Recently Viswa Group had published a detailed analysis of liner wear issues that shippers witnessed using new VLSFO fuels. Now, they have furthers clarified the situation with some added facts and questions and answers.
Be sure of this when you use the new VLSFO blends. Read the previous white paper articles here
What is the effect of fuel characteristics on liner wear and scuffing issue?
We tested fuels from different perspectives and have not identified any clear pattern in the parametric values of these VLSFO fuels which caused excessive liner wear. Here are a few points related to fuel’s characteristics:
- Most of the fuels are paraffinic or paraffinic‐naphthenic (Based on Paraffin, Naphthene and Aromatic content analysis).
- Most fuels have a low viscosity below 120 cSt (at 50°C).
- The average CCAI for these fuels is 830.
- The average catfine content in the manifold
- sample for the 11 problem cases was only 15 ppm.
- Ignition properties of these fuels are normal based on FIA ignition combustion tests (ECN is greater than 40 for some of the fuels tested by IP 541).
- From the GCMS screening test, we do not see the presence of any chemical contaminant at a significant level.
- In a lot of cases the problems occurred over a period of time and using different VLSFOs from different regions indicating that the fuel may not have a role.
Are VLSFOs mainly paraffinic or aromatic fuels? How does VLSFO ignition combustion properties effect on liner wear issues?
VLSFOs are a blend of Aromatic and Paraffinic components. Viswa Lab has been testing VLSFOs falling into both categories. If they are more aromatic, they will have less stability issues but may be more difficult to burn (longer ignition delay and after burning period).
Most of the VLSFO fuels tested for liner wear problem issues are paraffinic or paraffinic‐naphthenic (Based on Paraffin, Naphthene, and Aromatic content analysis). Ignition properties of these fuels are normal based on FIA ignition combustion tests (some fuels had an estimated cetane number (ECN) greater than 40).
How can we make sure that the fuel is not mixed with light cycle oil?
With regard to cycle oil, we have a reasonable understanding of what this does when blended with VLSFO fuels. Apart from the low‐cost benefits, cycle oil being highly aromatic provides stability and fewer problems with regards to stability and compatibility.
Effect of cycle oil blending ‐ We have noticed that by using cycle oil the CCAI goes up because cycle oil typically has low viscosity and high density (aromatic). The higher the cycle oil in the blend, the higher the CCAI. Cycle oil alone has a CCAI greater than 900. However, the average CCAI values for these 11 fuels is only 830. We are therefore excluding an excess blending of cycle oil in these 11 fuels. We have checked a few fuels blended with cycle oils and have found them to have poor ignition and combustion
Since the liner wear and scuffing issue is happening only on some ships, can it be caused by the fact that VLSFOs characteristics vary while using CLO with different formulations?
Viswa Lab findings based on 15,000 VLSFO samples tested did not indicate any specific pattern in the fuel properties that could cause the liner wear and this led us to infer that the problem possibly lies with the cylinder lube oil characteristics.
What are the MAN guidelines related to cylinder lube oil alkalinity and feed rate?
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.
- As per MAN guideline, the minimum feed rate for proper cylinder lube oil distribution and oil film thickness has been set down to 0.6 g/kWh. Going by the below plot & using the “Dosage flow rate*Sulfur content%” equation, the 0.6 g/kWh feed rate is applicable for a BN 70 CLO for fuel with 3% sulfur. This means that the theoretical limit of the sulfur content of the fuel, using an ordinary BN70 oil, is 3% when using the minimum feed rate of 0.6 g/kWh.
- As another 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 lubricated. 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.
What are the recommended cylinder lube oil feed rate and alkalinity for very low sulfur fuels?
There are two aspects related to cylinder lube oil ‐ namely, feed rate, and required alkalinity to neutralize the acids formed during combustion. Based on MAN guideline Sep 2014 “Operation on low‐sulfur Fuels”, the minimum cylinder lube oil feed rate is 0.6 g/KwH to achieve proper lubrication. In the case of using CLO with 40 TBN for fuels with less than 0.5% sulfur, the feed rate based on the below figure should be less than 0.2 g/KWH which is lower than the minimum required feed rate. Therefore, using CLO with a
feed rate of 0.6 g/KWH and 40 TBN will be too high for the fuels with less than 0.5% sulfur content. The feed rate and alkalinity of CLO should be chosen carefully based on the sulfur content of the fuel in order not to create excess alkalinity.
Why were no liner wear problems reported while using 0.1% LSMGO with high TBN (70 to 100) CLO?
In the past, 0.1% LSMGO was used for short ECA passages. Using the high TBN CLO for short periods of time does not pose any significant operational risks (refer to CIMAC‐035 article).
Do the present‐day CLOs create too much alkalinity?
