The oil has to serve two purposes;
- Reduce friction
- Cool bearings
A good quality mineral oil will serve the purpose of reducing the friction to the required level depending upon the metals involved & other conditions such as the temperature. All the oils will oxidise & this reduces its effectiveness as a lubricant. Oxidation will also cause deposits that can block passageways & coat working parts. The rate of the oxidation will depend upon temperature, the higher the temperature the more rapid the rate. Anti oxidants are available which reduce the rate, also additional properties can be achieved by the use of the additives.
Under the high temperatures oil is liable to thermal degradation that can causes discoloration & changes the viscosity. Additives cannot change an oils susceptibility to this degradation. The refining process are able to remove compounds that effect the thermal stability of the oil & also those that lower oxidation resistance. Most of the chemicals found in the oil will react more or less with the oxygen, The effects of this oxidation is always undesirable. Hence, a major objective of this refining process of a mineral oil is to remove those hydrocarbons i.e. the aromatics, the small amount of the unsaturates together with the molecules containing sulphur, oxygen & nitrogen.
Unfortunately these same molecules are those that enhances the boundary lubrication performance. Hence, a careful balance should be struck. The use of the anti-oxidants makes a slightly better balance although there usefulness is limited.
Tin based white metal is susceptible to hardening as an oxide layers from on the surface.
These tin oxides are grey-black in appearance & are extremely hard. There formation decreases the bearing clearance, as the oxide layer is thicker than the original white metal material from which it is formed. The oxide has a lower coefficient of the friction than the original white metal but it will cause problems if it brakes up as fragments will become embedded edge on in the white metal & can score the pin.
- Sea via coolers
- leaky seals or washing in purifiers
Problems caused by water contamination:
- Water leads to corrosion especially if there is sulphur present due to the fuel contamination
- forms emulsions which are not capable of withstanding high loads
- removes water soluble additives when centrifuged out
- leads to the possible bacterial attack
May be heavy residual or light diesel/gas oil & can be sourced to faulty to cylinder combustion or faulty seals on the fuel p/ps.
- Increases viscosity for the HFO, reduces viscosity for the D.O.
- reduces flashpoint
- Introduces impurities such as sulphur
- dilutes lub oil when in large quantities
carbon from the cylinder combustion process, particularly of importance with trunk piston engines but also for the crosshead engines with inefficient diaphragm. The carbon can lead to restrictions & blockages of oil ways causing bearing failure. Straight mineral oils hold 1% carbon in suspension, dispersant oils hold about 5%.
Certain bacteria will attack the oil but water must be present. The bacteria may exist in the dormant state in the oil but water is needed if they are to reproduce.. The bacteria digest the oil causing breakdown emulsions to be formed, acidity increases, dead bacteria block filters & corrosive films form on the working surfaces.
In summary there are three essential conditions for the microbiological growth;
- There must be a source of carbon- present in the oil
- There must be some bacteria or the fungal spores present-these are almost universally present in the atmosphere
- There must be free water present
Two other factors which helps in the growth are a slight acidity in the water (pH 5 or 6) & a slightly raised temperature (20 to 40oC) which can lead to the rapid growth.
Biocide additives are available but they are not always compatible with the other desired additives & can lead to the large organic blockages if treated in the machinery. The best solution is to avoid the presence of the water. If mild attack takes place the oil may be heated in the renovating tank to above 90 degree Celsius for the 24hrs before being returned to the sump via the centrifugal separator. For a severe attack the only solution is complete replacement of the charge followed by the sterilization of the system. It may be noted that on replenishment the bacteria may be present in a dormant state in the new charge.
Test results of the crankcase oils
Viscosity – Increases due to the thermal degradation or the HFO contamination, decreases with diesel oil contamination, corrective action needed if it increases by about 25% from new oil.
Water content – Corrective action required at 1%
Insoluble Sediments – basically the result of wear & oxidation, corrective action at 1% by weight
Ash – a measure of the incombustibles in the oil sample, corrective action at 0.13% by weight
TAN – (Total Acid Number) – consists of strong acids (mainly sulphuric acid) formed in the combustion process & weak acids resulting form oxidation of the lub oil.
SAN – (Strong Acid Number) – the oil should be renewed if any is detected
TBN – (Total Base Number) – indicates the alkaline reserve particulary important for the trunk piston engines
Closed flash point – highlighted fuel contamination, corrective action if reduces by 30°C from new
Cylinder lube oil
The type of cylinder lube oil required will depend upon the cylinder conditions & the engine design e.g crosshead or trunk piston. However, the property requirements are basically the same but will vary in degree depending upon the fuel & operating conditions.
Normal properties required are;
- adequate viscosity at the working temperature so that the oil spreads over the liner surface to provide a tough film which resists the scrapper action of the piston rings
- the oil must provide an effective seal between the rings & liner
- only a soft deposit must be formed when the oil burns.
- alkalinity level (total base number or TBN) must match the acidity of the oil being burnt
- detergent & dispersant properties are required in order to hold deposits in the suspension & thus keep surfaces clean
All oils for all the purposes can be designed to give particular properties through the careful use of the additives to the base mineral oil stock.
Common additives are;
- Antioxidants-these are used in all oils to decrease the rate at which oxidation occurs & are especially useful were the lub oil cools the piston
- Extreme pressure agents these are compounds of the phosphorus, Sulphur or Chlorine which increase the strength of the oil film under the conditions of high temperature or pressure.
- Dispersants or detergents-found in trunk piston engine oils & cylinder lube oil these keep surfaces clean by holding deposits in suspension.
- Viscosity index improvers- these prevent excessive changes in the viscosity with change in the temperature
- Other additives can be defined by the name such as anti-wear, anti-corrosion, anti-bacteria, anti-foaming etc.
When running in, the cylinder L.O. injector pumps may be filled with a straight mineral oil without anti-wear properties typically the crankcase oil- once this small reserve of the oil is exhausted, running in carries on with normal cylinder L.O.. The flow of the oil is increased to carry away metallic particles.
Problems caused by stuffing box leakage oil entering crankcase
Low speed engines are particularly at risk from crankcase lubricant contamination caused by the cylinder oil drainage past the piston rod gland & combustion products. This can lead to severe damage of the engine crankcase components & reduction of life of the oil which is normally expected to last the lifetime.
There has been a general increase in the viscosity & Base number of the crankcase oils over recent years particularly for the engines built since the early 1980’s. Increased alkalinity, viscosity & insolubles, fuel derived elements such as the vanadium & oil additive derived elements such as calcium, suggest that the contamination is from the cylinder oil drainage.
Deterioration of the crankcase oil resulting in the expensive necessity of replacing up to 50% of the sump, this is particularly of the concern as it is often only a temporary measure.
Four causes are put forward:
1. New crankcase oil contaminated with new cylinder oil-unlikely
2. Cylinder oil drainings being recycled & returned to the sump very likely as it is a common practice to purify oil leaking through the gland, tests done on this purified oil found high amounts of the insolubles
3. Leakage past rod gland- very likely, high pressure scavenge air can blow cylinder oil & dirt past the top scrapper ring & sealing rings into the piston rod drain tank, & even possibly directly into the sump. A problem that worsens with age & wear.
4. leakage of the exhaust valve lubrication system-unlikely
From above the suggestion is the most likely cause for the contamination is leakage past the piston rod. It is seen that maintenance of the stuffing box is of the utmost importance. Tell tales & drainage lines should be proved free & use of oil drained from the uppermost drain should not be allowed even after purification due to the high level of the contamination which can deteriorate the properties of the oil in the sump.