Diesel fuel supply systems must ensure continuous and clean supply of fuel to the engine’s…
Fuel Piping Design Considerations
Fuel Supply Piping
Using shutoff valves in the delivery line may pull air into the system during shutdown and cause hard starting. The engine control system provides adequate shutdown options, but, if a shutdown solenoid is specified in the supply line, it should be timed to close after the engine stops rotating.
The pressure measured in the fuel supply line should be kept below the values shown in TMI.
Fuel Return Piping
Fuel return piping should normally enter the tank at the top and extend downward, exiting above the fuel level. Inlet and return lines should be separated in the tank as far apart as possible to allow fuel warmed in the engine to dissipate excess heat. Fuel tanks can function as a radiator of sorts, especially in engines that are not equipped with a fuel cooler or engines that use fuel to cool the injectors. Placing return lines and suction lines as far apart as possible provides the most opportunity for cooling. Return line placement is particularly important on smaller tanks and day tanks where the fuel volume is allowed to run down.
The fuel return line is under pressure, although not as high as the supply line.
Note: Shut-off valves should not be used in fuel return lines. Engine operation with the valve closed will cause damaging pressures.
Engine fuel pressure measured in the fuel return line should be kept below 27 kPa (4 psi), except for the
3300 engine family, which is 20 kPa (3 psi), C175 engine family, which is 60 kPa (8.7 psi) and the 3600 or C280 family, which is 350 kPa (51psi). The location of the day tank and the design of the pipes should accommodate these requirements.
Purging should take place both in the supply and the return line.
Siphoning & Check Valves
Siphoning can occur in full fuel pipes when the one end of the pipe is placed in the fuel and the other end is below the level of fuel.
Siphoning is a flow of fuel in the pipe without the help of pumps. It can occur in supply and return lines.
Siphoning is most likely to occur after a fuel line failure, which can be due to corrosion, fire or a cut from foreign objects or collision force.
The consequences of fuel line siphoning are fuel loss and the creation of a fire hazard. If the fuel ignites and the flow is not stopped, the fire will be more difficult to extinguish.
The fuel supply line has a fuel transfer pump. To avoid siphoning, the pump must be equipped with a check valve. This is in case the pump has been deactivated and the fuel supply line is breeched. For certain C175 installations, a check valve may be necessary.
Black iron pipe is best suited for diesel fuel lines. Steel or cast iron valves and fittings are preffered.
CAUTION: Copper and Zinc, either in the form of plating or as a major alloying component, should not be
used with diesel fuels. Zinc is unstable in the presence of sulfur, particularly if moisture is present in the fuel. The sludge formed by chemical action is extremely harmful to the engine’s internal components.
Pipes, hoses and fittings must be mechanically strong and resistant to deterioration due to age or
environmental conditions. They must also be airtight to avoid entry of air into the suction side of the fuel system. A joint, which is leak-tight to fuel, can sometimes allow air to enter the fuel system, causing erratic running and loss of power.
Sizing of pipes, hoses and fittings must be adequate to minimize flow loss.
Sizing for a particular application is determined by the supply and return line restrictions. This can be
estimated with help from the Piping System Basic Information section of the Application & Installation Guide.
The maximum allowable restrictions are published in the TMI.
Generally, the supply line carrying fuel to the fuel transfer pump and the return line carrying excess fuel back to the tank should be no smaller in size than the connection fittings on the engine. In addition, the return line should be at least as large as the supply line.
If the fuel tank supplies multiple engines over 9.14 m (30 ft) from the tank, or ambient temperatures are low, larger fuel supply and return lines should be considered to ensure adequate flow. The overflow line from the day tank (or, if no day tank is used, the engine fuel return line) should be one size larger than the supply and return lines.
Fuel lines should be well routed and clipped with flexible hose connections where relative motion is present. Lines should be routed away from hot surfaces, like manifolds and turbochargers, to avoid fuel heating and potential hazard if a fuel line should fail.
Fuel lines should be routed to avoid formation of traps, which can catch sediments, or pockets of water, which will freeze in cold weather.
Whenever possible, route fuel lines down low, so any potential leakage will be confined to the fuel tank base or floor space. Leaks from overhead fuel system components may fall onto hot machinery, increasing the likelihood of fire danger.
Route fuel lines to avoid crossing paths and walkways. Protect fuel lines from abrasion and damage.
Whenever possible, route fuel lines so they are visible for leak checking.
For electronic unit injector fuel systems, supply line pressure must decay to atmospheric pressure after engine shut down. Any sustained static pressure on the fuel system when the engine is not operating will cause excessive fuel to oil dilution.