ASTM G94-22

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: G94 − 22
Standard Guide for
Evaluating Metals for Oxygen Service
ThisstandardisissuedunderthefixeddesignationG94;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginal
adoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscript
epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
2.1 ASTM Standards:
1.1 This guide applies to metallic materials under consider-
ation for oxygen or oxygen-enriched fluid service, direct or D2512Test Method for Compatibility of Materials with
indirect, as defined in Section 3. It is concerned primarily with Liquid Oxygen (Impact Sensitivity Threshold and Pass-
the properties of a metallic material associated with its relative
Fail Techniques)
susceptibility to ignition and propagation of combustion. It
D2863Test Method for Measuring the Minimum Oxygen
does not involve mechanical properties, potential toxicity,
Concentration to Support Candle-Like Combustion of
outgassing, reactions between various materials in the system,
Plastics (Oxygen Index)
functional reliability, or performance characteristics such as
D4809Test Method for Heat of Combustion of Liquid
aging, shredding, or sloughing of particles, except when these
Hydrocarbon Fuels by Bomb Calorimeter (Precision
might contribute to an ignition.
Method)
G63Guide for Evaluating Nonmetallic Materials for Oxy-
1.2 This document applies only to metals; nonmetals are
gen Service
covered in Guide G63.
G72Test Method for Autogenous Ignition Temperature of
NOTE 1—The American Society for Testing and Materials takes no
position respecting the validity of any evaluation methods asserted in
Liquids and Solids in a High-Pressure Oxygen-Enriched
connection with any item mentioned in this guide. Users of this guide are
Environment
expresslyadvisedthatdeterminationofthevalidityofanysuchevaluation
G86Test Method for Determining Ignition Sensitivity of
methods and data and the risk of use of such evaluation methods and data
Materials to Mechanical Impact in Ambient Liquid Oxy-
are entirely their own responsibility.
gen and Pressurized Liquid and Gaseous Oxygen Envi-
NOTE 2—In evaluating materials, any mixture with oxygen exceeding
atmospheric concentration at pressures higher than atmospheric should be
ronments
evaluated from the hazard point of view for possible significant increase
G88Guide for Designing Systems for Oxygen Service
in material combustibility.
G93GuideforCleanlinessLevelsandCleaningMethodsfor
1.3 Units—The values stated in SI units are to be regarded
Materials and Equipment Used in Oxygen-Enriched En-
as the standard.
vironments
G124Test Method for Determining the Combustion Behav-
1.4 This standard does not purport to address all of the
ior of Metallic Materials in Oxygen-Enriched Atmo-
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro- spheres
priate safety, health, and environmental practices and deter- G126Terminology Relating to the Compatibility and Sensi-
mine the applicability of regulatory limitations prior to use. tivity of Materials in Oxygen Enriched Atmospheres
G128Guide for Control of Hazards and Risks in Oxygen
1.5 This international standard was developed in accor-
Enriched Systems
dance with internationally recognized principles on standard-
ization established in the Decision on Principles for the
2.2 ASTM Special Technical Publications (STPs) on the
Development of International Standards, Guides and Recom-
Flammability and Sensitivity of Materials in Oxygen-Enriched
mendations issued by the World Trade Organization Technical
Atmospheres:
Barriers to Trade (TBT) Committee.
ASTM STPs in this category are listed as:812, 910, 986,
1040, 1111, 1167, 1197, 1319, 1395, and 1454
ThisguideisunderthejurisdictionofASTMCommitteeG04onCompatibility
and Sensitivity of Materials in Oxygen Enriched Atmospheres and is the direct
responsibility of Subcommittee G04.02 on Recommended Practices. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
CurrenteditionapprovedMay1,2022.PublishedJuly2022.Originallyapproved contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
in 1987. Last previous edition approved in 2014 as G94–05(2014). DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/G0094-22. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
G94−22
2.3 CGA Documents: 3.1.12 qualified technical personnel, n—persons such as
G-4.4 (EIGADoc. 13)Oxygen Pipeline and Piping Systems engineers and chemists who, by virtue of education, training,
G-4.8Safe Use of Aluminum Structured Packing for Oxy- or experience, know how to apply physical and chemical
gen Distillation principles involved in the reactions between oxygen and other
G-4.9Safe Use of Brazed Aluminum Heat Exchangers for materials (see Terminology G126).
Producing Pressurized Oxygen
3.1.13 reaction effect, n—the personnel injury, facility
P-8.4 (EIGA Doc. 65) Safe Operation of Reboilers/
damage, product loss, downtime, or mission loss that could
Condensers in Air Separation Plants
occur as the result of an ignition (see Terminology G126).
2.4 ASTM Adjuncts:
3.1.14 threshold pressure, n—there are several different
Test Program Report on the Ignition and Combustion of
definitions of threshold pressure that are pertinent to the
Materials in High-Pressure Oxygen
technicalliterature.Itisimportantthattheuserofthetechnical
3. Terminology literature fully understand those definitions of threshold pres-
sure which apply to specific investigations being reviewed.
