CYLINDER LUBRICATION UPDATE

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2095

Introduction:

In this update, we will be discussing the following points

  1. Simplifying  the lubrication process on-board the ships
  2. process of inventing a single-cylinder lube oil that can lubricate the cylinders regardless of the sulphur content in the fuel.
  3. Optimizing the ACC factor
  4. Cylinder oils with different BN levels
  5. New guiding values
  6. Part-load operation (slow steaming)
  7. Familiarization of the ACC factor
  8. Breaking in & Running in opearation

As per the current market needs MAN Diesel & Turbo has concentrated on further enhancing the fuel efficiency while at the same time fulfilling demanding emission regulations.

On the newest engine design especially to increase SFOC(special fuel Oil consumption) the pressure in combustion chamber is increased . This increased pressure and the increased operating time at low load, has led to increased water and acid condensation on the cylinder walls, leading to cold corrosion in the combustion chamber.

Most recently developed part load, and the low load tuning option utilizes increased combustion chamber pressure  as main tool to ensure a low SFOC at part load and has experienced same results.

Therefore its necessary that to obtain appropriate cylinder oil feed rates and ACC (Adaptable Cylinder oil Control) values the basis of service inspections, measurements and wear data from combustion chamber parts (piston rings, liner, and crown)

Cylinder lubrication

Cylinder oil is essential for the two-stroke engine. Today’s cylinder oils are made with a complex chemistry and the individual feed rate  are assessed for each oil brand, viscosity class and BN level.

 To achieve the necessary level of detergency and dispersancy to keep the piston rings and crown clean, and the necessary base number (BN) to neutralize the acids formed during combustion.

The cylinder oil  serves as  lubricant for the moving parts and also is  designed to control the degree of corrosion on the liner surface.

This is illustrated by MAN B & W feed rate guide by setting the minimum feed rate to the level needed to keep the parts moving within a safe margin. But, to ensure the necessary lubrication effect, an increased formation of acid would be called for a higher BN level than specified at the minimum feed rate. This is compensated by calculating a feed rate on the basis of an ACC factor within the guide shown in Fig. 1.

For clarifying the lubrication process onboard the ships, as well the logistics of supply, the oil companies have started the process of developing a single cylinder lube oil that can lubricate the cylinders despite of the sulphur content in the fuel.

■ Such oils have BN levels that are lower than the conventional BN 70 cylinder lube oils. ■ Such oils have performed that is acceptable in the service tests carried out.

■ Such oils can very well be used on the vast majority of earlier-type MAN B&W engines that are not affected by cold corrosion, but  are not applicable on newer engine designs with higher levels of cold corrosion.

MAN Diesel & Turbo recommends use of cylinder lube oils characterised primarily by its BN number and SAE viscosity and to use a feed rate according to the BN in the cylinder oil & sulphur content of the fuel.

MAN Diesel & Turbo suggest that some engines may be operated satisfactorily at even lower feed rates. Thus, feed rates are, just as before, based on practical experience rather than pre calculated figures.

The above shows the importance of the fact that the crew should challenge the cylinder oil feed rate ACC factor, so as to find the correct ACC value that suits the actual engine configuration and engine load.

Optimising the ACC factor

The best way to establish the optimum ACC factor is to measure the engine wear. If the wear rate of the liner and piston rings is too high, because of corrosion, the ACC factor is increased to reduce the wear.

As per MAN B&W recommendation  start out with an ACC factor in the upper end of the range, and then slowly reduce it when the engine wear response has been confirmed by measurements.

But, the ACC factor can only be assessed when the fuel sulphur level is high enough to ensure that the lubrication has been in the ACC active area (the blue area marked in Fig. 1), at low fuel sulphur levels the engine is excessively protected against corrosion because of the active minimum feed rate.

Drain oil analysis

Drain oil analysis is a strong tool for judging the engine wear condition. Drain oil samples taken in active ACC operation will show if the oil feed rate is optimised while keeping the BN between 10-25 mg KOH/kg and the iron (Fe) contents under 200-300 mg/kg in the drain oil (for loads above 50%).

Used oil taken from the engine through the scavenge bottom drain is used for cylinder condition evaluation. Onboard sets exist, however, it is important to get a valid test result that shows the total content of iron (Fe). Laboratory testing as per the ASTM D5185-09 is the only certain measuring method. The BN is tested in accordance with ISO 3771:2011(E).

A cylinder oil is  degraded to a certain level where the corrosion level begins to increase. The level of depletion is different among oil brands as well as among engines, and an individual evaluation of each engine is carried out.

Table 1 Drain oil Values

One probability is to perform a stress test called “feed rate sweep”. This will shorten the ACC familiarization period considerably. The sweep test is depends on a fast six-day test at steady load and, preferably, running on fuel in the high sulphur range of 2.8-3.5 % sulphur content. The feed rate is regulated to set values, i.e. 1.4, 1.2, 1.0, 0.8 and 0.6 g/kWh.

