Chapter 13 Exhaust Systems 293 Copyright Goodheart-Willcox Co., Inc. Cast iron, wrought iron, or steel exhaust pipes are preferred for industrial applications because they can stand up to hot and cold cycling. However, cast iron and wrought iron are generally considered too heavy for mobile diesel applications. Stainless, galvanized, and alu- minized steels are preferred in mobile applications. Hangers, brackets, and clamps must be sturdy enough to withstand expansion forces and severe vibration, shock loads, and heat. In industrial installations, the design of the hangers must allow for thermal expansion of the pipe (10′ of pipe will lengthen almost one inch when hot). Heavier exhaust system components, such as silencers, DPF, and DOC should be independently supported. The design of the installation must allow for expansion on both sides of all devices. The exhaust system must be supported from below when overhead cranes or booms will be used in the engine room. Flexible (flex) connections capable of absorbing vibration, shock, and expansion forces should be installed when needed in the exhaust system. These connections must be large enough to isolate engine vibra- tion and to compensate for thermal expansion. If the con- nector is not large enough, thermal expansion would compress the connection, resulting in damage to the engine manifolds and turbochargers, and in excess exhaust system vibration. The piping of a marine diesel engine system, like a muffler, can be either wet or dry. Wet exhaust systems are characterized by the following: ❑ Generally the exhaust gases are mixed with the water, which is discharged from the water side of the engine’s heat exchanger. ❑ Particulate and condensable/soluble gaseous emissions from the exhaust system are effectively scrubbed from the exhaust gases, reducing the possibility of atmospheric pollution. ❑ Exhaust piping that is cool enough to be made of uninsulated fiberglass reinforced plastic (FRP) or rubber. ❑ The moisture of exhaust gases and water is dis- charged from the boat at or slightly below the ves- sel’s waterline. ❑ With the relatively small elevation difference between the engine’s exhaust discharge elbow and the vessel’s waterline, it is difficult to design a system that will always prevent water from entering the engine through the exhaust system. While a number of proprietary exhaust components are available to help avoid this problem, the most common generic methods are exhaust risers and water lift mufflers. Preventing Water Intrusion in Marine Applications One way to minimize the possibility of water entering the engine from backflow in the wet exhaust system is to have a steep downward slope on the exhaust piping, downstream of the engine. Exhaust risers are pipes that elevate the exhaust gases, allowing a steeper slope in the downstream piping. The risers must be insulated or water- jacketed to protect personnel in the engine compartment from the high-temperature exhaust gas. The water is not injected into the exhaust gases until downstream of the top of the riser, so the upward-sloping portion is dangerously hot if not insulated or water-jacketed. Another way to mini- mize the possibility of water entering the engine from backflow in the wet exhaust system is by using a wet muf- fler, described earlier in this chapter. The traditional method of preventing water from entering an idle engine is to locate the engine far enough above the waterline that breaking waves do not reach the height of the exhaust elbow, Figure 13-25. While the rela- tive elevation of the engine to the waterline is fixed and unchangeable, it is possible for the exhaust system to pro- tect the engine from ingesting water. Features of such an exhaust system include the following: ❑ Sufficient elevation difference between the water- line and the highest point in the exhaust piping to prevent even small amounts of water from reaching the engine. Turbocharger Figure 13-24. Typical turbocharger mounting on a mobile diesel engine. (David Gaylor/Shutterstock.com) End cover plate Engine Water cooled elbow Flexible connector Flexible connector Exhaust pipe Figure 13-25. Marine wet exhaust systems use water that actually contacts the exhaust gases. (Caterpillar Trucking Co.)