3.1 Definitions:
Twodefinitionsforthresholdpressure,basedoninterpretations
3.1.1 autoignition temperature, n—the lowest temperature
of the bulk of the current literature, appear below.
at which a material will spontaneously ignite in oxygen under
3.1.14.1 threshold pressure, n—in a promoted ignition-
specific test conditions (see Terminology G126).
combustion test series conducted over a range of pressures,
3.1.2 direct oxygen service, n—in contact with oxygen
this is the maximum pressure at which no burns, per the test
during normal operations. Examples: oxygen compressor pis-
criteria, were observed and above which burns were experi-
ton rings, control valve seats (see Terminology G126).
enced or tests were not conducted.
3.1.3 exemption pressure, n—the maximum pressure for an
3.1.14.2 threshold pressure, n—the minimum gas pressure
engineering alloy at which there are no oxygen velocity
(at a specified oxygen concentration and ambient temperature)
restrictions (from CGA 4.4 and EIGA doc 13/02).
that supports self-sustained combustion of the entire standard
3.1.4 impact-ignition resistance, n—the resistance of a ma-
sample (see Test Method G124).
terial to ignition when struck by an object in an oxygen
atmosphere under a specific test procedure (see Terminology
4. Significance and Use
G126).
4.1 The purpose of this guide is to furnish qualified techni-
3.1.5 indirect oxygen service, n—not normally in contact
cal personnel with pertinent information for use in selecting
with oxygen, but which might be as a result of a reasonably
metals for oxygen service in order to minimize the probability
foreseeable malfunction, operator error, or process upset.
of ignition and the risk of explosion or fire. It is intended for
Examples: liquid oxygen tank insulation, liquid oxygen pump
use in selecting materials for applications in connection with
motor bearings (see Terminology G126).
the production, storage, transportation, distribution, or use of
3.1.6 maximum use pressure, n—the maximum pressure to
oxygen. It is not intended as a specification for approving
which a material can be subjected due to a reasonably
materials for oxygen service.
foreseeable malfunction, operator error, or process upset (see
5. Factors Affecting Selection of Materials
Guide G63).
3.1.7 maximum use temperature, n—themaximumtempera-
5.1 General:
ture to which a material can be subjected due to a reasonably
5.1.1 The selection of a material for use with oxygen or
foreseeable malfunction, operator error, or process upset (see
oxygen-enriched atmospheres is primarily a matter of under-
Terminology G126).
standing the circumstances that cause oxygen to react with the
material. Most materials in contact with oxygen will not ignite
3.1.8 nonmetallic, adj—appliestoanymaterial,otherthana
without a source of ignition energy. When an energy-input
metal, or any composite in which the metal is not the most
exceeds the configuration-dependent threshold, then ignition
easilyignitedcomponentandforwhichtheindividualconstitu-
and combustion may occur. Thus, the material’s flammability
ents cannot be evaluated independently (see Terminology
propertiesandtheignitionenergysourceswithinasystemmust
G126).
be considered. These should be viewed in the context of the
3.1.9 operating pressure, n—the pressure expected under
entire system design so that the specific factors listed in this
normal operating conditions (see Terminology G126).
guidewillassumetheproperrelativesignificance.Insummary,
3.1.10 operating temperature, n—the temperature expected
it depends on the application.
under normal operating conditions (see Terminology G126).
5.2 Relative Amount of Data Available for Metals and
3.1.11 oxygen-enriched, adj—applies to a fluid (gas or
Nonmetals:
liquid) that contains more than 25 mol% oxygen (see Termi-
5.2.1 Studies of the flammability of gaseous fuels were
nology G126).
begun more than 150 years ago.Awide variety of applications
have been studied and documented, including a wide range of
Available from Compressed Gas Association (CGA), 8484 Westpark Drive,
importantsubtletiessuchasquenchingphenomena,turbulence,
Suite 220, McLean, VA 22102, http://www.cganet.com.
cool flames, influence of initial temperature, etc., all of which
Available from ASTM International Headquarters. Order Adjunct No.
ADJG0094. Original adjunct produced in 1986. have been used effectively for safety and loss prevention. A
G94−22
smaller, yet still substantial, background exists for nonmetallic explain the emphasis on using the most fire-resistant materials
solids. In contrast to this, the study of the flammability of and Guide G93 which deals with the importance of system
metals dates only to the 1950s, and even though it has cleanliness.
accelerated rapidly, the uncovering and understanding of 5.3.3 Since metals are typically more fire-resistant and are
subtleties have not yet matured. In addition, the heterogeneity used in typically less fire-prone functions, they represent a
of the metal and oxidizer systems and the heat transfer second tier of interest. However, because metal components
properties of metals, as well as the known, complex ignition are relatively so large, a fire of a metal component is a very
energy and ignition/burning mechanisms, clearly dictate that importantevent,andshouldanonmetalignite,anyconsequen-
cautionisrequiredwhenapplyinglaboratoryfindingstoactual tial reaction of the metal can aggravate the severity of an
applications.Inmanycases,laboratorymetalsburningtestsare ignition many times over. Hence, while the selection of
designedonwhatisbelievedtobeaworst-casebasis,butcould nonmetalsbyGuideG63andthecarefuldesignofcomponents
the particular actual application be worse? Further, because so by Guide G88 are the first line of defense, optimum metal
many subtleties exist, accumulation of favorable experience selection is an important second-line of defense.
