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4 NBS TECHNICAL NOTE 690

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U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

Is Hydrogen Safe?

QC

100

U5753

NO

1976

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NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

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the Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

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1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

- Located at Boulder, Colorado 80302.
Is Hydrogen Safe?

-e

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J. Hord

National Bureau of stands

JAN

2 6 1977 w 0"V <*G

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Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

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U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward 0. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

^NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued October 1976
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 690

Nat. Bur. Stand. (U.S.), Tech Note 690, 38 pages (October 1976)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1976

For sale by the Superintendent of Documents, U.S. Government Printing Office , Washington, D.C. 20402

(Order by SD Catalog No. C13. 46:690). Price 85 cents (Add 25 percent additional for other than U.S. mailing)
CONTENTS

Page

1.0 INTRODUCTION 1 2.0 GENERAL PHYSICAL AND CHEMICAL PROPERTIES 1 3.0 FIRE HAZARDS 7 4.0 FIRE DAMAGE 9 5.0 EXPLOSIVE HAZARD 11

6.0 EXPLOSIVE DAMAGE 12 7.0 COMPARISON OF FUEL STORAGE METHODS 15

8.0 FUTURE HYDROGEN APPLICATIONS 19 9.0 SUMMARY 20

10.0 REFERENCES 22 Appendix: Hydrogen Safety Guidelines and Applicable Regulatory Codes ... 29

Appendix References 31

LIST OF FIGURES

Figure 1. Quantity-distance relationships for the protection of personnel near

liquid hydrogen storage and experimental areas (unbarricaded) 16

Figure 2. Quantity-distance relationships for liquid hydrogen dewar separation in

storage and experimental areas (unbarricaded) 17

Figure 3. Industrial storage standards (quantity-distance) for hydrogen,

liquefied natural gas (LNG) and gasoline 18

LIST OF TABLES

Table 1. Properties of Hydrogen, Methane and Gasoline 5-6

Table Al. Hydrogen Safety and Equipment Guidelines (nonmandatory) 29

Table A2. Regulations (mandatory) for Distribution of Hydrogen 30

iii

IS HYDROGEN SAFE?

J. Hord

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

The safety aspects of hydrogen are systematically examined and compared with

those of methane and gasoline. Physical and chemical property data for all three

fuels are compiled and used to provide a basis for comparing the various safety

features of the three fuels. Each fuel is examined to evaluate its fire hazard,

fire damage, explosive hazard and explosive damage characteristics. The fire

characteristics of hydrogen, methane and gasoline, while different, do not largely

favor the preferred use of any one of the three fuels; however, the threat of fuelair explosions in confined spaces is greatest for hydrogen. Gasoline is believed

to be the easiest and perhaps the safest fuel to store because of its lower volatility

and narrower flammable and detonable limits. It is concluded that all three fuels

can be safely stored and used; however, the level of safety risk for each fuel will

vary from one application to another. Generalized safety comparisons are made herein

but detailed safety analyses will be required to establish the relative safety of different fuels for each specific fuel application and stipulated accident. The technical

data supplied in this paper will provide much of the framework for such analyses.

Hydrogen safety guidelines, regulatory codes applicable to the distribution of

hydrogen, and safety criteria for liquid hydrogen storage are compiled and presented.

Key words: Explosion, fire, fuel, gasoline, hydrogen, methane, safety.

1.0 INTRODUCTION

For many years now hydrogen has been considered a suitable, if not ideal, synthetic fuel

for future generations. Its clean-burning, rapid-recycling characteristics are lauded by

hydrogen advocates and its explosive characteristics are emphasized by hydrogen opponents.

The safety aspects of hydrogen are the most common concern of the man-on-the-street when he

is first introduced to the hydrogen fuel concept. This spontaneous fear reaction is probably

attributable to the automatic association of hydrogen fuel with the 'hydrogen-bomb' and the

Hindenburg fire. Subsequent to the Hindenburg incident this country has experienced more

tragic and devastating fires involving natural gas, yet natural gas is a fuel that is commonly

accepted by the general public. Also, there is no connection whatsoever between the chemical

explosive potential of hydrogen fuel and the thermonuclear explosive potential of hydrogen as it is used in 'hydrogen-bombs.' Thus, even the novice will recognize that, the wide-eyed fear

of hydrogen is unjustified. Simultaneously we must realize that hydrogen is one of the most

flammable and explosive fuels available to us and it must be handled with appropriate respect

and safeguards.

Evaluation of the safety hazards of a particular fuel is a highly complex task requiring

interpretation of specific technical data and intercomparisons with other fuels. Fire and

explosion hazards must be carefully assessed to determine the relative safety of a fuel in

each potential application. Therefore, hydrogen can be safer than conventional fuels in some applications and more hazardous in other applications. Because of the complexity and depth

of this topic, it is treated rather superficially in page-limited technical articles dealing

with hydrogen-energy concepts. The purpose of this paper is to systematically examine the

safety aspects of hydrogen and to determine if hydrogen is sufficiently safe for use as a future fuel. It will be demonstrated herein that the answer is overwhelmingly, YES, although

its use may be restricted in some future applications.

2.0 GENERAL PHYSICAL AND CHEMICAL PROPERTIES

To permit insight and provide perspective for the relative safety of hydrogen, comparative data are given in table 1 for methane and gasoline. Methane is the major constituent
of most compressed natural gases and of liquefied natural gas (LNG) ; therefore, methane and

LNG are used interchangeably in this document. The significance of the technical data, listed

in table 1, is discussed in considerable detail throughout the remainder of this paper. These

data were obtained from numerous sources and are believed to be the best available. Some of

the data given in table 1 were obtained by giving weighted consideration to several sources

of data and by performing appropriate computations.

Most of the properties listed in table 1 will be familiar to the average reader; however,

each of the combustion properties will be briefly described because of their importance to this safety analysis and because some of them are not commonly used properties. The thermophysical properties conform with the conventionally accepted definitions; however, explanatory

notes are provided at the bottom of table 1 to explain the bases for some of the "gasoline"

properties. The heats of fusion and sublimation are taken at the triple point (at the

freezing point for gasoline) and the heats of vaporization are taken at the normal boiling

point. The combustion properties are defined in the following paragraphs.

Limits of flammability in air : The range of concentrations of gaseous fuel in air at which the mixture will burn, i.e., a homogeneous fuel-air mixture will propagate a flame

within a limited range of compositions. These limits are determined experimentally [40]

by visually observing upward propagation of flames through flammable mixtures in vertical

tubes.

Limits of detonability in air : The range of concentrations of gaseous fuel in air at which the mixture will detonate, i.e., the flame front will propagate at supersonic speed.

These limits are usually determined by measuring the rate of propagation [19,35,41] of

combustion-supported pressure waves in long horizontal tubes filled with combustible

mixtures of fuel in air.

Stoichiometric composition in air : The concentration of fuel in air at which all of

the fuel and oxygen in the mixture are consumed by the reaction and maximum combustion

energy is realized.

Minimum energy for ignition in air : The minimum spark energy required to ignite the

most easily ignitable concentration of fuel in air. The most easily ignited mixture of

fuel in air is usually not the stoichiometric composition. To determine the minimum

ignition energy, a calibrated capacitor is charged to a specified voltage and discharged

through an arc between parallel plate electrodes that are immersed in the combustible

2 mixture of gases. Ignition energy is calculated from (1/2)CV , see references [42,43].

Autoignition temperature : The minimum temperature at which a combustible mixture of

fuel and air can be ignited by a hot surface, [8,18,44].

Hot air-jet ignition temperature : The temperature of a jet of hot air as it enters pure

fuel vapors or a combustible fuel-air mixture at NTP and causes ignition to occur. The data

given in table 1 represent the jet temperature of hot air as it enters pure fuel vapors at

NTP. The jet diameter for these data is 0.4 cm. This ignition temperature decreases with

increasing jet diameter [22,45,46] and for a given jet diameter the hot gas jet ignition temperature increases if hot jets of nitrogen gas (rather than air) are squirted into combustible

fuel-air mixtures [45]. The hot gas jet ignition temperature is dependent upon the composition of the combustible mixture and the velocity of the jet of hot gas.

Flame temperature in air : The temperature within the flame as determined by burning

premixed stoichiometric fuel-air mixtures in the open air. Fires at the scenes of fuel

spills or leaks more closely simulate stationary gaseous diffusion flames which burn at lower temperatures. Detailed discussions on this topic and experimental techniques are reviewed by Lewis and von Elbe [47].

Percentage of thermal energy radiated from flame to surroundings : The percentage of

the heat of combustion (high) that is radiated from the combustion zone to its surroundings.

The higher heating value of every flame is eventually dissipated by radiative processes.

The data given in table 1 are for flames fueled by vaporization of pools of liquid fuels in an air environment. These data are similar to those obtained in laboratory experiments

with stationary gaseous diffusion flames [25,26]. Atmospheric moisture absorbs thermal
energy radiated from a fire and can reduce the values recorded in table 1. Hydrogen fires

benefit most from this absorption effect [25], e.g., it is estimated that 45 percent of radiant hydrogen flame energy is absorbed within a distance of 8 m in 25°C air containing water

vapor at 15 mmHg partial pressure.

Burning velocity in NTP air : The subsonic velocity at which a flame propagates through a flammable fuel-air mixture. This velocity is usually determined in the laboratory by measuring the velocity of gas flow to a bunsen burner [27].

Detonation velocity in NTP air : The supersonic velocity at which a flame (and

accompanying pressure wave) propagates through a detonable fuel-air mixture.

Diffusion coefficient in NTP air : The diffusivity or coefficient of diffusion is a parameter used in Fick's equation to describe the rate at which a gaseous fuel diffuses

through air. For a given fuel concentration gradient the mass diffusion flow rate is proportional to the diffusion coefficient. This coefficient can be computed from an empirical

expression [48] that is based on the kinetic theory of gases.

Diffusion velocity in NTP air : The velocity at which a gaseous fuel diffuses through

air. For a specified fuel concentration gradient the diffusion velocity is proportional to the diffusion coefficient and can be estimated from Stefan's equation [49]. Diffusion velo3/2 city varies with temperature according to T and consequently low temperature gases produced

by cryogenic liquid fuel spills will diffuse more slowly than NTP fuel gases. The values re- corded in table 1 are based on NTP fuel gas and NTP air densities and fuel concentrations that

vary from 99.99 percent to 0.0 percent over path lengths of 3 cm to 30 m.

Buoyant velocity in NTP air : The velocity at which gaseous fuels rise in air under the

influence of buoyant forces. This velocity cannot be determined in a direct manner as it is

dependent upon drag and friction forces that oppose buoyant forces acting on the rising vol- ume of gaseous fuel. Atmospheric turbulence as well as shape and size of the rising volume

of gas can affect the terminal velocity of the buoyant gas. Buoyant forces are related to the difference in air and fuel densities; therefore, cold, dense fuel gases produced by cryogenic fuel spills will rise more slowly than NTP fuel gases. The buoyant velocities recorded

in table 1 were estimated from fundamental principles of dynamics and with the aid of empirical data [50,51]; it was assumed that the radii of the buoyant masses of NTP fuel gas varied

from 3 cm to 1.5 m.

Maximum experimental safe gap (MESG) in NTP air : The maximum permissible clearance,

between flat parallel steel surfaces, that prevents the propagation of dangerous flames or sparks through the gap. The MESG is measured by igniting a combustible fuel-air mixture

inside of a test enclosure and observing a similar combustible mixture surrounding the

enclosure to detect its ignition. The MESG is the largest gap size that does not permit

ignition outside of the test enclosure. The MESG is dependent upon the test gap flange

width, composition of combustible mixtures inside of and surrounding the test enclosure, the

initial pressure and temperature of the mixture, location of the ignition source and enclosure

geometry [31,52]. The hydrogen and gasoline MESG data recorded in table 1 were obtained using

parallel steel flanges that were 1.905 cm wide and 10.16 cm long. The methane MESG data were

obtained using an 8 liter chamber with equatorial flanges that were 2.54 cm wide. MESG data

are of vital importance to the design and manufacture of explosion-proof equipment.

