not

incorporate a notification system. This is not felt to be a significant

detriment in light of the primary hazard scenario of the yard fire. It

is difficult to think of a situation where a large yard fire could occur

without fairly rapid discovery. If a maximum expected severity for a yard fire can be estimated and the amount of insulation provided is

designed accordingly, sufficient time should be available for both

discovery and extinguishment of the fire prior to the approach of

critical conditions within the car. By design, the thermal barrier can maintain temperatures below selected critical values for any time period

desired

.

17
11. RECOMMENDED FURTHER WORK

The feasibility of a thermal barrier approach to the protection of

a heat sensitive load from an exposure fire has been demonstrated.

There is, however, insufficient data to build a mathematical model which

would allow accurate prediction of the performance of other insulating

materials or different thicknesses of the insulating material tested

with any degree of confidence. Also, the model currently assumes the

load consists of MK-82 bombs. However, this type of protection could

also be applied to any thermally sensitive load in any type of container

or configuration with additional modifications to the model. Such

modifications should be made, so that the model can handle other explosive

loads such as shells or bulk explosives.

It is proposed that additional tests be carried out in full-scale

using different thicknesses of calcium silicate board and various thick- nesses of other types of insulators as well as to increase the numbers

and types of measurements taken (for example flux measurements under the

car) which would allow the development of a mathematical model capable

of accurate prediction of the performance of these insulating materials

for bombs and other thermally sensitive load configurations. The tests

would provide the data necessary to improve the accuracy of the existing

model and its verification. Ideally the proposed model would allow the

designer to select any critical load temperature and configuration

(horizontal cylinder, vertical cylinder, metal drum, etc.) and compute

the time to reach hazardous conditions as a function of fire size and

characteristics of the insulating barrier.

12 . REFERENCES

[1] Pakulak, J. , Naval Weapons Center, China Lake, CA; private

communication.

[2] National Transportation Safety Board, Accident Report No.: NTSBRAR-75-2, February 26, 1975.

[3] The Roseville Train Explosions: Final Litigation Report, by Lanny

Winberry Associates, Sacramento, CA, prepared for the U.S. Department of Justice, U.S. Department of Army, and MTMC, April 30, 1980.

[4] FRA Emergency Order Regarding Class A Explosives, Federal Register

Vol . 38, No. 156, August 14, 1973.

[5] Kenna, J., USADACS, Savanan, IL, private commune iat ion.

[6] Brooks, J. L. , Heat activated alarm systems for railroad boxcars

carrying explosives, U.S. Navy Civil Engineering Laboratory, Port

Hueneme, CA, TN No. N-1512, December 1977.

[7] Zinn, J. and Rogers, R. N. , Thermal initiation of explosives,

Journal of Phys. Chem 66.2646, 1962.

18
19

20

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21

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floor

22

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steel

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PREDICTED

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10.

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1U
NBS-114A (REV. 2-60

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBSIR 80-2170

2. Performing Organ. Report No. 3. Publication Date November 1980 4.

TITLE AND SUBTITLE

FIRE PROTECTION SYSTEMS FOR RAIL TRANSPORTATION OF CLASS A

EXPLOSIVES: INTERIM REPORT

5.

AUTHOR(S)

Richard W. Bukowski

6.

PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

Interim

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

U.S. Department of Transportation

Federal Railroad Administration

Washington, D.C. 20590

10.

SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here) As a result of several accidents involving fire induced detonation of military

explosives during rail shipment, a research project, funded by the Federal Railroad

Administration (FRA) , was initiated at the Center for Fire Research (CFR) at the Nation4l

Bureau of Standards (NBS). This project was initiated to evaluate various methods of

protection of Class A explosives from fire, and to identify one or more cost-effective

approaches which could be explored in greater detail in later studies.

Active systems (detection, notification, and extinguishment) and passive systems

(thermal insulating barriers) were evaluated regarding couc, feasibility and level of

protection provided for the major hazard scenarios involved in rail shipment of explosives. The passive, thermal barrier approach was selected as the most reliable and less

costly of the options studied while providing an acceptable level of protection.

Small-scale and full-scale tests were conducted to obtain performance data on one specific thermal barrier material. Based on this data, a computer model was developed

which can predict temperatures of the boxcar floor, top surface temperature of a thermal barrier, and casing/ explosive interface temperature of a wood-pallet mounted

bomb for a range of fire sizes. The model predictions compare favorably with measured

results from a limited number of experiments. Further experimental data are needed to

refine the model and establish an acceptable confidence level in the predicted values. Tho nrnpncoH tjm-lr * ’ ’ ' " * * ‘ ' necessary to pr ovide thi s ref inement and ver if ica t ion i s de s c r ibed.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Bombs (ordnance); boxcars; computer models; fire detection systems; fire suppression;

full-scale fire tests; heat transfer; railroad accidents; small-scale fire tests;

thermal protection,

13. AVAILABILITY

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