(no metal fires) in some particular application may not be as 5.3.4 Contaminants and residues that are left in oxygen
fully relevant to another application as might be the case for systems may contribute to incidents via ignition mechanisms
gaseous or nonmetallic solids where the relevance may be such as particle impact and promoted ignition-combustion
more thoroughly understood. (kindling chain). Therefore, oxygen system cleanliness is
5.2.1.1 ASTM Symposia and Special Technical Publica- essential. Guide G93 describes in detail the essential elements
tions on these symposia have contributed significantly to the for cleaning oxygen systems.
study of the flammability and sensitivity of materials in
5.4 Differences in Oxygen Compatibility of Metals and
oxygen-enriched atmospheres. See section 2.2 for listing of
Nonmetals:
STP numbers and the References Section for key papers.
5.4.1 Thereareseveralfundamentaldifferencesbetweenthe
5.3 Relationship of Guide G94 with Guides G63, G88, and oxygen compatibility of metals and nonceramic nonmetals.
G93: These principal differences are summarized in Table 1.
5.3.1 This guide addresses the evaluation of metals for use 5.4.2 Common-use metals are harder to ignite. They have
in oxygen systems and especially in major structural portions high autoignition temperatures in the range 900 to 2000°C
ofasystem.GuideG63addressestheevaluationofnonmetals. (1650to3600°F).Incomparison,mostcombustiblenonmetals
Guide G88 presents design and operational maxims for all haveautoignitiontemperaturesintherange150to500°C(300
systems. In general, however, Guides G63 and G88 focus on to 1000°F). Metals have high thermal conductivities that help
physically small portions of an oxygen system that represent dissipate local heat inputs that might easily ignite nonmetals.
the critical sites most likely to encounter ignition. Guide G93 Manymetalsalsogrowprotectiveoxidecoatings(see5.5)that
covers a key issue pertinent to actual operating oxygen interfere with ignition and propagation.
systems; cleaning for the service. 5.4.3 Once ignited, however, metal combustion can be
5.3.2 The nonmetals in an oxygen system (valve seats and highlydestructive.Adiabaticflametemperaturesformetalsare
packing,pistonrings,gaskets,o-rings)aresmall;therefore,the much higher than for most polymers (Table X1.7).The greater
use of the most fire-resistant materials is usually a realistic, density of most metals provides greater heat release potential
practical option with regard to cost and availability. In fromcomponentsofcomparablesize.Sincemanymetaloxides
comparison, the choice of material for the major structural do not exist as oxide vapors (they largely dissociate upon
members of a system is much more limited, and the use of vaporization), combustion of these metals inherently yields
specialalloysmayhavetobeavoidedtoachieverealisticcosts coalescingliquidmetaloxideofhighheatcapacityintheflame
and delivery times. Indeed, with the exception of ceramic zoneattheoxideboilingpoint(theremaybeverylittlegaseous
materials, which have relatively few practical uses, most metal oxide). In comparison, combustion of polymers yields
nonmetals have less fire resistance than virtually all metals. gaseous combustion products (typically carbon dioxide and
Nonmetals are typically introduced into a system to provide a steam) that tend to dissipate the heat release.
physical property not achievable from metals. Nonmetals may 5.4.4 Contact with a mixture of liquid metal and oxide at
serve as “links” in a kindling chain (see 5.6.5), and since the high temperature results in a massive heat transfer relative to
locations of use are typically mechanically severe, the primary thatpossibleuponcontactwithhot,low-heat-capacity,gaseous
thrust in achieving compatible oxygen systems rests with the combustion products of polymers. As a result, metal combus-
minor components as addressed by Guides G63 and G88 that tion can be very destructive. Indeed, certain metal combustion
flames are an effective scarfing agent for hard-to-cut materials
like concrete (1).
TABLE 1 Comparison of Metals and Nonmetals Flammability
5.4.5 Finally, because most polymers produce largely inert
Metals Nonmetals gas combustion products, there is a substantial dilution of the
oxygen in the flame that inhibits combustion and if in a
Combustion products molten metal oxide hot gases
Autoignition temperatures 900–2000 °C 150–500 °C
stagnant system, may even extinguish a fire. For many metals,
Thermal conductivities higher lower
Flame temperature higher lower
Heat release higher due to density lower
Surface oxide can be protective negligible Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this guide.
G94−22
A
TABLE 2 Pilling and Bedworth Ratios of Metal Oxides
combustion produces the molten oxide of negligible volume
Nonprotective Oxides Potentially Protective Oxides
condensing in the flame front and, hence, oxygen dilution is
Oxide P&B < 1 Oxide P&B$ 1
much less.
