The fuel system on a diesel engine is a highly specialized set of components which…
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
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.
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.
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.
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).
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.