Using Diesel Exhaust to Purge Gas Mains

When new gas mains are installed or existing mains removed from service, crews must purge the mains with an inert gas to eliminate the potential hazard of a combustible mixture. KeySpan Energy, as with most gas companies, typically purges mains with nitrogen. Exhaust from cars or trucks is a less costly inert gas than nitrogen and does not require the space or logistics needed for nitrogen tanks. However, when calculations by The Polytechnic University raised questions about the safety of using diesel or automotive exhaust, under KeySpan sponsorship GTI set out to determine the safety of using diesel exhaust to purge gas mains. GTI established that diesel exhaust can be used safely for such purges if the oxygen content is below 11%. Typically, exhaust is 9.5% oxygen at high speed.

Objective
The objective of this project is to determine the range of diesel exhaust/natural gas ratios that will support combustion over the temperature range of diesel exhaust.

The exhaust test system determines the safe range of temperature and diesel/gas ratios.

Background
Nitrogen is a safe purge gas, provided that it flows into the main quickly as a "slug," avoiding the risk of stratification. Pure nitrogen is safe because it contains no oxygen and thus cannot support combustion. Diesel exhaust, however, can contain from 9% to 13% oxygen, depending on engine operating conditions. Diesel exhaust may also contain unburned hydrocarbons and may have temperatures as high as 700°F; both conditions that can extend the range of flammability.

Even with very rapid purge gas flow in a gas-filled main, there will be a continuous transition region between pure natural gas and pure purge gas — allowing all possible ratios of methane to exhaust in the main being purged. Field crews typically have equipment to measure both temperature and oxygen content of a gas, and it is desirable to use these parameters to define safe conditions.

Technical Approach
GTI has constructed a unit that mixes natural gas with exhaust from a diesel engine. The gas streams can be mixed in any ratio, and the gas and exhaust streams can be heated or cooled as required to span the temperature range from ambient to the 700°F maximum when a compressor is running wide open. Gases can be sampled and their compositions analyzed to trace component levels.

Measurements are made to establish that the gas streams are well mixed. The mixed gases are then passed through a continuously active spark to determine whether they will support combustion.

KeySpan Energy has developed a simple, low-cost exhaust gas heat exchanger that reduces the exhaust gas temperatures to about 140°F. GTI is testing the full range of gas temperatures and compositions to develop a simple guideline for field crews to determine if it is safe to purge with diesel exhaust. The determination will be based solely on measurement of the temperature and oxygen content of the diesel exhaust.

Benefits
The ability to purge gas mains with diesel exhaust gas — which is readily available, low in cost, and easily generated — will reduce the overall cost and complexity of purging operations. Also, with no need for a compressed nitrogen tank at the purge site, purging operations can be conducted in a smaller area. And because the only measurements needed to determine safety are the exhaust gas temperature and oxygen concentration, this operation can be conducted easily by any field crew.

July 2004