NBSIR 73-208

Residential Buildings and

Gas-Related Explosions

E. F. P. Burnett, N. F. Somes and E. V. Leyendecker

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

June 1973

Final Report

Prepared for Office of Policy Development and Research

Department of Housing and Urban Development

Washington. D. C. 20410

NBSIR 73-208

RESIDENTIAL BUILDINGS AND

GAS-RELATED EXPLOSIONS

E. F. P. Burnett, N. F. Somes and E. V. Leyendecker

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

June 1973

Final Report

Prepared for Office of Policy Development and Research

Department of Housing and Urban Development

Washington, D. C. 20410

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

IMATIOIMAL BUREAU OF STANDARDS, Richard W. Roberts. Director

RHSIDKNTIAL BUILDINGS AND GAS-RELATED EXPLOSIONS

by

Eric F. P. Burnett,* Norman F. Somes

and

Ed.^ar V, Leyendecker

Structures Section

Structures, Materials, and Life Safety Division

Center for Buildint^ Technology

ABSTRACT

The findings of an analysis of available statistics

concerning the frequency of gas-related explosions in residential

buildings are presented. The study was confined to incidents

involving piped gas systems as they affect residential and

commercial buildings. Though due regard has to be taken of

the limitations inherent in the available statistics, it is

concluded that in the USA the probability of occurrence of an

explosion capable of causing significant structural damage

could be 2.2 per million housing units per year.

Key Words: Building; explosion; frequency; gas; gas industry;

progressive collapse; risk; statistics; structure.

* Guest Worker from the University of Waterloo, Ontario, Canada,

October 1971 to April 1972.

i
: ' CONTENTS

Page

1; Introduction 1

2. Background to the Problem - the Distribution and Usage 3

of Gas

3. Current Statistics ..... 6

3.1 American Gas Association 7

3.2 Office of Pipeline Safety 10

4. Implications 14

5. Conclusions ............ 18

Acknowledgements 19

References 20

Tables 22

Appendix 1 25

Appendix 2 27

ii
1. Introduction

Since 1968 there has been growing international concern

that buildings, particularly multistory residential buildings,

may be subjected to loading conditions not normally considered

in design, i.e. abnormal loadings. In that year there occurred

the much-publicized collapse of apartments at Ronan Point in

England (see figure 1). In this 22-story building of precast

concrete panel construction, collapse was triggered by an

accidental explosion of gas that leaked from the connection of

a gas range located in an apartment on the 18th floor.

A Commission of Inquiry Report [1]—^ and subsequent

studies revealed a number of deficiencies in existing codes

and standards, particularly as they applied to multistory

construction. In the United Kingdom interim criteria for

the appraisal and strengthening of existing buildings and the

design of new structures were quickly implemented. Several

other countries in Europe introduced additional design criteria

to deal explicitly with the risks exposed by the Ronan Point

incident.

While Ronan Point was of precast concrete panel construction,

there is no basis for assuming that other forms of multistory

— Figures in brackets designate literature references listed

at the end of the report.

1

Figure 1 Ronan Point Apartment Building

after the collapse, with a second

identical building in the

background. Reproduced from [1]

construction are free from the potentially disastrous effects of

abnormal loading. This point has been demonstrated by a number

of incidents in recent years e.g. the collapse [2] of an 11

story load-bearing masonry apartment building in Barcelona,

Spain, as the result of an explosion. Allen and Schriever [3]

have attempted to summarize other recent incidents of progressive

collapse, particularly those occurring in North America in recent

years.

In November 1971, the US Department of Housing and Urban

Development requested the National Bureau of Standards to make

a study of all aspects of abnormal loading and the problem of

progressive collapse. A discussion of the recognized sources

of abnormal loading is given elsewhere [4]. Several of these

types of loading are considered to occur frequently enough

to warrant particular attention. The domestic gas explosion

is one of these. In an attempt to assess the probability

of gas-related explosions that might be structurally significant,

a study of relevant ns data was initiated. This report

is confined to explosive incidents -"nvolving piped gas systems

insofar as they affect I'esidential and commercial buildings.