Each feed rate is applied for 24 running hours before taking a sample and switching to the next feed rate. A detailed feed rate sweep test discussed http://marineinbox.com/2019/08/25/sweep-test/

Cylinder oils with different BN levels

The various oil suppliers provide cylinder oils with a broad range of BN levels. MAN B&W engine design is based on the 70 BN oil traditionally used, however, as new oil products are introduced, BN levels have changed.

When switching to a different BN level, start out with scaling the ACC factor from 70 to the new BN level by multiplying the ACC factor with the fraction of70/BN oil.

e.g.: Using a BN 45 and ACC (BN 70) = 0.26

ACC(BN 45) = 0.26 × 70/45 = 0.40

When changing to a new oil brand or type, the ACC factor is needed to be reassessed as described above, starting with an ACC factor in the upper range. After this, a gradual reduction is carried out based on actual observed conditions or the sweep test.

Low-sulphur HFO and Distillates

When running on low-sulphur residual fuel (HFO), the feed rate is set at the minimum feed rate. High-BN cylinder oils leads to over-addition in the aspect of controlling the corrosion as well as lead to increased build-up of piston crown deposits.

We therefore switched  to a low-BN cylinder oil at the same time as switching to a low-sulphur heavy fuel. Continuous running on high-BN cylinder oils  in special cases,but not for more than 1 to 2 weeks.

When switching to distillate fuels (MGO/MDO), we switched to a low-BN cylinder oil at the same time as the switching of the fuel. We do not use a high-BN cylinder oil when running on distillate fuels.

Part-load operation (slow steaming)

When operating the engine at part load, the cold corrosion behavior deviates from operation at normal load. When the vessel is slow steaming, the engine is operated at low load, and the liner surface becomes cooler and therefore, the risk of corrosion increases. Waste heat recovery and various part-load optimization possibilities, e.g. Turbocharger cut-out, variable turbine area (VTA) turbocharger, and exhaust gas bypass (EGB), calls for a re-assessment of the ACC factor to accomodate the new corrosion level.

New guiding values

New proposal for standard operation with standard 70 BN cylinder oil. Viscosity range: SAE 40-SAE 50 (SAE 50 for Mk 9 and newer engines)

Table 3: Guiding values

Familiarization of the ACC factor

After the breaking-in period, the engine ACC factor is assessed over a period of steps of 600 hours.

The Breaking in period which is followed by the familiarization period, where the crew assesses the engine wear and cylinder condition to select the right ACC factor for the engine application.

To be able to asses the engine wear, the steps  are completed with a fuel sulphur content that is high enough to assure that the cylinder oil feed rate is in the ACC active range. This means that the feed rate is above minimum 0.60 g/kWh. Before moving to the next step, the cylinder condition and wear is assessed through a scavenge port inspection.

In some cases, this familiarization period extends substantially. However, the period is substantially shortened by means of scavenge drain analyses, where the laboratory results shows the remaining BN and Fe (iron) content. If the samples taken during the ACC active feed rates repeatedly show high BN and acceptable (Fe) levels, the ACC factor is lowered.

A feed rate sweep is also performed to  quickly find the correct ACC range for the given engine configuration and load.

Breaking-in (0-500 rh)

Cylinder liner & piston ring breaking-in takes 500 running hours maximum. During this breaking-in period, the running-in coating on the piston rings gradually wears off, and the wave cut shape on the cylinder liner surface smoothens. During this process, additional lubrication oil is needed to flush away wear particles and assure a satisfactory oil film between the relatively rough sliding surfaces.

During breaking-in, we  checked  piston rings and cylinder liners through scavenge air port inspections for every 100 hours. Do not proceed to the next lubrication step if the scavenge air port inspection reveals seizures or other irregularities.When the steps in the breaking-in reaches 1.20 g/kWh, the feed rate depending on the fuel sulphur content is also be taken into account. The set feed rate is to be the highest of the two.

Hours                                                                             g/kWh

5-100 hours                                                               1.50 g/kWh

0-5 hours                                                                   1.70 g/kWh

100-200 hours                                                           1.30 g/kWh

200-300 hours                                                           1.10 g/kWh*

300-400 hours                                                           0.90 g/kWh*

400-500 hours                                                           0.70 g/kWh*

* Only if the ACC dependent (fuel sulphur x ACC factor) feed rate is lower than the step, if not then the ACC dependent feed rates are to be used.

Running-in operation

MAN B&W two-stroke engines require extra attention & lubrication during their first running hours.

The first 500 running hours are the most demanding. This is the period where the liners are run in, which is also referred to as the breaking-in period. The motive of the breaking-in period is to flush away wear particles and facilitate running-in of the liner surface and rings.