Introduction to Process Design Engineering

A: Closing HAZOP on an unverified level signal can leave a vessel blind during an upset, risking overfill and relief lifting. An MOC forces confirmation of actual installed devices, loop purpose, and any safety role before documentation is changed. That avoids locking in a paper fix that hides a real gap between control intent and field hardware.

A: Driving the pump harder under cavitation eats impellers and fails seals fast. Reducing flow demand restores NPSH margin that was lost due to higher vapor pressure, stopping vapor collapse at the eye. That protects rotating parts while you assess upstream heat input.

A: Ignoring thermal expansion in a blocked-in liquid line can split tubes or shells as pressure climbs quietly. Providing thermal relief addresses a real overpressure mechanism even without process heat, aligning with why the standard treats trapped liquid as a credible hazard.

A: Letting a valve choke the line without checking extremes can cause high ΔP, noise, and control instability that show up during SAT. Revalidating all cases ensures the selected size handles start-up and upset flows, not just normal operation.