Respondent owns and operates a chemical manufacturing facility that has an Acrylonitrile Manufacturing process. On or about December 1-3, 2015, an EPA inspector conducted an inspection of Respondent’s facility. A contractor was removing a six-inch blind and installing four-inch and three quarters inch blinds to prepare the Heads Column in the Acrylonitrile Unit for caustic cleaning. The contractor controlled the method and manner for removing and replacing the blinds. The contractor opened and removed the six-inch flange blank, inserted gaskets, closed the flange, and tightened the bolts. He opened and installed the four-inch flange blank, inserted gaskets, closed the flange, and tightened the bolts. During this work, the contractor was unexpectedly exposed to elevated concentrations of hydrogen cyanide that seeped through an “unseated” gate valve on the four inch line. The gate valve appeared to be fully closed and was confirmed to be locked with a chain.