OIL ANALYSIS

Regular testing of crankcase lub oil is important to ensure that

deterioration has not taken place. The results of in service deterioration

could be a reduction in engine protection or actual attack on working

points by corrosive deposits. Oil samples are generally tested every 3 to

4 months depending on the system and experience. Shipboard testing is

taking a rising prominence to allow monitoring of oil condition between

testing.

To ensure good representation, care should be taken where

the sample is drawn

Correct

Main supply line

inlet or outlet from l.o. cooler

Outlet from main l.o. pump

Incorrect

standpipes

purifier outlet

purifier direct sump suction

Samples should be drawn over a period of several minutes

Viscosity

The viscosity is the most important property of the oil. Oil of correct

viscosity will provide optimum film strength with minimum friction losses

and leakage.

The viscosity of a L.O. may fall due to fuel dilution if running on gas oil,

and rise if running on heavy f.o. Viscosity may also increase due to

heavy soot loading if purifiers and filters not operating efficiently. Oil

ageing caused by oxidation and thermal degradation increases viscosity.

A simple shipboard test is the Mobil flow stick where drops of new and

used oil are placed in separate channels on an inclined 'stick'. The rate

the oil flows down the stick is proportional to its viscosity.

Water content

Initially determined by 'crackle' test. The presence of Na and Mg in a 4:1

ratio indicates salt water contamination.

Limits are laid down by the manufacturer, but as a rule of thumb a limit of

0.2% should cause investigation into source and remedial action at 0.5%

Gross contamination can be remedied by placing the charge in a

separate tank and heating to 70oC and circulating through purifier.

Metals Content

Indicates the presence of metal element composition and identifies

additive and contaminant levels.

Zinc(Zn),Phosphorus(P)- are components of many oils such as diesel

engine oils, hydraulic oils and gear oils, to enhance anti wear and over

properties of the oil

Calcium(Ca)- primarily a component of engine oils, provides

detergency, alkalinity and resistance to oxidation. Residual fuel engine

oils have higher Ca levels

Nickel(Ni)- Bearings, Valves, gear plating, fuel derivative

Barium(Ba)- Multi purpose additive, declining importance

Chromium(Cr)- Piston rings, hydraulic actuator cylinders

Manganese(Mn)- Cylinder wear

Aluminium(Al)- generally comes from wearing piston skirts, levels rise

where new piston fitted to old engine. Typically 10ppm, but rises during

bedding in. May also indicate the presence of catalytic fines in residual

fuels.

Iron(Fe), Molybdenum(Mo), Chromium(Cr)- metals alloyed for piston

ring etc, a rise in level may indicate ring pack/liner wear.

Copper(Cu), Lead(Pb) , Tin(Sn), Silver(Ag) - soft metals used in the

overlay of shell bearings, and phosphor bronze gears. Note that high

copper content can also occur when samples are drawn from copper

pipes which have not been flushed as well as gear wear.

Silicon(Si)- Indicates poor air filtration, possible fuel derivative

Sulphur(S)- May indicate the presence of greases

Sodium(Na)- With Mg indicates the presence of sw contamination,

possible coolant system and fuel derivative

Vanadium(V)- Usually indicates the presence of fuel oil

Alkalinity and acidity

TBN-TOTAL BASE NUMBER- measure of alkaline additives available

for the neutralization of acids from combustion products and oxidation.

Level governed by type of fuel.

For crosshead engines the TBN will tend to rise due to

contamination by liner lubrication, it should not be allowed to raise more

than twice that of the new charge.

As a guide, the TBN of fresh oil should be at least:

10 x fuel sulphur content (%) for trunk piston engines

(10mgKOH/g)

20 x fuel sulphur content (%) for cyl oil in x-head engines

(20mgKOH/g)

Purple:Good level of TBN

Green:Borderline

Yellow:Low level of TBN

TAN-TOTAL ACID NUMBER-measure of organic acid and strong acid

content of oil. Where SAN is nil, the TAN represents the acidity in the oil

due to both the acids in the additives and the oxidation of the

hydrocarbons in the oil. The TAN of fresh oils varies with oil type, and

tends to climb with age. A high TAN may indicate that an oil should be

changed or freshened by top up. A high TAN may be accompanied with

increased viscosity.

SAN-STRONG ACID NUMBER-indicates the prescience of strong,

highly corrosive (inorganic) acids, usually formed from combustion

products. If SAN is non zero the oil should be changed immediately