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2.6. BEARINGS

2.6.1. GENERAL

Introduction :

The MS 9001 E gas turbine unit contains three main journal bearings used to support the gas turbine rotor. The unit also includes thrust bearings to maintain the rotor-to-stator axial position. These bearing assemblies are located in three housings : one at the inlet, one in the compressor discharge casing, and one in the exhaust frame. All bearings are pressurelubricated by oil supplied from the main lubricating oil system. The oil flows through branch lines to an inlet in each bearing housing.

BEARINGS

BEARING NO.

CLASS

TYPE

1

Journal

Elliptical

2

Journal

Elliptical

3

Journal

Elliptical

1

Loaded thrust

Self-aligned

1

Unloaded thrust

Tilting pad

Lubrication :
The three main turbine bearings are pressure-lubricated with oil supplied by the 12540 liters capacity lubricating oil reservoir. Oil feed piping, where practical, is run within the lube oil reservoir drain line, or drain channels, as a protective measure. This procedure is referred to as double piping and its rationale is that in the event of a pipe-line leak, oil will not be lost or sprayed on nearby equipment, thus eliminating a potential safety hazard.

When the oil enters the bearing housing inlet, it flows into an annulus around the bearing liner. From the annulus the oil flows through machined slots in the liner to the bearing surface. The oil is prevented from escaping along the turbine shaft by labyrinth seals.

Oil seals :
Oil on the surface of the turbine shaft is prevented from being spun along the shaft by oil seals in each of the three bearing housings. These labyrinth packings and oil deflectors (teeth type) are assembled on both sides of the bearing assemblies where oil control is required. A smooth surface is machined on the shaft and the seals are assembled so that only a small clearance exists between the oil and seal deflector and the shaft. The oil seals are designed with two rows of packing and an annular space between them. Pressurized sealing air is admitted into this space and prevents lubricating oil from spreading along the shaft. Some of this air returns with the oil to the main lubricating oil reservoir and is vented through a lube oil vent.


2.6.2. DESCRIPTION

• No. 1 bearing
The no. 1 bearing assembly is located in the center of the inlet casing assembly, and contains three bearings ; (1) active (loaded) thrust bearing, (2) inactive (unloaded) thrust bearing, and (3) journal bearing. Additionally, it contains a floating or ring shaft seal, labyrinth seals, and a housing in which the components are installed. The components are keyed to the housing to prevent rotation. The housing is a separate casting.

The no. 1 bearing assembly is centerline supported from the inner cylinder of the inlet casing. This support includes ledges on the horizontal and an axial key at the bottom centerline. The upper half of the bearing housing can be removed for bearing liner inspection without the removal of the upper half inlet casing. The lower half of the bearing assembly supports the forward stub shaft of the compressor rotor.

The labyrinth seals at each end of the housing are pressurized with air extracted from the compressor 5th-stage. The floating ring seal and a double labyrinth seal at the forward end of the thrust bearing cavity are to contain the oil and to limit entrance of air into the cavity.

• No. 2 bearing
The no. 2 bearing assembly is centerline supported from the inner cylinder of the compressor discharge casing. This support includes ledges on the horizontal and an axial key at the bottom centerline permitting relative growth resulting from temperature differences while the bearing remains centered in the discharge casing. The lower half of the bearing assembly supports the forward wheel shaft of the turbine rotor assembly. This assembly includes three labyrinth seals at both ends of the housing. The no. 2 bearing is located in a pressurized space between the compressor and the turbine, and air leaks through the outer labyrinth at each end of the housing. The space between the two other seals is cooled by air extracted from the 5th compressor stage.

Air flows through this seal into the drain space of the housing and is vented outside the machine via the inner pipe connecting to the bottom of the housing. This drain space vent piping continues to the lubricating oil tank. The middle labyrinth prevents the hot air leakage from mixing with the oil. The mixture of hot air and cool air is vented outside the unit via the outer pipe connected at the top of the bearing housing.

• No. 3 bearing
The no. 3 bearing assembly is located at the aft end of the turbine shaft in the center of the exhaust frame assembly. It consists of a tilting pad bearing, five labyrinth seals, and a bearing housing. The individual pads are assembled so that converging passages are created between each pad and the bearing surface. These converging passages generate a high-pressure oil film beneath each pad, that produces a symmetrical loading or "clamping" effect on the bearing surface. The clamping action help maintain shaft stability. Because the pads are point-pivoted, they are free to move in two directions, which makes them capable of tolerating both offset and angular shaft misalignment.

The tilting pad journal bearing comprises two major components pads and a retainer ring. The retainer ring serves to locate and support the pads. It is a horizontally split member that contains the pad support pins, adjusting shims, oil feed orifice, and oil discharge seals. The support pins and shims transmit the loads generated at the pad surfaces and are used to set the bearing clearance. An antirotation pin extends from one edge of the lower half of the rectangular ring. This pin locates the bearing within its housing and serves to prevent the bearing from rotating with the shaft.


N° 1 Bearing Assembly

No1 Bearing

Schematic Diagram of the Equalizing Means

Leveling Plates

Pads

Load Thrust Bearing

Load Thrust Bearing

Unloaded Thrust Bearing

Unloaded Thrust Bearing

No2 Bearing Assembly

No2 Bearing

No3 Bearing Assembly

No3 Bearing

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