In an earlier technical update, Viswa Lab has highlighted the issue of liner wear that occurs while using the new blends of VLSFO fuels. Now, they have taken it further and done another set of detailed testing which revealed the extent of it. Here we are putting up that report.
What is it?
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 11 cases. This technical update presents Viswa’s findings based on an in‐depth pattern recognition study of these 11 fuels and their effect on the engines.
We 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 for these 11 fuels is about 15 ppm only in the manifold samples.
- Ignition properties of these fuels are normal based on FIA ignition combustion tests (ECN is greater than 40).
- From the GCMS screening test, we do not see the presence of any chemical contaminant at a significant level.
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.
What’s the actual mechanical phenomenon causing this?
Based on the observations and limitations listed by MAN guideline Sep 2014 “Operation on low‐sulfur Fuels”, it is possible to explain the actual mechanical phenomenon which caused excessive liner wear (examples are shown in the below figure).
We hypothesize that the mechanism of damage happens in two stages.
First stage of damage:
Based on the recommendation listed by MAN it is possible to explain the actual mechanical
phenomenon which caused excessive liner wear. Please see the sketch below:
Please observe the following:
- The piston crown has a slight taper towards the top.
- 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.
Important Factors To Consider
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. 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;
- Base number in the CLO of 40 or above
- 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.
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.
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Source: Viswa Group