Quenching gap in NTP air : The spark gap between two flat parallel plate electrodes at which ignition of combustible fuel-air mixtures is suppressed, i.e. , smaller gaps have the

effect of totally suppressing spark ignition [16,33]. Other types of electrode geometries

are also used in laboratory measurements [16,33] of quenching distance. The experimental

data [16,33,53] are dependent upon the temperature, pressure and composition of the combustible gas mixture and electrode configuration. Experiments have shown that the quenching

distance is relatively independent of the mode of ignition. The quenching distance is

sometimes determined by measuring the smallest diameter tube through which flame will propagate in a flammable NTP mixture of fuel-air that fills the tube. Similarly, quenching

distances can be obtained by measuring the gap required to quench propagation of flame

between flanges or plates that form a narrow channel and are filled with a flammable fuelair mixture at NTP. In the latter two experiments the explosive gas mixture is ignited at one end of a long tube or channel by a pilot flame.
Detonation induction distance in NTP air : The distance required for a deflagration to transit to a detonation in a detonable fuel-air mixture. This distance is usually experimentally determined in a long cylindrical tube with a spark or hot wire ignitor on one end

of the tube. The tube is instrumented along its length to sense the velocity of the flame

front as it propagates through the detonable mixture of gases. The distance from the

ignitor to the axial position in the tube where the flame front first attains the detonation

velocity is reported as the induction distance. This distance is dependent upon the combustible mixture constituents, the pressure, temperature and concentration of the gaseous

mixture, the enclosure geometry [54] and strength of the ignition source [18]. A deflagration is a low order explosion resulting from subsonic flame speed, relative to the unburned

gas. It is conventionally defined as a propagating reaction in which the energy transfer

from the reaction zone to the unreacted zone is achieved through ordinary rate-limiting

transport processes such as heat and mass transfer. A detonation is a high order explosion

resulting from supersonic flame speed, relative to the unburned gas. It may be defined as

a propagating reaction in which energy is transferred from the reaction zone to the unreacted

zone on a reactive shock wave.

Limiting oxygen index : The minimum concentration of oxygen that will support flame

propagation in an unknown mixture of fuel, air and nitrogen, e.g., no mixture of hydrogen,

air and nitrogen at NTP conditions will propagate flame if the mixture contains less than

5.0 volume percent oxygen [14]. Use of diluents other than nitrogen results in different

values for the limiting oxygen index of each fuel [14,34].

Vaporization rates of liquid pools without burning : The rate at which the liquid level

decreases after a pool of liquid fuel has been formed by spilling fuel onto a warm surface

such as sand or soil. These evaporation rates are measured after subsidence of the violent

boiling that accompanies the initial liquid spill. Vaporization rates of the cryogenic

fuels can be expected to vary widely with the conductivity and heat capacity of the soil or other material confining the spilled liquid fuel. In the case of gasoline, vaporization

rates will vary with the volatility of constituents (blend), age, fuel temperature, ground

surface texture and temperature, etc. Wind velocity influences the vaporization rate of

all fuels considered herein.

Burning rates of spilled liquid pools : The rate at which the liquid level decreases

after a pool of liquid fuel has been formed by spilling fuel onto a warm surface and the

resultant vapor-air mixture has been ignited. Again, these burning rates are measured

after the initial-spill violent boiling has subsided and the vapor is mixing and burning in

air above the pool of spilled fuel. These burning rates may also be obtained by adding the

vaporization rate (without burning) and the liquid level regression rate attributable to the burning of vapors in the open air over liquid fuels that are contained in open-mouthed

insulated vessels. Steady state burning rates increase with liquid pool diameter while vaporization rates continuously decrease with time, irrespective of pool size. Burning rates can

be expected to vary with pool diameter and wind velocity [25,26,118].

Flash point : The minimum liquid fuel temperature required to provide a fuel vapor-air

mixture above the surface of the liquid that will propagate a flame. The liquid fuel is

heated in an open or closed cup [55] until the vaporization rate is sufficient to form a mixture of fuel vapor and air that is within the flammable limits near the liquid surface.

Ignition is accomplished with an open flame near the surface of the liquid.

Toxicity : The quality or condition of being harmful, destructive, deadly or poisonous

to living organisms. The toxicity of a fuel can only be described in terms of its effects

on living organisms and specifically in terms of its effects on humans [56]. Toxicity

ratings [56] are (of necessity) relative values and threshold limit values for exposure to toxic substances are published annually — see reference [57].

Energy of explosion : The theoretical maximum energy available from a chemical explosion.

This maximum energy release is determined by computing the isothermal decrease in the Helmholtz free-energy function. Explosive energies listed in table 1 are expressed in terms of

equivalent quantities of TNT (symmetrical trinitrotoluene) and may be converted directly to energy units by multiplying by 4602 J/ (g TNT).
Table 1. Properties of Hydrogen*, Methane and Gasoline**.

Property Hydrogen

Molecular weight 2.016 [1] Triple point pressure, atm 0.0695 [1] Triple point temperature, K 13.803 [1] Normal boiling point (NBP) temperature, K 20.268 [1] Critical pressure, atm 12.759 [1] Critical temperature, K 32.976 [1]

Methane

16.043 [5] 0.1159 [5] 90.680 [5] 111.632 [1] 45.387 [5] 190.56 [5]

Gasoline

-107.0 [7]

180 to 220 1 [7] 310 to 478 [7,8] 24.5 to 27 [7] 540 to 569 [7]

Density at critical point, g/cm 0.0314 [1]

3 Density of liquid at triple point, g/cm 0.0770 [1]

3 Density of solid at triple point, g/cm 0.0865 [1]

3 Density of vapor at triple point, g/m 125.597 [1]

3 Density of liquid at NBP, g/cm 0.0708 [1]

3 Density of vapor at NBP, g/cm 0.00134 [1]

3 Density of gas at NTP, g/m 83.764 [1] Density ratio: NBP liquid- to-NTP gas 845 [1]

0.1604 [5] 0.4516 [5] 0.4872 [1] 251.53 [5] 0.4226 [1] 0.00182 [1] 651.19 [1] 649 [1]

0.23 [9]

~.702 [7] 0.0045 2 [9] -4400 [9] 156 3 [7,9]

Heat of fusion, J/g Heat of vaporization, J/g Heat of sublimation, J/g

Heat of combustion (low), kJ/g

Heat of combustion (high), kJ/g

58.23 [i; 445.59 [1] 507.39 [1] 119.93 [2] 141.86 [2]

58.47 [1] 509.88 [1] 602.44 [1] 50.02 [2] 55.53 [2]161 [12] 309 [7,9]

44.5 [2,4,9]

48 [2,4,9]

Specific heat (C ) of NTP gas, J/g-K 14.89 [1] Specific heat (C ) of NBP liquid, J/g-K 9.69 [1] Specific heat ratio (C /C ) of NTP gas 1.383 [1] P v Specific heat ratio (C /C ) of NBP liquid 1.688 [1] Viscosity of NTP gas, g/cm-s 0.0000875 [1] Viscosity of NBP liquid, g/cm-s 0.000133 [1] Thermal conductivity of NTP gas, mW/cm-K 1.897 [1] Thermal conductivity of NBP liquid, mW/cm-K 1.00 [1] Surface tension of NBP liquid, N/m 0.00193 [1] Dielectric constant of NTP gas 1.00026 [3] Dielectric constant of NBP liquid 1.233 [3] Index of refraction of NTP gas 1.00012 [1] Index of refraction of NBP liquid 1.110

Adiabatic sound velocity in NTP gas, m/s 1294 [1] Adiabatic sound velocity in NBP liquid, m/s 1093 [1] Compressibility factor (Z) in NTP gas 1.0006 [1] Compressibility factor (Z) in NBP liquid 0.01712 [1]

3 Gas constant (R) , cm -atm/g-K 40.7037 [1]

2 Isothermal bulk modulus (a) of NBP liquid, MN/m 50.13 [3] Volume expansivity (g) of NBP liquid, K

_1

0.01658 [3]

2.22 [1] 3.50 [1] 1.308 [1] 1.676 [1] 0.000110 [1] 0.001130 [1] 0.330 [1] 1.86 [1] 0.01294 [1] 1.00079 [1] 1.6227 [1] 1.0004 [1] 1.2739 [1] 448 [1] 1331 [1] 1.0243 [1] 0.004145 [1] 5.11477 [1] 456.16 [6] 0.00346 [6]1.62 2 [9] 2.20 2 [9] 1.05 2 [7]

0.000052 [9] 0.002 [9] 0.112 [9] 1.31 [9] 0.0122 [9] 1.0035 5 [10] 1.93" [9] 1.0017 5 [10] 1.39" [9] 154 1155 6 [12] 1.0069

0.00643 2

0.77

763 s [11] 0.0012 2 [9]
Table 1. Properties of Hydrogen*, Methane and Gasoline** (continued)

Property

Limits of f lammability in air, vol. % Limits of detonability in air, vol. % Stoichiometric composition in air, vol. % Minimum energy for ignition in air, mJ Autoignition temperature, K Hot air-jet ignition temperature, K Flame temperature in air, K Percentage of thermal energy radiated from

flame to surroundings, % Burning velocity in NTP air, cm/s

Detonation velocity in NTP air, km/s

2 Diffusion coefficient in NTP air, cm /s Diffusion velocity in NTP air, cm/s

Buoyant velocity in NTP air, m/s

Maximum experimental safe gap in NTP air, cm Quenching gap in NTP air, cm Detonation induction distance in NTP air

Limiting oxygen index, vol. % Vaporization rates (steady state) of liquid

pools without burning, cm/min 2.5 to 5.0 [26] Burning rates of spilled liquid pools, cm/min 3.0 to 6.6 [25,26]

Flash point, K gaseous Toxicity

Hydrogen

4.0 to 75.0 [13,14] 18.3 to 59.0 [15] 29.53

0.02 [16] 858 [17] 943 [22] 2318 [23]

17 to 25 [25,26] 265 to 325 [27,28] 1.48 to 2.15 [29,35] 0.61 <2.00

1.2 to 9 0.008 [31] 0.064 [16,33]

L/D«100 [35,36] 5.0 [34]

nontoxic [37] (asphyxiant)

Energy 10 of explosion, (g TNT)/(g fuel)

Energy of explosion,

(g TNT)/ (cm

3 NBP liquid fuel)

Energy of explosion,

(kg TNT)/(m3 NTP gaseous fuel)

~24

1.71

2.02

Methane

5.3 to 15.0 [13,14] 6.3 to 13.5 [19] 9.48

0.29 [16] 813 [8] 1493 [22] 2148 [23]

23 to 33 [25,26]

37 to 45 [18,19] 1.39 to 1.64 [19] 0.16 <0.51

0.8 to 6 0.12 [32] 0.203 [16,33]

12.1 [34]

0.05 to 0.5 [26] 0.3 to 1.2 [25,26] gaseous nontoxic [38] (asphyxiant)

~11

4.56

7.03

Gasoline

1.0 to 7.6 [8,18] 1.1 to 3.3 [20] 1.76 0.24 [16,21] 501 to 744 [8,18] 1313 7 [22] 2470 [24]

30 to 42 [25,26]

37 to 43 [18,24] 1.4 to 1.7 8 [30] 0.05 <0.17

nonbuoyant

0.07 [31] 0.2 [33]

11.

6

9 [34]

0.005 to 0.02

.2 to 0.9 [25,26

-230 [8] slight [39] (asphyxiant)

~10

7.04

44.22

NBP = Normal boiling point.

NTP = 1 atm and 20 C (293.15 K)

.

* Thermophysical properties listed are those of parahydrogen.

** Property values are the arithmetic average of normal heptane and octane in those cases where

"gasoline" values could not be found (unless otherwise noted).

Freezing temperatures for gasoline @ 1 atm.

@ 1 atm and 15.5 C. Density ratio @ 1 atm and 15.5 C.

@ 1 atm and 20 C.

@ 1 atm and 100 C.

@ 1 atm and 25 C. Based on the properties of butane. Based on the properties of n-pentane and benzene. Average value for a mixture of methane, ethane, propane, benzene, butane and higher hydrocarbons.

Theoretical explosive yields.
3.0 FIRE HAZARDS

By considering fire hazards, fire damage, explosive hazards and explosive damage

attributable to each fuel we can expose the relative safety merits of each fuel for potential

fuel applications. By definition, a hazard identifies a pending risk or peril. Of course,

the existence of a hazard does not assure the occurrence of a fire or explosion or that

damage will be sustained if a fire or explosion does occur. Thus, the safety hazards and

damage potential of candidate fuels must be compared to provide the requisite safety criteria

for fuel selection. The degree or extent of hazard and damage potential are frequently

difficult to express in indisputable scientific terms. Consequently, scientific data are usually tempered with experienced judgment to formulate safety evaluations for different

fuels. Hence, the perpetual controversy over the relative safety of fuels.

By referring to the combustion properties listed in table 1 we can systematically step

through the fire hazards of hydrogen, methane and gasoline. First we will consider the

accidental or inadvertent means of obtaining flammable mixtures of fuel in air. Usually,

such mixtures are the result of fuel leakage or spillage which may be attributable to mechanical failure of equipment, material failure, erosion, physical abuse, improper

maintenance, collision, etc.

