The Role of ASTM E27 Methods in

Hazard Assessment Part II:

Flammability and Ignitability

Laurence G. Britton,a

a

Consultant, Charleston, WV25314

Kenneth L. Cashdollar,b

b

Pittsburgh Research Laboratory, National Institute for OccupationalSafetyand Health, Pittsburgh, PA15236

William Fenlon,c

c

SHE Technologies Inc., Webster, NY14580

David Frurip,d

d

The Dow Chemical Co., Midland MI48667

John Going,e

e

Fike Corporation, BlueSprings, MO 64013

B. Keith Harrison,f

f

Dept.of Chemical Engineering, UniversityofSouth Alabama, Mobile,AL36688

Jeff Niemeier,g

g

EliLillyand Co.,Lilly Technology Center, Indianapolis, IN 46285

and Erdem A. Uralh

h Loss Prevention Science & Technologies, Inc.,Stoughton, MA02072; erdem.ural@lpsti.com, www.lpsti.com (for correspondence)

Accurate flammability and ignitability data for

chemicals form the cornerstone of procedures used to

assess the hazards associated with commercial chemical production and use. Since 1967 the ASTM E27

Committee on the Hazard Potential of Chemicals has

issued numerous, widely used consensus standards

dealing with diverse testing and predictive procedures

used to obtain relevant chemical hazard properties.

The decision to issue a standard rests solely with the

membership, which consists of representatives from industry, testing laboratories, consulting firms, government, academia, and instrument suppliers. Consequently, the procedures are automatically relevant,

timely, and widely applicable. The purpose of this paper

is to highlight some of the widely used standards, complemented with hypothetical but relevant examples describing the testing strategy, interpretation, and application of the results. A further goal of this paper is to

encourage participation in the consensus standards

development process.

The paper is published in two parts. The first part (in

the preceding issue of Process Safety Progress) dealt

with the E27 standards pertaining to thermodynamics,

thermal stability, and chemical compatibility. The second part, published here, focuses on the flammability,

ignitability, and explosibility of fuel and air mixtures.

© 2005 American Institute of Chemical Engineers Process Saf Prog 24: 12–28, 2005

1. INTRODUCTION

Committee E27 of ASTM International, which deals

with the Hazard Potential of Chemicals, has been in

existence since 1967. During that time, its members

have developed and approved over 20 consensus standards relating to ignitability, flammability, and chemical

reactivity. The Committee was formed because of the

need for consistent, scientifically based, accurate, easy

to apply, and most of all, consensus methodologies for

the determination of parameters and properties that

allow industry to design safe processes properly. The

E27 standards are widely recognized and used in hazard assessment and loss prevention engineering.

This paper1

1 The two parts of this paper are based on a presentation at the 38th Loss

Prevention Symposium held at the AIChE Spring National Meeting in New

Orleans, LA, in April 2004.

is published in two parts. Part I dealt

with the E27 methods pertaining to thermal stability,

compatibility, and energy release estimation [1]. Part I

also included a background description of the E27

committee including its scope, membership, and organization. Part II, published here, focuses on the ignitability, flammability, and explosibility of fuel and air

mixtures. In this paper and in the E27 standards, the

terms “flammability” and “explosibility” are used inter­
changeably to refer to the ability of a gas mixture or

dust cloud to propagate a deflagration after it has been

initiated by a sufficiently strong ignition source. Historically, the term “flammability” has been used more

often for gases, and “explosibility” more often for dusts.

A comprehensive listing of the various active standards currently administered by E27 was previously

given in Tables 1A, 1B, and 1C of Part I.

Table 1A. Standard test methods currently administered by ASTM Subcommittee E27.04 on the Flammability

and Ignitability of Chemicals.

ASTM

Designation Title Scope, Citations, Applications, or General Comments

E 502 Selection and Use of ASTM

Standards for the Determination

of Flash Point of Chemicals by

Closed Cup Methods

E 582 Minimum Ignition Energy and

Quenching Distance in Gaseous

Mixtures

E 659 Autoignition Temperature of

Liquid Chemicals

E 681 Concentration Limits of

Flammability of Chemicals

(Vapors and Gases)

E 918 Determining Limits of

Flammability of Chemicals at

Elevated Temperature and

Pressure

E 1232 Temperature Limit of

Flammability of Chemicals

E 2021 Hot-Surface Ignition Temperature

of Dust Layers

E 2079 Limiting Oxygen (Oxidant)

Concentration in Gases and

Vapors

Guides in the selection of the appropriate ASTM closed-cup

flash-point test method. Repeatability and reproducibility

for different test methods. Effect of ambient pressure on

flash point. Referenced in NFPA 30, 49, 77, 325, 497, and

704.

Tests are performed on specified gas mixtures in a 1-L vessel

using capacitance sparks and flanged electrodes. Cited in

NFPA 77 and 497.

Vapor–air mixtures are tested in 0.5-L heated glass flask.

Cited in NFPA 70, 497, and 921.

Lower flammable limit (LFL) and upper flammable limit (UFL)

are determined in 5-L spherical vessel. Cited in NFPA 69

and 921.

This practice covers the determination of the lower and

upper concentration limits of flammability of combustible

vapor–oxidant mixtures at temperatures up to 200° C and

initial pressures up to 1.38 MPa (200 psia).

Minimum temperature at which a liquid in equilibrium with

its vapor forms a flammable mixture with air at

atmospheric pressure. The lower temperature limit of

flammability (LTL) is more relevant than flash point for

large process vessels.

Minimum surface temperature capable of igniting a dust

layer. Cited in NFPA 70, 499, and 654.

Limiting oxidant concentration (LOC) determined in 5-L

vessel. Cited in NFPA 69.

Note: These standards are available from ASTM online (www.astm.org) or in print in reference 2.
 

ASTM E27

standards [2] relevant to this Part are recapitulated in

Table 1A, which lists the standards of Subcommittee

E27.04, and Table 1B, which lists the standards of

Subcommittee E27.05. Subcommittee E27.04 on the

Flammability and Ignitability of Chemicals deals with

gases, vapors from liquids and solids, and nondispersed dusts, such as layers. Subcommittee E27.05 on

the Explosibility and Ignitability of Dust Clouds deals

only with dispersed dust clouds. In addition to the

ASTM numerical designation and title, the tables include a short description of the scope of each standard

and some of the citations where the standard may be

recommended for use. (These standards are available

from ASTM International at the website www.

astm.org.) International consensus standards such as

NFPA 69, Standard on Explosion Prevention Systems,

and other NFPA documents [3] provide guidance for

the practical application of the data from the ASTM E27

test methods.

To give the reader a further sense of the general

applicability of E27 standards, Figure 1 shows a flowchart for dust explosion protection, which was modified from Figure 7.2 of Eckhoff [4]. Within each appropriate box, relevant ASTM E27 test methods are listed.

Clearly, the methods administered by ASTM E27 are

well aligned with these and other commonly used

hazard/consequence evaluation methodologies.

These ASTM E27 ignitability, flammability, and explosibility standards are used to assess the gas, vapor,

and dust explosion hazard properties of materials; to

establish safe operating conditions; and to perform

consequence analysis for hypothesized accidents. Generally speaking:

• Ignitability test methods determine the minimum

energy or temperature required to ignite the most

sensitive mixture of fuel, air (oxidant), and diluent.

• Flammability limit test methods determine the

concentration limits of fuel, air (oxidant), or diluent (suppressant) in the presence of a sufficiently strong ignition source. The flammability
limit is the boundary between regions that can

propagate a flame and regions that cannot. As will

be seen below, this concept can also be extended

to temperature limits of flammability, as related to

commonly used flash-point values.

• The test methods for maximum explosion pressure and rate of pressure rise assume the presence

of both a sufficiently strong ignition source and

the worst case or most energetic mixture of fuel

and oxidizer. They aim to characterize how fast

the combustion reaction can proceed in a particular combination of fuel, air (oxidant), and diluent.