BaO 0.685 Al O 1.29
l2 3
5.5 Protective Oxide Coatings:
CaO 0.663–0.637 CuO 1.71–1.77
MgO 0.806 Cu O 1.68
5.5.1 Oxides that grow on the surfaces of metals can play a
Cr O 2.02
2 3
roleinthemetal’sflammability.Thosefilmsthatinterferewith
FeO 1.78
ignition and combustion are known as protective oxides. Fe O 2.15
2 3
Fe O 2.09
3 4
Typically, an oxide will tend to be protective if it fully covers
CoO 1.76
the exposed metal, if it is tenaciously adherent, and if it has a
MoO 2.10
NiO 1.70
high melting point. Designers have very limited control over
PbO 1.28–1.52
the integrity of an oxide layer; however, since oxide can have
SnO 1.15–1.28
significant influence on metal’s test data, an understanding of
SnO 1.19–1.33
its influence is useful. TiO 1.76–1.95
ZnO 1.59
5.5.2 A protective oxide provides a barrier between the
A
The Pilling and Bedworth (P&B) ratio is the ratio of the volume of a metal oxide
metal and the oxygen. Hence, ignition and combustion can be
compared to the volume of metal from which it was grown. A P&B ratio $ 1
inhibited in those cases where the oxide barrier is preserved.
suggests the potential for an oxide to be protective if it is also conformal and
tenaciously adherent. All data are calculated and do not always agree with P&B
For example, in some cases, an oxide will prevent autogenous
ratios in the literature (1-5).
ignition of a metal up to the temperature at which the metal
melts and produces geometry changes that breach the film. In
other cases (such as anodized aluminum wires), the oxide may
The P&B ratio suggests whether a grown metal oxide is
be sufficiently sturdy as either a structure or a flexible skin to
sufficient in volume to thoroughly cover a metal surface, but it
contain and support the molten base metal at temperatures up
does not provide insight into the tenacity of the coating or
to the melting point of the oxide itself. In either of these cases,
whetheritdoesindeedgrowinaconformalfashion.Theratios
autogenous ignition may occur at much lower temperatures if
in Table 2 have been segregated into those oxides that one
the metal experiences mechanisms that damage the oxide
would suspect to be nonprotective (P&B < 1) and those that
coating.Oxidedamagingmechanismsmayincludemechanical
mightmorelikelybeprotective(P&B≥1).Notealsothatifthe
stresses, frictional rubs and abrasion, or chemical oxide attack
P&B ratio >> 1 (as in the case of Fe O ), the volume of the
2 3
(amalgamation, etc.). Depending upon the application, a high
oxide can increase so dramatically that chipping, cracking, or
metal autoignition temperature, therefore, may be misleading
breakingcanoccurthatmayreduceits“protection.”Theeffect
relative to the metal’s flammability.
of protective oxides on alloys is a still more complex aspect of
5.5.3 One criterion for estimating whether an oxide is
a metal’s flammability.
protective is based upon whether the oxide that grows on a
5.6 Operational Hazard Thresholds:
metal occupies a volume greater or less than the volume of the
metal it replaces. Pilling and Bedworth (2) formulated an 5.6.1 Most practical oxygen systems are capable of ignition
and combustion to some extent under at least some conditions
equation for predicting the transition between protective and
nonprotective oxides in 1923. Two forms of the Pilling and of pressure, temperature, flow, etc. The key to specifying
oxygen-compatible systems is avoiding the circumstances in
Bedworth(P&B)equationappearintheliteratureandcanyield
differentresults.ASTMCommitteeG04hasconcludedthatthe whichignitionislikelyandinwhichconsequentialcombustion
may be extensive.This often involves avoiding the crossing of
most meaningful formulation for the P&B ratio in oxide
hazard thresholds. Guide G128 is very useful in assessing
evaluations for flammability situations is:
hazards and risks in oxygen systems.
P&B Ratio 5Wd/awD (1)
5.6.2 For example, many materials exhibit a bulk system-
where the metal, M, forms the oxide M O , a and b are the related ignition temperature that represents a hazard threshold.
a b
oxide stoichiometry coefficients, W is the formula weight of When a region of a system is exposed to a temperature greater
theoxide,disthedensityofthemetal,wistheformulaweight than its bulk in-situ autoignition temperature, the likelihood of
of the metal, and D is the density of the oxide. The other form an ignition increases greatly; a hazard threshold has been
oftheequationtreatsthestoichiometrycoefficientasunityand crossed.
thus for those oxides that have a single metal atom in the 5.6.3 Hazard thresholds can be of many types. Ignition may
formula, the two equations yield the same results. Pilling and depend upon a minimum heat energy input, and the threshold
Bedworth ratios should always reference an oxide rather than may be different for heat inputs due to heat transfer, friction,
the metal of oxide origin, because for many metals, several arc/spark, etc. Propagation may require the presence of a
different oxides can form each having a different P&B ratio. minimum oxygen concentration (the oxygen index is one such
For example, normal atmospheric corrosion of iron tends to flammability limit) or it may require a minimum oxygen
producetheoxide,Fe O ,whereastheoxidethatformsforiron pressure (a threshold pressure below which propagation does
2 3
at the elevated temperatures of combustion is Fe O . In cases not even occur in pure oxygen). It may also require a specific
3 4
where a mixture of oxides forms, the stoichiometry geometry.