2. Background to the Problem - The Distribution and Use of

Gas

Tt has been reported [5] by the American Gas Association

(AGA) that there are more than 915,000 miles of gas pipeline

3
within the continental USA. This total is composed of approximately

288,000 miles [6] of pipeline in gathering and transmission-'

systems and approximately 627,000 miles [6] of pipeline involved

in the subsequent distribution of gas. Since the gathering

and transmission systems occur largely in rural or non-builtup areas, it is with gas distribution systems that this study

is primarily concerned. It is significant that within the

gas distribution system, gas leaks occur at an annual rate

of 1 per 1.1 mile of gas line; i.e., a total of more than

560,000 leaks in 1972. More than 300,000 of these leaks occurred

as a consequence of normal ^^;ear and tear; i.e., corrosion,

fatigue, material failure, etc. Leaks are reported [6] to

occur with com.parable frequency in both the mains and service

lines. About two-thirds of all leaks were associated with

the pipeline itself, the remainder being associated uith the

fittings and other attachments.

Including both single and multiple dwelling units, almost

34,700,000 housing units in the USA used gas for house heating

in 1970 [5] . It is also estimated [5] that natural gas serves

about 55 percent of all housing units as a fuel for residential

space heating. Of all new customers for gas house heating

in 1970, 37 percent were conversions to a gas service

system. Because a single customer, meter, or furnace

may involve more than one dwelling unit these figures may

2_/ Appendix 1 contains a definition of terms commonly used

by the Gas Industry.

4
not be fully representative of the total number of dwelling

units to which gas is supplied.

The Gas Appliance Manufacturers Association indicated

[7] that as of December 1970 there were: 39,840,400 gas

domestic ranges in use (as opposed to 35,534,00 electric)

and 31,964,300 gas water heaters in use (as opposed to 20,232,000

electric). Based on the number of gas ranges in use and the

1970 total housing stock of 67,711,029 housing units, it

appears that approximately 59 percent of all dwelling units

have gas supplied to them. The AGA estimates that, including

appliance usage, natural gas is supplied to upwards of 60 percent

of the total residential market.

A gas-related explosion occurs as a consequence of a leak and the accumulation of gas. Considering the number of

miles of distribution pipeline, the number of leaks, and the

proportion of the population served directly with gas, it is

quite possible that gas service system explosions constitute

a structural hazard for buildings. That a gas related

explosion can and does occur in buildings that are in the

neighborhood of a distribution line but are not directly

served by gas, suggests that gas explosions may be a structural hazard for the majority of buildings in built-up

or urban areas wherever a gas distribution system exists.

5
3. Current Statistics

The most common forms of gas distributed by pipeline

are natural gas (principally methane), manufactured gases,

liquefied petroleum gas (propane and butane), and mixed gas

(usually manufactured plus natural or liquefied petroleum

gas). In terms of residential consumption in 1970, 98.5

percent of all gas utility customers used natural gas [5]

and for all practical purposes the data for natural gas may

be considered to be representative of the industry.

Irrespective of the specific cause of the explosion,

there are three categories of structurally relevant gas

related explosions; i.e, (1) explosions that occur within the

individual dwelling or useable unit; (2) explosions that

occur within the building but not in a dwelling unit or

habitable area of the building - this category would include

an explosion in a boiler room or in a laundry facility; and

(3) explosions that occur outside the building but are so

located or of such ^ magnitude that the structural integrity

of the building would be affected.

Ideally, the probability, severity, and the consequences

of each type of explosive incident should be evaluated.

To assess the probability, relative structural significance,

and risk, it would be useful to know the types of building

involved; i.e., single family, row housing, office building,

and in particular, multi-unit apartment buildings. Unfortunately,

the available statistics are somewhat limited and those that
do exist were gathered for a purpose that had little, if any,

concern for the actual nature and nagnitude of the explosion

and its structural implications. The two principal sources

of information on the gas industry are the AGA and the Office

of Pipeline Safety of the US Department of Transportation (OPS)

Information from each of these sources is evaluated in the

following sections.

. 1 American Gas Association

A prime source of information is the summary of the

research report "Public Safety and Gas Distribution" [8]

prepared for the American Gas Association by Arthur D. Little

Inc., dated December 1967. This was a survey of the

gas distribution industry in which 140 companies and systems,

representing 83.3 percent of all gas distribution meters,

participated. The report provides yearly averages for gasrelated incidents over the 10-year period 1957-66. Yearly

averages of incidents involving payment of compensation

are based on the 7-year period 1957-1963. Both in terms

of the time periods and the number of companies involved,

this survey constitutes the most comprehensive, if not the

only, study of the overall safety of the gas distribution

industry within the continental USA. Obviously, either the

original data or the report itself would be invaluable in any

study of gas-related incidents. However, the AGA said the full

7
report could not be released because it contained confidential

material from member companies and the raw data of the 1967

study had since been routinely destroyed by the consultants.

