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.
Pressure
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.
Pressure
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
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.
Material
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
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.
Routing
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.
Fuel Transfer Systems
The diesel engine fuel supply, delivery and governing systems are designed to deliver clean fuel in the
precise quantity and time needed to produce the required engine performance.
All connection lines, valves and tanks should be thoroughly cleaned before making final connections to the engine. The entire fuel system external to the engine should be flushed prior to connection to engine and startup.
Caterpillar supplies the engine with a transfer pump and the secondary filter. The customer must provide the
primary filter and, if needed, an auxiliary transfer pump. The auxiliary transfer pump is required when the distance, vertically or horizontally, between the day tank and engine exceeds the requirements discussed in Auxiliary Fuel Tanks. An example of a fuel transfer system is shown in Figure 6.
Fuel Transfer Pumps
Engine Driven
Cat engine-mounted transfer pumps are positive displacement gear-type or piston-type pumps, with a limited prime and lift capability.
The pump lifts the fuel by displacing air from the suction pipe to the discharge pipe. Low pressure (vacuum) develops in the suction pipe and atmospheric pressure [101 kPa (14.5 psi) at sea level] moves the fuel into the vacuum.
However, a perfect vacuum cannot be maintained, and the maximum that a pump can lift is about 5 m (17 ft).
Cat fuel pumps’ prime and lift capability is 3.7 m (12 ft), but pipe size, routing, and ambient temperature will impact this capability.
To determine if a pump can perform the required lift, the following items must be considered.
1. The vertical distance from the tank to the pump. The distance should be measured from the inlet pump port of the pump to the bottom of the tank.
2. Internal piping system losses reduce the lifting capability. This is based primarily on the size and the total length of the pipes, but also includes the various fittings and valves. As the temperature goes down the resistance goes up. The internal losses can be estimated using the Piping System Basic Information section of the Application & Installation Guide.
3. Elevation has a big impact on the pump’s lifting capability. As described above the atmospheric pressure is helping the fuel into the vacuum, but as the elevation gets greater, the atmospheric pressure decreases and the available lift will also decrease.
Refer to Table 3.
Auxiliary
An auxiliary transfer pump is required when the service tank or day tank is located further away,
horizontally or vertically, than the engine driven pump’s lift capability.
Special considerations must be given to the auxiliary transfer pump when dealing with electronic engines and the 3500 engine family. Refer to technical data for the engine’s fuel pump capacity to determine sizing auxiliary fuel transfer pumps.
A primary filter must be installed before the auxiliary pump and as close as possible to the tank.
In many cases, the auxiliary pump will be driven by an electric motor and therefore needs a regulator valve so that the fuel flow can match the engine speed.
Example:
A power plant with one (1) 3516B diesel generator set, rated for 1145 bkW (1560 bhp) at 100% load. The fuel rate for the engine is 284 L/hr (75 G/hr) as found in TMI.
The time between tank refills is based on weekly fuel tanker truck deliveries, so refill time is 168 hours.
The fuel tank for this genset is located 22 m (72.2 ft) horizontally and 2.5 m (8.2 ft) vertically (below) from the engine. This situation exceeds the fuel system requirements discussed in Auxiliary Fuel Tanks, therefore, an auxiliary
pump is needed.
Solution:
TMI indicates that the fuel flow at rated speed is 1260 L/hr (333 G/hr) @ 1200 rpm.
The auxiliary transfer pump required for this sample installation must be able to deliver fuel at 1260 L/hr (333 G/hr) at a pressure of 34.5 kPa (5 psi).
Emergency
Many marine applications require the capability to connect an emergency fuel oil transfer pump into the engine’s fuel oil system. Cat engines can be provided with these optional connections when necessary.
This is a specific requirement of marine classification societies for seagoing single propulsion engine applications. The purpose is to ensure fuel oil supply in the event of an engine fuel oil pump failure. The emergency fuel oil pump allows the single propulsion engine to operate and the ship to reach port for engine repairs.