coefficients,aandb,maybeweightedtoreflectthisfact.Table 5.6.4 Forafiretooccur,itmaybenecessarytocrossseveral
2 presents numerous P&B ratios for a number of metal oxides. thresholds of hazard simultaneously. For example, brief local
G94−22
exposure to high temperature above the ignition temperature 5.8.1 Ease of Ignition—Although metals are typically
might not produce ignition unless the heat transferred also harder to ignite than nonmetals, there is a wide range of
exceeds the minimum energy threshold. And even if a local ignition properties exhibited among potential structural
ignition results, the fire may self-extinguish without propaga- materials, and, indeed, some metals are difficult to ignite in
some ways while being relatively easy to ignite in others. The
tion if the pressure, oxidant concentration, or other conditions,
are not simultaneously in excess of their related hazard principal recognized sources of metal ignition include:
5.8.1.1 Contaminant promotion where the contaminant it-
threshold. It is desirable to operate on the conservative side of
as many hazard thresholds as possible. self may be ignited by mechanical impact, adiabatic
compression, sparks, or resonance.
5.6.5 Kindling Chains—A kindling chain reaction can lead
5.8.1.2 Particle impact ignition in which a particle may
to the crossing of a hazard threshold. In a kindling chain,
ignite and promote ignition of the metal.
ignitionofaneasilyignitedmaterial(suchasacontaminantby
5.8.1.3 Friction ignition where the friction results from
adiabaticcompression)maynotreleaseenoughheatto,inturn,
mechanical failure, cavitation, rubs, etc.
igniteavalvebody,butmaybesufficienttoigniteavalveseat,
5.8.1.4 Bulk heating to ignition.
which, in turn, may release sufficient heat to ignite the larger,
5.8.2 Ignition may also result from the following
harder-to-ignite valve body.
mechanisms, though these are not thoroughly studied nor
5.7 Practical Systems Considerations:
understood for metals, nor have they been implicated in
5.7.1 It is not always possible to use the most fire-resistant
significant numbers of incidents relative to those in 5.8.1.
metals in practical systems.As a result, operation below every
5.8.2.1 Mechanical impact.
hazard threshold may not always be used to minimize the
5.8.2.2 Resonance.
chance of a fire. Additional conservatism is often used to
5.8.2.3 Fresh metal exposure.
increasethesafetymarginswherepossible.Forexample,ifthe
5.8.2.4 Crack propagation.
pressure and temperature of an application are such that
5.8.2.5 Electric arc or spark.
particle impact may cause an ignition, the remedy has been to
5.8.2.6 Puncture.
limit the severity of particle impacts by limiting gas velocity
5.8.2.7 Trapped volume pressurization.
and filtering or screening of particles.This, in effect, limits the
5.8.2.8 Autoignition—Intheprecedingmechanisms,heating
application severity by constraining the operation conditions.
to the autoignition temperature can result. For some of them,
5.7.1.1 The Compressed Gas Association (CGA) and the
the achievement of ignition also can result from the material
European Industrial GasAssociation (EIGA) have published a
self heating as the freshly exposed metal oxidizes.
“harmonized” document, CGA G-4.4 (EIGA Doc. 13), which
5.8.3 Ignition can result from bulk heating to the autoigni-
provides a prescriptive metal selection method considering
tion temperature, but this is rare in oxygen systems unless an
flammability (based on pressure and thickness limits) and
environmental fire is present or unless electrical heaters expe-
ignition risk (based on gas pressure-velocity limits).
rience runaways. Autoignition temperatures are often used to
The harmonized CGA G-4.4/EIGA Doc 13 document in- compare metals, but they can yield rankings that disagree with
cludes design and installation requirements and recommenda-
observed experience. This is because ignition is a very com-
tions for the use of metal alloys in oxygen pipelines and plex process. For example, where a metal grows a protective
equipment for gaseous oxygen with a temperature range
oxide, the autoignition temperature can vary widely depending
between–30°Cand200°C(–22°Fand400°F),andpressures upon such things as the adherence of the oxide, its degree of
up to 21MPa (3000psi). This publication includes guidance
protection (as indicated in part by its Pilling and Bedworth
for selection of metals based on exemption pressures (EP), number), and its melting point. A more likely effect of
minimum application thickness, and pressure-velocity curves.
temperature on the ignition of a metal is via a promoted
The prescribed EP is defined as the maximum pressure at ignition-combustion mechanism.
whichametalisnotsubjecttooxygengasvelocitylimitations.