The rate at which the fuel vapors mix with air is indicated by their diffusion velocities and their buoyant velocities. The buoyant effect is dominant for hydrogen and methane

and from the data listed in table 1 it is apparent that hydrogen can be expected to mix with

air more rapidly than methane or gasoline — the latter is obviously the slowest mixing fuel

of the three fuels considered. In the event of a fuel spill, one could expect hydrogen to form combustible mixtures more rapidly than methane because hydrogen has a higher buoyant

velocity and a slightly lower flammable limit. Again, gasoline would be orders of magnitude

slower than hydrogen or methane in forming combustible mixtures in air. In some fuel applications these relative mixing times may be important while in others they are meaningless,

e.g., an instantaneous fire hazard exists for the impact rupture of an auto fuel tank

irrespective of the type of fuel carried.

Because of their higher buoyant velocities, hydrogen and methane can also be expected

to disperse more rapidly than gasoline and thus shorten the duration of the flammable

hazard. Even though the upper flammable limit (UFL) of hydrogen is much higher than that

of methane the higher buoyant velocity of hydrogen permits it to disperse to concentrations

below the lower flammable limit (LFL) more rapidly [58] than methane. Thus, one could expect

a fire hazard to exist most readily with hydrogen, methane and gasoline respectively, and

to persist in the inverse order.

We must exercise some caution in analyzing fuel mixing and dispersion rates by comparing

relative buoyant and diffusion velocities of the NTP fuel gases. In large cryogenic liquid

fuel spills, the vaporization of liquid and warming of the vapor can cool large masses of

air. In addition, NBP hydrogen vapor density approaches that of NTP air while NBP methane

vapor density is greater than the density of NTP air. Consequently, for some finite period

of time these cold vapor-air mixtures are nonbuoyant and may extend to appreciable distances

from the spill. Therefore, both the range and duration of the fire hazard may be extended

somewhat when cryogenic liquid fuels are spilled. More definitive experimental data are needed in this area to supplement existing knowledge [25,26,96]. Some experimental data and

analyses for LNG spills on land are available [118]; these data treat the dispersion and

drift characteristics of vapor clouds that form over LNG spills.

The low value of limiting oxygen index reflects the high value of the UFL for hydrogenair mixtures. The wide flammability limits of hydrogen are of practical significance only

when fuel leakage into enclosed spaces is a major concern. In this case the flammable limits

of hydrogen are sufficiently wide to enhance the probability of combustion from a random

ignition source. This flammability characteristic should not preclude the use of hydrogen

because the LFL is the vital one in most applications. The LFL is important because ignition

sources are nearly always present when a leaking fuel first reaches combustible proportions

in air.

The rate of vapor generation and burning over spilled liquid pools is of interest for

the various fuels. As indicated in table 1 the volumetric vaporization rates and burning

rates are highest for hydrogen, methane, and gasoline, respectively. Consequently, for a
given liquid spillage volume, gasoline fires will last the longest and hydrogen fires the

shortest while all of the fuels burn at about the same flame temperature. The thermal energy

radiated from these pool-fed fires may be computed by multiplying the appropriate (burning

rate) X (NBP liquid density) X (high heat of combustion) X (percentage of thermal energy radiated from the flame to its surroundings). The radiated thermal energy should not exceed

2 2 2 276 W/cm of pool liquid-vapor surface area for hydrogen, 155 W/cm for methane or 212 W/cm

for gasoline. Then, the scene of a hydrogen fire may be hotter than that of a hydrocarbon

fire, but the hydrocarbon fires will last five to ten times longer than hydrogen fires (for

equivalent fuel spillage volumes). Additional data for LNG vaporization and burning rates

and for radiant heat flux emitted from fires over LNG pools are presented in reference [118].

Another matter of concern in evaluating fire hazards is the rate of leakage flow of

liquids or gases through leakage paths of varying geometries, e.g., leakage through cracked

welds, improperly mated flanges, threaded fittings, damaged seals, etc. Previous work [59]

has shown that volumetric leakage rates are inversely proportional to the square root of the

density, or inversely proportional to the absolute viscosity of the leaking fluid. Using

the data given in table 1 we can estimate the relative volumetric leakage rates of the NBP liquid fuels and of the gases at NTP. Under these conditions the volumetric leakage flow

of liquid hydrogen will be 2.4 to 8.5 (viscous flow) times as large as liquid methane

leakage; volumetric leakage flow of liquid hydrogen will be 3.1 to 15 (viscous flow) times

as large as liquid gasoline leakage; volumetric leakage flow of hydrogen gas will be 1.3

(viscous flow) to 2.8 times as large as gaseous methane leakage; volumetric leakage flow of

hydrogen gas will be 0.6 (viscous flow) to 7.3 times as large as gasoline vapor leakage.

Since care is usually taken to minimize fuel leaks the viscous leakage flows are considered

to have the greatest practical significance. Thus, we conclude that liquid hydrogen is

much more difficult to contain than liquid methane or liquid gasoline and that NTP gasoline

vapors are more difficult to contain than either NTP hydrogen or NTP methane. In spite of

these unfavorable leakage characteristics, industry has proven that both liquid and gaseous

hydrogen can be safely and easily contained.

Techniques for detection of hydrogen leakage are effectively summarized by two documents

[60,61] and general procedures for leak testing are detailed in the handbook by Marr [60].

The minimum spark energy required for ignition of hydrogen in air is about an order of

magnitude less than that for methane or gasoline; however, the ignition energy for all three

fuels is sufficiently low that ignition is relatively assured in the presence of thermal

(weak) ignition sources, e.g., sparks, matches, hot surfaces or open flames. Even a weak

spark due to the discharge of static electricity from a human body may be sufficient to ignite any of these fuels in air — 10 mJ sparks may be produced in such electrostatic

discharges [62].

Although hydrogen has a higher autoignition temperature than methane or gasoline, its

low ignition energy characteristic makes it more readily ignitable than either of the

hydrocarbon fuels. The hot air-jet ignition temperature is highest for methane and lowest

for hydrogen; therefore, hydrogen is easiest to ignite by jets of hot combustion products

emitted from an adjacent enclosure. The flash point is meaningless for the cryogenic fuels,

hydrogen and methane, within the temperature range of interest because these fuels will

flash at all temperatures above their normal boiling points. The boiling points of the

cryogenic liquid fuels are so low that these fuels are considered to behave like gases.

The flash point of gasoline is also well below room temperature; therefore, all three fuels

must be considered volatile and will generate sufficient vapor to create a fire hazard at earth surface temperatures. Then, all three fuels are relatively easy to ignite. Hydrogen

is most susceptible while methane and gasoline appear to be equally susceptible to ignition.

The burning velocity is a fundamental property of a combustible gas mixture and should

not be confused with the flame speed [63]. The burning velocity influences the severity of

the explosion and along with quenching gap is important in the design of flame arresters [63].

Higher burning velocities indicate a greater tendency for the combustible gas mixture to support the transition from deflagration to detonation in long tunnels or pipes. The

instantaneous pressure rise due to the ignition of a fuel-air mixture contained in a spherical

vessel is proportional to the cube of the burning velocity [64]. In general, faster-burning

gases have smaller quenching gaps and flame arresters for faster-burning gases must have

smaller apertures [63]. The quenching gap is the passage gap dimension required to prevent

8
propagation of an open flame through a flammable fuel-air mixture that fills the passage and

it is clearly distinguishable from the MESG. The latter is the maximum permissible clearance

between flanges to assure that an explosion does not propagate from within an enclosure to a flammable mixture surrounding the enclosure. Because of the high explosion pressures the

MESG is always smaller than the quenching distance.

Available data [65] indicate that the pressure rise ratios for adiabatic combustion of

stoichiometric mixtures of hydrogen-air and methane-air in closed vessels are nearly identical. Similar data [31] produced pressure rise ratios for hydrogen-air that were 20 to 40

percent higher than those for gasoline-air. In long tubes or tunnels, hydrogen-air mixtures

will transit to detonation more rapidly than methane-air or gasoline-air mixtures; therefore,

overpressure hazards in confined spaces are enhanced in hydrogen systems. Thus, the high

burning velocity of hydrogen is an' indication of its high explosive potential and of the

difficulty of confining or arresting hydrogen flames and explosions. Industrial equipment is

currently available to safely confine hydrogen explosions but pipeline flame arresters, though

successful in certain hydrogen-air combustion applications, are not yet considered reliable

[87].

4.0 FIRE DAMAGE

Some of the combustion characteristics discussed in the previous section are also applicable to comparisons of fire damage by the different fuels. Similarly, many of these characteristics are applicable to explosive hazard and explosive damage criteria as will be revealed

in subsequent sections of this paper. In the previous section we discussed the risk or likelihood of having a fire. In this section we examine the damage potential of a fire.

The main fire damage parameters are thermal radiation, flame engulfment (fireball), smoke

inhalation, fire detection and extinguishment. Explosion overpressure, impulse and shrapnel

damage are related parameters that are reserved for detailed discussion under the section

entitled explosive damage. Thermal radiation characteristics of the three fuels under consideration have already been discussed so we turn our attention to fireballs.

Fireball damage is the direct result of combustion of materials initiated through contact

or engulfment by flames that are consuming fuels. A fireball may result from the ignition of

fuel-air mixtures or from the explosion of solid or chemical explosives. The explosion

fireball is short-lived but flames will persist until all of the fuel is consumed in fuel-air

fires. Ignition of fuel-air mixtures above pools of spilled liquid fuels produces flames

with dimensions that vary with the volume of spilled liquid, rate of spillage, nature of

spillage containment surface, wind velocity, location of ignition source and time delay before ignition. A simple mathematical expression [66] seems to adequately predict the maximum

equivalent spherical radii of fireballs for a wide variety of explosives including hydrogenair and rocket bipropellants. The diameter of the fireball is given by

1/3

D ss 7.93 W

f /J

,

where D = diameter in meters and W

f = weight of fuel in kilograms. The fireball duration may be

estimated from

t w 0.47 W

f 1/3

,

where t is in seconds and Wf is the weight of fuel in kilograms. See reference [66] for limitations of these approximations. Various fireball and fire radiation criteria are summarized

along with overpressure criteria on figures 1 and 2. The development and discussion of these

figures has been treated elsewhere [66] and will not be repeated here. Although these figures were developed specifically for hydrogen they are believed to provide conservative safety

criteria for methane and gasoline. It must be emphasized that these figures are based upon

potential fire and explosion hazards in unbarricaded storage and experimental areas and are not indicative of industrial storage standards for fuels. Industrial hydrogen storage standards are much less stringent — see figure 3.

Smoke inhalation is one of the major causes of injury and death in any fire. When fuelair fires cause buildings and other combustible materials to burn, smoke inhalation is of concern for hydrogen, methane and gasoline. When fuels burn in the open air only gasoline

can cause severe smoke inhalation damage as both hydrogen and methane are clean burning
fuels. Inhalation of the combustion products from hydrogen-air or methane-air fires is

considered less serious because both of these fuels are buoyant and require a large influx

of fresh air to sustain the fire. Also, the combustion products of hydrogen-air fires

(mainly nitrogen and water vapor) and methane-air fires (mainly nitrogen, carbon dioxide

and water vapor) are not foreign to the lungs as are the sooty combustion products of

gasoline-air fires. Of course, the lungs can be seared by breathing hot combustion gases

produced by burning any of these fuels. Breathing any of these fuels or their combustion

products in sufficiently rich concentrations can also cause asphyxiation. The physiological

effects of breathing aerosols, toxic gases (such as CO and C0„), hot gases and oxygendeficient gases are reviewed by Custer and Bright [67].

Current fire detection technology is summarized in a recent document [67] and hydrogen

fire detection is reviewed in an older publication [61]. Hydrogen flames are nearly

invisible in daylight but their visibility is improved by the presence of moisture and/or

impurities in the air. Hydrogen fires are readily visible in the dark or in subdued light

and large hydrogen fires are quite detectable in daylight because convective "heat ripples"

are visible in the air at near-range distances and thermal radiation heats the skin. Small

hydrogen fires are more difficult to detect and require that certain precautions be taken

to avoid personnel and equipment damage. Methane flames, though clean-burning, are yellowish in color and quite visible in daylight. Gasoline flames are similar to those

of methane but are mixed with large volumes of soot and smoke so that fire detection is

obvious.

Two types of sensors, thermal and optical, are used to detect hydrogen fires.

Thermal sensors are the conventional type and are fully discussed by Custer and Bright

[67]. These conventional sensors (including smoke and ionization detectors), coupled

with flame visibility, are adequate for detection of methane or gasoline fires and not

quite adequate for detection of all hydrogen fires. The aerospace industry has advanced

the use of optical sensors for detecting hydrogen fires in bright-field environments.

The most common optical sensors detect ultraviolet or infrared radiation and several

detection schemes exist [61, 67]. Closed-circuit infrared and ultraviolet television

sets, equipped with appropriate filters, have been successfully used to detect hydrogen

fires on rocket engine test stands [61]. Intumescent paints have also been used to detect hydrogen fires. These paints char and swell at low temperature (~200°C) and

emit pungent gases. Hydrogen fires are obviously more difficult to detect than methane

or gasoline fires; however, the availability of modern detection equipment makes it

possible to quickly and reliably detect the flames of all three fuels.

