VISWALAB WHITEPAPER: Excessive Liner Wear While Using the New VLSFO Fuels

Piston rings. Image Credit: Viswalab

Viswa Lab has so far received and tested over 20,000 number of VLSFO fuels. These fuels are from multiple sources, multiple blending procedures, and multiple global bunkering ports.

Out of tested VLSFOs, there are cases of abnormal cylinder liner wear in 12 cases. This paper presents Viswa's findings based on an in-depth pattern recognition study of these 11 fuels and their effect on the engines. The data studied has been provided by the fuel users who have experienced the excess liner wear problems when using VLSFO fuels.

We have not identified any clear pattern in the parametric values of these VLSFO fuels which caused excessive liner wear. Therefore, we started to investigate other factors and focused on the cylinder lube oil (CLO), its properties, its base number, feed rate, and other details. We identified that:

1. The piston in these cases had a high landing. This means that the space on the piston crown above the ring and up to the top of the crown is quite large.
2. The Cylinder lube oil had a uniform viscosity of 20 cSt (at 100°C) in all the cases and therefore we have ruled out viscosity as a factor.
3. The Base Number of these cylinder lube oils were 40 and above. The higher the base number, the better the ability to neutralize the sulfuric acid formed as a by-product of the combustion of the sulfur in the fuel. In high S fuels, there is a lot more SO2, SO3 and therefore MORE sulfuric acid. In low S fuels, there is less SO2, SO3, and sulfuric acid.
4. The feed rate of CLO varied between 0.6-1.5 g/kWh. In some cases, it was increased to about 2.5 g/kWh after noticing the problems.

While all the fuels conformed to the upper limit of sulfur at 0.5 wt.%, we came across some fuels where the sulfur content was as low as 0.28 %. The interesting observation was that in all the 11 cases the CLOs fell within the pattern values listed above.

MAN technical document on operations with low sulfur fuel oil makes the following observations and recommendation (refer to Figure 1):

• The required cylinder lube oil feed rate is a function of the sulfur content in the fuel and is calculated by using the equation: Dosage F x Sulfur content in % wt.
• The minimum feed rate for proper cylinder lube oil distribution and oil film thickness has been set down to 0.6 g/kWh, which by using "Dosage flow rate*Sulfur content%" equation the sulfur content should be at 3%. This means that the theoretical limit of the sulfur content of the fuel, using an ordinary BN70 oil, is 3%.

As an example, an engine using 1% sulfur fuel will require less dosage and a lower BN. At a dosage of 0.6 g/kWh would, therefore, be over additivated. A fuel with a sulfur content as low as 0.5% could call for a combination of a low cylinder oil dosage and a low-BN oil (BN40 or even lower).

• By using the lower cylinder lube oil feed rate, many engines can use low-sulfur fuel and still use BN70 cylinder oil, going in and out of ECA areas.
• The complexity of designing a low-BN cylinder oil consists in achieving the proper detergency
  level, which is seldom at the same high level as for BN70 oils. Therefore, MAN recommends that the low BN cylinder oil type should be selected very carefully. All the major oil companies have low-BN cylinder oils available today.

Reasons for liner damage when using VLSFO's

Based on the observations and limitation listed by MAN it is possible to explain the actual mechanical phenomenon which caused excessive liner wear. We hypothesize that the mechanism of damage happens in two stages.

First stage of damage:

Based on the observations and limitation listed by MAN it is possible to explain the actual mechanical phenomenon which caused excessive liner wear. Please see the sketch below (Figure 2):

Please observe the following:

• The piston crown has a slight taper towards the top.
• Most fuels in question have a low viscosity below 120 cSt (at 50°C).
• The temperature in the combustion chamber is around or higher than 350 C. The viscosity of CLO at that temperature goes down to 1 cSt so the lubrication film thickness at the top is less than 3micron.

