Description Of The Design Of Airframe Boost Pumps

Figures 3 and 4 present a cross section view and a photograph of an electric motor-driven boost pump assembly that is used on a commercial airliner. This pump utilizes a snorkel inlet. The design of this pump incorporates cartridge type pump and discharge valve modules that greatly enhances the maintainability of these pumps. The elliptical shaped main housing is a semi-permanent assembly in the airframe fuel tank. The pump modules are easily removable from it without draining the fuel tank through an interlocking inlet valve mechanism that is actuated by the removal action of the pump modules. Except for a screwdriver to remove the cover plate, no tools are required for this maintenance function. The removal of the discharge valve modules are similarly enhanced by this type of design. The immense fuel loads carried by commercial airliners clearly emphasizes the desirability of these maintenance features.

In the interest of weight, all of the housing and structure of the pump assembly are made out of aluminum. Steel is used for high stress parts such as shafts and fasteners and the electric motor laminations. Fuel is circulated throughout the pump cartridge to lubri-

FIGURE 4 Main boost pump assembly (Courtesy Hamilton Sundstrand)

cate the bearings and cool the motor. This fuel is returned to the tank through a port that incorporates a flame arrestor.

In the normal operation of the aircraft, fuel tanks will be run dry by the selective usage of fuel from the various fuel tanks to maintain aircraft balance. Under these conditions, the fuel pumps in these tanks will run dry. To simplify the management of the fuel system, it is normal practice to let the pumps run dry for the remainder of the flight. It is therefore necessary to provide a bearing system for the pump, which will accept continuous dry running without detrimental results to the pumps. This is generally accomplished through special configurations of carbon journal and thrust bearings with chrome plated journal and thrust running surfaces.

Because these pumps are located in the aircraft fuel tanks and are totally immersed in fuel, their safety and explosion-proof features are of utmost importance. All openings that communicate the pump electric motor cavity with the interior of the fuel tank must incorporate flame-arresting features. This includes communication through the pumping elements to the pump inlet. Also non-resetable thermal fuses are incorporated in the motor end turns to ensure the motor is disconnected before it can reach the minimum auto-ignition temperature of jet fuel (approximately 390°F/199°C) through various failure modes. It is general practice to provide two electrical insulation barriers between all points of different electrical potential.

Electric motor driven boost pumps are in use in sizes up to 200 gpm (45.42 m3/h) flow and at pressure rises ranging from 10 to 50 lb/in2 (.69 to 3.45 bar) with a motor out power of up to 6.5 hp (4.85 kw).

Electric motors of 8, 6, and 4 poles designs are in general use. The synchronous speeds of the motors with 400 Hertz power are 6000, 8000, and 12000 rpm respectively. The constant demand for smaller and lighter components has seen the increased application of 6- and 4-pole motors in recent years.

The overall weight per pump element (impeller, motor and housings) ranges from 7 to 25 pounds (3.17 to 11.34 kg). The number of pump elements used on large commercial aircraft in service today ranges from four to sixteen.

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