A brief review of fire extinguishment methods and recommended fire extinguishing

procedures for various combustible materials has been published by the NFPA [8], Class B extinguishing agents [8] are generally suited for gasoline fires and water deluge, or water

spray, are usually useful in fighting gasoline, methane or hydrogen fires. The water is

used to cool and protect adjacent exposed combustibles and may not extinguish the fire unless

it is used in a prescribed manner by skilled personnel. Water may be particularly ineffective

in extinguishing liquid gasoline fires as the gasoline is -less dense than water and will

float on top of the water and continue to burn.

It is sometimes advisable to permit hydrogen and methane fires to burn until gas-flow is

stopped or liquid-spills are consumed because of the potential explosive hazard created by

extinguishing such flames. If the fuel source is neither depleted nor shut off, an explosive

fuel-air mixture may be formed with far greater damage potential (if ignited) than the original

fire. In those instances where extinguishment is judged imprudent, a water deluge may be used

to cool surrounding combustibles and control fire damage.

Recent experiments [118] have evaluated the effectiveness of commercially available dry

chemical agents and high-expansion foams in controlling and extinguishing LNG pool fires.

These experimental results show that dry chemicals can be used to extinguish LNG fires and

foams applied to the pool surface reduce the radiant heat flux to surroundings while reducing

vapor evolution. Thus, fire control and fire extinguishment methods and equipment are commer- cially available to combat LNG fires.

It is anticipated that these same fire-fighting procedures would be effective in controlling liquid hydrogen fires but no such data exist. Inert gas-flooding and C0„ extinguishers

have been successfully used to extinguish gaseous hydrogen fires.

10
Thus, we may conclude that: 1) water deluge or water spray is useful in fighting hydrogen,

methane or gasoline fires, 2) gasoline or methane (LNG) fires may be controlled or extinguished

using commercial dry chemical or high-expansion foam agents and 3) the effectiveness of dry

chemicals or foams in controlling or extinguishing liquid hydrogen fires has not been evaluated.

5.0 EXPLOSIVE HAZARD

The ignition of a combustible mixture of fuel-air can result in a fire or an explosion.

An explosion is always accompanied by a fireball and a pressure wave (overpressure). The

fireball may ignite surrounding combustibles or fuel released by the explosion so that a fire may follow an explosion. If the fuel-air mixture is partially or totally confined

the explosion may propel fragments of the enclosure material at high velocities over great

distances. By its nature an explosion hazard constitutes fire, overpressure, impulse

and shrapnel hazards. The extent of overpressure, impulse (overpressure-force multiplied

by the time interval of explosive overpressure) and shrapnel hazards is dependent upon the

severity of the explosion. Detonations cause more damage than deflagrations.

The same combustion parameters that influence fire hazards also influence the explosive hazards associated with each fuel; therefore the discussion on fire hazards, section

3.0, is equally applicable to this section on explosive hazards. Two of the combustion

properties reviewed in section 3.0, MESG and hot air-jet ignition temperature, are vitally

important to the containment of an explosion and to prevent propagation of the explosion

to explosive mixtures of fuel-air surrounding the enclosure. The containment vessel must

be sufficiently strong to withstand the explosion pressure without emitting jets of

combustion products that are larger or hotter than those specified by the 'hot air-jet

ignition temperature' and simultaneously the MESG must not be exceeded.

The limits of detonability are important in an evaluation of the explosive hazards of

fuel-air mixtures. The wider these limits, the greater the probability of a high-order

explosion with attendant high overpressures and severe shrapnel hazards — detonation. The

flammable limits define the fuel-air concentrations that will burn and low-order explosions

may occur within these limits. Such explosions are called deflagrations and they result in

lower overpressures and less shrapnel hazard than those associated with detonations. In order to have a fire or an explosion there must exist in combination an oxidant, a fuel,

and an ignition source. The fuel and oxidizer are supplied and mixed by the release of

fuel into the air. Hydrogen has by far the widest limits of detonability of the three fuels

considered herein; therefore, it presents the greatest hazard to explosion damage. The

explosive potential of all three fuels is discussed in the next section.

The ignition source may be a mechanical or electrostatic spark, flame, impact, heat

by kinetic effects, friction, chemical reaction, etc. The strength of the ignition source influences whether a detonable mixture deflagrates or detonates. Weak (thermal) ignition

sources initiate deflagrations in open and closed systems; however, a deflagration may

develop into a detonation in a closed system due to the influence of the confining walls.

Strong (shock-wave) ignition sources tend to initiate detonations in open or closed fuel-air

systems. Matches, sparks, hot surfaces and open flames are considered to be thermal (weak)

ignition sources while shock-wave (strong) ignition sources are blasting caps, bursting

vessels, TNT, high voltage-capacity shorts (exploding wires), lightning, and other explosive

charges.

The geometry of an enclosure has a strong effect on the transition from deflagration to detonation. Experimental data [111] indicate that a U-shaped enclosure plus the ground com- prise sufficient confinement to support 'strong' explosions in detonable hydrogen-air mixtures

that are ignited by thermal ignition sources. Geometrical changes in the confining walls

that induce turbulence also enhance transition to detonation. The distance in a tunnel,

pipeline, heating duct, or hallway that it takes for a reaction to progress from a deflagration to a detonation is related to the detonation induction distance. Hydrogen is a rapid

burning fuel and the flame front has a tendency to accelerate in long enclosures. Consequently, detonation induction distances have been experimentally observed using hydrogen-air

mixtures but no such data exist for the slower burning gasoline-air or methane-air mixtures.

Transition to detonation occurs because compression of the unburned fuel-air mixture by

deflagration increases the mixture temperature and pressure, both of which increase the burning

velocity of the mixture. Recent experiments [87] indicate that it is difficult to design

flashback arrestors that successfully disrupt deflagrative or detonative combustion in

11
hydrogen-rich mixtures of hydrogen, methane and air that are contained within or flowing in

cylindrical pipelines. Burgess [20], Zabetakis [68], and Carhart [30] agree that methane-air

and gasoline-air mixtures will transit from deflagration to detonation if the pipe is long

enough and its diameter is large enough. Experimental apparatus used to determine detonation

induction distance is usually small and consequently these data are yet to be determined

for methane and gasoline. The largest detonation experiments conducted to date were performed

by Kogarko [19] and Burgess et al. [71]. Kogarko used a 30.5 cm inside-diameter tube and

Burgess et al. used a 61 cm diameter pipe and earthen tunnels with cross sectional areas

2 ranging from 0.025 to 1.39 m . We have no assurance that gasoline-air or methane-air mixtures

will not detonate from spark ignition in long tunnels or corridors; however, we can be quite

sure that hydrogen-air mixtures will detonate under the proper circumstances (L/D ^100).

We are belaboring the fuel detonation characteristics as they are believed to be of

vital importance in future fuel applications. The tendency of hydrogen to detonate from

spark ignition is perhaps the most significant deterrent to its widespread use. The pressure

rise ratio of a detonation may easily be an order of magnitude higher than that of a deflagration — see detailed discussion in reference [66]. The static pressure rise ratio will not

exceed 8:1 in a confined deflagration [65] of hydrogen-air or methane-air. These explosion

pressures are about 20 to 40 percent higher than those measured [31] for confined deflagrations of gasoline-air. Overpressures due to deflagrations in open air are usually considered

negligible; however, open-air deflagrations can cause structural damage if they are close to the structure and are of sufficiently large volume, e.g. see reference [69].

Shrapnel hazards relate directly to explosion overpressures so that all of the foregoing

arguments concerning overpressure apply to the evaluation of shrapnel hazards for the different

fuels. Thus, we see that overpressure and shrapnel hazards associated with ordinary

enclosures (L/D £ 30) are about the same for hydrogen-air and methane-air and somewhat less

stringent for gasoline-air. In long tunnels, etc. hydrogen is a greater explosion threat

than either of the other two fuels because it has a greater tendency to transit to detonation.

The wider flammable limits and detonable limits of hydrogen also tend to make hydrogen a greater explosive threat than methane or gasoline.

A number of preventive measures can be enacted to minimize the explosive threat of all

three fuels and are particularly helpful for the rapid-dispersion fuels, hydrogen and methane.

Roof vents and forced ventilation, where practical, are accepted methods of minimizing accumulations of gaseous fuel within enclosures. In some applications the quantity of fuel permitted

within an enclosure can be restricted. Ignition sources can be minimized but seldom are they

eliminated. Frangible (weak) walls can be used to relieve deflagration overpressures within

enclosures — rupture discs can be used to provide the same protection for pressure vessels.

Frangible walls and discs are of little value in relieving detonation overpressures, although

in some instances they may prevent or lessen the effects of detonation [66]. It appears that

weak but pressure-wave reflecting walls will support transition from deflagration to detonation — the use of elastic membranes (plastic curtains) may inhibit or prevent transition to detonation. Fuel storage tanks can be buried or storage areas can be diked (for fuel containment), barricaded and confining structures minimized. Major spillage can be avoided by using

storage vessels constructed of ductile materials and by adherence to established safety

procedures.

6.0 EXPLOSIVE DAMAGE

The elements of explosive damage, fireball, ensuing fires, overpressure and shrapnel, have

already been discussed and only overpressure and shrapnel warrant additional attention.

Explosions that create overpressures and shrapnel may be rated in terms of the amount of

energy that is released. This energy release may be evaluated directly in energy units such as

kJ although it is commonly expressed as an equivalent quantity of TNT (symmetrical trinitrotoluene). The explosive strength of TNT is well-known and reproducible and it is a good standard

for rating the explosive potential of various substances. Expressing explosive potential in

terms of an equivalent mass of TNT is a good technique for evaluating damage potential at distances well-removed from the explosion; however, at distances inside or near the reaction

zone, this procedure is less accurate because of the differences in shape and peak magnitude

of the impulse diagrams for TNT and fuel-air mixtures. A fuel-air explosion may deliver a considerably lower overpressure, relative to TNT, over a longer time interval and thus have

12
less crushing effect on some structures, but a greater overturning moment. Although there is

general dissatisfaction [85] with the TNT concept, it will continue to be used until non-ideal

explosions can be characterized more definitively. To provide conservative results the TNT equivalent concept can be used [66] to evaluate impulse and overpressure effects at distances

far from the explosion source, and to evaluate impulse effects in the near-combustion zone.

The theoretical TNT equivalent, of various fuels, can be determined [70] by using the

decrease in Helmholtz free energy to compute the maximum energy available for explosive yield.

Following this procedure we obtain the theoretical limiting values of explosive potential for

hydrogen, methane and gasoline as recorded in table 1. Note that hydrogen is the most potent

on a mass basis and the least potent on a volumetric basis. The explosive potential per kJ

of stored heating value (based on the high heat of combustion) is 0.17 (g TNT)/kJ for hydrogen,

0.19 (g TNT)/kJ for methane and 0.21 (g TNT)/kJ for gasoline. Thus, for equivalent energy

storage, hydrogen has the least theoretical explosive potential of the three fuels.

It must be emphasized that only a fraction of this theoretical explosive yield can be

realized in an actual open-air mishap because it is virtually impossible to spill or release

a large quantity of fuel and have all of it mixed in proper proportions with air prior to ignition. Experimental data and computations indicate that the fraction [66, 85, 96] of fuel

within the combustible range at any time following a massive or continuous fuel spillage will

be less than ten percent of the quantity spilled. Such explosive yield data are meager for

all three fuels; however, the vapor or gas phase mixing limitations are equally applicable

to all fuels. Hydrogen disperses much more rapidly than methane or gasoline, but it also has

much wider flammable and detonable limits, etc. Thus, in the absence of more definitive

experimental data it is impossible to accurately assess the probable explosive yield attributable to accidental release and ignition of hydrogen, methane or gasoline in air. The

'energy of explosion' values listed in table 1 should be considered theoretical maximums and

yield factors of ten percent are considered reasonable for fuel-air explosions. Higher yield

factors are possible in liquid bipropellant explosions [74].

A recent document [119] provides a new approach to the prediction of hazards and damage

from explosions of lightweight bipropellant tanks and from bursting lightweight gas storage

vessels. This approach evaluates explosive yield, overpressure, impulse and fragment characteristics directly and avoids the use of conventional TNT equivalency. This study also assesses the damage effects of blast and fragments on structures, facilities and humans, and

appears to be the first attempt to provide more realistic explosion-damage predictions than

can be obtained using TNT equivalents.