Table 1 was assembled from information contained in

the summary report prepared by A. D. Little Inc. It is

evident that in each year of the survey an avera^^e of 1,508

gas explosions occurred; 329 required monetary reparation.

A total of 151 (i.e., 60 percent of 253) explosions involved

payment of more than $1,000. It is recognized that only

a proportion of these explosions could have been severe enough

to cause structural damage, but unfortunately, there is no

record of:

1. the relative proportion of personal and property damage;

2. the numbers and types of l^uildings involved;

3. the relative severity of each of the incidents

involving payment greater than $1,000.

4. whether gas v/as supplied to the building

Presumably very few, if any, of the explosions incurring

payment of less than $1,000, were likely to have caused

significant structural damage. In order to evaluate a probability

based upon the AHA statistics, it will be assumed that only

one building was involved in eac)i of the 151 incidents and

that each suffered significant structural damage.

OPS figures for 1971 [6] indicate that of the total number

of reported explosions, 50 percent occurred in residential

8
buildings and 7 percent in commercial buildings. The remainder

occurred largely in manholes, regulator pits, etc., and it

is unlikely that these involved reparation in excess of $1,000.

Furthermore, it is unlikely that the remainder involved the

gas company in claims for payment since in most instances

these incidents probably occurred on or within gas company

property. Accordingly, it will be presumed that 50/57,

namely 87.5 percent of the 151 incidents; i.e., 131, were

likely to have involved residential buildings. In 1960 the

occupied housing stock in the US totaled 58,314,784 units

[9] . According to the American Gas Association approximately

32,530,200 of these units were supplied with gas. All things

being equal, the probability of occurrence of an explosion

must be greater for users of gas than for non-users. However,

it is also evident that gas can accumulate and explosions

do occur in buildings not serviced or supplied with gas.

Therefore, the probability of a gas-related explosion should

be determined for both the user and the non-user of gas.

Unfortunately the available estimate of 131 structurally significant

explosions in residential units cannot be separated into user

and non-user categories. If it were to be presumed that all

131 explosions occurred in dwelling units supplied with gas

then, for users of gas, the annual probability of occurrence

of a structurally significant gas-related explosion is

131/32,530,200 x 100 = .000403 percent or 4.0 structurally

significant explosions per million dwelling units per year.

9
/ /

Since (i) tlie figure of 131 includes a number of non-user

explosions and (ii) .^as distribution networks exist in most

if not all urban areas, a more representative figure would

be the annual nrobability of occurrence of a structurally

significant gas-related explosion in any residential unit

i.e.

131/58,314,784 x 100 = .00022 percent

or 2.2 structurally significant explosions per million dwelling

units per year.

During tlie period 1957-1963 , the average annual natural

g gas sales to residential customers totaled 300 x 10 therms.

In terms of energy, the annual rate of structurally significant

o gas-related explosions is 0.44 for every 1 x 10 therms of

gas supplied. If all gas-related explosions and the total

o amount of natural gas sales (^^05 x 10 therms) are considered,

the probability of explosion could be 1.67 explosions per

1 X 10 therms per year.

3.2 Office of Pipeline Safety

In accordance with the provisions of the Natural Has

Pipeline Safety Act of 1968, a nationwide reporting system

for gas-related incidents was initiated in 1970 by the Office

of Pipeline Safety. Detailed reports are required of individual

incidents that involve one or more of the follo\\fing criteria:

10
1. caused a death or a personal injury requiring

hospitalization;

2. required any segment of transmission pipeline to be

taken out of service;

3. resulted in gas igniting;

4. caused estimated damage to the property of the

operator, or others, or both, of a total of

$ 5 ,000 or more

;

5. required immediate repair and other emergency action

such as evacuation of a building, blocking off an

area, rerouting of traffic to protect the public;

6. were deemed significant but did not meet the criteria

2 , 3 or 4

.

