Description Of The Design Of Main Fuel Pumps

In this subsection, the design of a main fuel pump utilizing an external spur gear high-pressure element for application on engines powering modern airliners will be discussed. The designs of main fuel pumps for engines powering other classes of aircraft such as military fighters, executive aircraft, and helicopters are similar and differ mainly in the

FIGURE 8 Vapor/liquid ratio versus inlet pressure (m = ft X 0.3048; bar = psi X 0.06895; °C = (°F - 32) X 0.556; mm Hg = in Hg X 25.4)

details of size and specific operating requirements. The flow and component schematic of a typical commercial engine main fuel pump is presented by Figure 10. The components encompassed by the dotted square are contained within the pump. As shown, the fuel from the airframe system enters the boost stage inlet. It then passes through the engine oil-to-fuel cooler where the fuel absorbs rejected engine heat, thereby fulfilling its function as a heat sink. This positioning of the oil-to-fuel cooler also provides the de-icing function for the next component in the system, the engine fuel filter. Using the engine oil-rejected heat for the de-icing function eliminates the need for fuel heaters using engine compressor bleed air as the heat source. These types of systems were used on previous generations of engines incurring weight and engine efficiency penalties.

Fuel filters are usually rated at 10 microns nominal and 40 microns absolute. The minimum filter surface area is usually determined by the worst icing condition. A filter bypass valve is provided to ensure continuous engine fuel flow in the event the filter becomes blocked. An indicator is provided to indicate impending filter bypass so maintenance actions can be initiated.

The fuel then enters the high-pressure gear stage element and is delivered to the engine fuel control unit at a pressure determined by the downstream resistance characteristics. A high-pressure relief valve is utilized for burst protection for the high-pressure components of the fuel system in the event of a downstream blockage.

VERTICAL ptPK PLC¥ CONDITIONS HORIZONTAL PUB FLOW CONDITIONS

FIGURE 9 Possible multiphase flow conditions at pump inlet

VERTICAL ptPK PLC¥ CONDITIONS HORIZONTAL PUB FLOW CONDITIONS

FIGURE 9 Possible multiphase flow conditions at pump inlet

The control of the engine fuel flow is achieved in the fuel control system by bypassing the excess fuel back to the fuel pump upstream of the fuel oil cooler. This returned flow represents waste heat that is proportional to the quantity of the bypassed flow and the pressure differential between the gear stage and boost stage discharge pressures. This waste heat further increases the gear stage fuel inlet temperature.

The volumetric flow characteristics of positive displacement pumps closely match the fuel flow requirements of aircraft gas turbine engines. The ideal volumetric flow rate of a positive displacement pump is directly proportional to its rotational input speed. In actual practice, this is somewhat modified by the internal leakage flows that occur from the high-pressure discharge to the low-pressure inlet. These leakage flows are a combination of flow through the clearances between the pump parts and the necessary bearing lubrication and cooling flows. Also, leakage flows are proportional to the pressure rise across the pump and essentially constant over the operating speed range of the pump. Therefore, the leakage flows are a higher percentage of the ideal flow for lower percentages of rated pump speed. The percentage of the ideal flow that the delivered flow represents is the volumetric efficiency of the pump. Figure 11 depicts the basic relationship between the output characteristics of a positive displacement pump and the engine fuel flow requirements.

As previously discussed, the main fuel pump is required to provide both the engine burn flow requirements and the engine geometry actuation flow requirements. For a given class and type of engine (for example, commercial airline turbo fan engines), the engine geometry actuation flows are usually an essentially constant value for all engine speeds for a given number of actuator servos. Therefore, because of the volumetric characteristics of positive displacement pumps, the displacement sizing point of the high-pressure element for larger engines will tend to be the rated take-off power high pump rotational speed condition. For smaller engines, it will tend to be the starting low pump rotational speed condition. The example in Figure 11 is sized at the starting condition. The disadvantage of this type of pump concept, clearly shown in Figure 11, is the significant overcapacity of the high-pressure element for low engine power conditions such as the idle condition. This over-capacity represents the quantity of waste heat in the form of throttling loss in the bypass control loop that complicates the engine heat management system. The task is to avoid reaching the fuel thermal stability temperature limit (about 325°F/163°C) in the combustor fuel nozzles. Exceeding this limit will result in clogging the nozzles with fuel "coking" deposits. Significant effort has been expended on alleviating this problem through the application of variable displacement pumps. These efforts have not achieved general acceptance because of reliability, safety, and cost disadvantages.

FIGURE 10 Main fuel pump system schematic

Figures 12 and 13 present a photograph and a cross-section of a main fuel pump incorporating an external spur gear high-pressure stage and centrifugal boost stage. The pump is mounted directly to the engine gearbox and the rotational input power is transmitted directly to the spur gear stage by a spline coupling. The boost stage is driven by a secondary splined coupling. The drive priority is selected to reflect the power input order of the elements and that the high-pressure element is the primary pumping element. The housings are aluminum castings that provide the minimum weight and also provide the necessary structural integrity and stiffness for all specified conditions.

A face-type dynamic mechanical shaft seal is provided at the drive end of the pump. A wide range of design configurations have been employed, but all modern pumps utilize the sealing faces as the primary seal and "O" rings for the secondary seal. These seals are required to operate with pressure differentials in both directions. Special design features must be employed to accommodate this requirement. The drive shaft seal is vented to a

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Survival Treasure

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