Fig 8- Radial shaft motion VS. turbocharger speed measured with a TPS57E
ABB has conducted extensive investigations on engine lubrication oil behavior under extreme running conditions and has also evaluated field experience from the preceeding turbocharger type, i.e., the RR.. 1.
The results were used to optimize the design of the TPS..D/E turbocharger and to define operation limits and give installation recommendations.
4.2.1 OIL COKING IN THE BEARINGS
With the RR.. 1 type turbocharger oil coking has occasionally been observed on engines running on HFO and on gas engines with very high exhaust gas temperatures. In ABB owned oil laboratories oil coking tests were conducted with samples of new and used lubrication oil from relevant HFO applications. In these tests, the oil flows under controlled conditions over a metal surface heated to different temperatures, and the weight of the coking residues on this surface is registered.
The main results are summarized in Fig.9.
The condition of the used oils was analysed and characterised by the BP Predict Oil Monitoring Service.
The results showed that considerably lower amounts of oil coking residues are created with a new oil, compared to a used oil, at high temperatures. However, at temperatures below approx. 240℃ the used oil also creates only relatively small amounts of oil coking if the condition of the oil is still good.
All tested oil brands in a used but still good condition showed a considerable increase of oil coking above approx. 240℃ and relatively little oil coking below this temperature.
This temperature can therefore be considered an upper limit for the turbocharger bearings in HFO operation. In addition used lubrication oils in critical condition were tested. Such lubrication oils show a completely different behavior and large amounts of oil coking residues are also created even at low temperatures. Under these conditions serious oil coking is possible even in normal steady state operation. The investigations have also shown that the condition of the lubrication oil can turn from a good to critical condition within less than 1,000 rhrs and that, therefore, frequent oil condition monitoring can considerably increase the operation reliability of critical applications.
Temperatures of above 120℃ and particularly in the critical range of 240℃ can be created in the TPS..D/E turbocharger bearings only after a hot shutdown. In the bearing support design of the TPS..D/E this has been taken into consideration by optimization of the heat transfer after the shutdown, and the bearing casing itself includes oil jet cooling on the turbine side to reduce the temperatures after a hot shutdown.
Several test runs on turbocharger test beds, as well as on engines, were conducted with the TPS..D/E turbocharger to determine the component temperatures after a hot shutdown under different conditions. For these tests, thermocouples were fitted to different critical locations inside the turbocharger. Before the shutdown, the turbocharger was operated at constant exhaust gas temperature. Tests were performed under insulated as well as non-insulated conditions. The influence of post lubrication was also determined. Fig.10 shows the relative temperatures measured with the TPS57E at the turbine end bearing which is aside from the piston ring area the most critical zone.
The 100% temperature value in Fig.10 varies strongly with exhaust gas temperatures and installation specific parameters such as insulation type.
It can be noted that the highest temperatures are reached after more than 30 minutes after the shutdown and that post lubrication of 20 minutes can reduce the temperature in the critical zone by approx. 18%. A general reliability benefit can be achieved with post lubrication and on some installations post lubrication is essential to prevent unacceptable amounts of coked oil inside the turbocharger.
Based on the hot shutdown tests and lubrication oil investigations operation limits were defined for the TPS..D/E turbocharger to ensure reliable operation on different engine types.
