The heat of the engine is taken care of by the jacket cooling system of the main engine, however, the exhaust produced by the engine needs to be drawn out to the heat recoveries systems such as the turbocharger and economizer, and eventually to the atmosphere. This is done by fitting an exhaust valve.
The main engine of the ship, which is normally a Two-stroke crosshead marine engine, has a single exhaust valve per unit, mounted in the centre of the cylinder head. The exhaust valve of the marine diesel engine consists of a casing that is made of cast iron and contains different components and driving arrangements for the exhaust valve.
It also has jackets, through which, colling water passes for maintaining the valve temperature. The spindle is responsible to draw the exhaust gasses out from the engine cylinder when operated in an open and closed position.
The spindle can be a molybdenum chrome alloy with a layer of stellite, welded onto the seating face, or alternatively, a heat-resistant Nimonic alloy valve head, friction welded to an alloy steel shaft.
The valve spindle is fitted with a winged valve rotator. The exhaust valve seat is a renewable valve seat that can be reconditioned or replaced in case of damage and is usually made from hardened molybdenum steel with stellite coating for good temperature-resistant property
Check this video lesson on the Construction and Working of the Exhaust Valve –