Overpressure damage is highly dependent upon the nature of the explosion. A confined

and unvented deflagration [65] of hydrogen-air or methane-air will produce a static pressure

rise ratio of less than 8:1. Explosion pressures [31] for confined deflagrations of gasolineair are about 70 to 80 percent of those for hydrogen-air. Unconfined deflagration overpressures

are usually less than 7 kPa; however, 3 to 4 kPa is sufficient side-on pressure to cause structural damage [69] to buildings and unconfined large volume gas-phase explosions can be

destructive. Ordinary glass window panes fracture under pressures of 3 to 7 kPa, non- reinforced masonry walls fail at pressures below 55 kPa, and human eardrums rupture at pressures of approximately 35 kPa. Thus, it is apparent that confined deflagrations (even

if relieved) can be very devastating — up to 8 atmospheres (811 kPa) of explosion pressure —

and unconfined deflagrations can cause slight to moderate structural damage and injure people

via fire, window-glass shrapnel, etc.

Detonations, whether confined or unconfined, can be expected to severely damage or totally destroy ordinary buildings in the near vicinity of the explosion. TNT pressure-distance data [74] can be used [71, 74] to estimate overpressures resulting from fuel-air

detonations. The applicability of TNT equivalence to vapor or gas-cloud explosions is fully

reviewed in references [74, 85]. Determination of blast yield for liquid-phase explosions

was recently reviewed by Baker et al. [86,119]. The pressure accompanying detonation of any

fuel is approximately double [72] that obtained by adiabatic combustion of a stoichiometric

mixture of the fuel in air at constant volume. Consequently, we could expect static pressure

rise ratios of ~15:1 for hydrogen-air or methane-air detonations and a ratio of ~12:1 for a gasoline-air detonation. Much higher reflected pressure rises can be attained if the explosion transits from deflagration to detonation because the deflagration compresses the unburned

fuel-air mixture prior to transition to detonation, e.g., a reflected pressure rise ratio of

120:1 (8:1 X 15:1) could be achieved where a hydrogen or methane deflagration transits to detonation. Such transitions are more easily accomplished with hydrogen than with methane or gasoline.

13
The impulse created by explosion overpressures is of concern in evaluating explosion

damage and in the design of barricades or structures to withstand explosions. Although the

overpressure created by a gas-phase explosion is of lower peak magnitude and longer duration

[73] than the overpressure due to an equivalent quantity of TNT, the extensive TNT data may

be used [71, 73, 74] for design purposes. The fundamentals of dynamic blast loads and

structural response to shock waves is treated elsewhere [28, 70, 71] as is the applicability

[70, 71, 73, 74, 85, 86] of TNT explosive data to the design of structures to withstand

gas-phase explosions.

Unbarricaded distances required for the protection of personnel in inhabited buildings

that are exposed to shrapnel from TNT explosions are indicated by curve 6 on figure 1. These

data also predict the maximum observed fragment distances for space vehicle explosions [74]

and are more restrictive than the unbarricaded distances required for shrapnel protection of

personnel on roadways — see curve 2 on figure 1. Fletcher [75] has suggested that TNT shrapnel hazard data may be used to estimate propellant-explosion shrapnel hazards if the

appropriate TNT equivalent is used. There is evidence [75] that large low-velocity fragments

emitted from such explosions may exceed the TNT shrapnel scatter limits. This situation

results because propellant explosions endure longer and can impart more impulse to the

projectile; however, the range of high population-density projectile scatter is normally

greatest [75] for an equivalent quantity of TNT. An an interim measure the Department of

Defense and NASA have adopted [74] the TNT shrapnel hazard data for propellant explosives

at range launch pads and rocket engine test stands. These TNT data adequately predict [74]

maximum fragment distances for known incidents involving propellant explosions. A thorough

compilation, analysis and correlation of available fragmentation data for liquid propellant

tank explosions was recently published by Baker et al. [86,119].

Design for shrapnel protection is difficult because it is necessary to estimate the

size, mass, and velocity of fragments emitted from explosions of varied type, strength, and

location. Some guidance on this subject may be found in references [75-86,119]. In brief,

shrapnel shields made of materials with large modulus of elasticity (Young's) are the

most effective; for example, steel is more shrapnel resistant than copper and copper is more resistant than aluminum, etc. The opposite is true for shrapnel projectiles themselves.

A projectile having a given size and momentum will penetrate deeper if it has a lower

density — aluminum will penetrate deeper than copper and copper deeper than steel, etc. Recall that momentum is mass X velocity and a constant momentum and size require a higher

velocity for the lower density projectiles — see reference [82].

Figures 1 and 2 summarize much of the fire and explosive hazard data presented herein.

These figures illustrate the variation in conservatism of various authorities that generate

safety criteria. Obviously, when in doubt, the more conservative criteria should be used.

The overpressure band on figure 1 corresponds to breakage of ordinary window glass

(3.5 to 7 kPa) and the overpressure band on figure 2 relates to the estimated external

pressure capability of liquid hydrogen storage dewars. The derivation and use of data

shown on these figures (safe unbarricaded distances) are fully discussed in a summary document [66] treating the explosive hazards of hydrogen. Safe barricaded distances for TNT

and fuel-aii. explosions may also be estimated from data made available in references

[66, 74, 84]. A single series of documents [74] offer comprehensive treatment of over- pressure, impulse, fireballs, shrapnel, barricades, structural response and physiological

effects, as they relate to propellant explosions. The author feels that these documents

are applicable to fuel-air explosions where the TNT equivalent is properly estimated.

Care should be exercised when attempting to assess the damage potential of large-volume

gas-phase explosions because line-of-sight from such explosions to vulnerable targets may

pass over or around barricades that were erected to provide protection from concentrated

explosives.

Figure 3 provides a ready comparison of industrially accepted fuel storage standards for

hydrogen, LNG and gasoline. By comparing curves 1 and 4 on figure 3 With the data given on figures 1 and 2, we find that the industrial quantity-distance standards (figure 3) for hydrogen are less demanding than those suggested for experimental areas (figures 1 and 2). Also,

by comparing curves 1, 2, 3 and curves 4, 5, 6 on figure 3 we observe that industrial storage

standards are more restrictive for hydrogen, methane and gasoline, respectively.

14
Curve 3 on figure 3 represents the minimal distance for separation of two adjacent aboveground gasoline storage tanks. The distance [117] between such tanks shall not be less than

three feet and not less than one-sixth the sum of the diameters of two adjacent tanks. When

the diameter of one tank is less than one-half the diameter of the adjacent tank, the distance

[117] between the two tanks shall not be less than one-half the diameter of the smaller tank.

Curves 6A and 6B on figure 3 represent the variation in quantity-distance standards for

the protection of personnel in buildings adjacent to gasoline storage tanks. These curves bound standards [117] that vary with type of tank construction, fire control measures and

protection for exposures, tank operating pressure and emergency venting equipment.

The distance [117] from any part of an underground tank (storing gasoline) to the

nearest wall of any basement or pit shall be not less than one foot, and not less than three

feet from inhabited buildings.

It is apparent that industrial storage standards are least restrictive for gasoline;

however, the industrial storage standards for LNG and hydrogen fuels are not prohibitive

and should not limit their use.

7.0 COMPARISON OF FUEL STORAGE METHODS

Hydrogen may be stored in the compressed gas, liquid or hydride forms. The relative

costs [88] of storage in these various forms are dependent upon the quantity of hydrogen

stored and upon desired storage pressure and storage duration.

Currently, the most promising alloys for metallic hydride storage contain titanium and

magnesium. Storage data [89] for magnesium-nickel (Mg-Ni) and iron-titanium (Fe-Ti) hydrides

are available and hydride storage system evaluations are in progress [90]. Safety standards

for the production, processing, handling and storage of titanium and magnesium are well

documented [91]; however, the safety hazards are much less certain for the candidate Fe-Ti

and Mg-Ni ores in combination with stored hydrogen. A recent experimental evaluation [92]

indicates that Fe-Ti hydride can be considered a safe method of storing hydrogen. Hydride

storage appears attractive [90] for certain short-term storage applications but excessive

weight, volume or cost restraints could limit the use of hydrides in transportation applications.

Hydrogen is routinely stored as a compressed gas in industry and this practice must be

considered safe. It is usually stored in metal containers at pressures ranging from near- ambient to more than 20 MPa. It may also be possible to store compressed hydrogen in abandoned natural gas fields, caverns, aquifers, etc. Metal storage containers are normally

constructed of a hydrogen- compatible ductile steel and are not generally susceptible to catastrophic failures. Such containers are normally equipped with pressure and thermallyinduced pressure relief devices. Vessel fracture would most likely be accompanied by

autoignition of the released hydrogen and air mixture with an ensuing conflagration lasting

until the contents of the ruptured vessel are consumed. Considerable experimental data

substantiate this statement — see references [66, 93]. Adjacent metal storage vessels are relatively shrapnel and fire resistant and may be water-cooled or buried in sand for

additional fire and blast resistance.

Hydrogen is also routinely stored as a liquid in industry and in university and government laboratories supporting the U.S. space exploration program. Again, catastrophic

failures are not technically plausible as dewars are constructed of ductile metals and are diked to confine the liquid contents of the dewar in the event of spillage. The purpose of

the dike is to reduce the liquid evaporation rate and to confine the potential conflagration

to the vicinity of the defective dewar. The double wall construction of liquid storage

dewars provides good protection against fire and shrapnel and additional fire resistance for

adjacent storage dewars can be provided with a water deluge system. Liquid hydrogen storage

dewars are usually built with carbon steel vacuum jackets and aluminum or stainless steel

inner vessels. Liquid hydrogen has been safely stored in large metal dewars for nearly

20 years.

15
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Hydrogen continues to be stored and used in both the compressed gas and liquid forms in an industrially safe manner. Hydride storage should prove to be equally safe.

Gasoline is normally stored in simple steel containers at near-ambient pressures both

above and below ground. The principles of diking and water-cooling to protect adjacent

buildings or storage vessels are equally applicable to aboveground gasoline, liquefied natural

gas (LNG), and liquid hdyrogen storage. The use of ductile steel tanks virtually precludes

catastrophic failures and diking confines potential conflagrations. Gasoline has been safely

stored in large quantities for over half a century.

Methane is commonly and safely stored in large quantities as a constituent of compressed

gas in natural gas fields, caverns, abandoned coal mines, etc. The safety record of the natural

gas industry is exceptionally good.

LNG, whose primary constituent is methane, has been stored in large quantities [94] in

the U.S. since 1941. The early tanks were of double-walled steel construction (3.5 percent

nickel steel inner shell and carbon steel outer shell) and a disastrous fire accompanied the

failure of one of these tanks in 1944. Changing the inner shell material from 3.5 percent

nickel steel to 9 percent nickel steel or aluminum has cured the early storage problems

associated with LNG and it has been safely stored [94] in large quantities since the midsixties. Storage vessels with concrete inner shells and carbon steel outer shells have also

been placed in service within the last several years. LNG is stored in aboveground and belowground containers and the space between the double walls is usually filled with a foam, powder,

or fibrous insulation material and gas-filled with nitrogen or natural gas. All of the foregoing arguments concerning the storage of gasoline and liquid hydrogen apply to the storage of LNG. In fact, a comprehensive fire and explosion hazard study [26] by the Bureau of Mines

concluded that LNG could be safely stored in much the same manner as gasoline.

Of the three fuels examined, gasoline is certainly the easiest and perhaps the safest fuel

to store because of its higher boiling point, lower volatility and narrower flammable and

detonable limits. All of the previous discussion in this paper concerning fire and explosion

hazards support this generalized conclusion; however, we must recognize that hydrogen and

methane (or LNG) can also be safely stored. The degree of risk associated with the storage

of each fuel cannot be specified at this time so that we must conclude that all three fuels

can be safely stored using current technology.

8.0 FUTURE HYDROGEN APPLICATIONS

Hydrogen is being considered as a replacement fuel [95] in all of the major fuel markets

and will continue to penetrate these markets as time passes. It is anticipated that all of

the major fuel markets (industrial, commercial, transportation, and residential) will ultimately

rely on electricity or hydrogen. A detailed safety analysis covering the use of hydrogen fuel

in each of these market areas is needed but well beyond the scope of this paper. Credible

accidents must be postulated and detailed fault trees must be developed to provide meaningful

hazards data. A resourceful analyst is required because some of the experimental data required

for thorough hazards evaluation are nonexistent, e.g. the effects of partial confinement on explosion overpressure and transition to detonation, the effects of elastic weak-walls on deflagration overpressures and the effects of atmospheric dispersion rates on vapor or gascloud accumulations — see references [66, 85, 96]. A current review article [85] summarizes

the state-of-the-art concerning characterization and evaluation of accidental explosions

and is a useful guide to current research efforts in this area. The interested reader or analyst may find additional information and assistance with hydrogen safety problems by re- ferring to the recent compilations by Ludtke [113,114].