Guidelines for emergency fuel oil system operation:
1. Keep pressure drops to a minimum by using short, low-restriction lines.
2. Use a line size at least as large as the engine connection point.
3. Install a low-restriction strainer in front of the emergency oil pump.
4. Install a low-restriction check valve between the emergency pump discharge and the engine inlet connection.
5. Use a pressure-limiting valve in the emergency system set at the maximum oil pressure limit of the engine.
6. TMI contains flow rates and pressure limits to fulfill minimum engine requirements for full power at rated speeds for Cat engines.
Fuel Tank Design Considerations
Fuel Tank Sizing
The fuel tank is typically one of the least expensive items in an installation, and it is wise to provide too much, rather than too little, storage capacity.
However, while the minimum required capacities of fuel tanks can be estimated, as outlined in the previous discussion of fuel tanks, some applications may need to meet the requirements of outside organizations, such as the U.S. National Electrical Code (NEC) or National Fire Protection Association (NFPA).
Fuel Tank Material
Fuel tanks made from low carbon rolled steel are best.
CAUTION: 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.
Fuel Tank Installation
Large capacity storage tanks allow bulk purchases and minimize dirt contamination. Maintaining full tanks reduces condensation, particularly if fuel is seldom used.
Tanks may be above or below ground level, but high fuel level generally should not exceed the engine injector’s height. This prevents possible fuel leakage into cylinders.
Above ground tanks provide accessibility, allowing for easy draining of impurities and reducing the danger of ground water contamination.
Underground tanks allow the earth to work as an insulator, limiting radical temperature changes which can cause flow restrictions, condensation, and possible power loss. Seasonal settlings are also avoided when burying the tank below frost line. In underground tanks, the water must be removed by pumping through a tube placed down the fill pipe.
Regulations governing the installation and maintenance of both above and below ground fuel tanks may apply.
Locate storage tank fill tubes for convenience and safety of filling operations. Vents are necessary to relieve air pressure created by filling and prevent vacuum as fuel is consumed.
Fuel Tank Drains
All fuel tanks should have easily accessible drain connections. Water and sediment that collects in the bottom of the tank must be eliminated regularly. Provide clean out openings for periodical removal of sediment and trash that settles out of fuel tanks.
Well-designed tanks have large enough clean-out openings so the lowest part of the fuel tank can be accessed with cleaning equipment.
Fuel Tank Grounding
Fuel tanks, both bulk and auxiliary, need to be grounded. This is to improve personal safety and reduce the fire hazard of sparks discharged from static electricity build-up during refueling operations.
If the auxiliary tank is mounted to the base of the engine, it will be grounded at the same time as the engine. If the fuel tank is placed away from the engine, the tank must be grounded separately.
Fuel Tank Maintenance
Fuel has a storage life of approximately one year. This period may vary widely depending upon initial fuel quality, contaminant levels and storage conditions.
To remove water, scale and bacteria growth, periodic exchange of fuel and filtering/treating is recommended to extend fuel life.
Water contamination of fuel during long-term storage provides a medium for bacterial growth, forming a dark slime which:
- Plugs filters
- Deposits on tank walls and pipes
- Swells rubber products that it contacts
Sulfur compounds are natural antioxidants, so low sulfur fuels (0.05 percent by weight) degrade quicker in storage.
Diesel fuels oxidize and form gums and varnishes which can plug fuel filters and injectors.
Because microorganism growth occurs in the fuel/water layer, the tank should be designed to minimize this interface, and water bottoms should be drained regularly.
Microbiocide additives, either water or fuel soluble, can be added to fresh fuel to inhibit microorganism growth. Consult your local fuel supplier for recommended additives.
In warm climates, large bulk storage diesel fuel requires full filtering every six months to one year.
Every two years the fuel should be completely changed to remove water, scale, bacteria growth, oxidized gums/resins, and minimize filter clogging due to fuel separation into components such as asphaltenes.
Fuel Storage Systems
Bulk fuel is usually stored in large main storage tanks and transferred to smaller auxiliary tanks (service
tanks or day tanks) near engines by electric motor-driven pumps as shown in Figure 3.
If auxiliary tanks are not necessary, the main fuel tank must provide a ready fuel supply to the engine-mounted transfer pump.
