Main Turbine Lubricating Oil System

Tin oxide corrosion occurs mainly on tin-based white-metal (Babbitt) bearing surfaces, particularly when there is water or salt-water contamination, combined with high temperature and pressure.

Effects

Formation of a hard oxide film

• A black or dark-brown oxide film forms on the white-metal bearing surface. Unlike normal Babbitt, this oxide layer is very hard.

Loss of embedability

• The hard oxide layer destroys the embedability of the white metal. As a result, dirt and wear particles can no longer become safely embedded in the soft bearing surface.

Reduction in bearing clearance

• The oxide layer builds up on the bearing surface and reduces the bearing clearance, interfering with the formation and maintenance of the proper lubricating oil film.

Abrasive damage from detached oxide particles

• Pieces of the hard oxide layer may break away and circulate with the lubricating oil. These particles can cause abrasive scoring of the thrust collar/journal and other bearings.

Overheating and bearing failure

• The disturbed oil film can cause local overheating, wiping and eventual bearing seizure or failure. In a thrust bearing, detached oxide particles may become trapped in the oil wedge, further restricting the oil film and causing overheating.

Main Causes

The important causes to consider are:

• Water or salt-water contamination of the lubricating oil
• Presence of chlorides
• High bearing temperature and/or load
• Unsuitable lubricating oil or additives

If tin oxide corrosion is detected in the high-pressure (HP) turbine thrust bearing, the following actions should be taken:

Reduce or stop the turbine as necessary

• Reduce the turbine load or stop the turbine as required by the manufacturer's instructions to prevent further bearing damage. Closely monitor the thrust-bearing temperature and lubricating-oil pressure.

Inspect the thrust bearing and pads

• Inspect the thrust bearing and pads to determine the extent of tin-oxide formation. Also check the thrust collar/runner for scoring or other damage.

Drain and replace contaminated oil

• Drain the contaminated lubricating oil and replace it with clean oil. The source of water or salt-water contamination must be identified and eliminated.

Thoroughly clean and flush the complete oil system

• Clean and flush the bearing housing, oil reservoir, oil lines and associated lubricating-oil system to remove tin-oxide particles and other contamination.

Renew damaged components and check clearances/alignment

• Renew badly affected thrust pads and repair or replace any damaged thrust collar/runner. Check the bearing clearances and alignment before returning the turbine to service. Lowering the oil temperature may also help prevent further formation of tin oxide.

Important Point

Simply scraping or polishing the visible black deposit is not sufficient. The complete lubricating-oil system must be cleaned and flushed because detached hard oxide particles may remain in the system and continue to cause abrasive damage.

The filtration size is not selected simply to obtain the finest possible filtration. It is selected according to the component being protected, required oil cleanliness, and the flow and pressure characteristics of the system.

1. Bearing Clearances

The filter must be capable of removing particles that are large enough to damage the journal or thrust-bearing oil film.

Therefore, smaller bearing clearances require finer filtration.

2. Type and Sensitivity of the Component

Different components have different tolerances and sensitivity to contamination. These may include:

• Main turbine journal and thrust bearings
• Reduction gears
• Hydraulic and governing equipment
• Servo and control valves

Precision hydraulic and control components generally require finer filtration than large and more robust components.

3. Minimum Oil Passage or Orifice Size

The filter must prevent particles large enough to block small drilled passages, restrictors and orifices from reaching these components.

Turbine lubricating-oil systems contain many relatively small oil passages, which can readily become blocked by contamination.

4. Required Oil Cleanliness

The required oil cleanliness level, such as the specified NAS or ISO cleanliness level, determines the degree of filtration required.

Turbine oils are generally maintained to very high cleanliness standards because contamination can cause damage to bearings and control systems.

5. Oil Flow and Permissible Pressure Drop

A very fine filter provides better particle removal, but it also produces a greater pressure drop and may become blocked more quickly.

Therefore, the selected filter size must be compatible with:

• The required oil flow
• The allowable differential pressure
• The expected contamination level

The filter must not restrict the oil supply to the machinery.

6. Location and Purpose of the Filter

Filters at different locations in the lubricating-oil system may have different filtration requirements:

• Pump suction/strainers: Relatively coarse, mainly to protect the pump.
• Main LO supply: Finer filtration to protect the turbine bearings.
• Control/governor/servo oil: Often still finer because of the small clearances and sensitive valves.
• Oil purification/off-line filtration: Can use very fine filtration because it is not necessarily restricted by the full operating oil flow.

Overall Principle

The filtration should be as fine as necessary to protect the most sensitive downstream component, but not so fine that excessive pressure drop or premature filter blockage compromises the lubricating-oil supply.

For this reason, turbine filter elements are available in different mesh and micron sizes, allowing the filtration level to be selected according to the requirements of each part of the system.