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U. 9. BUREAU OF MINES, PITTSBURGH, PA.G a s companies using pipeline' natural gas generally provide emergency facilities which can replace or supplement their normal supply in the event of breakdown or abnormal demand. Oil gas and certain propane-air mixtures have been used for these purposes by the gas industry. A major limitation in the choice of a supplemental or substitute gas is the varying degree of interchangeability on appliances that had been adjusted on natural gas. A means of predicting interchangeability is therefore desirable. Recent studies have developed the fundamental theory underlying the stability of flames on burners and the entrainment of air by fuel jets. These principles were used to outline new concepts whereby burner performance with interchanged fuel gases may be predicted by caIcuIation. The body of data needed for HE interchangeability of fuel gases on consumer appliances T has long been a major problem of thegasindustryandisbeconiing more important as pipeline natural gas is being made available over the country. Gas companies using pipeline natural gas generally provide emergency facilities, which can replace or supplement their normal supply in the event of breakdown or frequent above-normal demand. They require means of predicting the behavior of new fuels on their burners, so that they may provide for satisfactory emergencv send-out gases. A method of predicting burner performance with interchanged fuel gases was outlined in two previous publications ( 2 , 5 ) . This method is based on theoretical principles making use of the concept of critical boundary velocity gradients that define the flamestability region of a fuel and the principles of air entrainment by a fuel jet. These are combined into approximate relationships, which predict the change in the over-all performance of burners on gas-utilities lines when fuel gases are changed. Data required are the density, gas-line pressure, and flame-stability diagrams of the fuel gases involved in the exchange.Oil gases and propane-air fuels are being used or considered by many gas utilities as a replacement or supplement for natural gas. The suitability of thecie gases for such purposes cannot be evaluated accurately at this time, because pertinent factors, such as the effects of hot flame ports, shallow ports, noncircular ports, insufficient secondary air, and yellow tipping on the interchangeability of gases, are not considered. These remain to be studied in the future. Furthermore, natural gases, oil gases, and propaneair fuels vary in composition, and therefore only the method of reasoning is strictly applicable to each exchange. This paper is essentially limited to a prediction of the likelihood of flash back and blowoff on consumer appliances when oil gases or propane-air fuels are interchanged with natural gases. Figure I may be considered to be the flame-stability diagram for natural gases (3). It was measured with the natural gas described in Table I. Diagrams such as Figure l and their use in application of this method to pr...

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