Creating a TBN equilibrium
Creating a TBN equilibrium in a medium speed engine is depending on the engine system lubricant consumption (SLOC= Specific Lubricating Oil Consumption in gr/kWh), the fresh lubricant TBN (Total Base Number), the engine power, the engine load, and the fuel Sulphur % of the heavy fuel. The first three parameters are generally stable, but the engine load is changing constantly, and the fuel Sulphur % is varying at each fuel bunker.
To be safely within the OEM (Original Equipment Manufacturer) TBN limits in medium speed engines running with heavy fuel, a TBN equilibrium of approximately 23-24 is recommended. While creating a TBN equilibrium, the total SLOC (engine consumption + partial replacements) and Fuel Sulphur % should be in balance. As the fuel Sulphur % is fluctuating with the bunker fuel, the total SLOC should be controlled to create a stable TBN of the used lubricant. The closer the TBN is stabilized towards the OEM’s TBN lower limit, the lower the overall lubricant consumption. The more frequent the partial replacements are done, the better the TBN equilibrium can be matched with the lower TBN limit, thus leading to overall lubricant savings.
The total SLOC is combined from the engine lubricant consumption plus the additional consumption created by partial replacements. The engine lubricant consumption is part of the engine’s construction, the additional consumption (partial replacement) is created by the Lubetimizer unit, according to the fuel Sulphur fluctuations and engine load, resulting in a stable TBN at the required level, close to the OEM’s TBN limit.
Saving by Partial Replacement
To create an acceptable TBN equilibrium or lubricant viscosity level within the engine builders limits, a regular full or partial lubricant sump replacement is needed. More frequent and small partial replacements result in a stabilized lubricant quality, optimized towards the engine builders limits, resulting in the lowest overall lubricant consumption. In combination with the constant rise of lubricant cost, the frequent partial replacement method is certainly a way to cut costs in the overall lubricant and engine maintenance budget.
The graph on the left shows the TBN trending of full sump replacement vs. frequent partial replacement. The TBN of the frequent partial replacement is stabilized close to the minimum lubricant analysis caution limit (TBN 24), whereas the average TBN of the full sump replacement is far higher, resulting in a higher overall lubricant consumption. The same is valid for viscosity.
The graph on the right shows the viscosity trending of full sump replacement vs. frequent partial replacement. The viscosity of the lubricant with frequent partial replacement is stabilized close to the maximum lubricant analysis caution limit, whereas the average viscosity of the full sump replacement is far lower, resulting in a higher overall lubricant consumption and fluctuating lubricant quality.
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