Chemical Safety and Process Risk Management

A: The 12% duty step is the trigger. That amount is enough to push vapor velocity toward the flooding limit on the lower trays, even if overhead purity looks stable. Higher vapor traffic degrades efficiency first, not viscosity. Cutting reflux is a first-instinct move that worsens separation. The safe response is checking hydraulic margin before touching setpoints.

A: The hard boundary is sub-atmospheric pressure. PSVs are sized for overpressure cases like fire and thermal expansion, not vacuum. A stuck-closed blanket with pump-out creates inward buckling risk, and the PSV does nothing there.

A: The 30 K rise is the catch. 2,500 × 2.1 × 30 gives 157,500 kJ, or 157.5 MJ. The other answers all come from dropped temperature span, wrong participating mass, or a unit slip.

A: The tag prefix is the boundary. LT versus LG changes whether the signal is used in control logic. A mismatch between P&ID and instrument list is a classic as-built gap and must be resolved before claiming the CAPA eliminated the cause.