Fault Investigation

APU fuel cluster oil leakage


During operation, the differential pressure between the fuel pump inlet and outlet presses the gear set and bearing against the inlet side of the pump housing. Excessive wear of bearings (60) and (65) can lead to the tilt of gear shaft (105), which results in the displacement of the gear shaft end and the improper working of packing (100B) and (115B). The gear set and bearings are preloaded with compression wave spring (70) and (85). This maintains a preload on the gear set to limit play and prevent fretting and impact damage due to vibration. Malfunction of wave spring (70) and (85) can cause the centerline of gear shaft (105) to be out of parallel with the gear bore centerline. Either of the two above mentioned effects can cause side-loading of gear shaft resulting in failure of packing sealing. Also, tilt of gear shaft (105) can result in the tilt of boost pump impeller (40) causing contact of impeller vane and plate (50) and excessive wear of plate (50).

The third failure mode is the failure of packing (120) between fuel pump outer surface and fuel pump bore in fuel cluster.

Test results indicate that the worn and chipped seal is the primary contributor to the fuel cluster CU-A1 oil leakage. Severe wear on impeller outlet plate (50) indicates that there existed side-loading in fuel pump gear shaft (105), which resulted from excessive wear of bearings and failure of wave spring after fuel cluster runs for a long time. Side-loading of the gear shaft caused the improper packing sealing at the spline shaft, which is another contributor to the fuel cluster CU-A1 oil leakage. Aging and extrusion of packings can also contribute to the oil leakage.

Conclusion
It is can be concluded that oil leakage from APU fuel cluster CU-A1 was caused by the excessive wear and chipping on plain encased carbon seal, wear on bearings, failure of the wave spring and aging and extrusion of the packings.

Corrective/preventive actions
Root causes were not detected the first two times when fuel cluster CU-A1 was returned from field for oil leakage. The reason is that our facility didn’t have pressurizing lubrication system on an old test stand. The third time fuel cluster CU-A1 was tested, on a new test stand which has a pressurizing lubrication system, the test stand had just been set up and had no draining oil collecting system. So the root cause was not detected due to the difficulty in visually checking draining oil in the test stand. A draining oil collecting system was designed and installed to improve the oil leaking inspection on the current test stand. Draining oil can be measured with a measurement cup.

If oil or fuel leakage is reported, the unit will be disassembled such that all seals will be inspected and replaced as required, thus not just limited to the external packings (120). See Figure 1. The fuel pump will be removed from the fuel cluster, disassembled, the seal, seal rotor, bearing and wave spring will be inspected. All packings will be replaced.

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