5.8.4 Properties and Conditions Affecting Potential Resul-
Thus, at pressures below an alloy’s EP, an alloy may be used
tant Damage—A material’s heat of combustion, its mass, its
for gaseous oxygen applications regardless of gas velocity, as
geometry (thick versus thin), the oxygen concentration and
long as the application meets the minimum thickness criteria.
pressure, the presence of gaseous versus liquid oxygen, the
At pressures above an alloy’s EP, the alloy is limited to
flow conditions before and after ignition, and the flame
pressure-velocity (PV) combinations as dictated by corre-
propagation characteristics affect the potential damage if igni-
sponding PV curves, which include impingement and non-
tion should occur. They should be taken into account in
impingement versions, depending on the application. The PV
estimating the reaction effect in 8.5. Since so much damage in
curves evolved over time based on the experience of the
metal fires is attributable to direct contact with the molten
industrial gas community and experts. The EPs were more
oxideandfromradiationduetoitsextremelyhightemperature,
recently established based on Test Method G124 test data and
theprobableflowpathortrajectoryofthemoltenoxideshould
guidance of industry experts. If an alloy is desired for use
be considered in predicting the zones of greatest damage.
above its EP and corresponding PV curve, CGA G-4.4/EIGA
5.9 Extenuating Factors:
Doc. 13 allows for an oxygen hazard analysis to be performed
5.9.1 In choosing major structural members of a system,
to evaluate the risk of fire.
practicality becomes a critical factor. Frequently, the more
5.8 Properties of the Metal: fire-resistantmaterialsaresimplyimpracticaloruneconomical.
G94−22
For example, their strength-to-weight ratios may not meet decrease. Therefore, greater latitude may be exercised in the
minimummechanicalstandardsforturbinewheels.Thecostor selection of materials. For all metals, there is an oxygen
availability of an alloy may also preclude its use in a long concentration (a flammability limit analogous to the oxygen
pipeline. Corrosive environments may preclude still other index), below which (in the specific metal combustion tests
materials. In contrast, there may be a base of experience with undertaken)propagatingcombustionwillnotoccur,eveninthe
traditional metals in oxygen service, such as carbon steel presence of an assured (very high energy) ignition. This
pipelines, that clearly demonstrates suitability for continued concentration decreases with increasing pressure above a
service with appropriate safeguards. As a result, where these threshold pressure (below which the metal will not burn even
extenuating factors are present, less than optimum metals are in pure oxygen). The concentration may approach an asymp-
frequently selected in conjunction with operational controls tote at high pressures, Fig. X1.2, Fig. X2.1, and Fig. X2.3.
(such as operating valves only during zero-flow), established
NOTE 4—Some metals are extremely sensitive to oxygen purity. Since
pastpractice(suchasCGAPamphletG-4.4forsteelpiping),or
manymetaloxidesdonotexistasgases,thecombustionproductsofsome
measures to mitigate the risk (such as use with a shield or
metals do not interfere with the combustion as is the case with polymers.
removal of personnel from the vicinity). Therefore,smallamountsofinertgasesintheoxygencanaccumulateand
controlthecombustion.Inaresearchproject,Benningetal. (6)foundthat
5.10 Operating Conditions:
as little as 0.2% argon could increase the minimum pressure at which
5.10.1 Conditions that affect the suitability of a material
6.4mm(0.25in.)diameteraluminumrodssustainedcombustionfrom210
include the other materials of construction and their arrange- kPa(30psiabsolute)to830kPa(120psiabsolute).Thiseffectisbelieved
to be most significant for “vapor-burning” metals such as aluminum and
ment and geometry in the equipment and also the pressure,
less significant for “liquid-burning” metals such as iron. Theory is found
temperature, concentration, flow, and velocity of the oxygen.
in Benning (6) and Glassman (7-9).
For metals, pressure, concentration or purity, and oxygen flow
5.10.4 Flow and Oxygen Inventory—The quantity of oxy-
rate are usually the most significant factors. Temperature is a
gen present and the rate at which it can flow to an ignition site
much less significant factor than is the case for nonmetals
affects the intensity and scale of a metal fire. Since many
because ignition temperatures of metals are all significantly
metals do not form gaseous combustion products, self extin-
higher than those of nonmetals. The effects of these factors
guishment through accumulation of combustion products can-
show up in the estimate of ignition potential (8.2) and reaction
not occur as it does with polymers. However, accumulation of
effect assessment (8.5), as explained in Section 8.
inert gases in the oxygen may cause extinguishment. Since the
5.10.2 Pressure—The oxygen pressure is important, be-
densityofoxygengasismuchlowerthanthemetaldensity,the
cause it generally affects the generation of potential ignition
quantity of metal that can burn is often limited by the quantity
mechanisms, and because it affects the destructive effects if
of oxygen present or the rate at which it can be supplied.
ignitionshouldoccur.Whilegeneralizationsaredifficult,rough
scales would be as given in Table 3. 5.10.5 Temperature—Increasing temperature obviously in-
creasestheriskofignition,aswellastheprospectforsustained
NOTE 3—While the pressure generally affects the reaction as given in
combustion. Indeed, an increase in temperature may enable
Table3,dataindicatethatithasvaryingeffectsonindividualflammability
properties. For example, for many metals, increasing pressure results in combustion in cases where propagation would not be possible
the following:
atlowertemperature.Theinfluenceofenvironmentaltempera-
(a) A reduction in the oxygen concentration required to enable
ture on metals is much less significant than for nonmetals; this
propagation;
is because the autoignition temperature of the most sensitive
(b) Differing effects on autoignition temperature, with many metals
bulk metal (perhaps carbon steel at (~900°C (~1650°F)) is
having invariant autoignition temperatures, many metals having decreas-
ing autoignition temperatures, and some metals having increasing autoi- significantly greater than for the most resistant polymers (for
gnition temperatures;
example PTFE at (~480°C ( ~900°F)).