Table 1B. Standard test methods currently administered by ASTM Subcommittee E27.05 on Explosibility and

Ignitability of Dust Clouds.

ASTM

Designation Title Scope, Citations, Applications, or General Comments

E 789 Dust Explosions in a 1.2-L

Closed Cylindrical Vessel

E 1226 Pressure and Rate of Pressure

Rise for Combustible Dusts

E 1491 Minimum Autoignition

Temperature of Dust Clouds

E 1515 Minimum Explosible

Concentration of

Combustible Dusts

E 2019 Minimum Ignition Energy of a

Dust Cloud in Air

WK1680 Limiting Oxygen (Oxidant)

Concentration of

Combustible Dust Clouds

Historical value only, obsolete for design. Occasionally used as

a qualitative screening tool.

Maximum pressure, Pmax, and maximum rate of pressure rise,

dP/dtmax, in closed vessel of at least 20-L volume. Cited in

NFPA 68, 69, 484, 654, 921.

Minimum temperature at which a dust cloud will autoignite

when exposed to air heated in a furnace at local

atmospheric pressure. Cited by NFPA 654.

Minimum concentration of dust dispersed in air that will

propagate a flame. Cited in NFPA 69, 77, 484, 654, and 664.

Minimum ignition energy (MIE) of dust cloud in air. Cited in

NFPA 61, 77, 654, and 921.

Draft test method that will apply to NFPA 69, 484, 654. Note:

WK1680 refers to the ASTM Work Item of the E27.05

Subcommittee.

Note: These standards are available from ASTM online (www.astm.org) or in print in reference 2.
 

Usage Survey of ASTM E27 Standards

In preparation of this paper, an informal survey was

conducted in 2003–2004 to estimate the frequency of

use of ASTM E27 test methods for ignitability, flammability, and explosibility. The survey was sent to E27

committee members, which included commercial testing laboratories, government laboratories, chemical

manufacturers’ laboratories, and explosion protection

equipment manufacturers’ laboratories. These laboratories include most of the major U.S. facilities for testing

the ignitability, flammability, and explosibility of chemicals, particularly for dusts. The results are listed in

Table 2 in terms of total number of times a method was

used per year by all the laboratories, using the past 3– 4

years as a base. (Note that this table has been updated

from the one in the Loss Prevention Symposium paper.) The results are listed by ASTM standard number

and abbreviated title. It appears from Table 2 that the

dust test methods are used more frequently than the

methods for gases. Of the E27.04 standards, E 502 for

flash point is used most extensively. This particular

standard is probably also used by numerous industrial

laboratories that were not part of the survey. For the

E27.05 standards, E 1226 on pressure and rate of pressure rise and E 2019 for minimum ignition energy of

dusts are used very extensively.

There are two apparent reasons for the greater use

of E27.05 standards than E27.04 standards. First, the gas

and vapor test methods have been around for a long

time, and test data have been compiled and published

for a large number of pure chemicals (for example,

NFPA 325, Fire Hazard Properties of Flammable Liquids, Gases, and Volatile Solids). In addition, existing

mixing rules or correlations (for example, Le Chatelier’s

linear mixing law; see Kuchta [5], p. 14) are considered

adequate for most loss prevention applications involving gaseous fuel mixtures. On the other hand, a large

database for dusts that had been obtained using E 789

is now considered obsolete, and is being regenerated

using E 1226. The second reason for the large frequency of dust cloud tests is the strong sensitivity of

flammability, ignitability, and explosibility values to the

particle size distribution of the sample. That is why

different powder products made up of the same pure

chemical need to be tested individually.

2. IGNITABILITY TEST METHODS

Minimum Ignition Energy (MIE)

Measured MIE values are highly sensitive to the test

method, particularly the spark electrode geometry and

the characteristics of the electrical discharge circuit.

Therefore, standard test methods had to be developed

so that comparable data could be obtained by different

laboratories. ASTM standard test methods E 582 and E

2019 determine the lowest energy (stored by a capacitor and released as a spark) that will ignite gas– oxidant

and dust cloud– oxidant mixtures, respectively. At any

given fuel concentration, the minimum energy to ignite

can be determined. By testing over a range of fuel
concentrations, the lowest MIE for a material can be

determined for the optimum or most easily ignited

mixture.

Figure 1. Dust explosion protection philosophy (adapted from Figure 7.2 of Eckhoff [4]).

Minimum Ignition Energy of Gaseous

Mixtures—E 582

The ASTM standard test method E 582 for “Minimum

Ignition Energy and Quenching Distance in Gaseous

Mixtures” is based on research conducted at the U.S.

Bureau of Mines in the 1950s and 1960s. E 582 for gases

uses a simple high voltage (up to 30 kV) spark circuit

designed to minimize inductance, stray capacitance,

and current leakage from the power supply. These

design features are implicit in the procedure described

in the Bureau of Mines publication by Litchfield et al.

[6].

The test procedure starts with addition of gases into

the test vessel. When the proper gas mixture has been

formed, a spark is generated between electrodes. The

spark gap distance can also be varied. Ignition or nonignition is determined by visual observation. A series of

tests over a range of fuel concentrations determines the

MIE at the most easily ignited concentration. The test

method also allows determination of the parallel-plate

ignition quenching distance, which is comparable in

magnitude to the quenching diameter and the maximum experimental safe gap (“MESG”). Among various

provisions of the ASTM E 582 standard method is that

the spark circuit should have negligible inductance and

low capacitance. The smallest MIE values are typically

found using low-capacitance circuits with correspondingly low time constants. A discussion of these features,

with reference to experimental data, can be found in

Chapter 3-5.4 of a recent CCPS Concept Book by Britton [7]. It is also shown that, although the lowest MIE

values should in principle be obtained with pointed

electrodes and a very low circuit capacitance, the

higher voltages needed to compensate for a very low

capacitance tend to cause corona losses from pointed

electrodes. Thus, pointed electrodes are not generally

used for MIE measurements of gases.

The reported MIE is normally the stored energy that

is just sufficient to ignite the optimum mixture at the

optimum gap length, although the option exists to also

report the MIE calculated by integrating the measured

voltage and current across the spark gap. In principle,
this would yield a smaller MIE value because of energy

losses in the circuit.

For most fuel gases and vapors, the MIE is <1 mJ,

and therefore ignition control strategies cannot be used

as the sole means of explosion protection. A paper by

Ural [8] discussed how the commercial aviation sector,

after a series of deadly fuel tank explosions, has rediscovered how very difficult it is to limit the spark energy

in practice. Commercial airplane manufacturers had

been attempting to design fuel tanks so that the maximum credible electrical spark energy is less than onetenth the jet fuel MIE, and relying on the lack of ignition

sources as the sole means of fire prevention. Jet fuel

has an MIE of <1 mJ.

For low MIE gases and vapors, ignition control efforts are still useful because they may reduce the frequency of ignition incidents. The MIE can vary by

several orders of magnitude, depending on the types

and concentrations of fuel and oxidant, plus the temperature and pressure. Further, the operating conditions might be such that the optimum mixture cannot

form, in which case some higher MIE value might be

taken to represent worst-case conditions for a particular process.

Example of Evaluating the Ignition Hazard of a Gas

Mixture

There is no accepted method of calculating the ignition energy of a mixture of two or more fuel gases

from the values corresponding to the separate components. A possible method for determining MIEs for

mixtures of “C + H + O + N” organic fuels has been

developed, but has not yet been fully evaluated [9]. If,

for example, the mixture contains easily ignited components such as hydrogen, plus hard-to-ignite components, such as ammonia, ASTM E 582 should be used to

determine at what hydrogen concentration the mixture

becomes ignitable at an assumed effective energy level.