The data and discussions presented in this paper are intended to aid the hazards analyst

in preparing safety analyses for each specific application of hydrogen fuel as the applications

arise. Without resorting to specific applications and specific accident criteria we can categorically analyze some of the safety hazards associated with the various fuel markets.

Hydrogen is a feedstock in the process industries [95, 97] and has been successfully

handled for decades. Thus, the use of hydrogen in industry is essentially routine and no major

safety problems are anticipated with its increased use in industrial processes. New processes

and new uses of hydrogen may pose new safety hazards that must be dealt with as they arise.

19
Hydrogen has not been used extensively in commercial applications although it has been

successfully piped cross-country as a compressed gas in a few locales. A demonstration project

[90] is currently in progress to evaluate the feasibility of using hydrogen in electrical

utility load-leveling operations. The electrical utility industry has also successfully used

hydrogen gas to cool the rotor and stator coils in large turbine-generators for more than

forty years. No significant hydrogen safety problems are foreseen in the commercial sector.

Hydrogen is a potential replacement fuel in the voracious transportation market. Conceptually, and technically, hydrogen can be used to fuel aircraft [98], ships [99, 100], trains

[101], trucks and buses [102, 103] and automobiles [104, 105]; however, the economics,

logistics and safety of supplying and distributing hydrogen to fuel these vehicles are as yet

undetermined. Excluding a few demonstration projects with automobiles, trucks, buses and

airplanes, we find that hydrogen is a relatively untried fuel in the transportation market.

Hydrogen was tried briefly as an auto fuel in the mid-thirties [106, 107] and as an inflatant

for balloons and dirigibles prior to the ill-fated Hindenberg fire of 1937. Considerable

safety analysis is in order prior to the widespread use of hydrogen fuel in transportation —

particularly in highway vehicles where personnel exposure is maximum and fuel handling procedures are most difficult or impossible to enforce. Hydrogen is already the accepted fuel

of the aerospace industry and is safely handled in large quantities. Preliminary safety

analyses of hydrogen-fueled aircraft for military [108] and civilian [109] applications are , currently in progress. Bowen [112] recently completed a review of some of the hazards asso- ciated with the use of hydrogen as a military fuel.

Hydrogen can also be used to supply residential fuel needs because appliances, furnaces,

etc. can be made to operate on hydrogen gas. From a technical viewpoint this application can be readily satisfied, but from a safety viewpoint there are a number of significant concerns.

The major concerns relate to the problems of gas leakage, detection and the potential severity

of explosions of hydrogen-air mixtures in confined or partially-confined spaces. More experimental data and safety analyses are needed to fully resolve these questions and to determine

the comparative risks of hydrogen and natural gas as residential fuels; however, hydrogenenriched gases (coal gas, town gas, producer gas, etc.) have been successfully used [106, 110]

in European countries to satisfy residential fuel needs during the last century. Thus, there

also appears to be ample precedent for acceptably safe use of hydrogen in the residential

sector.

9 . SUMMARY

The safety aspects of any fuel are intimately related to the fuel application and to the

postulated accident criteria. Thus, specific conclusions await hard comparisons of competing

fuels in applications where credible accidents can be specified; however, generalized conclusions

and judgments may be drawn from the comparative technical data and discussions presented herein.

A comprehensive list of thermophysical and combustion properties of hydrogen, methane and

gasoline was compiled and presented in this paper. These data provide the bases for future

safety analysis and for direct safety comparisons of hydrogen, methane and gasoline.

Liquid hydrogen is more difficult to contain than either liquid methane or liquid gasoline

and NTP gasoline vapors are more difficult to contain than either NTP hydrogen or NTP methane.

Industry has proven that all three fuels can be safely and easily contained in both gaseous

and liquid phases.

In the event of a fuel spill, we can expect a fire hazard to develop most rapidly with

hydrogen, methane and gasoline respectively, and the fire hazard should persist in the inverse

order. For a specified liquid spillage volume and ensuing fire we can expect gasoline fires

to last the longest and hydrogen fires to be the shortest lived, while all three fuels burn

at nearly the same flame temperature. The scene of a hydrogen fire may be hotter (1.3 X to 1.8 X) than that of a hydrocarbon fire, but the hydrocarbon fires will endure five to ten

times longer than hydrogen fires (for spillage of identical liquid fuel volumes).

All three fuels are easily ignited by weak ignition sources such as matches. Even a weak

spark generated by the discharge of static electricity from a human body may be sufficient to ignite any of these fuels in air. Hydrogen is more readily ignitable than either of the hydrocarbon fuels which appear to be equally susceptible to ignition.

20
Hydrogen fires are more difficult to detect than methane or gasoline fires but modern

detection equipment makes it possible to quickly and reliably detect the flames of all three

fuels. In some applications hydrogen and methane fires should be allowed to burn until gas

flow is stopped or until liquid spills are consumed because of the potential explosive hazard

created by extinguishing such flames; however, the fire should be controlled in all situations

and in many cases it is advisable to extinguish the fire. Water may be used to fight fires

of all three fuels and commercial dry chemicals and high-expansion foams can be used to extinguish LNG and gasoline fires.

The potential for smoke inhalation damage is judged to be most severe in gasoline, methane

and hydrogen fires respectively.

The wider flammable limits and detonable limits of hydrogen coupled with its rapid burning velocity tend to make hydrogen a greater explosive threat than methane or gasoline. Unconfined fuel-air explosions are not normally very destructive; however, confined fuel-air

explosions can be devastating and hydrogen presents the greatest confined-explosion threat of

the three fuels considered.

For equivalent energy storage or for equivalent volume storage, hydrogen has the least

theoretical explosive potential of the three fuels considered — even though it has the

highest heat of combustion (and explosive potential) on a mass basis.

Hydrogen is currently being safely stored and used in industry in both the compressed gas

and liquid forms and it is anticipated that metal hydride storage will be equally safe. Of the

three fuels examined, gasoline is the easiest and perhaps the safest fuel to store because of

its lower volatility and narrower flammable and detonable limits.

Personnel and equipment safety criteria (fuel quantity-distance) are concisely charted

on figures presented herein. It is believed that these figures provide safe exposure

distances for all three fuels.

Consideration of future hydrogen applications reveals no safety problems in the industrial

and commercial markets. Hydrogen safety problems may exist in the transportation and residential fuel markets and additional safety analyses are needed in these areas. Lower risk (or

lower cost) fuels will most likely be used to satisfy many of these markets over the next few

decades; however, hydrogen cannot currently be considered unsafe and cannot be excluded from

consideration in any of these applications on the grounds of safety. It is the author's

belief that fuel availability and cost will outweigh fuel safety in the selection of fuels

in the future and hydrogen must be considered a contender in the chemical fuel market.

Finally, hydrogen safety guidelines and applicable regulatory codes are summarized and

tabulated in the Appendix of this paper.

21
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22
[21] Magison, E. C. , Electrical Instruments in Hazardous Locations, 2nd edition, Book,

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[22] Ibid., p. 304.

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[38] Ibid., p. 903.

[39] Ibid., p. 785.

[40] Lewis and von Elbe, op. cit. , p. 312.

[41] Ibid., pp. 511-554.

[42] Magison, op. cit., pp. 30-36.

[43] Lewis and von Elbe, op. cit., pp. 323-346.

[44] Ibid., pp. 346-366.

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23
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[49] Ibid., pp. 397-401.

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[53] Ibid., pp. 37-92.

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[56] Sax, op. cit., p. 5.

[57] Ibid., pp. 12-13.

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[64] Zabetakis, Safety with Cryogenic Fluids, op. cit., p. 38.

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24
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25
[83] Siewert, R. D. , Evacuation areas for transportation accidents involving propellant

tank pressure bursts, NASA Tech. Memorandum NASA TM X-68277 (1972).

[84] Safety Manual, U.S. Army Materiel Command, AMCR 385-100 (Apr. 1970)".

[85] Strehlow, R. A., and Baker, W. E. , The characterization and evaluation of accidental

explosions, NASA CR-134779 (June 1975); (available from National Technical

Information Service, Springfield, VA 22151).

[86] Baker, W. E. , Parr, V. B. , Bessey, R. L. , and Cox, P. A., Assembly and analysis of

fragmentation data for liquid propellant vessels, NASA CR-134538 (Jan. 1974);

(available from National Technical Information Service, Springfield, VA 22151).

[87] Howard, W. B. , Rodehorst, C. W. , and Small, G. E. , Flame arresters for high-hydrogen

fuel-air mixtures, Loss Prevention 9_ (American Institute of Chemical Engineers,

345 East 47th Street, New York, NY 10017, 1975).

[88] Hord, J., and Parrish, W. R. , Economics of hydrogen (to be published as Chapter 1 of Volume 5, Implications of Hydrogen, in the book: Hydrogen, Its Technology

and Implications, CRC Press).

[89] Wiswall, R. H. , Jr., and Reilly, J. J., Metal hydrides for energy storage, Proc.

of 7th Intersociety Energy Conversion Engr. Conf., ACS, Washington, D.C.,

pp. 1342-1348 (1972).

[90] Salzano, F. J., Braun, C, Beaufrere, A., Srinivasan, S., Strickland, G., and

Reilly, J. J., Hydrogen for energy storage: a progress report of technical

developments and possible applications, presented at the Energy Storage Conf.,

Asilomar Conf. Grounds, Pacific Grove, California, Feb. 8-13, 1976 (Jan. 1976).

[91] NFPA National Fire Codes, Volume 3 (1975); (available from National Fire Protection

Assoc, 470 Atlantic Ave., Boston, MA 02210).

[92] Lundin, C. E. , and Lynch, F. E. , The safety characteristics of FeTi hydride, Proc.

of 10th Intersociety Energy Conversion Engr. Conf., p. 1386 (Aug. 1975).

[93] Ordin, P. M. , Review of hydrogen accidents and incidents in NASA operations,

Proc. of 9th Intersociety Energy Conversion Engr. Conf. , San Francisco,

California, p. 442 (Aug. 1974).

[94] LNG Information Book . . .1973, American Gas Assoc, 1515 Wilson Blvd., Arlington,

VA 22209 (1973).

[95] Parrish, W. R. , Voth, R. 0., Hust, J. G. , Flynn, T. M. , Sindt, C. F. , and Olien,

N. A., Selected topics on hydrogen fuel, Ed. by J. Hord, Nat. Bur. Stand.

(U.S.) SP 419 (May 1975).

[96] Burgess, D. S. , Murphy, J. N. , Zabetakis, M. G. , and Perlee, H. E. , Volume of

flammable mixtures resulting from the atmospheric dispersion of a leak or spill, 15th Int'l. Symp. on Combustion, The Combustion Institute, Pittsburgh,

PA, Paper No. 29 (in press).

[97] Commodity Data Summaries, 1976, Bureau of Mines (Jan. 1976); see also, Adkins, D. C.

,

and Jaske, R. J., Hydrogen, a chapter from Mineral Facts and Problems, Bureau

of Mines Bulletin 667 (1975).

[98] Brewer, G. D., Aviation usage of liquid hydrogen fuel, Symposium Proc. of Hydrogen

Energy Fundamentals, Miami Beach, Florida, Paper S4-3 (March 1975).

[99] Berkowitz, B. , DeVore, J., Harris, S., Haun, L. , McNamara, W. , and Slager, W.

,

Alternative, synthetically fueled, Navy systems: force element missions and

technology, General Electric Company-Tempo report GE74TMP-46 (Nov. 1974);

(available from Advanced Research Projects Agency, 1400 Wilson Blvd., Arlington, VA 22209).

26
[100] Carhart, H. W. , Affens, W. A., Boss, B. D. , Hazlett, R. N. , and Schuldiner, S.

Hydrogen as a Navy fuel, Naval Research Laboratory Report 7754 (June 1974);

(available from National Technical Information Service, Springfield, VA

22151, AD-781 262).

[101] Gier, H. L. , Hydrogen fuel for the railroads: a proposal, Proc. of the 1st World

Hydrogen Energy Conf., Miami Beach, Florida, Vol. Ill, Ed. by T. N. Veziroglu,

Paper 7C-43 (March 1976).

[102] Billings, R. E. , A hydrogen-powered mass transit system, Proc. of the 1st World

Hydrogen Energy Conf., Miami Beach, Florida, Vol. Ill, Ed. by T. N. Veziroglu,

Paper 7C-27 (March 1976).

[103] Wall, T. , Feeney, R. , Hollenberg, J., McAlevy, R. F. Ill, Engineering study of

hydrogen-fueled bus operation, Proc. of the 1st World Hydrogen Energy Conf.,

Miami Beach, Florida, Vol. Ill, Ed. by T. N. Veziroglu, Paper 7C-25 (March

1976).

