Crankcase Explosion Protection on Marine Engines
The IACS UR M9, M10, M66 and M67 regime for crankcase explosion relief valves, oil mist detection, bearing temperature monitoring and crankcase enclosure strength.
Crankcase explosion protection is the set of design, monitoring and operating requirements that keep an oil mist in an engine crankcase from reaching an ignitable concentration, and that contain the consequences when it does. The regime runs across four IACS Unified Requirements: UR M10 for the protection arrangement and the enclosure, UR M9 for explosion relief valves, UR M66 for relief valve type testing, and UR M67 for oil mist detection and alarm equipment.
The full article will cover the initiating mechanisms, hot spot formation and the mist generation that follows, the primary and secondary explosion sequence, relief valve count, free area and opening pressure, flame arrester duty, oil mist detection ranges and alarm set points, bearing temperature monitoring as the permitted alternative, crankcase ventilation and the vacuum limit, the dual-fuel regime introduced by UR M10 Rev.5 for engines certified or contracted on or after 1 January 2026, and the documented casualty record.
Relief valve and crankcase enclosure requirements as they apply to the frame box of a large two-stroke are treated today in A-frame and column design .