PR-312-16202-R03 Methane Emissions from Transmission and Storage Subpart W Sources

A 2018 PRCI report evaluated related emissions from compressor seals, isolation valves, and blowdown valves based on direct measurements required by Subpart W of the GHG Reporting Program. This report presents the methane emissions data from 2011 – 2016 for the balance of the Subpart W emission sources, including: facility leaks (other than from compressor isolation valves and blowdown valves), pneumatic controller venting, condensate tank dump valve leakage, and blowdown emissions from stations. Transmission pipeline blowdown emission reporting was added to the EPA regulation in late 2015, and 2016 and 2017 pipeline blowdown data are presented in this report. The objective of the project is to evaluate and analyze the dataset, and compare methane emission estimates from these sources to historical data used by EPA, primarily the emission factors (EFs) from the mid-1990s EPA/Gas Research Institute (GRI) study used by EPA in its annual GHG inventory (GHGi) report. The results and related EFs and analysis of relative contribution from different sources can be used: (1) as an alternative to GHGi EFs for estimating methane emissions for Transmission and Storage (T&S) operations; (2) to document the relative contribution of different T&S methane emission sources; and (3) to identify reductions relative to historical estimates and support more efficient methane mitigation strategies. The Subpart W data for leaks and pneumatic devices are consistently lower than GHGi estimates, and blowdown emissions from compressor stations and transmission pipelines are similar in magnitude to GHGi estimates.