4.2.2 EROSIVE WEAR FROM PARTICLES IN THE LUBRICATION OIL
ABB has vast field experience with the RR..1 type turbocharger using a similar bearing and lubrication arrangement as utilized in the TPS..D/E. This experience could directly be applied for the design of the TPS..D/E bearing system.
The axial bearing of the TPS..D/E was designed for higher axial forces created by the high pressure ratios of this turbocharger type and - compared to the RR.. 1 - similar or larger oil films between the bearing surfaces could be realized. The clearances in the radial bearings were optimized and are in the same range in both turbocharger types.
From the RR..I type turbocharger it is known that larger particles in the lubrication oil system can create wear of the axial thrust bearing as well as the radial bearing bushes. If the wear exceeds critical limits, instabilities can occur in the dynamic rotor behavior resulting in bearing damages.
Such damages are extremely rare and can be prevented by correct lubrication oil filtration and treatment. Based on field experience and similar bearing design a lubrication oil filter of 34μm (mesh size) has been specified for the TPS..D/E. In diesel and MDO operations courser filters can also be used if the turbocharger is not operated at the highest pressure ratio.
In field tests conducted with the TPS48 turbocharger on HFO engines using an automatic 34[tm filter no bearing wear has been observed either in the axial or in the radial bearings after 10,400rhrs (see Fig.11).
4.2.3 CORROSION OF THE BEARING MATERIAL
Experience has shown that corrosion of the bearings can occur, particularly on gas engines running with waste gas. This corrosion can be prevented by the use of resistant materials and also here the RR.. 1 experience could directly be applied.
In the TPS..D/E special brace and bronze alloys are used which are known to be well suited for all known lubrication oil conditions including those found in gas engines.
4.3 COMPRESSOR AND TURBINE END SEAL OIL LEAKAGE
The compressor and turbine end seals have to operate under extreme engine idling conditions without any oil leakage. Tests for this purpose were run under conditions ensuring extreme pressure drop across the corresponding seals.
The results showed that even after long running periods no oil leakage was detected. The same results were found under pre-lubricated conditions, on a test rig as well as on actual engines, after operating for hours at a time, which often occurs for stand-by applications. A further indication of proper seals was a measured air-blow-by of less than 0.2% of the compressor air flow.
4.4 TURBINE CLEANING AND EROSION OF NOZZLE RING
The TPS..D/E turbocharger series was initially designed for operation with heavy fuel oil and therefore special features such as turbine cleaning and nozzle ring protection were developed.
Compared to the operation on a standard diesel or MDO engine where mainly soot particles are present in the exhaust gases, the exhaust gases of HFO engines contain large amounts of different contaminant types which are transported to the turbine of the turbocharger. Particularly sodium (Na) and vanadium (V) compounds are known to be responsible for erosive wear and hard sticking contamination deposits which are mainly accumulated on the nozzle ring blades. Shorter component lifetimes, lower efficiencies and a reduced turbine flow area are the known consequences. In addition, contamination of the turbine blades results in increased unbalance levels which create higher loads on the radial turbocharger bearings.
In special test projects ABB has investigated the wear and contamination mechanisms of heavy fuel combustion residues on turbocharger components.
As a result of these tests it is known that contamination layers are accumulated mainly at higher exhaust gas temperatures and engine loads and are therefore more often observed on power plant applications running for extended periods of times at high loads. Depending on the amounts of sodium and vanadium in the fuel, compounds formed by these elements are present in the exhaust gases in a fluid state and can, under these conditions, solidify on the turbocharger components resulting in the accumulation of very hard deposit layers.
To prevent the accumulation of such contamination layers on the turbine side parts, wet cleaning of the turbine (water injection) is used in the TPS..D/E turbocharger. The cleaning effect is based on a thermal shock principle in conjunction with a subsequent flushing phase.
