At Shell Polymers Monaca, five safeguards were each tied to an operating mode. The furnace was sitting in a state none of them covered: isolated, pilots lit, halfway back to service. Walk your next return to service one state at a time and find the blank cells before the startup does.
One item per test. Pick the piece you're most nervous about bringing back.
List every state it passes through from full isolation to normal operation, including the awkward in-between ones. Then fill the five cells for each. If a state has no name in your procedures, that's your first finding.
The five cells cover the equipment. These cover the plan around it.
Alarm suppression, interlock logic, procedures and exclusion-zone rules each assume the unit is in a named mode. A state that isn't named inherits none of them.
Monaca kept pilots lit to protect refractory. Reasonable, but it pushed the job off the only start-up procedure they had, without the required review.
The same move had worked 19 times. Experienced people were quietly compensating for a job aid with steps out of order.
Source for the example: U.S. Chemical Safety Board, Furnace Explosion and Fire at Shell Polymers Monaca, Investigation Report No. 2025-05-I-PA (September 2026). The example grid is STO Intelligence's reading of the report's findings, for illustration. This self-check runs entirely in your browser; nothing you enter is sent anywhere.