All gas companies with more than 100,000 customers

(over 851 of the total number of gas customers) are required

to submit reports within 20 days of the incident. Records

for 11 months of 1970 and the whole of 1971 are publicly

available [6] . Since the standard of response was somewhat

better in 1971 than in 1970 and because the 1970 records do

not cover the whole year, only the 1971 figures vsrill be

considered here.

During 1971, a total of 164 explosions in distribution

systems and 11 explosions in transmission systems were reported

in individual leak reports. Eighty-three of these explosions

involved residential type buildings (houses, apartments, etc.)

11
and at least 38 of these could have been of structural

significance since either a fatality, serious injury or

extensive property damage was reported. Similarly, 12 gasrelated explosions occurred in commercial or educational

buildings, and of these, at least 4 were structurally significant.

The majority of the remaining 69 explosions were either industrial

or construction incidents and often occurred in manholes,

compressor stations, etc.

Whereas the AGA reported that over the 10-year period

1957-1966, the annual average number of explosions was 1,508

and of these 329 involved reparation of some sort, the OPS

has recorded a total of 175 for 1971. Possible reasons for

this variation are as follows

1. The OPS restricts itself to distribution and transmission

systems; i.e., the safety of all pipeline facilities up to

and including the customer's meter. While it appears that

some incidents that occur downstream of the meter are reported,

the statistical requirements of OPS do not extend past the

meter between the primary gas distribution system and the

appliances or services and their internal distribution systems.

As table 2 demonstrates, the majority of all incidents, but

not necessarily all significant incidents, occur after or

downstream of the meter.

2. Whereas the A. D. Little survey recorded all explosive

incidents (it has here been presumed that only those requiring

reparation of more than $1,000 were structurally significant)

12
the OPS reporting criteria are somewhat different. For

example, criterion (2) does not include the taking of any

segment of distribution pipeline out of service; criterion

(4) places a lower limit on total property damage of $5,000;

while criteria (5) and (6) are both open to various interpretations.

On the other hand the criteria for reporting incidents to

the OPS should insure that most significant incidents are

reported, irrespective of the source of the leak and the

location of the appropriate meter. At the very least, the

figure of 38 structurally significant gas-related explosions

should permit the evaluation of a lower bound criterion applicable

to residential construction. The 1970 Census of Housing [10]

gives the total number of units as 67,711,029. Accordingly,

a lower bound estimate of the annual probability of incidence

of a structurally significant gas-related explosion in any

dwelling unit is

38 X 100 * x 100 = .000066 percent

67,711,209 85

or 0.66 explosions per million dwelling units per year.

To emphasize that this figure is very much a lower bound

estimate, it is instructive to refer to the accident report

in Appendix 2 which records how one explosive incident destroyed

13 houses and fire damaged 93 others. Furthermore, if only

* OPS Statistics (Individual Leak Reports) only cover 85

percent of the gas distribution industry.

13
the proportion of dwellinc^ units served with f{3.s approximately

42 , 259 , 000 according to the A.r,.A.) were to be used, then

the value for this probability would be approximately

38 X 1_0_0 X 100 = .000106 percent

42,259,200 85

or 1.06 structurally significant explosions per million dwelling

units per year.

At present, the ACA and OPS statistics conflict.

Of the two, the ACA statistics appear to be the more

representative since:

1. they apparently cover all incidents, both upstream

and downstream of the meter in distribution systems,

2. they represent a 10-year average whereas the first

complete year of OPS statistics is 1971.

• ' 4. Implications

Four values for the probability of occurrence of

structurally significant gas-related explosions involving

residential type buildings have been determined. The most

reliable and representative of these is the AGA based figure

of 2.2 structurally significant (assuming damage in excess of

$1,000 indicates significance) gas-related explosions per million

dwellings where the total number of gas-related explosions has

been related to the total num.ber of dwelling units. In the

United Kingdom (UK) the figure 3.3 explosions per million

dwellings is often quoted [1] as being representative of the

ten-year period 1957-66. However this figure involves only

14
those gas-related explosions that occur in dwelling units

supplied with gas i.e. the probability of a gas user being

subjected to a gas-related explosion. If the total number of

dwelling units (approximately 18 x 10^ [1]) were used instead,

then the related probability is approximately 2.2 per million

dwellings. Since the number of gas-related explosions involving

residential non-users of gas has not been included, this figure

is obviously on the low side. Comparison of British and

American figures is therefore difficult but it is evident

that they are of similar orders of magnitude and that the

probability of a gas-related explosion in the US is not as

great as in the UK.