For example, the latter might be taken as 10 mJ to

represent corona and brush discharges from plastic

surfaces, or 25 mJ to represent sparks from ungrounded

personnel [7].

Alternatively, ASTM E 582 might be used to evaluate

the benefits of reducing the oxygen concentration relative

to that available from ambient air. As nitrogen or other

inert gas is added to air, its strength as an oxidizer diminishes, and the MIE of optimum fuel–oxidant mixtures

increases by orders of magnitude. If the maximum effective energy of ignition sources present in a system can be

determined, partial inerting might be used to complement

existing explosion protection systems. For instance, the

frequency of activation of a suppression system might be

minimized. Alternatively, the ignition hazard resulting

from an upset in a nitrogen–air mixer serving an inhibited

monomer storage tank might be assessed.

E 582 can also be used to assess the increased

hazards if the oxygen concentration is increased. As

additional oxygen is added to air, for example by decomposition of a peroxide, the MIE of optimum fuel–

oxidant mixtures decreases by orders of magnitude.

Difficult-to-ignite gases may ignite readily and common

solvents may be ignited by corona discharges and other

very weak sources. NFPA 53 on “Recommended Practice on Materials, Equipment and Systems Used in Oxygen Enriched Atmospheres” and a series of ASTM

publications, such as G-128 “Standard Guide for Control of Hazards and Risks in Oxygen Enriched Systems,”

provide detailed information on the hazards of oxygenenriched systems.

Table 2. Survey results of ASTM E27 method usage by 14 laboratories.

ASTM

Method Title

Average

Annual Use

Subcommittee

E 502

E 582

E 659

E 681

E 918

E 1232

E 2021

E 2079

Subcommittee

E 789

E 1226

E 1491

E 1515

E 2019

WK1680*

E27.04

E27.05

on the Flammability and Ignitability of Chemicals

Flash Point of Chemicals by Closed Cup Methods

Minimum Ignition Energy and Quenching Distance

Autoignition Temperature of Liquid Chemicals

Concentration Limits of Flammability

Limits of Flammability at Elevated Temperature and Pressure

Temperature Limit of Flammability of Chemicals

Hot-Surface Ignition Temperature of Dust Layers

Limiting Oxygen (Oxidant) Concentration in Gases and Vapors

on Explosibility and Ignitability of Dust Clouds

Dust Explosions in a 1.2-L Closed Cylindrical Vessel

Pressure and Rate of Pressure Rise for Combustible Dusts

Minimum Autoignition Temperature of Dust Clouds

Minimum Explosible Concentration of Combustible Dusts

Minimum Ignition Energy of a Dust Cloud in Air

Limiting Oxygen Concentration in Dust Clouds

212

11

56

34

8

2

59

8

118

797

155

189

650

56

*Note: WK1680 is not a currently published standard, but a working draft.
 

Minimum Ignition Energy of Dust Clouds—

E 2019

Test method E 2019 for “Minimum Ignition Energy of

a Dust Cloud in Air” describes several electrical spark
circuits in current use that have been found to yield

comparable MIE results. Unlike E 582 for gases, E 2019

is a “performance standard,” whereby the methodology

adopted must produce data within the expected range

for a series of reference dusts. E 2019 describes several

suitable spark generating systems in an appendix, but

other circuits may be used if they produce the same

MIE values for the reference dusts. Attributed in part to

the high dependency of dust MIE on physical characteristics such as particle size distribution and (in some

cases) moisture content, plus the large ignition energies

involved relative to most gases, there has been considerable variability in published “dust ignition energy”

data even for pure chemicals such as sulfur. Apart from

physical differences between samples of the “same”

dust, the cause of much of the data scatter has been the

design of the electrical circuit and the calculation of the

energy released by the spark. It has been found that,

unlike the case with most gases, the ignitability of dusts

is dependent on the spark duration, which is directly

affected by the spark-generating system (see Eckhoff

[4], p. 407; and Eckhoff and Enstad [10]).

As alluded to above, many parameters affect the

measured MIE value. These include concentration,

electrical circuit, capacitors, inductance, electrodes/

spacing, ignition timing, turbulence, and the physical

characteristics of the dust (surface area, moisture level,

particle size). Therefore, it would be virtually impossible to find the absolute minimum energy required for

ignition. Given that, for all practical purposes, there is

no “absolute” value for dust MIE, it is difficult to assess

the accuracy of a particular value except by reference

to other values measured for an identical sample. Thus,

E 2019 is a performance-based standard to measure the

relative MIE of dusts. Calibration tests for each apparatus and circuit must be performed with at least three

different reference dusts. Reference data for four dusts

are listed in E 2019. The range of MIE values listed for

each dust provides some information on the typical

scatter in the data for different MIE circuits. With some

circuits, dust MIE values are determined only to within

a factor of about three. E 2019 allows the reporting of

a specific MIE value or the reporting of an MIE range,

whose ratio is a factor of about 3. For example, the MIE

of lycopodium powder can be reported as either about

20 mJ or 10 –30 mJ, using one of the circuits listed in E

2019. The method does permit interlaboratory differences as large as a factor of 10.

Examples of four suitable spark-generating systems

are described in Appendix X1 of E 2019. Three of these

are high-voltage capacitance circuits (10 –30 kV) triggered by (1) an auxiliary spark in a three-electrode

system, (2) a moving electrode, or (3) a slowly increasing voltage across the gap. The fourth circuit involves

energy storage at a lower voltage (<2.5 kV) and triggering by a low-energy, high-voltage pulse applied

across the gap. This pulse is generated by discharging

a small capacitor through a transformer whose secondary winding is connected in series with the spark gap.

Figure 2 shows the effect of particle size on the MIE

of a dust.

Figure 2. MIE data from E 2019 showing effect of particle size for two materials.

The aluminum MIE data (diamonds) are from

Eckhoff ([4], p. 33), and are plotted vs. the particle size

of the samples. The zinc stearate data (circles) are from

an unpublished ASTM E27.05 round-robin study

among four different labs, all using the slightly modified Eckhoff circuit, which is referred to in E 2019 as the

“Triggering by Auxiliary Spark.” The scatter of the data

points in Figure 2 is a good indication of the interlaboratory reproducibility of the MIE test results for samples

with narrow particle size distributions. A polydisperse

sample can be expected to show a wider range of MIE

values. As illustrated by the dashed lines through the
data points in Figure 2, the data sets for both dusts

appear to support the theoretical expectation that MIE

is proportional to approximately the cube of the particle size (see Eckhoff [4], p. 32).

Examples of Safety of Additives and Special Procedures

A choice of antioxidants for a granular polyolefin

production facility may represent different chemical

and physical options. One consideration is the MIE of

the additive compared with that of the polymer. If the

MIE is less than that of the polymer, the likelihood of

ignition will be increased in accumulation sites such as

baghouses. The likelihood of ignition may also increase in bins, especially if the additive MIE is <10 mJ.

Options include a different additive whose MIE is

greater when tested at the same particle size, or to

purchase an additive that is coarser. For example, a

sub-400 mesh additive, whose MIE is <10 mJ, could be

replaced by a coarser additive that contains no material

having an MIE < 20 mJ.

Many companies have adopted policies to restrict

handling of dusts below some prescribed MIE value.

They may also inert conveying systems and storage

bins for a dust with MIE below some value. The “action

level” adopted varies with operating experience but is

typically s10 mJ. This is the effective energy normally

ascribed to bulking brush discharges, which are believed responsible for dust ignition in properly

grounded equipment [11, 12].

Autoignition or Spontaneous Combustion

A series of ASTM E27 standards address spontaneous combustion of gas mixtures (E 659), dust clouds (E

1491), and dust layers (E 2021). There is no ASTM test

addressing spontaneous ignition of bulk solids. The

principal application of these test methods is to define

the maximum acceptable surface temperature in a particular area, usually for electrical classification purposes. Other applications include qualitative assessment of hazards inside process equipment rather than

external to it (such as bucket elevator bearings).