Research carried out by Viswa Lab indicates that CLO with TBN of 40 and a feed rate of 0.6 g/KwH may be too high for VLSFO fuels. Some VLSFO fuels have sulfur as low as 0.2% which means that the vessel has to avoid excess alkaline conditions at the rubbing surface by using cylinder lube oil with optimum feed rates and TBN. It is important to keep in mind that reducing the Base Number reduces the detergency of the lubricants which increases the deposit formation. Therefore, it is necessary to choose the correct CLO with TBN and feed rate in order to avoid any liner wear and scuffing issues which can result in deposit formation and accumulation. It is also necessary for lube oil manufacturers to formulate a CLO that has detergency properties matching to that of a high TBN lube oil.
What is the importance of the presence of acidity in the fuel sample with regards to liner wear problems?
Acidity in fuels is required to get some corrosion pits to aid lubrication. These pits act as reservoirs to retain the cylinder lube oil which allows for lubrication. With sulfur being low in VLSFOs there is a very low amount of acids formed (post‐combustion) as a result 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.
Under what conditions the scuffing will take place while using most fuels barring VLSFO fuels?
Severe adhesive wear or scuffing takes place when:
- The temperature, the sliding speed or the load exceeds a critical value. This usually starts at a very small part of the contact surface but spreads rapidly due to the significant deterioration of the surface. The friction is so intense that the surface is melted and forms “white layers” which are very hard and brittle. When they crack, small hard particles flack off and plow the surface producing typical scuffing appearance shown in the below figure.
- Excess alkalinity and feed rate. In this case, the alkalinity is higher than what is needed with regards to fuels with low Sulphur content. Excess of alkalinity resulted in The CLOs tend to accumulate on the piston top land leading to mechanical bore polish, and excess additives will suppress the corrosion completely leading to “chemical” bore polish. Both are seen leading to scuffing of rings and liners (Refer to technical information by Henrik Rolsted and Jesper Weis Fogh).
Technical information by Henrik Rolsted and Jesper Weis Fogh MAN engineers, JIME; We quote the relevant portions below:
Lubricating proportional to the sulfur at the factor 0.34 g/kWh x S% on the older low topland engines and 0.20 g/kWh x S% on our newer high topland engines will give more or less the ideal match between sulfur and neutralizing alkali additives. However, at lower sulfur percentiles, below the breakpoint where the minimum dosages at 0.6 g/kWh must be followed instead, excess alkalinity will be the result with the risk of getting bore‐polish problems. Lower BN cylinder oils are needed for that reason. For SECA zones where 1.5% S is maximum, BN40 oil is close to the ideal match. However, for the coming new SECA and CARB rules, where the sulfur will be limited to a maximum of 0.5% and lower, we need lower BN cylinder oils, BN20 or BN12, with today’s high detergency level.
What are the main causes of wear marks and scuffing on rings and cylinder liners while using VLSFO fuels?
An important factor to consider is the required minimum 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. With sulfur being low in VLSFOs there is a very low amount of acids formed (post‐ combustion) as a result 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 create enough acidity to generate corrosion pits
which will act as a reservoir for CLO. Whatever acidity was in the fuel has been neutralized by;
a. Base number in the CLO of 40 or above
b. The feed rate of 0.6 g/KWH or above
Both a and b 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.
The next stage of damage is 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. 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.
What is the role of the piston cleaning ring?
The piston cleaning ring is installed to protect against excessive deposit build‐up on the piston crown top land by scraping off the deposits when the piston approaches the top dead center (TDC).
Are the wear marks caused by the piston crown contact against the PC ring at the top?
It is difficult to determine if perhaps the scuffing started with the top ring. We do not see scuffed top rings in many of the damaged cases. The complete wear out of the top ring is also seen which could have facilitated the “leaning” of the piston.
What is the reason for deposition formation on the piston?
In general, the detergency property of CLO is responsible for keeping the piston clean by removing the combustion deposits from the piston. With the reduced base number, there is reduced detergency properties leading to incomplete removal of deposits.
What is the difference between high top land and low top land?
The high top land (HTL) piston designed has a higher top land compared to the low top land piston. In the HTL design, the liner temperature was lowered slightly. The low level of acid condensation is caused by the lower temperature and pressure in the combustion chamber at the time when the liner surface is exposed to the exhaust gases. There is a remarkable difference in acid condensation in the HTL design. This made it possible to lower the cylinder oil feed rate as the requirement for acid neutralization additives was reduced considerably. Therefore one important parameter while using the high top land
piston is the CLO feed rate. One of the advantages of using high top land piston is lowering the CLO dosage which is more crucial when using the fuels with low sulfur content. As a result, the CLO feed rate should be chosen carefully when using the fuels with low sulfur content in order to prevent the excess of alkalinity using a high top land piston.
Note: The above clarification supersedes what we have stated in our original white paper.
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Source: Viswa Group