Bearing - Case Sudies

The first case study is of a large instrument air 450 rpm reciprocating compressor for a large power plant. It supplies instrument air to an air-receiver vessel (downstream of compressor) which feeds instrument air for various continuous and intermittent requirements. The compressor train is started and stopped by means of a pressure of air-receiver (instrument air remaining volume), a classic design used on many units.

The compressor uses oil-free technology, which includes piston and packing work without any lubrication. However, the machine crankshaft system still requires a pressurized lubrication system, using a shaft-driven oil pump. Given its large size, the compressor uses sleeve-type bearings. In this example, the compressor suffers from bearing failure. The bearings were totally black and babbitt was not found on the bearing shells. At the same time, high vibration was not reported. It was confirmed that the bearings were correctly selected and installed. An extensive investigation showed that the power plant's actual instrument air consumption increased five times compared to its design value. This increase was mainly due to purge flow increases of the generators and other electrical equipment. It resulted in the compressor starting and stopping five times more often than the compressor design condition's specified value. In other words, since the flow produced by the compressor is constant, the increased consumption flow reduced the pressure in the air-receiver more rapidly and resulted in more frequent stops and starts. This in turn put the bearing under more transient stresses, resulting in an extended period of time when lubrication was lacking and loads were excessive (during transient situations). This in turn led to bearing failure.


The compressor vendor confirmed this compressor model was originally designed for continuous operation. It can also be operated for long durations of intermittent operation. But it cannot afford five times more starts/stops. The proposed solution involved changing the compressor control philosophy by eliminating stop/start and by operating the compressor continuously and using a bypass control valve to suction to the maintain air-receiver pressure.

A second case study involves oil contamination in the bearing brackets of a steam turbine. Water in the oil caused several bearing failure. The source of the water is from the carbon ring seal leakage of steam into the bearing bracket. Knowing that the main reasons for problems are usually based on operational changes or assembly/installation issues, these potential causes were checked first. A detailed analysis confirmed the trouble was design-related. Specifically, the carbon ring seal system (carbon ring seals and bearing bracket isolator) was not designed to prevent oil contamination in the bearing bracket. For this case, two solutions were proposed:

A bearing isolator to positively prevent steam condensate from entering the bearing bracket
An educator system to positively prevent leakage from the seal assembly.

Generally the first option is preferred. In this case, the first option was selected and implemented.