[104] VanVorst, W. D. , and Finegold, J. G. , Automotive hydrogen engines and onboard

storage methods, Symposium Proc. of Hydrogen Energy Fundamentals, Miami Beach,

Florida, Paper S4-65 (March 1975).

[105] Ecklund, E. E. , Kester, F. L. , Hydrogen storage on highway vehicles: update '76, Proc. of the 1st World Hydrogen Energy Conf., Miami Beach, Florida, Vol. II,

Ed. by T. N. Veziroglu, Paper 3B-3 (March 1976).

[106] Weil, K. H. , The hydrogen I.C. engine - its origins and future in the emerging

energy- transportation-environment system, Proc. of 7th Intersociety Energy

Conversion Engr. Conf., ACS, Washington, D.C., pp. 1355-1363 (1972).

[107] Billings, R. E. , and Lynch, F. E. , History of hydrogen-fueled internal combustion

engines, Energy Research Publication 73001; (available from Billings Energy

Research Corporation, P.O. Box 555, Provo, UT 84601).

[108] Mikolowsky, W. T. , and Noggle, L. W. , The potential of liquid hydrogen as a military aircraft fuel, Proc. of the 1st World Hydrogen Energy Conf., Miami

Beach, Florida, Vol. Ill, Ed. by T. N. Veziroglu, Paper 5C-31 (March 1976).

[109] Witcofski, R. D. , Langley Research Center, NASA, Hampton, VA, private communication

to the author (June 1976).

[110] Marchetti, C, Hydrogen and energy, CEER 5_, No. 1, pp. 7-25 (Jan. 1973).

[Ill] On an investigation of hazards associated with the storage and handling of liquid

hydrogen, Final Report C-61092, Contract No. AF 18(600)-1687, prepared by

Arthur D. Little, Inc., DDC Access. No. AD 324194, 137 pages (Mar. 1960).

[112] Bowen, T. L. , Investigation of hazards associated with using hydrogen as a military

fuel, Report 4541, Naval Ship Research and Development Center, Bethesda, MD

20084 (Aug. 1975).

[113] Ludtke, P. R. , Register of specialized sources for information on selected fuels

and oxidizers, NASA CR-134807 (Mar. 1975).

[114] Ludtke, P. R. , Register of hydrogen technology experts, NASA CR-2624 (Oct. 1975).

[115] Standard for Liquefied Hydrogen Systems at Consumer Sites, NFPA Pamphlet No. 50B

(ANSI Z292.3).

[116] Standard for the Production, Storage and Handling of Liquefied Natural Gas (LNG)

NFPA Pamphlet No. 59A (ANSI Z225.1).

[117] Flammable and Combustible Liquids Code, NFPA Pamphlet No. 30 (ANSI Z288.1).

27
[118] LNG Safety Program, Phase II, Consequences of LNG Spills on Land, American Gas

Association Project IS-3-1 (Nov. 1973); see also, Wesson, H. R. , Brown, L. E.

,

and Welker, J. R. , Vapor dispersion, fire control, and fire extinguishment of

high evaporation rate LNG spills, paper 74-D-36 in 1974 Operating Section

Proceedings of the American Gas Association, 1515 Wilson Boulevard, Arlington,

VA 22209 (1974).

[119] Baker, W. E. , Kulesz, J. J., Ricker, R. E. , Bessey, R. L. , Westine, P. S. , Parr,

V. B., and Oldham, G. A., Workbook for predicting pressure wave and fragment

effects of exploding propellant tanks and gas storage vessels, NASA CR-134906

(Nov. 1975).

28
Appendix: Hydrogen Safety Guidelines

and Applicable Regulatory Codes

Various industrial, university and government organizations have published guidelines

for safe production and handling of hydrogen. Some of the more substantial documents are listed in table Al.

Table Al: Hydrogen Safety and Equipment Guidelines (nonmandatory)

SAFETY ITEM REFERENCES

Handling procedures (includes safety

in production, transportation and

distribution, personnel training, etc.)

A1-A25

Storage Al, A16, A17, A21, A22, A23, Bl

Explosive Criteria Summarized in Al and CI

See also C2 and C3

Leak Detection & Fire Detection,

Prevention, Control

D1-D6, Al, G4 & G5

Facilities Al and CI

Mechanical Equipment F1-F18

Electrical Equipment G1-G10

Some of these documents are accepted as national standards and become mandatory and enforceable

when specified by regulatory bodies such as the DoT, state and local authorites, etc. Insurance

companies usually adopt or specify existing standards, practices and procedures as a condition

of insurability for industrial organizations. Examples of nonmandatory standards that may be

adopted by regulatory bodies are those issued by the ASME, ANSI, ISA, NFPA, UL, ASTM, etc. Current regulations that pertain to hydrogen are listed in table A2.

Equipment specifications and shipping and carrier regulations for gaseous and liquid

hydrogen are given in CFR, Title 49, Parts 170-179. Pipeline transmission of flammable

compressed gases (presumably including hydrogen) is regulated by CFR, Title 49, Parts

190-192. The DoT classifies and treats liquid hydrogen as a flammable compressed gas

(Part 172.5 of CFR, Title 49). Air transportation of compressed hydrogen gas is regulated

by CFR, Title 14, Part 103 and the commercial carrier IATA tariffs [H6]. Liquid hydrogen is

not currently accepted for air shipments. Transportation of compressed hydrogen gas by water

is regulated by CFR, Title 46, Part 146 and liquid hydrogen cannot currently be shipped by

water; however, the NASA has barged liquid hydrogen over distances of 84 km offshore

Louisiana-Mississippi under special permit of the U.S. Coast Guard.

Federal regulations apply to the interstate transportation of hydrogen and are administered by the various branches of the Department of Transportation: the Federal

Highway Administration for transport by highway; the Federal Railroad Administration for

transport by railway; the Federal Aviation Administration for air transport; and the U.S. Coast Guard for water transport. In addition to the regulations and standards set forth in

table A2, municipal and state regulations may be imposed for the storage and intrastate

transportation of hydrogen. „ q
Table A2. Regulations (mandatory) for Distribution of Hydrogen

Distribution of

Hydrogen by

Equipment

Specifications

Transportation

Regulations

Cylinders filled with liquid

(< 1000 pounds water capacity)-

by highway only

Railway tank cars filled

with liquid

Highway truck trailers filled

with liquid

Container filled with liquid

by air or waterway

Liquid pipeline

Cylinders filled with

compressed gas—by highway

or rail

Railway tank cars filled

with compressed gas

Highway truck trailers filled

with compressed gas

Container filled with

compressed gas—by air

Container filled with

compressed gas—by waterway

Gas pipeline

CFR, Title 49, Part 178.57

Ref. [H2]

CFR, Title 49, Parts 173.316,

177.823, 177.840 Ref. [H2]

CFR, Title 49, Parts 179.400, CFR, Title 49, Part 173.316,

179.401, Ref. [H2]

CGA*-341 and DoT Approval

prior to fabrication

Ref. [F17]

ANSI B31.8, B31.3*,

B31.10*, Ref's. [H1,F5,F6]

CFR, Title 49, Parts

173.34, 178.36, 178.37

Ref. [H2]

CFR, Title 49, Parts

173.31, 179.500

Ref. [H2]

CFR, Title 49, Parts

173.34, 178.36, 178.37,

Ref. [H2]

CFR, Title 49, Parts

173.34, 178.36, 178.37

Ref. [H2]

CFR, Title 49, Parts

173.31, 173.34, 178.36,

178.37, 179.500 and

CFR, Title 46, Part

146.24, Ref's. [H2,H3]

ANSI B31.8, B31.3*,

Ref's. [H1,F5]

Ref. [H2]

Special permit from Hazardous Materials Regulation

Board of DoT (see CFR, Title

49, Parts 173.33, 177.823)

Ref. [H2]

Not allowed under current

regulations (see CFR, Title

14, Parts 103.7, 103.9 and

CFR, Title 46, Part 146.24),

Ref's. [H3,H4]. Shipment by tank

barge has been allowed under

special permit of the U.S.

Coast Guard Commandant.

CFR, Title 49, Parts 190-192,

Ref. [H2]

CFR, Title 49, Parts 173.301,

173.302, 176.703, 177.823,

Ref. [H2]

CFR, Title 49, Part 173.314,

Ref. [H2]

CFR, Title 49, Parts 173.301,

173.302, 177.823, Ref. [H2]

CFR, Title 14, Part 103.9,

Ref. [H4] . DoT approved cylinders

on cargo aircraft only, with

a maximum charge of 300 pounds

(136 kg) of H2 per cylinder.

See also CAB No. 82 Ref's. [H2,H6]

CFR, Title 46, Part 146.24.

DoT approved cylinders on cargo vessels only and DoT

approved railway tank cars

on trainships only, Ref. [H3]

CFR, Title 49, Parts 190-192

Ref. [H2]

Storage of Hydrogen @ Consumer Sites as Equipment Specifications Industrial Standards

Compressed gas

Liquid

ASME*, Section VIII,

Divisions 1 & 2,

Ref's. [F1,F2]

ASME*, Section VIII,

Divisions 1 & 2,

Ref's. [F1,F2]

NFPA* 50A, Ref. [A21]

NFPA* 50B, Ref. [A22]

General industrial safety matters, including the production and handling of flammable

compressed and liquefied gases, are regulated by OSHA PL 91-596, Ref. [H5].

* Not mandatory but industrially accepted standards.

30
Appendix References

Note: In the references that follow,

CGA = Compressed Gas Association, Inc., 500 Fifth Ave., New York, NY 10036.

ANSI = American National Standards Institute, 1430 Broadway, New York, NY 10018.

NFPA = National Fire Protection Association, Publication Sales Department,

470 Atlantic Ave., Boston, MA 02210.

ASME = American Society of Mechanical Engineers, United Engineering Center,

345 East 47th Street, New York, NY 10017.

UL = Underwriters' Laboratories, Publication Stock, 333 Pfingsten Road,

Northbrook, IL 60062.

ISA = Instrument Society of America, 400 Stanwix Street, Pittsburgh, PA 15222.

[Al] General Safety Engineering Design Criteria, Vol. 1, CPIA public. 194, (Oct. 1971);

also, Liquid Propellant Handling, Storage and Transportation, Vol. 3, CPIA public.

194, (May 1972) — documents prepared by the JANNAF propulsion committee of the

JANNAF Hazards Working Group and are available from the Chemical Propulsion Information Agency (CPIA) of the Johns Hopkins University Applied Physics Laboratory,

8621 Georgia Ave. , Silver Spring, MD 20910; (also available to the public through

National Technical Information Service, Springfield, VA 22150: Vol. 1 Accession No. AD 889 763, Vol. 3 Accession No. AD 870 259).

[A2] Hydrogen safety manual, NASA Lewis Research Center, NASA Tech. Memo TM-X-52454 (1968).

[A3] On an investigation of hazards associated with the storage and handling of liquid

hydrogen, Final Report C-61092, Contract No. AF 18(600)-1687, prepared by Arthur

D. Little, Inc., DDC Access. No. AD 324194, 137 pp. (Mar. 1960).

[A4] Liquid hydrogen, a guide for the safe handling and storage of liquid hydrogen at LASL

Facilities, Los Alamos Scientific Laboratory, P.O. Box 1663, Los Alamos, NM 87544.

[A5] Weintraub, A. A., Control of liquid hydrogen hazards at experimental facilities: a review, health and safety laboratory, U.S. Atomic Energy Comm. , New York, NY (May

1965); also, Grumer, J., Strasser A., and Van Meter, R. A., Principles for safe

handling of liquid hydrogen, Bur. of Mines Tech. Rept. (June 1967).

[A6] Chelton, D. B. , Safety in the use of liquid hydrogen, Book, Technology and Uses of

Liquid Hydrogen, Ed. by R. B. Scott, W. H. Denton, and C. M. Nicholls, pp. 359-378

(The Macmillan Co., New York, 1964).

[A7] Connolly, W. W. , A practical safety standard for commercial handling of liquefied

hydrogen, Book, Advances in Cryogenic Engineering 12_, Ed. by K. D. Timmerhaus, pp. 192- 197, (Plenum Press, New York, 1967).

[A8] Recommended materials and practices for use with cryogenic propellants, Aerospace

Information Report, AIR 839B, 68 pp. (Soc. of Auto. Engrs. , Apr. 1969).

[A9] Cryogenics safety manual, a guide to good practice, (British Cryogenics Council,

16 Belgrave Square, London, SW1X8PT, 1970).

[A10] Hernandez, H. P., Safety guidelines for accelerator installations, Lawrence Radiation

Laboratory Rept. No. UCRL-18555 (Feb. 1969).

[All] Handbook of Compressed Gases, Compressed Gas Assoc, Inc., (Reinhold Publishing

Corp., New York, 1966).

[A12] Safe Handling of Compressed Gases in Containers, CGA Pamphlet P-l.

