Operational experience has shown that 4 stroke engines operating heavy fuels being bunkered from various ports globally and low load operation of the engine tend to cause fouling of the turbocharger.
The condition and cleanness of the turbine of an exhaust -gas turbocharger have a decisive influence on the efficiency, the performance of the combustion process and hence on the service data of the engine which will be negatively affected.
Possible consequences can be, but not limited to:
- overspeed of the turbine
- vibration excitation of the turbine caused by deposits
- reduced engine efficiency
- increased fuel consumption
Fouling of the turbine side of the turbocharger will, in its first phase, manifest itself in increasing turbocharger revolutions on account of increased gas velocity through the narrowed nozzle ring area. In the long run, the charging air quantity will decrease on account of the greater flow resistance through the nozzle ring, resulting in higher wall temperatures in the combustion chambers.
A high contamination level with deposits will cause an offset to the normal gas flow resulting in increase in temperature level before the turbine or in extreme case cause surging. This can lead to significant increase in unfavorable excitation orders of vibration which could advance a severe damage of the turbocharger by causing fatigue on the turbine rotor vanes.
Service experience has shown that the turbine side is exposed to increased fouling when operating on HFO.
The rate of fouling and thereby the influence on the operation of the engine is greatest for small turbochargers where the flow openings between the guide vanes of the nozzle ring are relatively small. Deposits occur especially on the guide vanes of the nozzle ring and on the rotor blades.
In accordance with the operating conditions of the engine, it is sometimes necessary to free both nozzle ring and the blading of the turbine from deposits and other adhesive particles from the combustion process.
This mechanical cleaning is normally carried out in the course of the specified maintenance intervals (general turbocharger overhaul). The turbine rotor and nozzle ring of the turbocharger have to be dismantled for the cleaning process.
Turbine side of the turbocharger can be cleaned effectively by combination of wet water washing and dry nut shell cleaning, hence it is recommended to carry out wet as well as dry cleaning of turbochargers.
Wet cleaning of the turbines during low-load engine operation has been used for a long time in order to achieve the specified time intervals between the turbocharger inspections and to protract the time-consuming manual cleaning of turbine and nozzle ring.
Wet cleaning is normally carried out about every 250 hours, but it can be executed in shorter or longer intervals depending on the operating conditions. Regular performance observations will show the trend in charge air pressure and exhaust gas temperatures, and define the cleaning intervals for the turbine.
To carry out the wet cleaning process, the engine load has to be significantly reduced and the exhaust gas temperatures have to be put into steady-state condition. The preparation and execution of the wet cleaning process easily adds up to approx. 1 hour during which the full engine load is not available.
The wet cleaning tool would be connected to the same connection as dry cleaning, through a snap coupling.
As all injected water is not evaporated, turbochargers need to be fitted with a drain from the exhaust gas outlet. The necessary water flow is dependent on exhaust gas flow and temperature and must be so high that all water does not evaporate, also the water flow must not be so high that the turbine wheel is drowned and stops rotating. The washing sequence should be in accordance with turbocharger manual and the engine load, exhaust gas temperature before turbine and the turbine speed must be as per same manual.
The wet washing kit comes with different nozzle orifice which should be choose as per engine size. If you want and are interested for more in depth information regarding nozzle selection and water flow adjustment you need to subscribe to Seafarer’s World Forum (powered by chiefengineerlog.com).
The dry cleaning of the turbine is carried out by blowing in granulate (nutshells or activated charcoal) of a specified particle size and volume – the volume depends on the turbocharger type – into the exhaust pipe before turbine under operating load by means of compressed air and permanently attached nozzles. The high kinetic energy produced during the impact of the granulate onto the nozzle ring and turbine blading causes the deposits to flake off. It is not necessary to reduce the engine load for the cleaning process. If you want and are interested for more in depth information regarding procedure and flow adjustment you need to subscribe to Seafarer’s World Forum (powered by chiefengineerlog.com).
Practical experiences gained over the years have shown that the intervals of wet cleaning can be extended when using the dry cleaning system.
It also became apparent that the increase of the cleaning interval for dry cleaning – from once a day to twice a day – results in a significant improvement of the cleaning result when using highly coating -forming heavy oil fuels.
Benefits of dry cleaning compared with wet cleaning:
- turbine can be cleaned at full load – thus no losses at all in the operational readiness of the engine;
- short execution time of the cleaning procedure (approx. 20 min);
- frequent use of the cleaning process effectively prevents deposits at nozzle ring and turbine blades
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