Q701. In Equation Y-3, startup, shutdown, or malfunction (SSM) events exceeding 500,000 standard cubic feet per day (scf per day) must be reported.  What time period and flare gas volume should be used to assess if this threshold was exceeded?

A701. When Equation Y-3 is used to estimate CO2 emissions from a flare, 98.253(b)(1)(iii)(A) requires a per event estimate for “each SSM event exceeding 500,000 scf/day”.  The time period is a consistently designated 24-hour period (see definition of “day” at 98.6), and the 500,000 scf threshold is assessed based on the flow directly attributable to the SSM event.  For example, if the total flare gas volume on a particular day was 700,000 scf, and, during that day there was a 12-hour SSM event that discharged an estimated 400,000 scf of gas to the flare, the SSM event would not be included in the Equation Y-3 calculation.  On the other hand, if the SSM event discharged 600,000 scf, then it would be included in the Equation Y-3 calculation.  In summary, the SSM event threshold is not determined based on either (1) the cumulative flare gas flow for a certain day, (2) the maximum instantaneous flow rate due to the SSM event, or, (3) the cumulative flare gas flow for a particular day minus the annual normal flare gas volume (Flare_Norm) expressed on a daily average basis. Rather, it is determined based on the best available estimate of the flow directly attributable to the SSM event during that day.