LUB OIL TESTING ON BOARD WITH PORTABLE TEST KIT :

Basic test kits for :

1 Viscosity determination.

2. Insoluble content.

3. Water and other aqneous contaminants.

4. Acidity determination.

VISCOSITY DETERMINATION

It is a 3 - tube rolling ball viscometer. One tube is filled with a minimum safe viscosity oil and the other with a high optimum viscosity oil and the sample oil, to be tested, is filled in the 3rd tube. All the three tubes are placed in a bucket of hot water until all three oils are at the same temperature. Three identical same size, same weight hollow balls are inserted one in each tube. The three tubes mounted on a tilted board are then inverted, hollow balls then start floating to the surface. If the time taken in the sample oil is between that of the lower and upper limit test oils, then the oil is fit for further use. If less than that of the lower limit oil, it is ready for replacing, similarly if the time is greater than upper limit test oil, the sample is too viscous and the oil should be replaced.

(Example : Sample SAE 30 grade, lower SAE 20 and higher SAE 40)

INSOLUBLE CONTENT - BLOTTER TEST

A single drop of oil is released from a given height onto a sheet of special filter paper. With straight mineral crankcase oils the insolubles should be kept as low as possible by efficient filtration. The upper limit should be about 1 to 1.5%. The spot is compared with standard spots with known carbon contents in the oilof about 0.4 and 0.8%; The sample result should fall between the two.

With detergent/dispersant oils, designed to keep carbon in finely divided suspension, the acceptable insolubles content can be as high as 5% depending upon the initial detergency level of the oil. Again the sample of oil is dropped on to the test sheet and the result compared with a similar oil with known varying insolubles content from 1 to 5%. Again the test oil should be below the upper limit.

WATER CONTAMINATION - CRACKLE TEST

A known amount of oil sample is taken into a test tube and heated over a spirit lamp, shaking the tube while so doing. If there is no crackling, the oil is dry. A slight crackling indicates trace of water, heavier crackling with a slight frothing indicates a heavy trace and if there is continuous crackling and frothing, the oil is wet and steps to be taken to remove the water.

ACIDITY DETERMINATION -

The sample oil is mixed with a known amount of distilled water and snaked for extracting acid with the water. The acidic extract is then placed on a watch - glass with an indicator solution of known strength. The mixture is then drawn upto a capillary tube and its colour is compared with a series of colour standards, each representing a known pH value. By comparison with the standards, the pH value of the sample can be determined quite accurately.

Cylinder Lubricating oil : Large diesel engine cylinders are subjected to three types of wear:

1. Frictional wear, caused by metal-to -metal contact under boundary lubrication conditions. This may be aggravated by an oil with inadequate load-carrying properties, too low a viscosity or an inadequate oil supply.

2. Abrasive wear ; caused by hard foreign matter, introduced with the induction air and by hard particles of carbon, asphalt, wear debris and ash-forming constituents present either in the fuel or lubricating oil.

3. Corrosive Wear, caused by acidic products of combustion , especially condensed sulphur oxides. This is especially troublesome when burning high sulphur content residual fuel.

There are many additional factors which can influence wear. Some of these are

(a) Unsuitable quality liner.

(b) Piston and piston ring materials.

(d)Distortion of liner and piston due to thermal & material stresses.

(e)Poor combustion causing deposits and uneven fuel spray pattern.

(f) Poor design or insufficient or too great piston ring clearance.

(g)Poor piston cooling causing excessive ring zone temperature.
(h) Under piston deposits increasing ring zone temperatures.

Quality of Oil:-

A premium quality cylinder lubricant for use in engines burning high sulphur content fuel, should have a TBN from 40 to 70 to combat corrosive wear, the alkaline salt must have rapid acid-neutralising properties, it must have good detergency/dispercency properties, good load-carrying and metal wettability properties, good spreadability properties, good thermal and oxidation properties and form the minimum amount of abrasive ash.

These demands have been met to a very large degree by the development of blends of suitable additives with carefully selected base oils. Amount of additives required is in the order of upto 33%.

The oil grade varies depending on the manufactures and ship owners. Some prefer high viscosity index paraffinic oils, others medium viscosity index napthenic oils or in some cases a blend of suitable napthenic oil with a high viscosity cylinder oil or bright stock. Paraffinic oils have the best natural oxidation resistance, good load-carrying properties at high temperatures because of their high Viscosity Index; form relatively little sludge when oxidized and have a low volatility. Unfortunately when subjected to high temperatures and oxidised in a diesel engine ring zone, they tend to form extremely hard, slate-like carbon deposits. Such deposits rapidly cause ring sticking, port blocking and in oil-cooled pistons, under-crown deposits, which adversely affect heat transfer. They do have a tendency to form weak organic acids when oxidized which can attack certain non-ferrous bearing metals.

Napthenic oils however, while not being so thermally stable and having poorer viscosity/temperature characteristics (low V.I.) , also being more volatile , form much softer carbon deposits when exposed to very high temperatures. They are therefore, much less prone to cause ring sticking, port blocking or under piston deposits.

While modern lubricants for both large crosshead engine cylinders and dual purpose lubricants in trunk-piston engines are now almost invariably blended with special additives to impart, or improve detergency/dispensancy properties, acid-neutralising properties, oxidation stability, corrosion resistance and load-carrying properties, nevertheless it is still most important to select the most suitable base oil. A successful lubricant is a careful blend of base oil and additives, good compatiability being essential.

LABORATORY TESTS OF LUB. OIL :

1. VISCOSITY TESTS :- Kinematic Viscosity is measured in the Laboratory by the IP 71, method. This involves timing the passage of the fixed volume of oil through a calibrated orifice immersed in a constant temperature bath. The result is obtained in terms of Centistokes.

2. FLASH POINT :- This is determined by the Pensky - Marten closed Cup test, IP 34, which involves gradual heating of the oil sample in a closed container which is periodically checked by an open flame for accumulated volatile matter.-The flash point is the temperature at which sufficient vapour has collected to ignite on application of the flame.

3. WATER CONTENT :- The exact amount of water content by volume, is determined by Dear and Stark Test IP 74 method, in which a sample of the oil is distilled in the presence of a suitable water - immiscible solvent.

4. INSOLUBLE CONTENT TEST :-The normal - heptane or pentane insoluble tests are generally carried out on a used detergent oil. These include the total solid matter present in suspension including fuel soot, incompletely burnt lub. oil carbonaceous matter, fuel, additive ash, and fine solid contaminents such as silica, core sand, dust, rust and wear particles.

5. NEUTRALISATION VALUE :- This measures the ability of an oil to react with a basic reagent which indicates the acidity expressed either as total acid number (T. A. N) and/or strong acid number (S.A.N). A modification of the test, carried out under prescribed condition, using an acidic reagent gives the Total Base Number.

The neutralisation value is expressed as mgm of KOH per gm of oil for both acid and base number.

6. CORROSION TEST :- Standard Corrosion.test used is. IP135, in which a cylindrical polished rod is immersed in a mixture of 300 ml oil and 30 ml distilled water or salt water at 60°C for 24 hours. After testing, the steel rod is examined for rusting.

INDEX ADV ENG NEXT