It should be noted that for the United Kingdom, Taylor

and Alexander have reported [11] that about 18 percent of

all gaseous explosions caused significant structural damage.

If this percentage were applied to the 1508 US explosive

incidents, the resulting number, 271, is greater than the

T 51 incidents reported "^o "'"he '^GA involving payments of

"'ore than $1 ,000.

That US practice with T-egard to the distribution and use

of gas is relatively safer ^-han that in the UK is further

borne out by the fact that for the period 1957-1963 the US

o had 1.67 explosions ver 1 x 10 therms per vear while in the

^^K for 1966, the comparable figure was 2.1 explosions

o '^er 1 X 10 therms. In the UK. nossibly as a result of the

process of conversion from the use of manufactured to '^atural

gas, the rates for 1969 and 1970-1971 are 5.0 and 6.5 per

15
1 X 10 therms respectively ''12].

Subsequent to the Ronan Point failure in 1968, the British

appear to have gathered the '"ost comprehensive statistics

regarding gas-related explosions and buildings. As table

^ demonstrates, the incidence of explosions from liquids

(e.g., gasoline, adhesives, etc.) and liquefied petroleum

gas (e.g., butane, methane) in domestic buildings s relativelyhigh. However, the probability of structural damage is much

lower than that for piped gas and presumably a similar

situation prevails in North America.

Where individual single-family dwellings are concerned,

the probability of incident may be considered to be proportional

to the risk. When multi-unit dwellings are involved, the

probability of structural damage is the same for all individual

dwelling units, but the probability of damage to the building,

and hence the possibility of structural collapse, is much

greater. For a 100-unit apartment building and a service

life of 50 years, the equivalent risk of a structurally significant

explosion occurring during the lifetime of the building supplied

with gas is therefore proportional to 1.67 percent in the UK.

Since the probability of a (piped) gas-related explosion in

a dwelling unit in a multistory residential building not

served with gas is very small the actual probability of a gas-related explosion in a multistory dwelling unit served

by gas is less than that in a similar single family residential

unit. It is therefore very difficult to predict accurately

the susceptibility of a multistory residential building to

16
gas explosion in the US. It would appear that for a ino

unit buildin.e the equivalent risk is less than 2.0 percent

' 4.0 X 100 X 50 X 100 = 2 0"^ 1,000,000

and greater than 1.1 percent.

2 . 2 X 100 X 50 X 100 _

' -, „

' — i . i '0

1 ,000 ,000

On the basis that service systems are apparently slightly

safer in the US than in the UK a figure of 1.5 percent would

appear to be a reasonable estimate of the probability of

an explosion in a representative apartment building over

its service life in the United States.

In addition to previously stated qualifications, these

numbers should be qualified to some extent because the proportion

of gas appliances per unit in multi-unit dwellings is most

probably less than the proportion of gas serviced units in

single - family residences. On the other hand, many apartmenttype buildings v/ill have common utility systems; i.e., water

heating, central heating plant, etc., serviced by gas. In

addition, more multiple dwelling units are being supplied

with gas and, given the relative economy of natural gas,

these proportions are likely to increase. Certain practices

such as internal distribution systems operating at high pressure

or service rooms located on the roof with a high pressure

gas supply, may increase the volume of gas that is released into

a given space in a given amount of time in the event of a leak.

This increased volume of gas may increase the potential severity

17
of any resulting explosion. It follows that if gas service

systems constitute a structural hazard then this applies

equally to multi-unit dwellings and the probability of a serious

explosive incident is likely to increase in the future.

^' 5 . Conclusions

Though due regard has to be taken of the limitations

inherent in the available statistics, it is concluded that

in the USA the probability of occurrence:

1. Of an explosion capable of causing significant

(assuming damage in excess of $1,000 indicates

significance) structural damage could be on the

order of 2.2 per million housing units per year.

2. Of an explosion capable of causing significant

structural damage in a hundred unit anartment

building during a 50-year service life could be on the

order of 1.5 percent.

