Q502. For transmission tank vent stacks routing emissions to a flare, the reporter is referred to the emissions source for flare stack emissions in 40 CFR 98.233(n), but guidance is only provided for upstream production and gas processing in this section.

A502.  For facilities in the onshore natural gas transmission compression industry segment in a reporting year (RY) prior to RY 2025, 40 CFR 98.233(k)(5)(i) specifies that the volume and gas composition of any tank emissions routed to a flare are to be determined using the procedures in 40 CFR 98.233(k)(1) through (4) (i.e., monitor the tank vent stack or the compressor scrubber dump valve(s)  at least annually for any flow, quantify the rate if there is continuous leakage, and determine gas composition according to procedures in 40 CFR 98.233(u)(2)(iii)).   40 CFR 98.233(k)(5)(ii) specifies that the method in 40 CFR 98.233(n) is to be used to determine the flare stack emissions from transmission storage tanks. This means that the volume and composition determined in 40 CFR 98.233(k)(1) through (4) are to be used in equations W-19, W-20, and W-40 to calculate the volumetric CH4, CO2, and N2O flare stack emissions from the transmission storage tanks, which are converted to mass emissions according to the procedures in 40 CFR 98.233(t) and (v), as referenced from 40 CFR 98.233(n)(4) and (6). 

In addition to calculating the flared emissions specifically from transmission storage tanks, transmission compression facilities must calculate total emissions from each flare using the procedures in 40 CFR 98.233(n). Specifically, total volume to a flare is to be determined according to 40 CFR 98.233(n)(1) using either a continuous flow measurement device on the total inlet stream to the flare or, for flow that is not measured, by using engineering calculations based on process knowledge, company records, and best available data. Gas composition is to be determined according to 40 CFR 98.233(n)(2) using either a continuous gas composition analyzer or by using a representative composition determined by engineering calculation based on process knowledge and best available data for hydrocarbon product streams as specified in 40 CFR 98.233(n)(2)(iii) (i.e., EPA’s intent is that gas routed through a transmission compression facility is a hydrocarbon product stream). Finally, according to 40 CFR 98.233(n)(9), a transmission compression facility must determine the flare stack emissions to report according to 40 CFR 98.236(n)(9) through (11) by subtracting the flared transmission storage tank emissions from the total flared emissions in order to prevent double counting of flared emissions.

Please note that according to the May 14, 2024, amendments to subpart W, transmission compression facilities are not required to calculate and report flared transmission tank emissions separately from total flare stack emissions in RY 2025 and future reporting years. Specifically, 40 CFR 98.233(k)(5) was removed by the amendments. In RY 2025 and future reporting years, transmission compression facilities are to calculate total emissions from each flare according to the procedures in 40 CFR 98.233(n) of the amended rule.