(c) An increase in sensitivity to mechanical impact;
5.10.5.1 Although autoignition temperatures of metals in
(d) A negligible change in heat of combustion;
oxygen atmospheres have been cited as a means of ranking
(e) An increase in the difficulty of friction ignition, apparently due to
increased convective heat dissipation;
materials for service in high temperature oxygen, promoted
(f) An increase in the likelihood of adiabatic compression ignition,
ignition-combustionofmetalsinhightemperatureoxygenmay
however, adiabatic compression is an unlikely direct ignition mechanism
be more appropriate. Zawierucha et al. (10) have reported on
for metals except at pressures in excess of 20000 kPa (3000 psi); and
elevatedtemperaturepromotedignition-combustionresistance.
(g) An increase in the rate of combustion.
5.10.6 LOX versus GOX—Combustion of aluminum in
5.10.3 Concentration—As oxygen concentration decreases
LOXhasledtoextremelyseriouscombustioneventsknownas
from100%,thelikelihoodandintensityofapotentialfirealso
ViolentEnergyReleases(VERs)inbothoperatingsystemsand
experiments. In GOX, aluminum will experience rapid com-
bustion but not VERs. The destruction caused by a VER is
TABLE 3 Effect of Pressure on Typical Metal Burning Reactions
more typical of an explosion than simple combustion. Numer-
A
kPa psi Pressure Effect Assessment
ous investigators have duplicated this phenomenon (11-24).
0–70 0–10 relatively mild
Key Aluminum-LOX incidents are referenced (25-27). Miti-
70–700 10–100 moderate
700–7000 100–1000 intermediate
gatingapproachesaredescribedinCGApamphletsG4.8,G4.9,
7000–20 000 1000–3000 severe
and P-8.4 for aluminum air separation plant components.
Over 20 000 Over 3000 extremely severe
A 5.10.7 Geometry—The geometry of the component can
See 5.10.2.
have a striking effect on the flammability of metals. Generally,
G94−22
thin components or high-surface-area-to-volume components higher heat capacity and thermal conductivity of significantly
willtendtobemoreflammable.Forexample,bothStoltzfuset sized metals greatly attenuates high temperature produced this
al. (28) and Dunbobbin et al. (29) have shown that materials
way. Example: a downstream valve or flexible lined pigtail in
such as thin wire mesh and thin layered sheets can become
a dead-ended high-pressure oxygen manifold.
much more flammable than might be expected on the basis of
5.11.7 Electrical Arc—Electrical arcing can occur from
tests of rods. In these works, copper and brass alloys that
motor brushes, electrical control instrumentation, other
typically resist propagation in bulkier systems were capable of
instrumentation, electrical power supplies, lightning, etc. Elec-
complete combustion. Zabrenski et al. (30) have found that
trical arcing can be a very effective metal igniter, because
thin-wall tubes of 6.4mm (0.25in.) diameter stainless steel
current flow between metals is easily sustained, electron beam
would propagate combustion at atmospheric pressure while
heating occurs, and metal vaporizes under the influence of the
solid rods required pressures of 5.0 MPa [740 psi absolute].
plasma.Alloftheseareconducivetocombustion.Example:an
Samant et al. (31) in promoted ignition-combustion studies of
insulated electric heater element in oxygen experiences a short
Nickel 200, Monel 400, Hastelloy C-276, Copper, and Stain-
circuit and arcs through to the oxygen gas.
less Steels at pressures up to 34.6 MPa show that Nickel 200
5.11.8 Resonance—Acoustic oscillations within resonant
was the most combustion resistant in thin cross sections while
cavitiesareassociatedwithrapidgastemperaturerise.Thisrise
316/316L stainless steel was the least.
ismorerapidandachieveshighervalueswhereparticulatesare
5.11 Ignition Mechanisms—For combustion to occur, it is
present or where there are high gas velocities. Ignition can
necessary to have three elements present: oxidizer, fuel, and
result if the heat transferred is not rapidly dissipated, and fires
ignition energy. The oxygen environment is obviously the
of aluminum have been induced experimentally by resonance.
oxidizer, and the system itself is the fuel. Several potential
Example: a gas flow into a tee and out of a side port such that
sources of ignition energy are listed below. The list is not
the remaining closed port forms a resonance.
all-inclusive or in order of importance or in frequency of
5.11.9 Other—Since little is known about the actual cause
occurrence.
of some oxygen fires or explosions, other mechanisms, not
5.11.1 Promoted Ignition—A source of heat input occurs
readily apparent, may be factors in, or causes of, such
(perhaps due to a kindling chain) that acts to start the metal
incidents. These might include external sources, such as
burning. Examples: the ignition of contamination (oil or alien
weldingspatter,orinternalsources,suchasfractureorthermite
debris) which combusts and its own heat release starts a metal
reactions of iron oxide with aluminum.
fire.
