2008/01/07 Caterpillar

Engine:

3512B (S/N: S2H1-UP; S2L1-UP; 4TN1-UP; 2EZ1-UP)

3516B (S/N: 8KN1-UP)

Introduction

This article deals with the cause and effect of certain engine operating conditions that lead to the formation of deposits (oil coking) on the turbine shaft on 3512B/3516B Marine Propulsion engines with GT60 turbochargers. Failure to follow the recommendations in this article will result in damage to the turbocharger.

Affected Components

The turbocharger shaft exhibits carbon buildup. The inside diameter and the outside diameter of the turbine bearing exhibit carbon buildup.

Illustration 1 and Illustration 2 represent examples of damage that can occur to the turbocharger shaft and the turbocharger bearing.

Illustration 1	g01446985
The arrows show carbon deposits on the turbocharger shaft.

Illustration 2	g01445138
Turbocharger bearing

Table 1 lists the parts that are affected by oil coking.

Potential Causes of Excessive Carbon Buildup

When the engine is operated in any environment that causes the temperature of the lubrication oil for the turbocharger to rise, the potential for the above conditions will exist. The lubrication oil will start to "crack" at a defined temperature. Carbon from this process will adhere to available hot surfaces. Carbon will begin to buildup on the shaft and on the bearings of the turbocharger. Elevated levels of copper and soot in the lubrication oil will cause the lubrication oil to "crack" at a lower temperature. This will cause the oil to adhere to the surrounding surfaces.

The following conditions can cause excessive carbon buildup:

• Operating with elevated copper and soot levels.

• Aeration of the cooling water

• Hot engine shutdown

• Short cool down periods

• Entrapped air in the cooling system for the turbocharger

Oil coking that is deposited on the shaft journal or on the bearing surface will result in a reduction in the clearance between the bearing and the journal. This will eventually cause the shaft to seize.

Corrective Measures

Oil coking only occurs in a very limited number of engines. Oil coking usually occurs in applications when the above listed causes for carbon buildup exist.

The recommended corrective measures include actions in order to remedy one or all of the parameters that create the formation of lube oil coke to the turbocharger shaft or to the turbocharger bearing.

Recommendations for the Elimination of Turbocharger Shaft or Bearing Failures Due to Oil Coking

• Pay increased attention to the maintenance of the lubrication oil. Use trend analysis in order to fully understand the effects of the engine operating conditions. Decrease the S·O·S oil analysis sampling interval in order to confirm increased levels of copper or soot. Maintain the lowest level of copper and soot in order to minimize the potential for carbon buildup on the shaft and the bearing components. The maximum allowable levels of copper and soot for your application are dependent on a combination of trend analysis and the amount of carbon deposit buildup on the shaft and on the bearing surfaces. The acceptable levels of copper and soot will vary because of engine operating conditions and maintenance practices. Under certain operating conditions, carbon deposits may form if the levels of copper are above 60 to 100 parts per million (ppm). Maintain Ultra Fine Mist (UFM) levels of soot below 35 UFM (1.2 percent by weight).

• Evaluate the cooling system for the turbochargers. Ensure that the water system is properly vented. Ensure that the expansion tank and the deaerator system are sized correctly. Ensure that these systems are functioning correctly.

• Talk with the operator about the occurrence of an engine shutdown under a high temperature condition and general engine stopping procedure. It may be beneficial to allow more time for the engine to cool down.

• Evaluate occurrences of warnings for high exhaust port temperature or for engine shutdown due to a high temperature condition. Determine if measures can be taken in order to lower these levels. Some of these measures would include the following: cleaner filters, cooler outside air and cleaner aftercoolers.

Failure to follow the recommendations in this article will result in damage to the turbocharger.