18
Acknowledgements

The cooperation of the Office of Pipeline Safety and

the American Gas Association in gathering these statistics

is gratefully acknowledged.

19
References

1. Griffiths, H. Pugsley, A., and Saunders, 0., Report of

the Inquiry into the Collapse of Flats at Ronan Point,

Cannington Town, Her Majesty's Stationery Office, London,

F.ngland, 1968.

2. Marshall, M. R. , Report on the Explosion at Nos . 10-12,

Callc Santa Amelia, Barcelona, Spain, on March 6, 1972,

The Gas Council, London, England, July 19 72.

3. Allen, D. E., and Schriever, W. R. , Progressive Collapse,

Abnormal Loads and Building Codes, National Research

Council, Ottawa, Canada, Presented at ASCE National

Structural Engineering '^teeting, Cleveland, Ohio, April 1972 , Submitted for Publication by ASCE.

4. Somes, N. F., "Abnormal Loadings on Buildings and Progressive

Collapse," a paper in "Building Practices for Disaster

Mitigation," National Bureau of Standards, Building Science

Series 46 , U . S . Government Printing Office, Washington, D.C.

20402, February, 1973.

5. Gas Facts, A Statistical Record of the Gas Utility Industry,

in 1970, American Gas Association, Department of Statistics,

Arlington, Virginia, 1971.

6. United States Department of Transportation, Office of

Pipeline Safety, Annual Reports and Statistics of the

Secretary of Transportation on the Administration of

the Natural Gas Pipeline Safety Act of 1968, Washington, D. C.

7. Gas Appliance Manufacturers Association, Inc., Statistical

Highlights, Arlington, Virginia, May 4, 1971.

8. Arthur D. Little, Inc., Public Safety and Gas Distribution

a Survey Prepared for the American Gas Association,

December 1967,

9. United States Department of Commerce, Bureau of the Census,

Social and Economic Statistics Administration, 1960

Census of Housing, Vol. 1, States and Small Areas, United

States Summary, Washington, D. C, August 1963.

10. United States Department of Commerce, Bureau of the

Census, Social and Economic Statistics Administration,

1970 Census of Housing, Series HC(1)-B, Detailed Housing

Characteristics, United States Summary Report,

Washington, D. C, August 22 , 1972 .

20
11. Taylor, N. , and Alexander, S. J., Structural Damage byExplosions, A Pilot Survey, Construction Industry,

Research and Information Association, Technical Note 22,

London, England, June 1971.

12. Alexander, S. J., Field Surveys of the Damage caused

by Gaseous Explosions, Ove Arup Partners, London,

England, October, 1971.

13. Alexander, S. J. , and Hambly E. C. , Design of Domestic

Buildings to V.'ithstand Gaseous Explosions, Ove Arup and

Partners, London, England, May 1969.

n
I

Table 1 . Gas-Related Incidents, Their Nature, Annual Incidence and Consequence [8]

10 YEAR AVERAGE 7 YEAR AVERAGE - 1957-63 INCLUSIVE

NATURE OF INCIDENT

INCIDENT

REPORTED

INCIDENTS

INVOLVING

PAYMENT

SIZE OF PAYMENT

>$1,000

1 <$1,000

> $ 2 5 < $25.

NUMBER % NUMBER % % % %

EXPLOSION 1508 12.8 329 13.3 60 10 4

FIRE 1835 15.6 274 11.1 36 9 7

PRODUCTS OF COMBUSTION 1228 10.4 109 4.4 7 5 3

UNIGNITED GAS 3118 26.5 1498 60.7 12 66 69

FLASHBACK 4122 35.1 217 8.8 6 9 11

OTHER 344 2.9 167 6.8 3 7 8

103.3^ 105. 1^ 12^ 106 2 1022

TOTAL NUMBER

OF INCIDENTS 11753 2466

253>/ 160^.^

/65.1%^

606^/ ^ /24.65^^

1 Total Payments by Gas Company exclusive of Company repair costs.

2 Some incidents may involve more than one of the listed phenomena.

3 Some incidents may have been involved in more than one of the listed phenomena.

4 Percent of the total number (2466) of incidents.

22
Table 2: Relative location of the cause of the various gas-related incidents in a typical year. [8]

10 YEAR AVERAGE 7 YEAR AVERAGE

LOCATION OF BASIC

CAUSE OF INCIDENT

TOTAL NUMBER OF

INCIDENTS

r- TOTAL NUMBER OF

INCIDENTS

INVOLVING PAYMENT

NUMBER % NUMBER

BEFORE OR AT THE METER

I.E., UPSTREAM

3 , 122 26.5 1 , 780 72 .