[A13] Method of Marking Portable Compressed Gas Containers to Identify the Material

Contained, CGA Pamphlet C-4; [also ANSI Z48. 1-1954 (R 1971)].

[A14] Guide to the Preparation of Precautionary Labeling and Marking of Compressed Gas

Containers, CGA Pamphlet C-7.

31
[A15] Hydrogen, CGA Pamphlet G-5.

[A16] Standard for Gaseous Hydrogen at Consumer Sites, CGA Pamphlet G-5.1.

[A17] Standard for Liquefied Hydrogen Systems at Consumer Sites, CGA Pamphlet G-5. 2.

[A18] Commodity Specification for Hydrogen, CGA Pamphlet G-5. 3; see also, Hydrogen

Commodity Specification, MIL-P-27201B (June 1971); (available from USN Supply Depot,

5801 Tabor Ave., Philadelphia, BA 19120).

[A19] Lightning Protection Code, NFPA Pamphlet No. 78 (ANSI C5.1).

[A20] Static Electricity, NFPA Pamphlet No. 77.

[A21] Standard for Gaseous Hydrogen Systems at Consumer Sites, NFPA Pamphlet No. 50A

(ANSI Z292.2).

[A22] Standard for Liquefied Hydrogen Systems at Consumer Sites, NFPA Pamphlet No. 50B

(ANSI Z292.3).

[A23] Standard for the Installation and Operation of Oxygen-Fuel Gas Systems for Welding

and Cutting, NFPA Pamphlet No. 51; see also, Standard on Safety in Welding and

Cutting, ANSI Z49.1.

[A24] Siewert, R. D. , Evacuation areas for transportation accidents involving propellant

tank pressure bursts, NASA TMX-68277 (Nov. 1972).

[A25] Emergency services guide for selected hazardous materials (spills, fire, evacuation

area), Dept. of Transportation, Office of the Secretary, Office of Hazardous

Materials, Washington, D.C. 20590 (Apr. 1974).

[Bl] Zabetakis, M. G. , and Burgess, D. S., Research on the hazards associated with the

production and handling of liquid hydrogen, Bureau of Mines Report, RI-5707 (1961).

[CI] Hord, J., Explosion criteria for liquid hydrogen test facilities, Nat. Bur. Stand.

(U.S.), NBS Report (Feb. 1972).

[C2] Explosion Venting, NFPA Pamphlet No. 68.

[C3] Explosion Prevention Systems, NFPA Pamphlet No. 69.

[Dl] Rosen, B. , Dayan, V. H. , and Proffit, R. L. , Hydrogen leak and fire detection, NASA

SP-5092 (1970).

[D2] Automatic Fire Detectors, NFPA Pamphlet No. 72E.

[D3] Fire Hazard Properties of Flammable Liquids, Gases, Volatile Solids, NFPA Pamphlet

No. 325M.

[D4] National Fire Codes, NFPA Volumes 1 and 2 (general fire-fighting and fire control).

[D5] Donnini, J. M. , Hydrogen fire blink detector, U.S. Patent Application S/N 51477

(July 1970).

[D6] Custer, R. L. P., and Bright, R. G. , Fire detection: the state of the art, Nat. Bur. Stand. (U.S.) Tech. Note TN-839 (June 1974).

[Fl] ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Pressure Vessels:

this reference applies to gaseous and liquid containers for portable and stationary

applications.

[F2] ASME Boiler and Pressure Vessel Code, Section VIII, Division 2, Pressure Vessels:

Alternative Rules: this reference applies to gaseous and liquid containers for

stationary applications only.

32
[F3] ASME Boiler and Pressure Vessel Code, Section IX, Welding and Brazing Qualifications.

[F4] ASME Boiler and Pressure Vessel Code, Section III, Nuclear Power Plant Components: a series of standards applicable to nuclear plants and installations.

[F5] American National Standard Code for Pressure Piping, Petroleum Refinery Piping, ANSI

B31.3, (ASME, New York): this reference applies to gaseous and liquid piping.

[F6] American National Standard Code for Pressure Piping, Cryogenic Piping Systems, ANSI

B31.10 (in preparation).

[F7] Vacuum insulated cryogenic pipes, some recommendations concerned with pipe and coupling

design, (British Cryogenics Council, 16 Belgrave Square, London SWIX8PT, 1972).

[F8] Safety Relief Device Standards, Part 1 - Cylinders for Compressed Gases, CGA Pamphlet

S-l.l; Part 2 - Cargo and Portable Tanks for Compressed Gases, CGA Pamphlet S-1.2;

Part 3 - Compressed Gas Storage Containers, CGA Pamphlet S-1.3.

[F9] Recommended Practice for the Manufacture of Fusible Plugs, CGA Pamphlet S-4.

[F10] Frangible Disc Safety Device Assembly, CGA Pamphlet S-3.

[Fll] Compressed Gas Cylinder Valve Outlet and Inlet Connections, CGA Pamphlet V-l (also

Canadian Standards Association CSA B96-1965 and American Standards Association

ASA B57. 1-1965).

[F12] Methods for Hydrostatic Testing of Compressed Gas Cylinders, CGA Pamphlet C-l.

[F13] Standards for Welding and Brazing on Thin Walled Containers, CGA Pamphlet C-3.

[F14] Cylinder Service Life, Seamless, High-Pressure Cylinder Specifications ICC-3,

ICC-3A, ICC-3AA, CGA Pamphlet C-5.

[F15] Standards for Visual Inspection of Compressed Gas Cylinders, CGA Pamphlet C-6.

[F16] Standard for Requalification of DoT-3HT Cylinders, CGA Pamphlet C-8.

[F17] Tentative Standard for Insulated Tank Truck Specification for Cold Liquefied Gases,

CGA Pamphlet CGA-341.

[F18] Voth, R. 0., Safety of hydrogen pressure gauges, Book, Advances in Cryogenic Engineering, 1]_, Ed. by K. D. Timmerhaus, pp. 182-187 (Plenum Press, New York, 1972).

[Gl] Standard for Intrinsically Safe Process Control Equipment for Use in Class I Hazardous Locations, NFPA Pamphlet No. 493.

[G2] Standard for Purged and Pressurized Enclosures for Electrical Equipment in Hazardous

Locations, NFPA Pamphlet No. 496.

[G3] National Electrical Code, Chap. 5, Articles 500-501, NFPA Pamphlet No. 70.

[G4] Fire Protection Equipment List, (Jan. 1975), available from UL.

[G5] Hazardous Location Equipment List, (June 1974), available from UL.

[G6] Electrical Instruments in Hazardous Atmospheres, ISA-RP 12.1 (I960).

[G7] Intrinsically Safe and Non-Incendive Electrical Instruments, ISA-RP 12.2 (1965).

[G8] Instrument Purging for Reduction of Hazardous Area Classification, ISA-S12.4 (1970).

33
[G9] Electrical Safety Practices, ISA Monograph 113 (1972).

[G10] Magison, E. C. , Electrical Instruments in Hazardous Locations, 2nd edition, Book, 321

pp. (Instrument Society of America, Pittsburgh, Pennsylvania, 1972).

[HI] American National Standard Code for Pressure Piping, Gas Transmission and Distribution

Piping Systems, ANSI B31.8.

[H2] Code of Federal Regulations, Title 49, Transportation, Parts 100 to 199, U.S.

Government Printing Office, Washington, D.C. 20402.

[H3] Code of Federal Regulations, Title 46, Shipping, Parts 140 to 149, U.S. Government

Printing Office, Washington, D.C. 20402.

[H4] Code of Federal Regulations, Title 14, Aeronautics and Space, Parts 60 to 199, U.S.

Government Printing Office, Washington, D.C. 20402.

[H5] Occupational Safety and Health Act of 1970, PL 91-596 (Dec. 29, 1970) in United

States Statutes at Large, 91st Congress, Second Session, 84_, Part 2, pp. 1590-1620,

(U.S. Government Printing Office, Washington, D.C. 20402, 1971).

[H6] Official Air Transport Restricted Articles Tariff No. 6-D, Civil Aeronautics Board

(CAB) No. 82, Section I (pp. 14-15), Section II (p. 53) and Section III - Part A

(p. 94), issued by the Airline Tariff Publishing Company, Dulles International

Airport, P.O. Box 17415, Washington, D.C. 20041.

34
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS-TN-690

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Is Hydrogen Safe?

5. Publication Date October 1976

6. Performing Organization Code

275.05

7. AUTHOR(S)

J. Hord

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2750900

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as item 9.

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

The safety aspects of hydrogen are systematically examined and compared with

those of methane and gasoline. Physical and chemical property data for all three

fuels are compiled and used to provide a basis for comparing the various safety

features of the three fuels. Each fuel is examined to evaluate its fire hazard,

fire damage, explosive hazard and explosive damage characteristics. The fire

characteristics of hydrogen, methane and gasoline, while different, do not largely

favor the preferred use of any one of the three fuels; however, the threat of fuelair explosions in confined spaces is greatest for hydrogen. Gasoline is believed

to be the easiest and perhaps the safest fuel to store because of its lower volatility

and narrower flammable and detonable limits. It is concluded that all three fuels can be safely stored and used; however, the level of safety risk for each fuel will

vary from one application to another. Generalized safety comparisons are made herein

but detailed safety analyses will be required to establish the relative safety of diffe

ent fuels for each specific fuel application and stipulated accident. The technical

data supplied in this paper will provide much of the framework for such analyses.

Hydrogen safety guidelines, regulatory codes applicable to the distribution of

hydrogen, and safety criteria for liquid hydrogen storage are compiled and presented.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Explosion; fire; fuel; gasoline; hydrogen; methane; safety.

18. AVAILABILITY [T Unlimited

I For Official Distribution. Do Not Release to NTIS

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• Physics and Chemistry (Section A)

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on

standards of physical measurement, fundamental constants, and properties of matter. Issued six times a year. Annual subscription: Domestic, $17.00; Foreign, $21.25. • Mathematical Sciences (Section B)

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design, numerical

analysis, theoretical physics and chemistry, logical de- sign and programming of computers and computer sys- tems. Short numerical tables. Issued quarterly. Annual

subscription: Domestic, $9.00; Foreign, $11.25.

DIMENSIONS/NBS (formerly Technical News Bulletin) —This monthly magazine is published to inform

scientists, engineers, businessmen, industry, teachers,

students, and consumers of the latest advances in science and technology, with primary emphasis on the work at NBS. The magazine highlights and reviews

such issues as energy research, fire protection, building

technology, metric conversion, pollution abatement,

health and safety, and consumer product performance.

In addition, it reports the results of Bureau programs

in measurement standards and techniques, properties of matter and materials, engineering standards and services, instrumentation, and automatic data processing.

Annual subscription: Domestic, $9.45; Foreign, $11.85.

NONPERIODICALS

Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and

industrial practice (including safety codes) developed

in cooperation with interested industries, professional

organizations, and regulatory bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other

special publications appropriate to this grouping such

as wall charts, pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists, engineers, chemists, biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides

quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a world-wide

program coordinated by NBS. Program under authority

of National Standard Data Act (Public Law 90-396).

NOTE: At present the principal publication outlet for these data is the Journal of Physical and Chemical

Reference Data (JPCRD) published quarterly for NBS

by the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints,

and supplements available from ACS, 1155 Sixteenth

St. N.W., Wash. D. C. 20056. Building Science Series—Disseminates technical infor- mation developed at the Bureau on building materials,

components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental

functions and the durability and safety characteristics

of building elements and systems.

Technical Notes—Studies or reports which are complete

in themselves but restrictive in their treatment of a

subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under proce- dures published by the Department of Commerce in Part

10, Title 15, of the Code of Federal Regulations. The

purpose of the standards is to establish nationally rec- ognized requirements for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS

administers this program as a supplement to the activities of the private sector standardizing organizations.

Consumer Information Series—Practical information,

based on NBS research and experience, covering areas

of interest to the consumer. Easily understandable language and illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order above NBS publications from: Superintendent

of Documents, Government Printing Office, Washington,

D.C. 20U02. Order following NBS publications—NBSIR's and FIPS

from the National Technical Information Services,

Spr-ingfield, Va. 22161. Federal Information Processing Standards Publications

(FIPS PUBS)—Publications in this series collectively constitute the Federal Information Processing Standards Register. Register serves as the official source of information in the Federal Government regarding stand- ards issued by NBS pursuant to the Federal Property

and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented

by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or final reports on work performed by NBS for outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey

bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A

literature survey issued biweekly. Annual subscription: Domestic, $20.00; Foreign, $25.00.

Liquified Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: $20.00.

Send subscription orders and remittances for the pre- ceding bibliographic services to National Bureau of Standards, Cryogenic Data Center (275.02) Boulder,

Colorado 80302.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. D.C. 20234

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