Main Fuel Tank
The main fuel tank represents the primary fuel reservoir in all applications, and must have adequate capacity for the intended application. Rule of thumb for tank size is to find the fuel consumption rate at 100% load factor (depending
on application: Prime, stand-by etc.) and multiply it with the number of hours between refills. Fuel consumption rates are shown on the Engine Technical Data Sheets for the specific engine. Additionally, 10% should be added to the result; 5% for expansion at the top of the tank, and 5% for sediment settlements at the bottom.
Example:
A power plant with one (1) 3516B diesel generator set, rated for 1145 bkW (1560 bhp) at 100% load. The fuel rate for the engine is 284 L/hr (75 G/hr) as found in TMI.
The time between tank refills is based on weekly fuel tanker truck deliveries, so refill time is 168 hours.
Solution:
Tank vol. = 284 x 168 x 1.1 = 52,583 L
Tank vol. = 75 x 168 x 1.1 = 12,600 gal
Auxiliary Fuel Tanks
Note: Additional clarification is needed for C175. Reference TMI and the special instructions REHS4726.
Auxiliary fuel tanks, service tanks and day tanks are secondary fuel tanks located between the main fuel
tank and the engine. These tanks are required in the following situations.
– The main fuel tank is located on the same level but more than 15 m (50 ft) away.
– The main fuel tank is located 3.7 m (12 ft) or more below the engine.
– The main fuel tank is located above the engine fuel injectors.
Any of the above conditions can cause unsatisfactory engine starting and operation. The purpose of an auxiliary tank is to relieve the fuel pressure “head” from the fuel transfer pump and injection equipment for efficient fuel flow.
A manual fuel priming pump, offered as an attachment, or an electric motor-driver boost pump may allow operation under conditions more severe than those previously described; but where starting dependability is required, Caterpillar recommends the use of an auxiliary fuel tank.
Auxiliary tanks offer convenient and ready fuel storage while providing a settling reservoir for water, sediment and sludge. An example of an auxiliary fuel tank is shown in Figure 4.
Fuel Service Tank or Day Tank
Auxiliary tanks such as fuel service tanks or day tanks can be quite simple. It usually consists of a small metal tank, either floor or wall mounted, in the immediate vicinity of the engine. The tank is usually sized to hold approximately eight hours of fuel, based on the engine’s fuel consumption rate at full load.
Refilling can be accomplished by gravity, a hand pump, or with a motor-drive pump. Motor-drive
pumps can be either manually or automatically controlled. For convenience and safety, automatic control is usually employed using a float-actuated, electric motor-drive pump.
The refilling pump can be positioned either at the auxiliary tank or at the main tank outlet. The performance capability of the pump must be considered during placement.
Features of the auxiliary tank, as shown in Figure 5, should include the following.
- Fill line – Located above the high fuel level, with outlet baffled to prevent agitation of sediment in the tank.
- Delivery line – Located near the bottom but not so low as to pick up collected sediment or condensation.
- Return line – To carry excess fuel back to the auxiliary tank. Should have its outlet baffled for the reason described above.
- Overflow line – Allows excess fuel to return to the main tank in event of overfilling of the auxiliary tank.
- Vent line – Allows air pressure to equalize as tank is drained or filled (vent cap should be located away from open flame or sparks).
- Drain valve – Allows removal of condensate and sediment.
- Sight glass or float-type gauge – Provides a positive check on fuel level.
To prevent damage to the fuel filter housings, the return line should have no valves or restrictions to allow dangerous pressure buildups.
Flexible rubber hoses, used as fuel return lines, should be supported to prevent closing off over time due to
weight of the hose and fuel. Hard fuel lines prevent this problem, but a flexible connection is still required to isolate vibration between the line and the tank.
A nonflammable tank mounting will maximize fire protection.
The overflow line should be at least two pipe sizes larger than the fill line. To simplify engine maintenance, a shut-off valve in the supply line is useful.
The delivery line, carrying the fuel to the engine-mounted fuel transfer pump, and the return line, carrying excess fuel back to the tank, should be no smaller in size than the respective fittings on the engine.