Autoignition Temperature of Liquids—E 659

ASTM standard test method E 659 measures the

“Autoignition Temperature of Liquid Chemicals.” A liquid or adequately volatile solid sample is added to an

open-neck glass flask held inside a furnace. The quantity of added sample is systematically varied to determine the lowest temperature at which hot-flame ignition occurs, based on visual observation. This is the

autoignition temperature (AIT). Additional test observations that may be reported are the lowest temperature at which cool flames are observed (cool flame

temperature (CFT) [13]) and the lowest temperature at

which exothermic, nonluminous preflame reactions are

observed [reaction threshold temperature (RTT)]. The

AIT is the most widely used of these parameters and is

commonly listed in data compilations.

Approximately 30 years ago, ASTM E 659, which

uses a spherical 500-mL flask, replaced ASTM D 2155,

which used a smaller 200-mL flask. AIT values measured in the 500-mL flask can be 210° C lower, largely

because of a higher ratio of heat generation to heat

removal in the larger flask. However, another reason

for adopting the larger 500-mL borosilicate glass flask

was to reduce catalytic wall effects, which could lead to

spuriously low AIT values for certain chemicals. Because the 500-mL flask size has not been adopted

everywhere, two disparate databases continue to be

generated.

In practice the AIT is affected by any factor altering

the relative rates of heat generation and removal, such

as the size, shape and surface material of an enclosure,

convective conditions and residence time, plus the

composition and pressure of the reacting mixture.

Therefore autoignition data must be applied carefully.

Several papers have been written on this subject [14–

22]. Large-scale equipment presents a greater hazard

than small-scale equipment because heat generation

increases faster than heat removal (or natural losses) as

the scale increases. For large-scale chemical processes,

the CFT and RTT parameters may also need to be

considered. For instance, if a solvent vapor–air mixture

is heated in a large atmospheric process vessel, “cool

flames” might trigger two-stage ignition at much less

than the “standard” AIT. The CFT may be on the order

50 –100° C less than the AIT, whereas exothermic preflame reactions may occur at appreciable rates at still

lower temperatures. The situation becomes more complicated if pressures other than atmospheric are involved because the effect of pressure can be highly

irregular. In general, expert advice should be taken

when assessing situations involving large scale or high

pressure.

Example of Applying Equipment “T Code” for Gases and

Vapors

Article 500.8 of NFPA 70, also known as the National

Electrical Code, provides that “Class I equipment shall

not have any exposed surface that operates at a temperature in excess of the ignition temperature of the

specific gas or vapor.” The AIT measured using E 659 is

often used directly for evaluating the hazard of autoignition arising from electrical equipment. In this case the

measured AIT is considered synonymous with “ignition

temperature” in the field because it is tacitly assumed

that hot surfaces in a classified area cannot trap stagnant volumes of gas–air mixture significantly larger

than those used in the standard test and that no significant surface catalysis occurs.

AIT data compilations are given in NFPA 325 and

NFPA 497, “Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of

Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.” However, these data

compilations do not address mixtures and there is no

available method for combining individual component

data. Also, data compilations often use the lowest published AIT value, which can unnecessarily increase

operational costs in cases where a commercial-grade

chemical (such as hexane) displays a range of AIT

values, depending on the components actually present.

Finally, most of the compiled data predate the current

ASTM E 659 method and can involve tests conducted in

either larger or smaller vessels. It is therefore assumed
here that a new AIT determination is conducted using

ASTM E 659.

Table 500.8(B) of Article 500.8 of NFPA 70 shows a

list of 14 possible temperature classes (T codes) that are

to be marked on nameplates for Class 1 equipment.

Each T code has a corresponding “maximum safe operating temperature.” For example, code “T1” corresponds to a temperature of 450° C and code T6 to 85°

C. Article 500.8 provides that the maximum equipment

temperature, as indicated by the T code, not exceed the

ignition temperature of the specific gas or vapor to be

encountered. A particular item of equipment is often

available with a range of different T codes, depending

on the needs of the purchaser. Thus, the AIT of the

specific gas, vapor, or mixture present in an area can

determine which manufacturer or style of equipment is

purchased and at what cost.

A similar AIT application is to determine the maximum safe operating temperature of other hot surfaces

in a classified area, such as process or utility pipelines.

It might be determined that additional insulation is

required to reduce the external temperature. Provided

large stagnant pockets of gas–air mixture are not

trapped by the hot surfaces involved, the AIT is normally a conservative measure of the ignition temperature. Exceptions include certain reactive chemicals

whose ignition temperatures are lowered by rusty surfaces.

Dust Cloud Autoignition Temperature—E 1491

In ASTM standard test method E 1491 for “Minimum

Autoignition Temperature (MAIT) of Dust Clouds,” dust

is dispersed into a heated furnace set at a predetermined temperature. The dust cloud is exposed to the

heated furnace walls for several seconds. A visible

flame exiting the furnace provides evidence for ignition. The dust concentration and temperature are systematically varied to find the lowest temperature at

which self-ignition occurs for the most easily ignited

dust concentration. Four different furnaces are described in ASTM E 1491 as suitable for MAIT measurement: the 0.27-L Godbert–Greenwald Furnace, the

0.35-L BAM Oven, the 1.2-L Bureau of Mines Furnace,

and the 6.8-L Bureau of Mines Furnace. As for the dust

MIE standard, this is a performance standard. Any apparatus used must produce MAIT values within the

range of data listed for several standard reference dusts.

Each furnace yields slightly different MAIT data, with

the largest deviations occurring at the greatest MAIT

values. However, the lower MAIT range is of more

practical importance and here the agreement is much

better (for example, 265 : 25° C for sulfur). Comparison MAIT data for the four furnaces are listed in E 1491

and in publications by Conti et al. [23, 24]. In most

practical cases, such as dryers, the operating temperature is significantly below the measured MAIT of the

dust cloud.

Dust Layer Ignition—E 2021

ASTM standard test method E 2021 for “Hot-Surface

Ignition Temperature of Dust Layers” uses a constanttemperature hot plate to heat the dust layer on one side

only. It is based mainly on a test method described in

a publication by Miron and Lazzara [25]. Routine tests

use a 12.7-mm (0.5-in.) thick layer, which might simulate a substantial build-up of dust on the outside of hot

equipment. However, because the ignition temperature

normally decreases substantially with increasing dust

layer thickness, the method allows layer thickness to be

varied according to the application. This can depend

on the user’s housekeeping standards. Typical hotplate data are shown in Figure 3.

Figure 3. Example data from hot surface ignition

temperature test E 2021.

In E 2021, ignition is

defined as a temperature rise in the dust layer of at least

50° C above the hot-plate temperature or visible evidence of combustion, such as a red glow or flame. In

Figure 3, the dust layer did not ignite in a test starting at

an initial temperature of 280° C, but did ignite at an

initial test temperature of 290° C.

The dust layer ignition temperature measured with E

2021 is usually less than the dust cloud MAIT measured

with E 1491. This is because the duration of the layer

test may be hours as opposed to the few seconds that

a dust cloud is suspended in a furnace. However, some

exceptions occur. One possible reason for the cloud

MAIT being less than the layer ignition temperature is if

melting occurs. This excludes combustion air from a

layer but not from a suspension of dust particles. If the

layer ignition temperature is near the softening or melting point, increasing the layer thickness may promote

ignition rather than melting. Another possible reason

for a lower cloud ignition temperature is a dust with a

high thermal conductivity, such as some metal powders

[26]. The high thermal conductivity prevents the localized build-up of temperature within the layer.

Because of the great dependency of dust layer ignition temperature on test conditions, especially layer

thickness, data of uncertain origin should not be used

for direct comparison purposes. Indeed, many “dust

layer ignition temperature” data reported in older publications were determined using a basket held in the

so-called modified Godbert–Greenwald furnace [27], in

which a small sample was heated from all sides.

