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Basic Blocks of the Lubricants
The mineral lubricating oils are made from petroleum crude after fractional distillation and refining. The chemical combinations of hydrocarbon atoms are infinite. For example 25 carbon atoms could produce more than 36 million different hydrocarbons, each with a different molecular structure. No two crude oils are alike; even from the same oil field they vary appreciably in their characteristics, molecular structure, and chain. So it is very difficult to classify them. In general, however crude oils may be classified by the type of faction produced when they are refined. Basically there are three main types of hydrocarbons and based on these basic ingredients the properties of the lubricating oil differs. They are as follows:
Paraffin are straight chains of carbon atoms each joined with two hydrogen atoms. However the carbon atoms at either end have three hydrogen atoms connected unless they are replaced by side chains. Paraffins are also stable saturated compounds without any double bonds between the carbon atoms. The general formula for saturated paraffin is C nH2n+2
These are also saturated hydrocarbons in which all four valances of carbon atoms are connected to hydrogen or adjoining carbon atom, but the carbon atoms in each molecule is closed ring or cycle.
Aromatics are also ring or cyclic compounds but unlike the naphthalenes, they are unsaturated with double bond between some of the carbon atoms. The general formula is CnH 2n-6 .
In general, paraffinic lubricants have good natural resistance to oxidation, good thermal stability, good lubricating properties, high pour points, high flash points; low volatility and high viscosity index (90 to 115). When used in lubricants for internal combustion engines, however, they tend to form hard carbonaceous deposits of very close grained nature. But they are suitably blended with naphthenic oils to remove the above drawbacks.
Beside the above classification the following type of classifications are used for cylinder lubricating oils. However in these classifications too it is generally mentioned whether the oil is predominantly paraffinic or naphthenic so as to get an idea of their basic nature.
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The classification of the cylinder lube oil is done on the basis of the basic factions like aromatic, paraffinic, etc. The cylinder oils can be further classified on the basis of the additives added and methods of adding them like emulsion type, dispersion type, and Single phase lubricants.
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Cylinder Oil Classification
Emulsion type cylinder lubricants
It is a water in oil emulsion in which a water soluble, metallic based, highly alkaline compound namely calcium acetone was blended into a suitable base oil by means of an emulsifying agent. This lubricant dramatically reduced linear wear by neutralizing the sulphuric acid in the cylinder. Using an additive based on calcium acetate, the metallic salt reacts with sulphuric acid to form solid amorphous calcium sulphate. During the reaction a weak organic acid, acetate acid, is formed which is in gaseous form at normal cylinder temperatures and passes out with the exhaust gases. The chemical reaction is Ca(CH3COO)2 +H2SO4-----------2CH3COOH + CaSO4
- Such emulsions are extremely difficult to keep stable for long periods particularly at temperatures exceeding about 32 degrees C.
- A breakdown into separate water plus additives and oil phase could have serious results.
- Furthermore a minor but nevertheless irritating problem was that when used in cylinder lubricators using conventional discharge sight glasses filled with water the lubricant tended to stream up the guide wore, instead of passing up in well-defined drops which could be counted and the cylinder oil feed set thereby.
- Like many other suitable alkaline compounds this one was insoluble in oil.
Dispersion type cylinder lubricants
It incorporates somewhat similar oil- soluble alkaline mineral salts.
- The problem of overcoming the insolubility of the additive was solved by using grease making techniques, so that the additive was maintained, in an extremely fine dispersion in the oil.
- In addition to processing good acid neutralizing properties this type of lubricant has good inherent load carrying properties which helped to reduce abrasive wear.
- A small proportion of the additive came out of dispersion when stored for long periods forming sludge like deposits in service tanks and sometimes in mechanical lubricator oil wells.
- Trouble was also experienced with sight glass fluids but this was overcome by the use of a special fairly simple chemical solution.
Single phase cylinder lubricants
- The solution of single phase lubricator, which is much more stable than the two described. These are complex compounds incorporating a high percentage of a metallic salt such as calcium carbonate (CaCO3). When such salts react with sulphuric acid, calcium sulphate is again formed at temperatures normal in a diesel engine cylinder.
- It is possible that at low temperatures and pressures at the end of the exhaust stroke the carbon dioxide goes into the solution with water to form weak carbonic acid. The chemical reaction is: CaCo3 +H2SO4-----H2SO4 + CaSO4 or more likely CaCO3 + H2SO4------H2O +CO2
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The running in of the new piston rings and the cylinder liner is also very important in context with cylinder oil. We have to allow for a little amount of corrosion to rapidly smoothen the peaks of the rubbing metal surfaces. Incorrect cylinder lubrication can lead to numerous problems like scavenge fire and blow by. Every marine engineer must recognize the symptoms and the troubleshooting of the same.
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Running in of crosshead engine piston rings and liners
Correct running in of an engine should produce a close mating of the piston rings with the cylinder liner surfaces to prevent combustion blow by. In addition it is likely that the metallurgical transformations involved in the smoothing the peaks in the metal surfaces do generate a surface layer whose wear properties are superior to those of the parent materials.
