Respondent is a limited liability company which owns a chemical plant where regulated substances and other extremely hazardous substances are produced, processed, stored, and handled, and from which an accidental release occured. Within the facility, the Respondent produces nylon intermediates (adiponitrile (ADN), hexamethylene diamine (HMC), adipic acid, C12, and C12 co-products). On May 9, 2015, the facility started-up the OP-1 cyclohexane oxidation unit. Unknown to the unit operator there had been a failure of the cooling water inlet supply valve for the wash oil stripper condenser. Because of this failure the condenser did not properly cool the steam/cyclohexane mixture. As a result cyclohexane was released to the ambient air. The failed cooling waler valve was discovered five (5) days later. The unit operator, once alerted to the failed valve, shutdown the unit, had the valve repaired, and re-started the unit, which functioned properly. Over the five days 18,102 pounds of cyclohexane was released. The failed cooling water inlet supply gate valve was subsequently analyzed to determine causes of the metallurgic failure. The engagement slot of the valve gate was heavily corroded so as to be non-functional.