It should be noted that the lowest ignition temperature for a dust corresponds to bulk heating under

conditions that cause the dust to be heated from all

sides. This is the situation for very thick dust layers,

accumulations inside heated equipment such as dryers,
or large piles of dust. Neither the dust cloud MAIT nor

the dust layer hot-surface ignition temperature is appropriate for these scenarios. Instead, either scaled

isothermal basket tests or adiabatic calorimetry (with

an appropriate mathematical model tying the reaction

rate to heat and air flow) should be used. The Frank–

Kamenetskii equation is often used to model this situation [28].

Example of Applying Equipment “T Code” for Dusts

This is similar to the example for gas/vapor T code

but with two important differences affecting data application. These involve first, the definition of “ignition

temperature,” and second, the provision of a maximum

allowable surface temperature for dusts that may dehydrate or carbonize.

Article 500.8 of NFPA 70 provides that “The temperature marking specified in 500.8(B) shall be less than

the ignition temperature of the specific dust to be

encountered. For organic dusts that may dehydrate or

carbonize, the temperature marking shall not exceed

the lower of either the ignition temperature or 165° C

(329° F).”

The user of NFPA 70 is referred to a compilation of

“dust ignition temperatures” presented in Table 2-5 of

NFPA 499, Recommended Practice for the Classification

of Combustible Dusts and of Hazardous (Classified)

Locations for Electrical Installations in Chemical Process Areas, which states: “The ignition temperature of a

dust layer is a function of the type of dust and its

physical and chemical properties and is often less than

the cloud ignition temperature. The ignition temperature shown in Table 2-5 is the lower of the two.” The

data compiled in Table 2-5 of NFPA 499 are not derived

from consistent test methods nor were both dust cloud

and layer ignition temperatures available in all cases.

Ideally, both ASTM E 1491 and E 2021 should be run to

determine the lowest ignition temperature of a representative dust sample, addressing both chemical and

physical properties. Assuming that the layer ignition

temperature is less than the cloud MAIT, the layer

tested should be thick enough to represent “worst credible case” accumulations. There is, however, no formal

guidance with respect to dust layer thickness used in

either NFPA 70 or NFPA 499. This is attributed to the

fact that, until recently, the majority of dust layer ignition temperature data were generated using the “modified Godbert–Greenwald furnace,” where the layer

thickness was not a test parameter.

3. FLAMMABILITY LIMIT TEST METHODS

The ASTM E27 Committee developed and now

maintains separate flammability test standards for gas/

vapor–air and dust–air mixtures. For gaseous fuel concentration limits, there is a lower flammability limit

(LFL) and upper flammability limit (UFL). For dusts,

only the lower flammability limit or minimum explosible concentration (MEC) is of practical importance

[29]. Although the term “LFL” has been more commonly

used for gases and the term “MEC” used for dusts, they

both refer to the lowest concentration of fuel that can

propagate a flame when mixed with air or another

oxidizer. In addition to the fuel concentration limits,

E27 has a standard for the limiting oxygen concentration (LOC). Oxygen concentrations above the LOC allow flame propagation and oxygen values below the

LOC prevent flame propagation. For the LFL, UFL,

MEC, or LOC, the flammability limit occurs when the

heat release rate is insufficient to allow self-sustaining

flame propagation through the fuel–air mixture. The

object of these E27 test methods is to determine scientifically sound values for flammability limits of materials. To apply these values to practical situations, appropriate safety margins are available in NFPA 69.

Flammability Limits of Gases and Vapors—E 681

and E 918

ASTM standard test method E 681 determines the

“Concentration Limits of Flammability of Chemicals

(Vapors and Gases)” at atmospheric pressure and temperatures up to 150° C. It aims to simulate the behavior

of large volumes of test mixture with respect to the

lowest (LFL) and highest (UFL) fuel concentrations that

can propagate a flame through a homogeneous mixture with air (or other oxidant no stronger than air). For

most fuels, a 5-L spherical glass test vessel is used. For

fuels that have large quenching distances and that may

be difficult to ignite, a special procedure in Annex A1 of

E 681 recommends the use of a 12-L spherical flask.

The gaseous mixtures are subjected to electrical spark

ignition source, and the absence or the presence of

flame propagation is determined visually. Recognizing

the subjectivity of the visual flame propagation assessment for near-limit mixtures, some laboratories use

video-recording systems for subsequent analysis. Although lower and upper flammability limits have already been determined for many common fuels (see

NFPA 325 or MSDSs), the method is still useful for new

products, such as the new ozone-friendly refrigerants,

and for some mixtures. In many cases, Le Chatelier’s

linear mixing law (see Kuchta [5], p. 14) is appropriate

for estimating the LFL values of gas mixtures. Appropriate safety margins for the practical application of LFL

and UFL data are available in NFPA 69.

ASTM standard practice E 918 is used for “Determining Limits of Flammability of Chemicals at Elevated

Temperature and Pressure.” It can be used at initial

temperatures up to 200° C and initial pressures to 1.38

MPa (200 psia). The pressure resistant metal test vessel

is a vertical cylinder, 1-L in size, which is smaller than

that used in E 681. However, vessel size effect may not

be as important for tests at high initial pressure because

the quenching distance decreases with increasing initial pressure. Electrically exploding fuse wire is used as

an ignition source. E 918 uses a pressure criterion for

flame propagation, which is much less subjective than

the visual method used in E 681. Flame propagation is

defined as a combustion reaction that produces at least

a 7% rise above the initial absolute test pressure. The

data from this test can be useful for evaluating the

hazards in process vessels that operate at elevated

temperature and pressure.

A review article describing the evolution of flammability test methods, recently published by Britton [30],

indicates that current European test methods are generating erroneously wide “flammable limits” by “over­
driving” small, vertical test vessels. The lower flammable limits (LFLs) of organic fuels in air agree quite well

with the predictions of a “heat of oxidation” model [9],

provided the LFL data are generated in a vessel of

adequate size such as required in ASTM E 681. The

absolute error caused by “overdriving” the ignition process in small test vessels is far greater at the UFL than at

the LFL. This can cause unnecessary expense when the

data are applied to large or high-throughput systems

that are intentionally operated above the UFL.

Example of Deflagration Prevention by Control of

Combustible Concentration

The objective is to operate outside the flammable

range (below the LFL or above the UFL) using appropriate safety factors (such as those in NFPA 69) to

mitigate errors in either the gas concentration monitoring or the flammable limit measurements. In some

process systems, the composition may vary both spatially and temporally, whereas there may be additional

monitoring instrumentation errors caused by calibration offset and/or time lag. Where such effects make it

impractical to operate above the gas mixture UFL, such

as in some vent-collection headers, it is common practice to add a sufficient concentration of “enrichment

gas” to cause the net fuel composition to always be

above the UFL, regardless of how the vent stream

composition may change. Similarly, some early suppression systems used fuel as the suppressant.

Chapter 6 of NFPA 69 provides that after the appropriate LFL for the combustible components has been

determined, addressing all operating conditions, the

combustible concentration shall be maintained at or

below 25% of the LFL. The exception for gas-phase

systems is where automatic instrumentation with safety

interlocks is provided, in which case the concentration

shall be maintained at or below 60% of the LFL. This

method is termed “deflagration prevention by combustible concentration reduction.”

An alternative control method described in Chapter

5.7.3 of NFPA 69 permits operation above the UFL. This

is a common strategy for atmospheric storage of liquids

whose vapor pressures are high enough to maintain

vapor concentrations above the UFL under all operating conditions. This method is included under “deflagration prevention by oxidant concentration reduction”

in NFPA 69 because, arguably, when operating above

the UFL the concentration of oxygen available from

ambient air is insufficient to produce an ignitable mixture.

