The heat of oxidation is the net heat of combustion per mole of oxygen consumed. For complete combustion it is given by 'AH/S" (kcal/mol oxygen), where 'AH, " is the net heat of combustion (kcal/mol fuel) and "S" is the stoichiometric ratio of oxygen to fuel as written in the stoichiometric equation. Although previously unrecognized, the heat of oxidation 4fiH,/S" is the single most powerful parameter for evaluating the flammability hazards of fuels. %his article derives and discusses eight simple rules for estimating flammabilityparameters of single or mixed organic fuels in air under atmospheric conditions. %e rules apply to fuels containing combinations of C+H+O+N atoms that burn to carbon dioxide plus water Apart from the trivial First Rule, each rule i~ e.?cpressed in term ofthe heat of oxidation.Two Rules are presented for lowerflammable limit (LFL) estimation. %e First Rule is approximate and requires only the stoichiometric ratio of oxygen to fuel "S. "Although it outperforms Lloyd's Rule, it similarly overestimates the LFLs of energetic. fuels such as acetylene and ethylene oxide.The Second Rule requires both 3" and the net heat of combustion "AH. "It is shown that the LFL is inverselyproportional to theproduct of heat of combustion and beat of oxidation. LFL predictions are typically within experimental error of reported values, including energetic fuels such as acetylene and ethylene oxide.%be Third Rule shows that lower limitflame temperatures decrease linearly with increased heat of oxidation and increase stepwise between hydrocarbons and other organic fuels containing oxygen and nitrogen atoms.The Fourth Rule shows that the Limiting Oxygen Concentrations (LOCs) of non-decomposable fuels decrease with the inverse square of beat of oxidation. It is proposed that the optimum fuel concentration at the LOC is related via diffusivity ratio to the stoichiometric concentration. A fuel-oxygen Cartesian flammability diagram is suggested forpresenting LOC data.The Fijth Rule shows that maximum flame temperatures increase linearly with increased heat of oxidation and decrease stepwise between hydrocarbons and other organic fuels containing oxygen and nitrogen atoms. l;be Sixth and Seventh Rules address fundamental burning velocity "S,." and lowest minimum ignition energy (MIE), second order polynomial expressions being presented for easily calculating these parameters. It is shown how the calculated S can be used to estimate normalized rate of pressure rise 'K." data. It is also demonstrated that many published LMIE and quenching distance values are too high.The Eighth Rule gives a tentative expression for quenching distance, which decreases lineady with increased heat of oxidation. A scheme is presented for ranking fuel hazards in terms of heat of oxidation. Demarcations are made in terms of S,,, LMIE and Class I NEC and Zone Groups. It is shown how the approach might be extended to deflagration and detonation arrester selection.

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.

Flammability: The “net heat of oxidation” technique described in an earlier publication is extended to predicting the lower flammable limits, lower limit flame temperatures, and limiting oxygen concentrations of chlorinated organic fuels having H:Cl ratios greater than unity. A new Rule is derived for predicting the effect of initial temperature on the lower flammable limits and limiting oxygen concentrations of organic fuels. It is suggested that this Rule be used in preference to the modified “Burgess‐Wheeler” Rule. The effect of initial pressure is discussed. Instability: Net heats of oxidation (kcal/mol oxygen) for a series of disparate fuel groups are compared with “ΔHD,” the maximum heat of decomposition (cal/g) calculated using CHETAH methodology. Given the reasonable assumption that CHETAH's “maximum heat of decomposition” cannot exceed the net heat of combustion “ΔHC,” examination is made as to whether the ratio of these parameters (each expressed in units of kcal/mol), coined the “Reaction Heat Ratio” (RH), provides a useful new indicator for instability assessment. Of these parameters, the net heat of oxidation (ΔHC/S) is the best indicator to help assign NFPA Instability Ratings. However, ΔHC/S cannot generally be used to assign ratings for organo‐peroxides. Also, its performance as an indicator for hazardous polymerization depends on the ΔHC/S difference between the reacting monomer and the polymer product, so it should become increasingly unreliable as the monomer ΔHC/S approaches ‐100 kcal/mol oxygen. The ranking method tacitly assumes organic polymers to have a constant heat of oxidation of about ‐100 kcal/mol oxygen. Errors in this assumption must invalidate the ranking approach where ΔHC/S differences are small. Finally, separate “cut‐offs” must be used at each NFPA Instability Rating for organo‐nitrates versus other organics containing combinations of CHON atoms. Additional materials need to be examined to extend this preliminary analysis. The net heat of oxidation would be a useful additional output parameter of the CHETAH program, if only for its application in flammability assessment. No conclusions are drawn regarding the usefulness of net heat of oxidation or RH in conducting CHETAH hazard assessments, since this procedure requires consideration of several variables. However, the analysis may be helpful to the ASTM E 27.07 subcommittee responsible for developing the program. For example, the ‐ΔHD ⩾ 700 cal/g cut‐off used to assign a “high” CHETAH hazard rating typically corresponds to organic materials rated NFPA 1, the second to lowest hazard rating.

l%is article discussesflammable limit test methods with r#erence to the past 200 years of research. Critical examination is made of the various pressure rise criteria currently used to determine whetherflame propagation has occurred in closed test vessels. Of these, the most appropriate, wilh respect to uesseki of 5-20 liters volume, is judged to be the "net 7%"pressure rise criterion described in AS71M E-2079. Since the tests are typically carried out in spherical vessels with central ignition, only a small fraction of the limit mixture bums and consequently a small pressure rise criterion must be used. Akio, it is impossible to determine the product distribution for near-limitflames. A small pressure rise criterion may cause thejlammable range to be overestimated, particularly at the upper limit. For non-routine and standardization pupses at least, use of a tall, large diameter, cylindrical vessel with bottom ignition is suggested.European test metboa3 are creating a database offlammable limit values outside the range of compositions at which flames can self-propagate. l%e objective offlammable limit measurement should be to obtain close agreement with largescale observations and, ultimately, with theoretical modeki for se@ropagatingflames. It is recommended that 'jlammable limits" measured using DIN 51649 and European Standard prEN18.39 (or closed vessel methods striving to obtain values consistent with these) not be mixed with the existing database because they corespond to fuel concentrations that do not sup-pofi&mepropagation in any orientation.

Britton discovered that with increased "net heat of oxidation" (DH ox ), the maximum flame temperatures of CH and CHO fuels in air increase linearly while flame temperatures at the lower flammable limit (LFL) decrease linearly. Maximum flame temperature is a major factor determining the combustion rate of optimum fuel-air mixtures and relationships were found between DH ox and "optimized" flammability parameters such as minimum ignition energy. The LFL is the fuel concentration needed to attain the lower limit flame temperature; since less fuel is needed to attain a smaller flame temperature, the LFL varies inversely with DH ox . Simple expressions derived between DH ox and parameters commonly used in process safety were previously published in this journal. The commercially available computer program "CHETAH TM " now solves these expressions and outputs the flammability parameters plus the internally generated thermodynamic data used in the solutions. This article updates the original expressions together with new findings and explanatory material.