Significance and Use
5.1 Knowledge of the limiting oxygen (oxidant) concentration is needed for safe operation of some chemical processes. This information may be needed in order to start up or operate a reactor while avoiding the creation of flammable gas compositions therein, or to store or ship materials safely. NFPA 69 provides guidance for the practical use of LOC data, including the appropriate safety margin to use.
5.2 Examples of LOC data applications can be found in references (3-5).
Note 2: The LOC values reported in references (6-8), and relied upon by a number of modern safety standards (such as NFPA 69 and NFPA 86) were obtained mostly in a 5-cm diameter flammability tube. This diameter may be too small to mitigate the flame quenching influence impeding accurate determination of the LOC of most fuels. The 4-L minimum volume specified in Section 7 would correspond to a diameter of at least 20 cm. As a result, some LOC values determined using these test methods are approximately 1.5 vol % lower than the previous values measured in the flammability tube, and are more appropriate for use in fire and explosion hazard assessment studies.
5.3 Much of the previous literature LOC data (6-8) were measured in the flammability tube.
5.4 Accepted LOC values (when nitrogen is the inert gas) determined for the five reference gases using these test methods in 20-L and 120-L test enclosures have been reported in Zlochower (9), and are summarized below:
Hydrogen—4.6 % in 120-L, 4.7 % in 20-L
Carbon Monoxide—5.1 % in 120-L
Methane—11.1 % in 120-L, 10.7 % in 20-L
Ethylene—8.5 % in 120-L, 8.6 % in 20-L
Propane—10.7 % in 120-L, 10.5 % in 20-L
Note 3: For carbon monoxide, results are sensitive to the humidity of the test mixture in the enclosure. Presence of a small concentration of water vapor facilitates combustion and promotes flame propagation by supplying the hydrogen (H) and hydroxyl (OH) free radicals for the chain branching reactions. For conservative results, provisions are made to humidify the test air to near saturation.
5.5 These test methods are often used to determine the LFL (lower flammability limit) and UFL (upper flammability limit) of gases and vapors initially at or near atmospheric pressure. Accepted LFL and UFL values determined for the five reference gases using these test methods have been reported in Zlochower (9).
5.6 These test methods are also used to determine the maximum content of flammable gas which, when mixed with specified inert gas, is not flammable in air (ISO 10156, CGA P-23).
5.7 A minimum purity of 99 % is recommended for the standard reference gases used for the commissioning (qualification) of the test apparatus and for the periodic verification of data quality.
Scope
1.1 These test methods cover the determination of the limiting oxygen (oxidant) concentration of mixtures of oxygen (oxidant) and inert gases with flammable gases and vapors at a specified initial pressure and initial temperature.
1.2 These test methods may also be used to determine the limiting concentration of oxidizers other than oxygen.
1.3 Differentiation among the different combustion regimes (such as the hot flames, cool flames, and exothermic reactions) is beyond the scope of these test methods.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 These test methods should be used to measure and describe the properties of materials, products, or assemblies in response to heat and flame under controlled laboratory conditions and should not be used to describe or appraise the fire hazard or fire risk of materials, products, or assemblies under actual fire conditions. However, results of this test may be used as elements of a fire risk assessment which takes into account all of the factors which are pertinent to an assessment of the fire hazard of a particular end use.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.7 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.