Specific guidance is given for near-atmospheric

vent-collection headers in Chapter 5.7.3.2 of NFPA 69.

Addition of at least 25 vol % of methane or natural gas

can be used to render the gas stream nonflammable,

provided no component of the stream has a UFL

greater than that of hydrogen in air (75 vol %) and the

vent header does not contain oxygen at a concentration

greater than can be derived from ambient air. Marine

vent-collection headers can also be rendered nonflammable by the addition of enrichment gas but the precise

requirements are covered separately in Title 33.CFR

(Part 154).

Flammability Limits of Dust Clouds—E 1515

ASTM standard test method E 1515 determines the

“Minimum Explosible Concentration (MEC) of Combustible Dusts.” Flammability limits, such as the MEC,

are used to characterize safe operating conditions in

industrial processes or plants. If the operating conditions and maintenance procedures can be maintained

well below the MEC, then the hazard may be considered to be minimal. In the extreme, the E 1515 test may

identify a dust as being inert or nonignitable at any

concentration at the energy level used in the test. In

practice, the MEC can be used to indicate the likelihood

of explosions arising from accumulations of fugitive

dust. These dust accumulations, if dispersed, may create a significant dust explosion hazard.

The testing procedure for E 1515 involves the dispersion of dust into a 220-L vessel and the initiation of

a pyrotechnic ignitor. To determine the MEC, the dust

concentration is reduced systematically until no deflagration occurs. The criterion for significant flame propagation is a pressure rise of at least 1 bar. This pressure

criterion for dusts is much higher than that for gases in

E 918 because a much stronger ignitor is used for the

dust tests. The stronger ignitor is required because

dusts are intrinsically much more difficult to ignite than

gases. The criterion used in the 20-L chambers is based

on comparison MEC tests in a much larger 1-m3

(1000-L) chamber in a publication by Cashdollar and

Chatrathi [31]. Examples of E 1515 data for two different

dust samples are shown in Figure 4, where the corresponding MEC values are 30 and 80 g/m3

.

In MEC testing, the selection of the appropriate ignitor energy can be difficult, especially for hard-toignite materials. The importance of differentiating flammability limits from ignitability limits is discussed in E

1515 and Cashdollar and Chatrathi [31]. Therefore, tests

may need to be repeated using different energy ignitors. If there is an uncertainty whether a strong ignition

source is overdriving a deflagration in a 20-L chamber,

E 1515 recommends that additional tests be conducted

in a larger 1-m3 chamber.

As seen in Table 2, the MEC method enjoys frequent

use. Chapter 6 of NFPA 69 discusses explosion protection by control of dust concentration, including the

appropriate safety margin below the measured MEC.

Although it does not specifically reference E 1515, this

would be the applicable test method for the MEC of the

dust cloud for NFPA 69. E 1515 is identified as the

standard test method in NFPA 654, Standard for the

Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible

Particulate Solids; NFPA 664, Standard for the Prevention of Fires and Explosions in Wood Processing and

Woodworking Facilities; and FM Data Sheet 7-76.

Example

Before installing vents in their dust collector, a company desired to determine whether an explosion hazard did exist. An MEC test was conducted on the explosive ingredient being used, and a value of 30 g/m3

was found (see Figure 4). This is relatively low for a

MEC and indicates the high hazard associated with this

material. Engineering calculations and material recov-
ery data indicated that the dust concentration in the

collector could exceed this level, particularly during

pulsing. Consequently, the decision was made to install

explosion vents on the dust collector.

Figure 4. Typical data for determination of MEC for two dusts, using E 1515.

Limiting Oxygen Concentration for Gases—

E 2079

ASTM standard test method E 2079 was published

recently to measure the “Limiting Oxygen (Oxidant)

Concentration in Gases and Vapors.” Being a new

method, E 2079 has been designed to address some

problems associated with the E 681 and E 918 test

methods [32]. E 2079 requires the test vessel to be at

least 4 L in volume and to be nearly spherical. The

ignition sources include a 10-J electric spark, a continuous electric arc, and a fuse wire. Where necessary, the

use of stronger igniters is permitted. A 7% pressure rise

criterion is generally used to establish flammability. E

2079 also provides advice on how to modify this pressure criterion when using the stronger igniters.

To determine the LOC, the fuel gas concentration

and the oxygen concentration are systematically varied

to find the limiting oxygen value at the most easily

ignited gas mixture. The reported LOC value is the

average of the lowest oxygen concentration that produced a deflagration and the highest oxygen concentration that did not. Chapter 5 of NFPA 69 addresses

deflagration prevention by oxygen control, specifying

appropriate safety factors when using LOC data, and it

references E 2079.

The E27.04 subcommittee is also considering the

development of a new standard test method to determine the lower and upper flammability limits of gases

by using vessel size, ignition strength, and pressure

criteria concepts similar to those in E 2079.

Limiting Oxygen Concentration of Dust

Clouds—WK1680

ASTM Work Item WK1680 is a proposed or draft

standard test method for “Limiting Oxygen (Oxidant)

Concentration of Dust Clouds.” A brief description and

rationale for the LOC Work Item are listed at the ASTM

E27 website. The proposed LOC for dusts is a working

draft document available only to E27.05 subcommittee

members. The LOC method is used to identify safe

operating conditions for dusts when the oxygen concentration is controlled. In brief, the test method involves the dispersion of a dust into a chamber containing a gas mixture with a reduced oxygen concentration.

The test procedures and propagation criterion are similar to those in E 1515. When determining the LOC, the

oxygen concentration is gradually reduced until the

dust cloud no longer ignites. To determine the LOC, the

dust concentration and the oxygen concentration are

systematically varied to find the limiting oxygen value

at the most easily ignited dust concentration. The proposed method is based on comparison LOC data from

20-L and 1-m3 chambers in a publication by Going et al.

[33]. For dust systems where explosion venting or suppression is not practical, the reduction of the oxygen

level below the LOC can be a useful protection measure. Advice on the appropriate safety margin to use in

industrial plants is available in NFPA 69.

Temperature Limits

Flash point is considered a material hazard property,

perhaps the most frequently used in fire protection

engineering. ASTM defines flash point as “the lowest

temperature, corrected to a pressure of 101.3 kPa, at

which application of an ignition source causes the

vapors of the specimen to ignite under specified conditions of test.” As this definition suggests, the flashpoint values can be significantly influenced by the test

apparatus and the test method. In general, the opencup flash-point tests yield higher flash-point values

than those in the closed-cup tests. It is also important to

recognize that significant differences may exist even

among the values obtained from different closed-cup

test methods. Unfortunately, flash-point data reference
tables (such as NFPA 325) are populated with values of

unknown origin, perhaps because of the varying influences of different industries in the consensus standardmaking processes. For example, NFPA 30, Flammable

and Combustible Liquids Code, defines flash point as

“the minimum temperature of a liquid at which sufficient vapor is given off to form an ignitable mixture

with the air, near the surface of the liquid or within the

vessel used, as determined by the appropriate test procedure and apparatus specified” in that standard. NFPA

704, Standard System for the Identification of the Hazards of Materials for Emergency response, on the other

hand, does not specify a test method, and defines flash

point as “the minimum temperature at which a liquid or

a solid emits vapor sufficient to form an ignitable mixture with air near the surface of the liquid or the solid.”

The ASTM E27 Committee has two standards to address

some of the problems in this arena.

Flash Point Test Method Selection and Use—

E 502

ASTM E27 Standard Test Method E 502 for “Selection

and Use of ASTM Standards for the Determination of

Flash Point of Chemicals by Closed Cup Methods” gives

advice for the use of flash-point methods developed by

other ASTM committees. Despite being one of the earlier standards of the E27 Committee, E 502 is still widely

used and referenced today (see for example, NFPA 1,

30, 35, 77, 115, 385, and 704). This test method covers

the determination of the flash point of liquid and solid

chemical compounds flashing from below -10 to 370°

C (16 –700° F). E 502 uses the procedures and apparatus in ASTM Test Methods D56, D93, D3278, D3828,

and D3941. It provides additional explanatory notes

and procedure modifications not contained in the individual methods. E 502 also permits determination of

flash point for solids and highly viscous liquids. For a

given fuel viscosity and anticipated flash-point range,

the user is advised which particular test is suitable.