Another important factor to consider is the required min acidity in the combustion chamber, this acidity is needed to generate small corrosion pits on the surface of the cylinder liner, perhaps around 50-micron diameter and 10- 15 microns deep. These pits act as reservoirs to retain the cylinder lube oil which allows for lubrication. If there is very low acidity in the VLSFO fuel, these pits will not form on the liner surface. The very thin lubrication film along with the absence of pits will cause a direct rubbing action between the piston and liner and the liner will have a smooth shiny surface, which is a prime condition for scuffing. So, we predict that the scenario looks like this: A low sulfur content in the fuel does not have enough acidity to generate corrosion pits which will act as a reservoir for CLO. Whatever acidity was in the fuel has been more than neutralized by;

1. Base number in the CLO of 40 or above
2. The feed rate of 0.6 g/KWH or above

Both 1 and 2 create an alkaline condition which is a prime precondition for scuffing. The high-top landing on the piston crown assists this process by exposing a large surface area with a very thin lubrication film. We were able to see in the 11 cases reported that the crown carries wear marks which could be caused by scuffing between cylinder liners and piston crown.

It is important to understand the phenomenon of scuffing which is caused because of the heat of friction during the rubbing and due to very little availability of the lubricant on the surface of the piston. The material of the piston and cylinder liner fuse together which is similar to welding. However, with the movement of the piston, the weld is broken and in the next stroke, again another weld is formed. This is the process of scuffing. By nature, since scuffing eats into the body of liner material the damage to the liner (liner wear) will be excessive.

Liner wear tendency is usually in the Port /Starboard direction of the liner. This causes ovality in the liner. Once the ovality exceeds certain limits, the rotation of piston rings in the groove is restricted and the ring may not wear evenly, leading to thinning of the piston ring in Port/Starboard direction and will break off prematurely.

Second stage of damage:

Let us now come to the next stage after the initial scuffing has taken place on the cylinder liner. The liner surface will be heavily scored, and scuffed material will be sticking out. When the piston ring comes into contact with scuffed portions of the cylinder liner, the piston ring wears out rapidly (this is why MAN recommends chromium coated piston ring which will cut scuffing material out and help in regaining a relatively smooth surface for the cylinder liner). The CLO detergency assists in removing the liner material carried by the piston ring. If this detergency does not happen efficiently, the scuffed material keeps adding on and promotes more wear. Excessive piston ring wears is seen as well.

Fine hard metallic particles dislodged from scuffed liner surface will penetrate in piston ring grooves and adhere in the 0.4mm axial clearance between a piston ring and piston ring grooves. This will wear off the piston ring grooves prematurely accelerating the piston ring breakage.

This results in the piston ring wearing out rapidly and when over 40 % of the piston ring width is worn out, the ring does not have the strength to withstand the shear stress and it breaks.

Data from some similar cases: These are new vessels and as the scuffing started, the vessel operator noticed that the iron content in CLO drain increasing. This iron content rose significantly from 500 to 1000 to 1500 to 2000 ppm. This happened so fast that the ship staff reacted by increasing the feed rate from 0.6 to 2.5 g/kWh. In fact, they flooded the area with CLO hoping to see the reduction in iron content in CLO drain happen. This did reduce further scuffing to some extent. When the iron content in CLO drain went down the ship staff reduced the feed rate to slightly above normal values. However, the root cause of this problem, the progression from 1 to 2 have all been identified now with our study. We are happy to present this to you; we do recognize that there may be a few variations in the scenario we have presented. However, the base root cause is what we feel we have identified in this report.

Example: Table 1 shows the feed rate and BN of CLO for marine gas oil (MGO) used on other ships that did not face any liner wear or scuffing issue and the iron content in the CLO drain sample was low for all these three cases.

Below you can find the summary of some of the cases with high liner wear and excessive scuffing problem.

Problem case: Scuffing and liner wear issues

Problem Reported:

In most of the liner wear problem cases, the vessels experienced scuffing and wear of cylinder liner and rings (Figure3). Also,the vessel reported very high iron content from the scavenge drain (Fuel sample was VLSFO "RMG 380-0.5% grade").

Summary and Findings:

• Regular full specification test of the bunker drip sample which was tested by our laboratory was found to be meeting the ISO 8217 specification for IFO-RMG 380-0.5% grade.
• Comparing before and after the purifier sample, we note the Purifier Efficiency is normal and the quality of fuel after purification is normal and Al+Si within an acceptable limit.
• From the additional test, we note the pH of the fuel sample indicated normal (~7-7.5).
• From the GCMS screening test, we do not see the presence of any chemical contaminant at a significant level.
• In cases that they had red color deposits; We have tested Debris samples collected from piston crown, a good portion of the residue was magnetically active which could mean abrasive iron wear particles which can generate from abrasive wear or scuffing (Figure 4).
• From the elemental analysis of the residue, we note a high level of iron, calcium, sodium, and silicon. Calcium is from the cylinder lubrication process. High silicon is probably originating from inlet air. High sodium deposition could be contamination during sampling through evaporated seawater salt. Sodium in the fuel is very negligible. Iron particles that were found could be responsible for creating the red color which is resulting from corrosion/oxidation of these particles.

