During operation of engines (especially trunk type engines) the lubricating oil will gradually be contaminated by small particles originating from the combustion and, in particular engines operated on heavy fuels which will normally increase the contamination due to the increased content of carbon residues and other contaminants (about four stroke lubricating system you can read in here).
Contamination of lubricating oil with either freshwater or seawater can also occur.
A certain amount of contaminants can be kept suspended in the lubricating oil without affecting the lubricating properties, but the condition of the lubricating oil must be kept under observation, on a regular basis, by analyzing oil samples.
The moving parts in the engine are protected by the built-on duplex full-flow lubricating oil filters, where usually the replaceable paper filter cartridges in each filter chamber has a fineness of 10-15 microns. The safety filter, at the centre of each filter chamber, is a basket filter element, with a fineness of 60 microns (sphere passing mesh).
The pressure drop across the replaceable paper filter cartridges is one parameter indicating the contamination level. The higher the dirt content in the oil, the shorter the periods between filter cartridge replacement and cleaning.
The condition of the lubricating oil can be maintained / re-established by exchanging the lubricating oil at fixed intervals or based on analyzing oil samples.
For engines exclusively operated on MDO/MGO is recommended to install a built-on centrifugal bypass filter as an additional filter to the built-on full flow depth filter.
It is advisable to run lube oil separator units continuously for engines operated on MDO/MGO as separator units present the best cleaning solution. Mesh filters have the disadvantage that they cannot remove water and their elements clog quickly.
The engines operated on HFO (heavy fuel oil) require effective lubricating oil cleaning and in order to ensure a safe operation it is necessary to use supplementary cleaning equipment together with the built-on full flow depth filter. In this case it is mandatory to run lube oil separator units continuously, so that the wear rates are reduced and the lifetime of the engine is extended.
- As a result of normal operation, the lubricating oil contains abraded particles and combustion residues which have to be removed by the lube oil purifying system and to a certain extent by the duplex full-flow lubricating oil filter as well.
With automatic mesh filters this can result in an undesirable and hazardous continuous flushing. In view of the high cost of cleaning equipment for removing micro impurities, this equipment is only rated for a certain proportion of the oil flowing through the engine since it is installed in a bypass.For cleaning of lubricating oil the following bypass cleaning equipment can be used:
- Separator unit
- Self cleaning automatic bypass mesh filter
- Decanter unit
- Built-on centrifugal bypass filter
The cleaning equipment must be operated continuously when the engine is in operation or at standstill and is mandatory during engine operation, as an optimal lubricating oil treatment is fundamental for a reliable working condition of the engine.
In case full flow filtration equipment is used, this is installed as in-line cleaning upstream to the duplex full-flow lubricating oil filter, built onto the engine.
If the lubricating oil is circulating without a separator unit in operation, the lubricating oil will gradually be contaminated by products of combustion, water and/or acid and, in some instances cat-fines may also be present. Therefore, in order to prolong the lubricating oil lifetime and remove wear elements, water and contaminants from the lubricating oil, it is mandatory to use a lube oil purifier unit.
The separator unit will reduce the carbon residue content and other contaminants from combustion on engines operated on HFO, and keep the amount within recommendation, on condition that the separator unit is operated according to manufacturer’s recommendations.
When operating a cleaning device, the following recommendations must be observed:
- The optimum cleaning effect is achieved by keeping the lubricating oil in a state of low viscosity for a long period in the separator bowl.
- Sufficiently low viscosity is obtained by preheating the lubricating oil to a temperature of 95°C – 98°C, when entering the separator bowl.
- The capacity of the separator unit must be adjusted according to manufacturer’s recommendations. Slow passage of the lubricating oil through the separator unit is obtained by using a reduced flow rate and by operating the separator unit 24 hours a day, stopping only for maintenance, according to maker’s recommendation.
- The heater can also be used to maintain an oil temperature of at least 40 ºC depending on installation of the lubricating oil system.
With multi-engine plants, one separator unit per engine in operation is recommended,
but if only one separator unit is in operation, the following layout can be used.
If a separator unit is installed, with one in reserve, it is possible, for operation of all engines through a pipe system, which can be carried out in various ways. The aim is to ensure that the separator unit is only connected to one engine at a time, thus there will be no suction and discharging from one engine to another. It is recommended that inlet and outlet valves are connected so that they can only be changed over simultaneously.
With only one engine in operation there are no problems with separating, but if several engines are in operation for some time it is recommended to split up the separation time in turns on all operating engines. With 2 out of 3 engines in operation the 24 hours separating time must be split up in around 4-6 hours intervals between changeover.
