Exhaust gas bypass arrangements
A bypass duct and damper set lets an engine keep running when the exhaust gas cleaning system or SCR reactor in its uptake is out of service.
An exhaust gas bypass is the duct and damper arrangement that routes exhaust around an aftertreatment unit in the uptake, so the engine can continue to run when the unit cannot. IACS UR M86 states the requirement directly: the bypass is provided in order to continue the operation of the engines, and it operates automatically on the conditions in its alarm table. It may be omitted only where the exhaust gas cleaning system is designed to run dry.
The safety-critical detail is the damper interlock. Where the bypass and uptake dampers are individually controlled, an interlock must prevent both being closed at the same time, which would dead-head the engine. A pressure sensor upstream that opens the bypass on high back pressure is one accepted implementation. Paragraph 3.2 of resolution MEPC.340(77) separately requires measures against leakage of exhaust gas from the damper to bypass lines.
The full article will cover damper types and their sealing, actuation and control, the interlock logic, the leakage and fire risk in a bypass duct, and the equivalent arrangements on SCR and waste heat recovery installations.