E 502 offers a valuable discussion, providing insight

into some of the dangerous or curious results obtained

from flash-point tests. First, the flash point does not

represent the minimum temperature at which a material can evolve flammable vapors. With the exception

of certain equilibrium test methods, most flash-point

tests are run at a finite heating rate, and thus vapor

concentrations are not representative of equilibrium

conditions. Flash-point testing uses downward and

horizontal propagation of flame. Flame propagation in

these directions generally requires slightly higher vapor

concentrations than those required for upward flame

propagation. The flame is introduced at a finite distance above the liquid surface. Because the vapors are

denser than air, the vapor concentration is slightly

higher at the liquid surface than that at the flame position. There are instances with pure materials where the

absence of a flash point does not ensure freedom from

flammability. Included in this category are materials

that have large quenching diameters (such as halogenated hydrocarbons or certain aqueous acetic acid solutions). Such materials will not propagate a flame in

any apparatus the size of a flash-point tester. However,

its vapors may be flammable and may burn when

ignited in a larger process or storage vessel. Some

materials having very dense vapors, a narrow range of

flammability, or the requirement for being somewhat

superheated to burn will not exhibit a flash point with

conventional test methods, but can form flammable

vapor–air mixtures if heating and mixing are optimum

and the temperatures are raised.

In specific instances, contrary to usual behavior, the

open-cup flash point may be at a lower temperature

than the closed-cup flash point. Conventional flashpoint tests can also introduce large errors when the fuel

is a mixture or contains flammable impurities. A liquid

containing flammable and nonflammable components

can evolve flammable vapors under certain conditions

while not exhibiting a closed-cup flash point. This

phenomenon is observed when a nonflammable component is sufficiently volatile and present in sufficient

quantity to inert the vapor space of the closed cup. In

many instances, liquids of this type will exhibit an

open-cup flash point. In certain cases the material may

exhibit no flash point, either open or closed, but when

spilled it may become flammable after the nonflammable component has evaporated. Therefore, it is important to test samples that have been weathered to an

extent comparable to the material in the process

stream. It is also important to keep in mind that liquids

containing a highly volatile nonflammable impurity,

which exhibit no flash point because of the influence of

the nonflammable material, may form flammable mixtures if totally flash vaporized in air in the proper

proportions. Some mixtures of water and hydrocarbons, or low-volatility halogenated hydrocarbons and

volatile hydrocarbons, may have low flash points but

will not of themselves sustain burning. These materials

can present explosion hazards in closed vessels but will

not burn as a pool fire out in the open. Therefore, for

process and storage vessels, E 502 recommends the use

of E 1232, which is described below.

Temperature Limit of Flammability of

Chemicals—E 1232

ASTM standard test method E 1232 measures the

“Temperature Limit of Flammability of Chemicals.” The

temperature limit of flammability test measures the

minimum temperature at which liquid or solid chemicals evolve sufficient vapors to form a flammable mixture with air under equilibrium conditions. This test is

designed to remedy limitations inherent in flash-point

tests, and yields a result closely approaching the minimum temperature of flammable vapor formation for

equilibrium situations in the chemical processing industry, such as in process vessels, storage tanks, and

similar equipment. This test also allows the use of

oxidant/diluent mixtures other than air, which need to

be evaluated in cost-saving partial-inerting applications.

The result of the E 1232 test is deliberately differentiated from the flash point by using the term “lower

temperature limit of flammability (LTL).” For standard

test method E 1232, the LTL is defined as the lowest

temperature (corrected to standard atmospheric pressure of 101 kPa) at which application of an ignition

source causes a homogeneous mixture of a gaseous
oxidizer and vapors in equilibrium with a liquid (or

solid) specimen to ignite and propagate a flame away

from the ignition source. Unlike flash point, LTL is a

true material property in most circumstances. The difference between the flash point and LTL increases as

the flash point increases. It is important to recognize

that LTL can be tens of degrees Celsius lower than the

closed-cup flash point.

The E 1232 test apparatus is similar to that described

in E 681. The 5-L test flask is placed in a heated

chamber to ensure a controlled and uniform temperature. A pool of liquid sample is stirred in a closed vessel

in an air atmosphere. The vapor–air mixture above this

liquid is exposed to an ignition source and the upward

and outward propagation of flame away from the ignition source is noted by visual observation. Temperature

in the test vessel is varied between trials until the

minimum temperature at which flame will propagate

away from the ignition source is determined. A striking

comparison of this apparatus with the common closedcup flash-point test apparatuses can be found in Table 3.

Table 3. Comparison of E 1232 test apparatus with commonly used closed-cup flash-point test apparatuses.

D56 Tag D93 D3828

Method/Apparatus Closed Pensky–Martens Setaflash E 1232 LTL

Test volume shape Cylinder Cylinder Cylinder Sphere

Nominal vapor space volume, mL 67 46 19 5000

Sample volume, mL 50 75 2 or 4 50 or more

Stirrer No Yes No Yes

Heating rate finite finite equilibrium equilibrium

Ignition source flame flame flame spark

Ignition location top top top below center

Temperature limit of flammability results obtained

by E 1232 are consistent with vapor pressure and concentration limit of flammability data. This provides a

built-in check of results on pure materials that produce

vapors obeying the ideal gas law at the test conditions,

and leads to a high degree of confidence in the results

obtained for mixtures. In Figure 5, from ASTM subcommittee data by Bodman et al. [34], experimental LTL

values for a number of pure chemicals obtained using

E 1232 show good agreement with the theoretical values, which are the computed temperatures at which the

vapor pressure equals the LFL.

Although not specifically listed in the E 1232 test

method, a modification of the E 1232 procedures has

also been used to measure the upper temperature limit

of flammability (UTL). Experimental data for UTL are

compared with the calculated values in Figure 5.

Figure 5. Comparison of experimental LTL values from E 1232 with theoretical values. UTL values are also

given, although the method for determining UTLs is not specifically covered by E 1232.

This

capability, which is not available with conventional

flash-point test methods, is useful for applications

where explosion protection is achieved by operating

above the UFL.

If the estimated LTL is significantly different from the

temperature at which the LFL was measured, a correction to LFL may be required in the prediction of LTL.

This is because LFL is somewhat dependent on tem­
perature. Certain chemicals (such as organic acids) exhibit a high degree of nonideal vapor-phase behavior,

being highly associated in the vapor phase. To properly

interpret the vapor pressure/volume percentage/

weight percentage/temperature relationships, knowledge of the degree of nonideality (association and

molecular weight) is necessary. (Concentration limits of

flammability for these materials are generally expressed

in terms of weight per unit volume.) The fact that E

1232 combines all-important aspects and bypasses

such difficulties associated with predictions makes E

1232 a useful tool for hazard and consequence analysis,

particularly for mixtures where accurate estimation of

vapor pressure is often difficult.

4. TEST METHODS FOR MAXIMUM EXPLOSION CHARACTERISTICS

There are two ASTM E27 test methods for measuring

the maximum pressure and rate of pressure rise (dP/dt)

for dusts. The first is E 789, which uses a 1.2-L chamber

and spark ignition. It can provide some information on

the ignitability of dust clouds by a continuous electric

arc, but the data are not recommended for explosion

protection design. It has been kept by E27 mainly for

historical reasons. The second method is E 1226, which

uses a 220-L chamber and much stronger ignition

sources. The data from it are recommended for use in

explosion vent sizing by NFPA 68, Guide for Venting of

Deflagrations.