In order to ensure that the centrifugal forces separate the heavy contaminants in the relatively limited time that they are present in the separator bowl, as stated above the inlet temperature of 95-98°C for lubricating oil must be kept. A control circuit including a temperature transmitter and a controller with accuracy of ±2°C is usually installed and it is essential to keep the incoming oil temperature to the separator unit steady with only a small variation in temperature allowed (maximum ±2°C). The position of the interface between oil and water in the separator bowl is a result of the density and the viscosity of the oil, which in turn depends on the temperature.
It is known that separation efficiency is a function of the separator unit’s flow rate. The higher the flow rate, the more particles are left in the oil and therefore the lower the separation efficiency. As the flow rate is reduced, the efficiency with which particles are removed increases and cleaning efficiency thus improves. It is, however, essential to know at what capacity adequate separation efficiency is reached in the specific case.
In principle, there are three ways to control the flow:
- Adjustment of the built-in safety valve on the pump. This method is NOT recommended since the built-on valve is nothing but a safety valve. The opening pressure is often too high and its characteristic far from linear. In addition, circulation in the pump may result in oil emulsions and cavitation in the pump.
- A flow regulating valve arrangement on the pressure side of the pump, which bypasses the separator unit and re-circulates part of the untreated lubricating oil back to the treated oil return line, from the separator unit and NOT directly back to the suction side of the pump.
The desired flow rate is set manually by means of the flow regulating valve. Further, the requirement for backpressure in the clean oil outlet MUST also be fulfilled, helping to maintain the correct interface position.
Proper maintenance is an important, but often overlooked operating parameter that is difficult to quantify. If the bowl is not cleaned in time, deposits will form on the bowl discs, the free channel height will be reduced, and flow velocity increases. This further tends to drag particles with the liquid flow towards the bowl’s centre resulting in decreased separation efficiency.
Oil seldomly loses its ability to lubricate (to form a friction-decreasing oil film), but it may become corrosive to the steel journals of the bearings in such a way that the surface of these journals becomes too rough and wipes the bearing surface. In that case the bearings must be renewed, and the journals must also be polished.
The corrosiveness of the lubricating oil is either due to far advanced oxidation of the oil itself (TAN) or to the presence of inorganic acids (SAN). In both cases the presence of water will multiply the effect, especially sea water as the chloride ions act as an inorganic acid.
If circulating oil of inferior quality is used and the oxidative influence becomes grave, prompt action is necessary as the last stages in the deterioration will develop surprisingly quickly, within one or two weeks. Even if this seldomly happens, it is wise to be acquainted with the signs of deterioration.
These may be some or all of the following:
- Sludge precipitation in the separator unit multiplies;
- Smell of oil becomes acrid or pungent;
- Machined surfaces in the crankcase become coffee-brown with a thin layer of lacquer;
- Paint in the crankcase peels off or blisters;
- Excessive carbon is formed in the piston cooling chamber.
In a grave case of oil deterioration the system must be cleaned thoroughly and refilled with new oil.
At normal service temperature the rate of oxidation is insignificant, but the following factors will accelerate the process:
- High temperature
If the coolers are ineffective, the temperature level will generally rise. A high temperature will also arise in electrical pre-heaters if the circulation is not continued for 5 minutes after the heating has been stopped, or if the heater is only partly filled with oil.
- Catalytic action
Oxidation of the oil will be accelerated considerably if catalytic particles are present in the oil. Wear particles of copper are especially harmful, but also ferrous particles and rust are active. Furthermore, the lacquer and varnish oxidation products of the oil itself have an accelerating effect. Continuous cleaning of the oil is therefore important to keep the sludge content low.
- High temperature
If the TAN is low, a minor increase in the fresh water content of the oil is not immediately detrimental while the engine is in operation. Naturally, it should be brought down again as quickly as possible (below 0.2% water content, which is permissible). If the engine is stopped while corrosion conditions are unsatisfactory, the crankshaft must be turned ½ – 3/4 revolution once every hour by means of the turning gear and you must make sure that the crankshaft stops in different positions, to prevent major damage to bearings and journals. The lubricating oil must be circulated and separated continuously to remove water.
Water in the oil may be noted by steam formation on the sight glasses, by appearance, or ascertained by immersing a piece of glass or a soldering iron heated to 200-300°C in an oil sample. If there is a hissing sound, water is present. If a large quantity of water has entered the lubricating oil system, it has to be removed, either by sucking up sediment water from the bottom, or by replacing the oil in the sump.
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Source and Bibliography:
- Alfa Laval purifying system
- Westfalia purifying system
- MAN Diesel & Turbo Service letter SL13-582/KEL