AFTER THE METER

I.E., DOWNS TREAM

8,296 70.6 639 25.9

OTHER 335 2.9 47 2.0

TOTAL NUMBER OF

INCIDENTS PER YEAR

11,753 100 2,466 100

23
Table 3. The Number of Explosions and their Probability of Occurrence in

Domestic Buildings in the UK for the Period 1957-1966 [1]

Explosions

10 years

1957-66

1. Total of

explosions

reported

2. Explosions

that caused

structural

damage

^ Fxn1 os 1 on s that caused

structural

damage and

could have

occurred in

Ntulti-story

blocks

4 . Explosions

that caused

severe structural

damage and

could have

occurred in

multi-story

blocks

Source Number Number Risk Number Risk Number Risk

1 . >4anufactured

Gas

747 393 3.3 393 3.3 152 1.3

2 . Liquids

(e.g. , spille

gasoline

methylated sp

adhesives)

298

1

72 0.4 36 0.2 7 0.0

T 1 ni ip» "Ft (^rl Petroleum Gas

(e.g., methan

butane)

QfS 71 0.4 30 0.2 21 0.1

4. Other and

Unknown

748 251 1.4 45 0.3 13 0.1

5 . Total of All

Sources

1889 787 5.5 504 4.0 193 1.5

24
Appendix 1

Definitions

The following definitions are consistent with those used in

the gas industry [8]

British Thermal Unit (Btu.): The amount of heat required to

raise the temperature of one pound water one degree

Fahrenheit under stated conditions.

Conversion to Natural Gas: Changing the gas service to

ultimate customers from gas other than natural gas to

natural gas, including adjustments of consumers'

appliances to perform satisfactorily with natural gas.

Distribution Company: Company which obtains the major portion

of its gas operating revenues from the operation of a retail gas distribution system, and which operates no

transmission system other than incidental connections

within its own system or to the system of another

company. For the purposes of A.C.A. statistics, a distribution company obtains at least 95 percent of its

gas operating revenues from sales to ultimate customers

and classifies at least 95 percent of mains (other than

service pipe) as distribution.

Manufactured Gas: A gas obtained by destructive distillation

of coal, or by the thermo-deconposition of oil, or by

the reaction of steam passing through a bed of Iieated

coal or coke. Examples are coal gases, coke oven gases,

producer gas, blast furnace gas, blue (water) gas,

carbureted water gas.

Natural Gas: A naturally occurring mixture of hydorcarbon and

nonhydrocarbon gases found in porous geologic formations

beneath the eartli's surface, often in association with

petroleum. The principal constituent is methane.

Pressure, Gauge, (PSIG) : Pounds per square inch above

atmospheric pressure.

25
Service Line: The pipe which carries gas from the main to

the customer's meter.

Sys tern

Distribution : Generally, mains, services, and equipment

which carry or control the supply of gas from the

point of local supply to and including the sales meters.

Low Pressur e: A system in which the gas pressure in the

mains and services is substantially the same as that

delivered to the customer's appliances; ordinarily a a pressure regulator is not required on individual

service lines

High Pressure : A system which operates at a pressure

higher than the standard service pressure delivered

to the customer; thus, a pressure regulator is required

on each service to control pressure delivered to the

customer. Sometimes this is referred to as Medium Pressure

Field and Gathering : A network of pipelines (mains)

transporting natural gas from individual wells to

compressor station, processing point, or main trunk

pipeline.

Transmi ss ion : Pipelines (mains) installed for the purpose

of transmitting gas from a source or sources of supply

to one or more distribution centers, or to one or more

large volume customers, or a pipeline installed to

interconnect sources of supnly. In typical cases transmission lines differ from gas mains in that they operate

at higher pressures, are longer, and the distance

between connections is greater.