Larger fuel supply and return lines ensure adequate flow if the fuel tank supplies multiple engines over 9 m (30 ft.) away from the tank or when temperatures are low. Consult general dimension drawings for the sizes for each model.
It is important that the fuel return line is sloped down to the tank with no traps or obstructions in the line.
If this is not done, the fuel system is prone to air-lock with consequent hard-starting.
The auxiliary tank should be located so that should be close enough the level of the fuel when the tank is full is no higher than the injection valves. On electronic unit injector fuel systems, static pressure on the fuel system will cause fuel to leak from the injectors leading to excessive fuel dilution of the engine oil. Static pressure would allow fuel
to leak into the combustion chambers in the event of injection valve leakage. The tank to the engine so that the total suction lift to the transfer pump with the fuel at low level, plus the line loss of the supply line, is less than the fuel pump’s maximum suction lift capability. This figure should be minimized for better starting. A float valve or solenoid valve in this type of day tank regulates the fuel level to keep it below the level of the injectors.
Note: For C175 installations that are set up such that excess fuel from the engine returns to the main tank and also require the fuel supply day tank to be located higher than the main tank, a check valve may be required to be installed in the return line to prevent fuel drainage and loss of prime. Reference REHS4726.
Fuel Head Limiting Tank
If overhead mounting is unavoidable, include an open/close solenoid shut off valve in the supply line and a 3.45 kPa (0.5psi) check valve in the return line. Be sure return restriction does not exceed 350kPa (51psi) at speed and load.
Base Mounted Tanks
Base mounted day tanks are sometimes used to provide a convenient and close source of fuel with adequate capacity for four to eight hours of operation. While minimizing the floor space needed for fuel storage, the height of the engine will increase significantly with this option designed to ease maintenance.
Fuel returning to the main tank may, because of its volume, aid with cooling, but returning to the day tank is permissible.
Diesel Fuel System Design Considerations
Diesel fuel supply systems must ensure continuous and clean supply of fuel to the engine’s fuel system.
The recommended diesel fuel supply system typically has three major components: a fuel storage system, a fuel transfer system and a fuel filtration system. The three component systems provide clean operating fuel to the engine.
ACERT Technology
Caterpillar ACERT Technology improves diesel engine performance.
This technology provides precise control over a range of combustion variables, which can be regulated to produce higher performance with fewer emissions. This new technology works with the MEUI, HEUI and Common Rail fuel
systems.
Common Rail Fuel System
Unlike the MEUI fuel system, in a common rail fuel system injection pressure is created external to the
unit injectors in a high-pressure fuel pump which is driven off the engine.
The pump pressurizes a high-pressure fuel manifold that runs along both sides of the engine feeding high pressure fuel to the injectors. The electronic fuel injectors at each cylinder control the delivery and timing of the fuel injection(s). Similar to some other systems, the common rail fuel system has capability of multiple injections for a given combustion event.
The main components of a common rail system include the high-pressure pump, the high-pressure lines and rail system, and the injectors. The low-pressure fuel system utilizes similar components to the unit injector fuel system. See Figure 2 for a schematic of the common rail fuel system.
The common rail fuel system does not continually circulate fuel through the entire system like the unit injector fuel system. Instead, small amounts of fuel are bypassed during the injection event. Due to the very high pressure in the fuel manifold, more heat is put into the fuel than on previous systems. Because of the additional heat added to the fuel, it is critical that the fuel inlet temperature is maintained within guidelines provided for the engine model. Recommended, and sometimes required, is the use of a fuel cooler to maintain the appropriate inlet fuel temperature.
Otherwise, the overheated fuel will have very low viscosity and film strength which makes the fuel system components, especially the injectors, more susceptible to damage from fuel contaminants and wear, hence the importance of proper filtration practices on common rail engines.
HEUI Fuel System
The Hydraulically actuated Electronically controlled Unit Injectors (HEUI) use a hydraulic pump and engine oil to generate fuel injection pressure, and an ECM to control the pressure and amount of fuel injected into the cylinders.
The operation of the HEUI fuel system is completely different from any other type of fuel system that is actuated mechanically. The HEUI fuel system is completely free of adjustment. Changes in performance are made by installing different software in the ECM.