M12 – Fire extinguishing systems for scavenge manifolds

APPLICATION FIELD

IACS UR M12, “Fire extinguishing systems for scavenge manifolds,” applies specifically to crosshead-type internal combustion engines

It an IACS rule essentially the large, slow-speed two-stroke diesel engines used for main propulsion on most deep-sea ships, and, within those engines, to the scavenge spaces that sit in open connection with the cylinder.

It requires those scavenge spaces to be connected to an approved fire extinguishing system, and it require that system be entirely separate from the fire extinguishing system serving the engine room as a whole.

PIPE CLASS / SYSTEM TYPE

M12 has nothing to do with the P2.2 Class I/II/III piping classification system. it’s a machinery fire-protection requirement, not a piping strength or classification rule.

It belongs to the same family as M9, M10 and M11, all of which govern protective and containment devices fitted around specific ignition or explosion hazards in internal combustion engines, rather than to the P-series logic that classifies pressure piping by service and pressure/temperature combination.

RELATED RULES

• Direct sibling: UR M9 and M10 address crankcase explosion relief valves and crankcase explosion protection, a closely related family built around the same underlying idea of containing and relieving an internal ignition event, applied to a different part of the engine.
• Direct sibling: UR M11, immediately preceding this one in the numbering, addresses a different ignition-propagation pathway in the same engine, explosion risk in the starting air main.
• Cross-reference (by design contrast): the general engine room fire-extinguishing arrangements required under SOLAS and the corresponding classification society fire protection rules, M12’s main requirement is precisely that the scavenge manifold system must stand apart from that general arrangement.

RESTRICTION

M12 targets crosshead engines with scavenge spaces open to the cylinder, and doesn’t extend automatically to trunk-piston engines lacking that construction.

The scavenge manifold’s fire extinguishing system has to be entirely separate from the engine room’s general fire extinguishing system, meaning the two can’t share extinguishing medium supply or piping, and one can’t be pressed into service as a substitute for the other if it’s momentarily unavailable. 

DESCRIPTION

Scavenge space fires are a a different hazard from the crankcase explosions M9 and M10 address, even though both sit close to the combustion process.

In a crosshead engine, the piston rod runs through a stuffing box that separates the enclosed, lubricated crankcase from the scavenge space and cylinder below the piston, a design that keeps crankcase oil largely isolated from combustion residue, which is part of what lets crosshead engines burn heavier fuels reliably.

The scavenge space itself, though is exposed to blow-by gas and unburned fuel residue over time, and that residue can build up as a combustible deposit of oil, soot and fuel on the walls of the scavenge trunk and manifold.

Given enough heat, from a hot liner, a leaking piston ring, or poor combustion, and the oxygen that’s naturally present in an air scavenge space can ignite into what’s known as a “scavenge fire”, an accident that’s not easy to catch early because it develops inside a space that isn’t directly visible. M12’s response is narrow and mechanical by design: it mandates a dedicated, approved means of extinguishing a scavenge fire once one starts, and it insists that this system be entirely independent of the general engine room fire fighting arrangement. 

IMPORTANT NOTE

• Scavenge fires remain a recognized, recurring casualty on crosshead-type main engines, often traced back to poor combustion, fuel quality, or a leaking piston or liner allowing residue to accumulate