Therm: A unit of heating value equivalent to 100,000 British

thermal units (Btu)

26
Appendix 2

NATIONAL TRANSPORTATION SAF r/Fi' BOARD

Washington, D. C. 20591

FIPELINF ACCIDU.NT REPORT

MOCIL OIL CORPORATION

HIGH rRESSURE NATURAL GAS

PIPELINE

Houston, Texas September 9, 1969

I. SYNOPSIS

At 3:40 p.m. on September 9, 1969. a 14-

inch pipeline carrying natur?J gas at a pressure

of more than 780 p.s.i.g. ruptured in a newly

constructed residential subdivision 3/-i miles

north of Houston, Texas. The escaping gas

created a duststorm-hke condition and sounded

lilce a jet engine. Electric and telephone utility

servicemen v/orking in the area, with the help

of local residents, immediately commenced to evacuate all residents in the vicinity of the

rupture. About 8 to 10 minutes later, the

escaping gas exploded violently. Tliirteen

houses, ranging from 24 {"cet to 250 feet from

the rupture, v.^ere destroyed by the blast. The

leaking gas caught fire and continued to burn

to heiglits of 125 feet for IVi hours until valves on cither side of the leak were clo'^cn hy Mobil

workmen dispatciicd to ilie valve locations.

The fire abated at tliat time, but some gas

burned for another 5 hours. In all, 106 nouses were damaged and property daui<i';e was

estimated at $500,000. Miraculously, there were no dcatlis, but nine people were injured,

two seriously.

PROliAr.LE CAUSE

The Nalion-i! Tran.',j.oii;;tion Safety iioaid

<ictermines that the prob..b!e cause of the

accident was the ruptuiing of a length of pipe

along a weak zone in the electric resistance

weld, made when the pipe was mianufactured in 1941, due to the subjcctinc; of the pipeline to pressures higher than ever before experienced.

Contributing causes to the rupture were: (1)

the setting of the regulators to control the gas

pressure at levels higher than the maximum

allowable operating pressure permitted by the

American Standards Association Code for

Pressure Piping, and higher than the pressure to v/hich the pipeline was tested, (2) the lack of

any written procedures tor making the tie-in,

(3) the failure of Federal or State regulations

to limit the maximum operating pressure.

Contributing to the extent of the damage

was the delay in shuttino; down the pipeline

after the runture occurred.

'2 7

FORM NBS-IMA

U.S. DE. PT. 0^ COMM.

BIBLIOGRAPHIC DATA

SHEET

1 . PL' RLICM IO:^ Ki-. ! j SO.

KBSIR 75-208

2. (-ov't AcCf^SK'Ti No.

4. TIT LI- A.vn SUBTITLL

Residential Buildings and Gas-Related Explosions

7. AUTHOIUS^

Eric F. P , Burnett. Noimian F. Soir.e s, ond ?x:}:\nx^_V_!_ Le yendecker

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPAR1MF,NT OF COM.MERCE

WASHINGTON, D.C. 20234

12. Sponsoring Org.amzanon Name and Address

Office of Policy Development and Research

Department of Housing and Urban Development

Washington, D. C. 20410

3. JU-fipiciu's Accession N(

5. Publication Date

6. Performing Organizaf ii^n Code

a. Performing Or g. n i /.ation NBSIR 73-208

10. Project/ Task/ V,'ork Unit No. 4618488

11. Contract/Grant No.

13. Type of HsTlort & Period, „ „.

Covered -OV . 1 , 19/1

Final Nov. 1, 19 7;

14, Sponsoring Agency Code

15, SUPPLEMENTARY NOTES

16. ABSIRACT ^A ."^OO-word or less factual summary of most si;;ni!ica:u information. If docum.ent includes a significant

bibiiographi' or literature survey, miention it here.)

The findings of an analysis of available statistics concerning the frequency of

gas-related explosions in residentia] buildings are presented. The study x^as

confi.ned to incidents involving piped gas systems as they affect residential and

commercial buildings. Though due regard has to be taken of the limitations

inherent in the available statistics j it is concluded that in the USA the probability

of occurrence of an explosion capable of causing significant structural damage

could be 2.2 per million housing units per year.

17. KEY U'ORDS (Alphabetical order, separated by semicMions)

Building; explosion; frequency; gas; gas industry; progressive collapse; risk;

statistics; structure.

18. AVAILABILITY STATEMENT

EH UNLIMITED.

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(Tins PAGE)

UNCL ASSIFIED

21. NO. OF PAGES

22. Pace

USCOMM-DC 6624'1-P71