There is currently no comparable ASTM test method

for maximum pressure and rate of pressure rise for

gases and vapors. Nonetheless, NFPA 68 uses the gas/

vapor deflagration index value (KG) for deflagration

vent sizing for gases and vapors. The KG value depends

on dP/dt as does KSt for dusts (see the definition of KSt

below).

It is well known that, when an atmosphere of a fuel

and air is ignited, rapid combustion can occur. If this

occurs in a confined space, the pressure increases in

response to the rapid temperature rise. The result can

be a destructive explosion. The requirements for this to

occur are well known: fuel, oxidant, mixing, ignition,

and confinement. The fuel needs to be dispersed or

mixed with the oxidant. In the case of a solid, the fuel

must be of sufficiently small particle size to be dispersed and burn rapidly. The maximum pressure developed and the rate of pressure rise are important

input data for explosion protection design.

Pressure and Rate of Pressure Rise for Dusts—

E 789 and E 1226

ASTM standard test method E 789 for “Dust Explosions in a 1.2-Liter Closed Cylindrical Vessel” is based

on a 1950s Bureau of Mines test method described in a

report by Dorsett et al. [27]. It is limited by the size and

cylindrical shape of the test vessel and by the relatively

weak electric ignition source. The data from this

method are used as part of the criteria to identify

combustible dusts for NFPA 70 area classification for

dusts. E 789 says that the data should not be used for

vent sizing or explosion containment design.

ASTM test method E 1226 for “Pressure and Rate of

Pressure Rise for Combustible Dusts” was developed to

correct the limitations of E 789. The data from E 1226

are recommended for deflagration vent design in NFPA

68. The test procedures are similar to those in ASTM E

1515. A well-dispersed dust cloud is formed in a 220-L

chamber, nearly spherical in shape, and subjected to a

strong pyrotechnic ignition source. A dust cloud is

formed using the discharge of a small cylinder of compressed air. After a prescribed time delay, the highly

turbulent dust cloud is ignited using a strong ignition

source of known reproducible energy release characteristics. The pyrotechnic ignitors cause a significant

pressure and this must be accounted for, especially

when using a strong ignition source in a small chamber.

For example, two 5-kJ igniters used in a 20-L chamber

by themselves will cause an increase in pressure of

about 1 bar. Appendix X1 of E1226 discusses corrections that can be made to Pmax data generated in 20-L

chambers using strong ignition sources. In some cases,

it may be necessary to perform larger-scale testing to

obtain reliable data.

The resulting pressure and rate of pressure rise are

measured by appropriate instrumentation. Figure 6

shows typical test data from E 1226 for an individual

test at a particular concentration of a particular dust.

The rate of pressure rise and the explosion pressure are

plotted vs. time. To obtain the maximum explosion

data for the dust, tests are conducted over a wide range

of dust concentrations (Figure 7). The peak values of

Pmax and (dP/dt)max from individual tests (for example,

as shown in Figure 6, which are from a test at 500 g/m3)

are plotted in Figure 7. Then the explosibility parameters Pmax (maximum pressure) and (dP/dt)max (maximum rate of pressure rise) are found at the worst-case

dust concentration. The rate of pressure rise data are

then size-normalized by the following equation:

K V1/3 St = (dP/dttmax

where KSt is the deflagration index and V is the chamber volume.

As with the other dust test methods, E 1226 is a

performance standard. One suitable test chamber is

listed in the appendix, but other chambers can also be

used if their data match the data from a series of

reference dusts. The test procedures in standard E 1226

are based on extensive comparison data between 20-L

and 1-m3 chambers by Bartknecht [35] and Siwek [36].

The primary use of the test data Pmax and KSt is for

the design of explosion protection systems: containment, venting, suppression, or isolation. Vent designs

provide a relief area that will eliminate damage or limit

damage to the process equipment to an acceptable

level. The required vent area is calculated using equations from NFPA 68 and requires knowledge of the

process (volume, temperature, operating pressure, design strength, and vent relief pressure) and of the fuel

characteristics (Pmax and KSt). The safety of the design

is only as good as the explosibility data. Suppression is

the active extinguishment of the combustion and again

limits the explosion pressure to an acceptable level.

Suppression designs require similar process and hazard

data to determine the hardware requirements such as

size, number, and location of suppressors; detection
parameters; and the maximum pressure caused by the

protected event. Isolation, which is the prevention of

flame propagation through interconnections, requires

the same process and hazard data to determine hardware needs and locations.

Figure 6. Data from E 1226 for pressure and rate of pressure rise for a dust explosion.

Figure 7. Rate of pressure rise and pressure vs. dust concentration from E 1226.

From Table 2, E 1226 appears to be the most fre­
quently used ASTM E27 method for dusts. It is cited in

numerous NFPA documents (1, 61, 68, 484, 654, 664,

and 921) and Factory Mutual documents (Data Sheets

7-36 and 7-76). The KSt and Pmax data are also used in

hazard assessment, forensic analysis, and consequence

modeling. They are also used to estimate the deflagration burning velocity, flame speeds, and induced gas

and particle velocities [37, 38].

Example

A pharmaceutical company is processing an explosible ingredient through a spray dryer, cyclone, and

dust collector. The cyclone is used for collection of the

bulk of the material and the dust collector retains the

fines. It is located outdoors and requires protection by

venting. The only material available for testing was

collected from the cyclone and gave a Pmax value of 8.1

barg and a KSt value of 153 bar-m/s when tested by E

1226. When sieved to sub-200 mesh (<75 [m) and

tested, the material gave Pmax and KSt values of 8.3 barg

and 242 bar-m/s, respectively. From the dirty volume of

the dust collector of 250 ft3 and a design strength of 3.0

psig, the NFPA 68 equation for L/D < 2 was used to

calculate the required vent areas using 0.1 bar Pstat

vents. The required area was 4.5 and 7.2 ft2 for the

coarser and finer material, respectively. Because the

finer particles could be expected to be present in the

dust collector, the larger vent area was selected for the

design.

5. CONCLUSIONS

ASTM Committee E27 has produced and continues

to produce important standard methodologies that are

used for hazard evaluation. Dedicated use of these

standards offers the following benefits:

• More reproducible data as a result of well-defined

test methods.

• Reduced probability of incorrect data or data interpretation stemming from warnings on pitfalls

that may negatively affect data generation and

interpretation.

• The ability to verify adherence to test procedures

by comparing test results on standard materials

with those published in the Precision and Bias

sections of the E27 standards.

• Simpler data interpretation and application attributed to the number of guides, standards (such as

NFPA), and technical papers based on data from

E27 methods.

• Reduced probability of accidents during testing

arising from warnings regarding issues that may

negatively affect safety.

The E27 standards greatly enhance the ability of an

organization to produce accurate, meaningful data.

Consequently, those organizations that practice these

methods and apply the data to their processes have a

competitive edge in that they are able to avoid reactive

chemical accidents, fires, and explosions.

New technologies will require modification of existing test methods and development of new ones, and

therefore the E27 Committee will continue to be active.

Individuals knowledgeable in process safety testing are

encouraged to become part of E27. Individuals that

participate in E27 are able to influence the modification

and development of standards and, perhaps more important, are able to establish contacts and interactions

with individuals at other organizations. These contacts

yield ideas on improvement of process safety testing

methods and instrumentation. As with any volunteer

organization, a key to success is to have active, enthusiastic, dedicated, and knowledgeable members participating. The current emphasis within companies to

enhance performance in Environmental Health and

Safety coupled with the Process Safety Management

regulations (OSHA) makes the relevance of ASTM E27

activities more important than ever.

ACKNOWLEDGMENTS

The authors are indebted to ASTM International for

facilitating and promulgating the E27 standards. The

contributions of all ASTM E27 members from the past,

present, and future are gratefully acknowledged.

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