The greatest assistance in the running in of any engine is given by the presence of sulfur in the fuel since it provides corrosive wear to help smooth the initial roughness of the liner surfaces. For this reason it is essential that the cylinder oil used should not have sufficient alkalinity to neutralize the combustion acids.
In the case of engines which are being run in while burning diesel fuel it is customary to use a straight mineral oil of SAE40 or SAE50 viscosity as the cylinder lubricant.
In case of running in while the engine is burning heavy fuel it is customary to use heavy duty oil of TBN around 10 and SAE 50 viscosity as the cylinder lubricant. For detailed running in procedures it is advisable that the particular engine builder recommendations should be observed.
All the above attempts to keep liner and ring wear within acceptable limits will, however be nullified if the oil feed rate, method of application and proper corrective action of troubleshooting is not done properly.
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Trouble Shooting Cylinder Lubrication problems
Blow by of combustion products and Scavenge fire
Indications – scavenge temperature high and sticking of piston rings
- Insufficient spreadability, detergency of lube oil and thermal degradation.
- Insufficient feed rate of cylinder lube oil.
- Leakage in the cylinder lube oil feed system.
- Lubricating oil discharge by lubricating quill is not uniform.
- Excess side clearance between piston ring and groove resulting in pumping action which causes the oil to be lost without providing lubrication.
Sticking of pistons rings and wear down takes place because of the above resulting in blow by of combustion products and eventually causing a scavenge fire.
- Analyze the cylinder lube oil and fuel used by chemical laboratory to find out compatibility or suitability of the cylinder oil for the residual fuel used and accordingly choose the proper grade of cylinder lube oil.
- Compare the theoretical and the actual feed rate (theoretical feed rate can be found out from the graphs given in the engine manual). Any discrepancy shows leakage or insufficient pressure build up in lubricating mechanism due to wear. So temporarily increase the feed rate but carry out the overhaul of the cylinder lubrication mechanism at the earliest.
- Locate the leakage and correct it.
- During maintenance if oil discharge by all quills is found to be non-uniform, then it is to be checked. Check whether the adjusting screw on the cylinder lubricating pump for the lubricating point is differently adjusted with other adjusting screws. Otherwise locate the defective quill and repair it or replace it.
- If side clearance between the piston ring and the groove is found to be excessive then replace the piston ring.
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Incorrect amount and the incorrect quality of the cylinder oil can lead to numerous problems in the long run. Insufficient lubrication is as dangerous as over lubrication. Enery marine engineer must be aware of the problems that can occur like excessive liner wear rates, excessive deposits on the pistons, and on the turbochargers. Having recognized the problem the troubleshooting has to be done.
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Trouble Shooting Cylinder Lubrication problems (Continued)
Excessive ring and cylinder liner wear
Indications: Excessive wear rate as compared to previous records of the same.
- Corrosive wear due to high sulphur percentage in fuel or low TBN of cylinder lube oil.
- Cylinder lube feed rate insufficient.
- Improper running in.
- Increase TBN of the cylinder oil as per the sulphur content of fuel oil.
- Increase the cylinder lube oil accordingly.
- Strictly follow the instructions given by the engine manufacturer during running in operation.
Piston crown (fire side) deposits inside the ring groove and on the body of piston
Indications: Visual inspection during maintenance
- High cylinder feed rate.
- Improper timing of cylinder lube oil injection.
- Adjust the feed rate of cylinder oil according to the guideline given in the manual.
- Overhaul lubricating mechanism at the earliest if the problem persists.
Indications: Visual inspection during turbocharger overhaul
Causes: Excessive feed rate of cylinder oil
- Adjust the feed rate of the cylinder oil according to the guide lines given in the manual.
- Overhaul lubricating mechanism at the earliest if the problem persists.
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Summing up, a premium quality cylinder lubricant for use in engines burning high sulphur fuel should have a TBN of from 40 to 70 to combat corrosive wear. The alkaline salt must have rapid acid neutralizing properties along with good detergency and dispersancy properties. The other properties that the cylinder lubricant must have are good load carrying, good metal wettability, good spreadability, good thermal, good anti oxidation properties, and form a minimum amount of abrasive ash. These demands emphasize the problems faced by the engineers developing the lubricants. Unfortunately due to the complexity and the amounts of additives required (up to 33%), the lubricants are expensive, and the cylinder oil is the most expensive lubricant in the engine room.
All the above good attempts to improve the conditions of cylinder lubrication will come to naught if the lubricant is not chosen by taking into account the result of the fuel analysis and if proper maintenance is not carried out according to the manufacturer’s instructions. Troubleshooting should be done at the earliest signs of the problem. Thus the marine engineer is the link between the researcher and the actual engine performance and the marine engineer should be well informed about the lubrication mechanism. I hope this article would be useful for junior engineers as well as the practicing marine engineers.
Cylinder Liner Lubrication of Marine Two Stroke Crosshead Diesel Engines Part 2
This series discusses the challenge of cylinder liner lubrication in Marine two stroke crosshead low speed diesel engines. Due to engine development to give higher outputs per cylinder and the commercial pressure to burn residual fuels, have all contributed to harsher conditions on the liner