of controlling solids in the 1-15 micron range and smaller should be used.
Ultrafiltration (UF) is a
variety of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane in a form of forced osmosis. The soot index is a linear measurement that measures the extent to which the oil has become contaminated by fuel soot which is an unwanted by product of combustion. The units reported depend on the spectrometer manufacturer. The measurement really applies only to compression ignition oil engines because the soot measurement on petrol engines is expected to be very low.
In compression ignition
oil engines, excessive amounts of soot can be generated by over- fuelling (air to fuel ratios), incorrect combustion temperatures, low operating revolutions, restricted intake and exhaust systems and faulty turbochargers. Dispersant additives are formulated in engine oils to hold soot in suspension. Unfortunately, there is a limit to how much soot a lubricant can carry. When the maximum amount is exceeded, sludge deposits start to form, which can harm the engine. The effects of severe sludging manifest themselves as increasing oil viscosity. These usually occur rapidly to the point where the oil can no longer be pumped and engine failure ensues. When interpreting the severity of the soot index measurement, the marine surveyor should take into account the soot readings on previous samples from the engine as well as the magnitude of the change in the oil’s viscosity.
It should also be
noted that high soot loading can negatively affect the accuracy of other infrared measurements.
Other than soot, there are several other types of contamination that must be dealt with in order to extend lubricating oil change times requiring additional filtration to be installed and proper service
intervals to be established to deal with contamination missed by the full flow filters and other types of contamination generated within the engine.
These types of contamination are explained below together with how additional by-pass filtration will effectively control them.
Moisture Contamination:
This form of contamination within the lubricating oil will cause viscosity increase, VI polymer decrease, TBN decrease, acid formation, accelerated sludge formation and corrosion of parts. To safely eliminate routine oil drains, e additional filtration that utilizes an adsorbent filter media which can remove suspended moisture from the lubricating oil must be used.
Water, when it contaminates hydraulic and lubricating oils, has a degrading effect to both the lubricant and the machine. Some additives adsorb to the water and are removed when the water separates from the oil. Others are destroyed by water induced chemical reactions. Water also promotes oxidation of the oil’s base stock. It also causes rust and corrosion of machine surfaces and reduces critical, load bearing film strength and represents a real risk to the equipment and should be aggressively controlled. Water coexists with oil in either a dissolved state or a free state. When single water molecules are distributed throughout the oil due to the water’s chemical attraction to the fluid, it is in a dissolved state. Numerous factors such as viscosity, base stock type and condition, impurities and additive package determine the volume of water that the oil will dissolve and the dissolved volume is a function of the oil’s temperature and relative humidity.
If the oil
has dissolved all the water it can at a given temperature, it is said to be saturated.
Dissolved water is
very difficult to place under control and does only minimal harm to the machine and the oil.
When the oil is saturated and experiences a temperature decrease, it reaches a point below which water will condense into a free form. This is called the dew point temperature. Free water is the other state in which water coexists with the oil. Water is in a free state when undissolved globules of water are physically suspended in the oil.
Large
globules tend to separate to the bottom of the reservoir or sump. However, in mechanical equipment, the shearing forces of gears, pumps, bearings, etc., tend to crush the water into such small globules that a stable emulsion exists. An emulsion is the stable state of physical coexistence of chemically insoluble substances such as oil and water. Additives and impurities that lower the oil’s surface tension can serve as agents to strengthen the emulsion. Free and emulsified water pose the greatest risk to the machine and the lubricant and they should be placed under strict control.
There are a number of ways to measure the presence of water in oil but most of them are complicated, expensive or difficult to use in the field because they employ wet chemistry. An easy way to detect the presence of free and emulsified water, the most dangerous forms of water in oil, is with the hot plate crackle test. This simple, tried and true method alerts the marine surveyor to the presence of any free water.
PART 2: Coming up in
Major condition caused contamination: acid, oxidation, nitration. Lubricating oil on site tests: the feel test, the drop test, the crackle test.... and more!
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