Root Cause Analysis of Water-Cooled Exhaust Manifold in High Speed Marine Diesel Engine

In marine diesel engines with high power to weight ratio, water-cooled exhaust manifolds are designed and manufactured to achieve a good heat transfer in exhaust system with appropriate safety and reliability in engine room. In one of high-speed marine diesel engine after 417 hours operation, water was leaked from aluminum alloy exhaust manifold due to cavity creation between coolant and gas port walls. This paper investigates the root cause analysis of this exhaust manifold failure using fishbone diagram. Results showed that film boiling was accrued on damaged surface. In this region, surface was completely covered by a continuous stable vapor film. So, heat flux of this area reached a minimum and surface temperature was increased locally. The root cause of this phenomena was decreasing or blocking of water flow due to some unwanted casting burr. Although the film boiling nucleated thermal crack on surface, but cracks and cavities propagated under an electro-chemical mechanism until coolant leakage.