New Measurement Data Has Implications For Quantifying Natural Gas Losses From Cast Iron Distribution Mains

EPA/GRI Study
By Carey Bylin, Luigi Cassab, Adilson Cazarini, Danilo Ori, Don Robinson and Doug Sechler | September 2009 Vol. 236 No. 9

Figure 2: Comgas inserts plastic pipe

The loss of natural gas from oil and natural gas systems is a global environmental concern given that methane, the primary component of natural gas, is a powerful greenhouse gas over 20 times more potent than carbon dioxide.

This loss has economic, efficiency, safety, and energy supply implications, in that a valuable, non-renewable hydrocarbon resource is being lost forever.

Natural gas is most valuable when it has been processed to pipeline quality and transported to local distribution systems for delivery to end users. A significant source of natural gas losses from distribution systems is cast iron distribution pipes. U.S. cast iron distribution mains are estimated to have leaked 9 billion cubic feet (Bcf) of natural gas in 2007. This equates to $150 million worth of gas, assuming the average U.S. distribution price in 2007 , , or $50 to $115 million if gas were valued between $3 and $7 per thousand cubic feet (Mcf).

In the U.S. and abroad, quantifying natural gas losses is the first step to mitigating losses and conserving this valuable resource. The U.S. Greenhouse Gas Inventory estimates natural gas losses from cast iron distribution mains utilizing leak factors determined in a study commissioned by the U.S. EPA and the Gas Research Institute (EPA/GRI). Notably, this study impacts global emissions estimates, as the EPA/GRI study leakage rates serve as default emission factors in the Intergovernmental Panel on Climate Change guidelines for 2006.

New data on leakage rates from cast iron distribution pipelines highlights the potential variability of actual leakage rates worldwide. Since 2005, Comgas, the largest natural gas distribution company in Brazil by distribution volume, has measured leak rates from 912 segments of cast iron pipelines and discovered significantly larger leak rates than the EPA/GRI study. These results point to the need for further study in order to accurately gauge emissions levels from individual distribution systems.

This article will compare methodology and results of the Comgas measurement studies and the EPA/GRI study. In both cases, the need to address leakages from cast iron distribution pipelines is apparent and mitigation options employed by Comgas and other distribution companies will also be discussed.

Background

Cast iron was the material of choice for low pressure distribution mains in the U.S. until the 1950s. Pipelines typically consist of 12-foot sections connected by bell and spigot joints, shown in Figure 1 , that are sealed by jute packing plus cement or molten lead. Leaks tend to develop in the packing over time due to heavy overhead traffic, freeze-thaw cycles, naturally shifting soil, and the switch to dryer natural gas.

Fugitive methane emissions from distribution mains account for 32% of methane emissions from the U.S. natural gas distribution sector. Cast iron pipelines contribute the most to these emissions, despite representing only 3% of the miles of the U.S. distribution mains 1 . These estimates are based on methane leak rates from an EPA-commissioned study to estimate emissions from all sources in the U.S. natural gas industry 4. GRI led the study and relied heavily on extrapolating emissions measurements conducted in 1992.