Engines vibrate relative to the boat, and therefore a fuel system made entirely of solid pipe would soon fracture. Part of the pipework between the tank and the engine must be flexible, purpose-made diesel fuel hose with a protective wire mesh cover. This is normally fitted next to the engine, between the water separator and the lift pump and in the return from the system to the tank, allowing the rest of the fuel line to be made of solid pipe and securely fastened to the boat.
The solid piping can be made of thickwalled copper, bundy or stainless steel. End fittings are either brazed-on ferrules or swaged – olive-type fittings should not be used. All sealing joints should be metal-to-metal, and sealing compound or PTFE tape must not be used because of the risk of particles entering the fuel system.
The fuel isolator or fuel shut-off valve must be as close to the fuel tank as possible, placed to allow someone to cut off the supply without entering the engine compartment in the event of a fire.
On the high pressure side, fittings using copper or aluminium face seals should be tightened just enough to start compressing the seal material. Be careful not to overtighten. The high pressure pipework between the high pressure pump and the injectors is carefully sized thick wall steel pipe. Both the length and the inside diameter are important relative to the dynamics of the system, to ensure that fuel delivered to the injectors is at the correct pressure. If the pipework needs to be replaced, make sure you use genuine replacement parts from the engine manufacturer.