Q557. For blowdown vent stacks, does the 40 CFR 98.233(i) reference to a volume of less than 50 cubic feet refer to a gas volume or a physical container volume?

A557. The specified 50 cubic feet refers to the physical volume of pipeline segments, vessels, or other equipment between isolation valves. As stated in 40 CFR 98.233(i), unique physical volumes smaller than 50 cubic feet are not subject to reporting under 40 CFR 98.233(i). Beginning in Reporting Year (RY) 2025, facilities in the natural gas distribution industry segment also must report emissions from blowdowns; the unique physical volume threshold for pipeline segments and other equipment at facilities in this industry segment is 500 cubic feet rather than 50 cubic feet.