Analysis of the problem

Based on the tested results, we do not see anything significantly wrong with the fuel samples' properties. The only issue was using CLO with BN higher than 40 with the feed rate greater than 0.6 g/kWh.

Problem case: Piston ring breakage and liner wear

Problem Reported:

Viswa Lab has received a report of piston ring breakage and liner wear (Fuel sample was VLSFO "RMG 380-0.5% grade"). We received a damaged piston ring for testing. Below is the photograph of the surface of the ring in contact with the liner.

• Piston ring showed evidence of heavy scuffing related damage as well as the rough running surface. • The ring was worn out with sharp edges.
• The thickness of the ring doesn't seem to have changed.
• This evidence is consistent with scuffing related damages.

Summary and Findings:

• Regular full specification test of the bunker drip sample which was tested by our laboratory was found to be meeting the ISO 8217 specification for IFO-RMG 380-0.5% grade.
• Reserve stability number result for all fuels is higher than 10 indicating low reserve stability. ASTM D 7061 says "if the separability number is above 10, the stability reserve of the oil is very low and asphaltenes will easily flocculate or have already started to flocculate".
• GCMS screening indicated the presence of Indene at low levels. Machinery problems have not been reported Indene at this level of contamination.
• Elemental analysis on the sludge sample was performed by IP 501 test method. The elemental analysis indicated the sludge to be composed mainly of calcium & iron. Calcium can be traced to cylinder lube oil.

Analysis of the problem

• Lubrication appears to be a little excessive in all 7 units based on reserve TBN values. There is further scope to reduce/optimize cylinder lubrication.
• The fuels tested had low reserve stability at greater than 10. ASTM D 7061 says "if the separability number is above 10, the stability reserve of the oil is very low and asphaltenes will easily flocculate or have already started to flocculate".
• Problem Fuel Identification Number (PFIN) for the manifold sample was found to be 206 which is higher than 130. When PFIN value is high (> 130) in 85%- 95% cases piston ring breakage reported. Based on the PFIN the fuel did have the potential to cause piston ring breakages.
• Sludge & deposit samples both indicated that they are mainly composed of inorganic materials primarily calcium & iron content.
• FTIR analysis on the under piston indicated the debris to be mostly composed of inorganic materials – carbonates, gypsum, etc. possibly sourced to cylinder lube oil.
• The piston ring received showed extensive damage to the surface due to scuffing.

Conclusion:

Traditionally high sulfur fuel (HFO) required cylinder lube oil with a base number of 70 to neutralize the sulfuric acid produced during the fuel combustion. While cylinder lubrication consumption should be carefully monitored by switching to very low sulfur fuel oil (0.5% sulfur). Choosing a cylinder lube oil with an optimum feed rate and the base number is critical. Since CLO with high base number and feed rate high will lead to excessive scuffing, wear of piston rings. The first stage of damage will happen when the BN of the CLO is excessive with a high feed rate. In this case, small corrosion pits will not form because of the excess of alkalinity, the CLO film thickness becomes thinner, the friction between two surfaces increase which causes liner wear and scuffing. The second stage of damage is caused by insufficient CLO detergency characteristics. CLO does not have enough detergency to wash all the scuffed liner material carried by the piston ring.

References:

1. MAN Diesel & Turbo, Operation on Low Sulfur Fuels, MAN B&W Two-stroke Engines.
2. Qyvind Buhaug, Deposit formation on cylinder liner surfaces in medium-speed engines, 2003.
3. MAN Diesel, Cylinder lubrication update Guiding ACC feed rates for Alpha lubricator and ME lube, Service
4. letter SL09-507/HRR, 2009.
5. MAN Diesel, Cylinder lubrication update Adjusting the ACC factor in service Replaces SL07-479 and SL09-
6. 507, SL2013-571/JAP, 2013.
7. CIMAC, Guideline for the operation of marine engines on low sulfur diesel, 2013.
8. ALFA LAVAL, Marine engine lubrication after 2020; what to expect in the next decade, 2018.