In March 2018, operators discovered that the thermal oxidizer had tripped due to low fuel gas pressure. It took operators 14 hours to discover and repair the source of the low fuel pressure, which they ultimately determined to be a partially failed upstream fuel gas regulator. During the thermal oxidizer outage, vents were rerouted to carbon cannisters, which serve as effective absorption for most emissions, but not for 1,3-butadiene. Operations personnel calculated that during the thermal oxidizer outage, approximately 143.1 pounds of 1,3-butadiene were released to the ambient air. Operations personnel discovered that approximately one month prior to this release, the failed regulator had been replaced by a rebuilt regulator during maintenance. Pursuant to 40 CFR 68.73(f)(3), the owner or operator shall assure that maintenance materials, spare parts and equipment are suitable for the process application for which they will be used. Respondent failed to assure that the rebuilt regulator was suitable for the process application for which it was used. Respondent?s failure to assure that its maintenance materials were suitable for the process application for which they were used constitutes a violation of 40 CFR 68.73(f)(3).