Abstract
This report describes a research program that was conducted to define natural gas contaminant levels necessary to insure that internal corrosion of compressed natural gas (CNG) cylinders does not constitute a hazard over the lifetimes of the cylinders. A literature search was performed and companies in the natural gas transmission and distribution industries were contacted: to identify and determine the composition ranges of contaminants in natural gases; and to obtain information regarding corrosion damage of CNG cylinders and cylinder materials. Corrosion and stress corrosion cracking (SCC) tests were performed on the cylinder materials most widely used in CNG cylinders in the United States (4130X and 15B30 steels and 6061-T6 aluminum alloy). Tests were conducted in: natural gases from several producing wells and from an interstate pipeline; and in aqueous solutions saturated with varying concentrations of natural gas contaminants. Also, metallurgical analyses of nine (eight steel and one aluminum), used CNG cylinders were performed. Limiting concentrations of hydrogen sulfide (H{sub 2}S), carbon dioxide (CO{sub 2}), and other CNG contaminants necessary to prevent internal corrosion of CNG fuel and storage cylinders were defined. This knowledge will minimize potential hazards of using CNG as a vehicle fuel. It should also lead to reduced costs of CNG use, since it has been shown that reduction of contaminants to the very low levels currently specified by the U.S. Department of Transportation (DOT) and the Canadian Transport Commission (CTC) is not necessary. A gas-quality standard based on program results is recommended. The National Fire Protection Association (NFPA) has adopted the recommended gas-quality standard.
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