Ethanol Bacteria Create Pipeline Cracks

Wednesday, August 10, 2011 @ 03:08 PM gHale

With the growth of ethanol for fuel rising, existing gas pipelines might be an efficient alternative for moving this renewable fuel around the country.

The problem is ethanol, and especially the bacteria sometimes found in it, can dramatically degrade pipelines, said researchers at the National Institute of Standards and Technology (NIST).

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Bacteria that feed on ethanol and produce acid boosted fatigue crack growth rates by at least 25 times the levels occurring in air alone, NIST researchers said.

The NIST team used a new biofuels test facility to evaluate fatigue-related cracking in two common pipeline steels immersed in ethanol mixtures, including simulated fuel-grade ethanol and an ethanol-water solution containing common bacteria, Acetobacter aceti. Ethanol and bacteria can cause corrosion, but this is the first study of their effects on fatigue cracking of pipeline steels.

Micrograph of crack in X52 steel after the sample was in an ethanol solution containing acid-producing bacteria, Acetobacter aceti, for several days.

Micrograph of crack in X52 steel after the sample was in an ethanol solution containing acid-producing bacteria, Acetobacter aceti, for several days.

“We have shown that ethanol fuel can increase the rate of fatigue crack growth in pipelines,” said NIST postdoctoral researcher Jeffrey Sowards. “Substantial increases in crack growth rates were caused by the microbes. These are important data for pipeline engineers who want to safely and reliably transport ethanol fuel in repurposed oil and gas pipelines.”

Ethanol, an alcohol made from corn, sees use as a gasoline additive due to its oxygen content and octane rating. Ethanol can also be a fuel by itself in modified engines. The NIST tests focused on fuel-grade ethanol.

The team performed tests on X52 and X70 pipeline steels, which are alloys of more than a dozen metals. Simulated fuel-grade ethanol significantly increased crack growth at stress intensity levels found in typical pipeline operating conditions, but not at low stress levels. The cracking relates to corrosion. The X70 steel, which is finer-grained than X52, had lower rates of crack growth at all stress levels. This was because larger grain size generally reduces resistance to fatigue. In the bacteria-laden solutions, acid promoted crack growth at stress intensity levels found in typical pipeline operating conditions.

Preliminary tests also suggested that glutaraldehyde, a biocide used in oil and gas operations, may help control bacterial growth during ethanol transport.

The findings are the first from NIST’s biofuels test facility, where material samples are in hydraulic test frames and subjected to load cycles while immersed in fuel inside a transparent polymer tank. Fatigue crack growth and other properties are under observation over a period of up to 10 days. NIST staff expects to continue and possibly expand the research to other potential biofuels such as butanol or biodiesel.



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