New Measurement Data Has Implications For Quantifying Natural Gas Losses From Cast Iron Distribution Mains
To evaluate leaks from cast iron mains, GRI visited 21 sites volunteered by distribution companies in North America. It followed the path of the cast iron mains on the surface, monitoring the air for elevated concentrations of methane using a flame ionization detector. Any increase of two to three parts per million methane concentration above background air concentration was considered a leak. Detected leaks were centered by boring holes in the ground on each side of the pipe, and the point with the highest concentration was considered the probable location of the leak.
Next, GRI excavated at least 10 feet downstream and upstream of the leak and isolated the segment. It equipped the segment to receive gas passing through a meter to measure the flow rate. Once the segment returned to operating pressure, GRI measured the flow rate necessary to sustain that pressure. A pipeline segment with no leaks requires a certain flow rate to maintain operating pressure. The leak rate is equivalent to the difference between the measured flow rate to the leaking segment and the flow rate at which a non-leaking segment would sustain operating pressure.
The average leak rate per distance of cast iron pipeline surveyed was calculated to be 428,123 standard cubic feet (scf)/mile-year 4. With an average methane concentration of 93.4%, the methane leak rate is 399,867 scf/mile-year.
Comgas Leak Studies
It is useful to compare measurement studies by Comgas with those from the EPA/GRI study. Comgas is Brazil’s largest distributor of natural gas, servicing over 700,000 customers in 67 municipalities in metropolitan Sao Paulo with daily throughputs exceeding 490 MMcf. Its cast iron network, which is 50 years old or older, was installed to supply town gas. In 1993 Comgas converted to natural gas when new sources of gas became available to Sao Paulo through the GASAN pipeline. This change to dryer natural gas has resulted in drying of joints and increased operating pressure – both of which increased natural gas leakage.
Comgas recognized that this leakage would be detrimental both financially and environmentally. The renewal of their cast iron pipeline network is one of the key aspects of the operational asset support projects undertaken since 1999, when the concession agreement was signed with the Sao Paulo State Government.
To calculate the magnitude of gas loss, Comgas developed a systematic approach to measuring losses from segments of pipeline prior to repair. To conduct the measurements, Comgas employees isolated segments of the cast iron network due for renewal and used a manometer to record the segment pressure every three minutes to watch the pressure decay in the normal operating pressure range. Using the ideal gas law, Comgas calculated the total volume of leaked gas and plotted it against time, then fitted an equation. The derivative, or rate of change, of the equation is equivalent to the volumetric leak rate at a given time and pressure. Comgas evaluated the derivative of its fitted data for each segment at the time when the segment was at operating pressure. In other words, Comgas calculated the leak rate for each segment at its normal operating pressure.
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