The original 1993 PHA for the ISOM unit identified the level instrumentation on the raffinate splitter as a safeguard against tower overfilling. However, the raffinate splitter instruments were not included on the list of critical instruments.
In addition, a safety review of the ISOM unit relief system and blowdown drum identified the raffinate splitter level transmitter and alarm as justifications for concluding that tower overfill was not a credible scenario. However, neither the PHA nor the relief system review resulted in the raffinate splitter level. instruments being designated as “safety-critical.” The result was a history of ineffective maintenance for the level instruments. Further, no work history was found for the tower level transmitter for the five years prior to the incident