5.11.2 Friction Ignition—Therubbingoftwosolidmaterials
5.12 Reaction Effect—The effect of an ignition (and subse-
results in the generation of heat and removal of protective
quent propagation, if it should occur) has a strong bearing on
oxide. Example: the rub of a centrifugal compressor rotor
the selection of a material. While reaction effect assessment is
against its casing.
an obviously imprecise and strongly subjective judgment, it
5.11.3 Heat from Particle Impact—Heat is generated from
must be balanced against extenuating factors such as those
thetransferofkinetic,thermal,orchemicalenergywhensmall
given in 5.9. Suggested criteria for rating the reaction effect
particles (sometimes incandescent, sometimes igniting on
severity have been developed in Guide G63 and are shown in
impact), moving at high velocity, strike a material. Example:
Table 4, and a method of applying the rating in a material
high velocity particles from a dirty pipeline striking a valve
selection process is given in Section 8. Note that, in some
plunger.
cases, the reaction effect severity rating for a particular
5.11.4 Fresh Metal Exposure—Heat is generated when a
application can be lowered by changing other materials that
metal with a protective surface oxide is scratched or abraded,
may be present in the system, changing component locations,
and a fresh surface oxide forms. Titanium has demonstrated
varyingoperatingprocedures,orusingshieldsandthelike(see
ignition from this effect, but there are no known cases of
Guide G88). The combustion of aluminum in LOX has
similar ignition of other common metals. Nonetheless, fresh
generatedcombustionphenomena,VERs,thatareexplosiveon
metal exposure may be a synergistic contributor to ignition by
systems and test facilities.
friction, particle impact, etc. Example: the breaking of a
5.12.1 Heat of Combustion—The combustion of a metal
titanium wire in oxygen.
releases heat, and the quantity has a direct effect on the
5.11.5 Mechanical Impact—Heat is generated from the
destructivenatureofthefire.Onamassbasis,numerousmetals
transfer of kinetic energy when an object having a large mass
andpolymersreleaseaboutthesameamountofheat.However,
or momentum strikes a material.Aluminum and titanium have
because of its much larger mass in most systems, combustion
been experimentally ignited this way, but stainless steels and
of many metals has the potential for release of the major
carbon steels have not. Examples: a backhoe rooting-up an
amount of heat in a fire. Combustion of aluminum in LOX is
oxygen line; a fork truck penetrating an oxygen cylinder.
an example of an explosive phenomenon.
5.11.6 Heat of Compression—Heat is generated from the
5.12.2 Rate of Combustion—Theintensityofafireisrelated
conversionofmechanicalworkwhenagasiscompressedfrom
to both the heat of combustion of the materials and the rate at
a low to a high pressure. This can occur when high-pressure
oxygen is released into a dead-ended tube or pipe, quickly which the combustion occurs. The rates of combustion of
various metals can vary more than an order of magnitude, and
compressing the residual oxygen that was in the tube ahead of
it. An effective ignition mechanism with polymers, the much for some metals can be so rapid as to be considered explosive.
G94−22
TABLE 4 Reaction Effect Assessment for Oxygen Applications
Rating
Effect on Personnel Safety Effect on System Objectives Effect on Functional Capability
Code Severity Level
A negligible No injury to personnel. No unacceptable effect on production, No unacceptable damage to the system.
storage, transportation, distribution, or
use as applicable.
B marginal Personnel-injuring factors can be Production, storage, transportation, No more than one component or
controlled by automatic devices, distribution, or use as applicable is subsystem damaged. This condition is
warning devices, or special operating possible by utilizing available redundant either repairable or replaceable on site
procedures. operational options. within an acceptable time frame.
C critical Personnel injured: (1) operating the Production, storage, transportation, Two or more major subsystems are
system; (2) maintaining the system; or distribution, or use as applicable damaged; this condition requires
(3) being in vicinity of the system. impaired seriously. extensive maintenance.
D catastrophic Personnel suffer death or multiple injuries. Production, storage, transportation, No portion of system can be salvaged; total
distribution, or use as applicable loss.
rendered impossible; major unit is lost.
and v is the linear velocity), and this is the ranking criterion used in Table
6. Test Methods
X1.2. Pressure affects friction ignition in that it has been harder to ignite
6.1 Promoted Combustion Test—Ametal specimen is delib-
metals at higher pressures above a minimum Pv value. In addition, in
eratelyexposedtothecombustionofapromoter(easilyignited
limited testing to date, the relative rankings of metals may change at
different linear velocities.
material) or other ignition source. Metal specimens reported in
theliteraturehavevariedinlengthandthickness.Thepromoter
6.3 Particle Impact Test—An oxidant stream with one or
may be standardized, in which case the test ranks those
more entrained particles is impinged on a candidate metal
materials that resisted ignition as being superior to those that
target. The particles may be incandescent from preheating
burned; varying the oxygen pressure, oxygen purity or speci-
(likely for smaller particles) due to earlier impacts. The
men temperature allows further ranking control. The promoter
particles may be capable of ignition themselves upon impact
mass may also be varied, in which case, the metals are ranked
(in this case, the test resembles a promoted ignition test under
according to the quantity of promoter required to bring about
flowing conditions with
...