Methods

This was a national study designed to obtain data for distribution systems across the United States.

Field measurements occurred May-November, 2013. Partners were from 13 local distribution systems across the U.S. The researchers traveled to each of those distribution areas to do the sampling, spending 7 to 10 days in each area.

• In each area, the researchers worked with companies to identify a portion of their system in which to concentrate efforts. Within these areas, the researchers randomly selected pipeline leaks from a list of leaks provided by companies and Metering and Regulating facilities to measure.
• At any given location, the researchers collected approximately 10-20 pipeline leak measurements and emissions data for 10-20 metering and regulating stations.
• Altogether, 230 measurements of pipeline leaks and 229 measurements of emissions from gas metering and regulating facilities were collected.
• The researchers revisited nine sites from the previous GRI/EPA study.

The research team carefully selected numerous sites in various regions around the country that met specific criteria to ensure that a comprehensive dataset was representative for national scaling.

Emission Types

The project focused on two different types of emissions of methane from the distribution system.

• Leaks from underground pipelines.
  • Randomly selected leaks to measure from the DOT required company leak surveys.
  • At each leak location, the researchers used a surface enclosure to measure how much methane is being released into the atmosphere.
• Gas metering and pressure regulating stations. These are operated by the distribution companies. Using a portable methane sniffer, the researchers screened all pipes, joints, and control devices within the facility looking for leaking or emitting components. In these facilities there are sometimes vented devices, flow controllers that, by design, emit a small amount of methane each time the device is activated. If leaking or emitting components were found, the researchers sampled and determined how much methane is lost. For each facility, the emissions from all of the leaking or emitting components are summed to give the total methane emissions for that facility.

Measurement Techniques

Two methods for measuring methane emissions from underground pipeline leaks and from natural gas facilities were used:

• High flow sampling method for measuring emissions component by component on site. The high flow sampler is like a large vacuum cleaner placed next to the leaking component. The sampler collects leaking natural gas and sends it through a hydrocarbon sensor. By measuring how much air is flowing through the sampler and the concentration of methane, the researchers can calculate how much methane is coming from that leak.
• Tracer ratio method, used at some leaks or facilities to spot check and determine accuracy of high flow sampling results. The researchers go to a facility and set up a tracer release, in which a tracer gas is released at a steady measured rate. With an instrumented van, the researchers then measure both the tracer and methane levels in the air downwind of the facility. By knowing the ratio of the concentrations and how much tracer is released, they can back calculate exactly how much methane is coming from that facility.
• Tracer ratio method provides a second, completely independent method for determining methane emissions from a given location and can be compared with what we get from a high flow sampling method.