Filtration of Fuel and Lubricating Oils Flashcards
What is the purpose of a filter?
To remove wear particles, dirt, debris and other contaminants from fuel and lubricating oil. If these contaminants were left in the fluid, they would seriously abrade the internal workings of the machinery and eventually lead to its failure
Three methods of filtration
- Full flow
- Bypass
- Batch
Modern machinery installations normally use two or more of these methods in order to achieve the desired cleanliness of the fluid.
- Full flow
Occurs when all of the fluid is passed through the filter. Since the pumps most commonly used in fuel and lubricating oil systems are of the positive displacement type, some sort of relief valve must be fitted to the filter in order to prevent over-pressurization of the system or, in the case of a lube oil system, starvation of the engine.
The draw back to this type of system is that if the relief valve opens due to excessive pressure, there will be no filtration of the fluid.
- Bypass
A filter is connected to the discharge line of the pump and 10%-50% of the total volume of fluid discharged is passed through the filter and back to the tank. Since the fluid passed through the filter does not re-enter the pressure system, the pump must have an additional capacity beyond that required in the system.
The major problem with this system is that there is no protection/filtration for the fluid that does not pass through the filter.
- Batch
Occurs when a system or circuit is set up parallel to the pumping circuit to clean or process a quantity of fluid.
Three types of filtering materials
- Mechanical materials
- Absorbent materials
- Adsorbent materials
The type of filter material used will depend on the design of the filter and its application.
- Mechanical materials
Filter by trapping particles between closely woven metal screens or discs and are only good for relatively course contaminants.
- Absorbent materials
Made of porous substances such as paper, cotton or cellulose and trap minute particles.
- Adsorbent materials
Filter media (e.g. activated charcoal) that remove the particles by electrostatic forces or by molecular attraction.
Not often used in filtering lubricating oil as they will remove the additives in the oil.
Types of Filters
- Basket Strainer
- Cartridge Filter
- Screw on Cartridge Filter
- Magnetic Filters
- Cleanable filters
- Coalescing Filters
- Automatic Full flow Lube oil Filters
- Centrifugal oil filters
- Basket Strainer
The wire mesh basket consists of a wire mesh element which is normally made of stainless steel so as to be corrosion resistant. The basket is located inside of a steel casing and inlet and outlet connections are fitted. This type of unit is normally fitted in pairs (duplux) so that one strainer can be cleaned by hand while the other is in use. Suitable for relatively course particles (50 micron or more), a strainer will be fitted upstream of other types of filters to trap coarse particles that would otherwise clog or damage fine filters.
- Cartridge Filter
A disposable cartridge filter consists of a steel housing, resembling a pressure vessel that has a welded steel bottom plate and a removable steel cap. A vertical tube is fitted to the bottom plate at its center and this helps locate the filter element inside the chamber.
The filter element consists of a perforated metal cylinder that houses the filter media (used to be steel wool, cotton, or linen but nowadays is felt, cellulose or paper). If paper is used, it is pleated to give a larger filter area within a smaller overall volume and also to create a large dirt retention capability. The pleated ends are jointed together to form a cylinder and are supported within the filter body by a central core (normally perforated steel cylinder). The end caps are steel and have a hole drilled at the center with a suitable gasket (felt or rubber) attached to the outside.
The filter cartridge is placed over the vertical tube and pushed down into the housing so that it firmly seats against the bottom plate. A cover and gasket are fitted in plate. Pressurized fluid enters the housing at the periphery of the top cover. Since the cartridge is sealed top and bottom, the only way for the fluid to exit is to pass through the filter and then through a hole in the bottom or top center of the housing.
This type of filter will reduce contaminants down to 8 microns for lubricating oil and 5 microns for fuel oil. Average life expectancy of 1000-2000 hours. As contaminants build up within the filter media, the efficiency of the filter will decrease.
- Screw on Cartridge Filter
Used when design or use makes it difficult to place a conventional cartridge filter on a machine. The housing is composed of a thin sheet of steel with sufficient thickness to withstand the pressure developed in the system. The end of the housing will be pressed with indentations or covered in a non-slip coating to allow easy installation and removal. A spring presses down against the bypass valve so that all the components are held firmly in place. The bypass valve is fitted so that the system will not be starved of fluid should the filter become excessively clogged.
A pleated paper or cellulose filter element, used to give a larger filter area within a smaller overall volume and to create a large dirt retention capability, is supported by a cylindrical perforated tube known as the support frame. Below the filter element is the anti-drain valve. This is an internal check valve that is used to ensure that fluid does not drain out of the system when it is shut down. If the fluid was allowed to drain out in an engine lube oil system, the engine may be started up without adequate lubrication.
Welded to the bottom of the housing is a cover plate with a groove for a gasket or sealing ring. The center of the cover plate is threaded so that it can be installed on the machine.
The oil enters the oil filter under pressure through the holes on the perimeter of the base plate. The “dirty” oil then passes through the filter media where it is “cleaned”. It then flow to the central tube and back into the engine through the threaded hollow center mounting stud.
- Magnetic Filters
Normally used in fluid pumping systems to remove very small particles. As these filters will only trap ferrous particles, they are often used in conjunction with cartridge or spin-on filters. Direct contact between the fluid and the magnets is required because when the oil is stationary, only the particles immediately surrounding the magnet are attracted. Therefore, this type of filter will be placed where flow of the fluid is present (in the pumping circuit) rather than in a position where the fluid is stagnant (sump or reservoir).
While a large number of configurations exist, most magnetic filters work by producing a magnetic field or loading zones that collect magnetic iron and steel particles. Various types of magnets can be used in these filters. Magnets used in some filters can have flux density (magnetic strength) as high as 28,000 gauss. The higher the flux density, the higher the potential magnetic gradient and magnetic force acting on nearby iron and steel particles.
The most common type of magnetic field is a drain plug, where a magnet in the shape of a disc or cylinder is attached to its inside surface (typically by adhesion). Periodically, the magnet plug is removed and inspected for ferromagnetic particles, which are then wiped from the plug. Magnetic plugs are commonly used in engine oil pans, gearboxes, and occasionally in hydraulic reservoirs.
In some instances, rod magnets are used. These are magnetic plugs that are inserted into the oil, either down from the reservoir top or within the center tube of a standard filter element. These collectors consist of a series of rings or toroidal-shaped magnets assembled axially onto a metal rod. Between the magnets are spacers where the magnetic gradient is the highest, serving as the loading zone for the particles to collect. Periodically, the rods are removed, inspected and wiped clean with a rag or lint-free cloth.
In some cases, the magnetic filter will be combined with a cartridge filter or a basket strainer into one unit. This allows filtration to occur in one unit rather than having multiple units installed. When the cartridge filter is replaced, the magnets will be wiped clean with a lint-free cloth.
- Cleanable filters
These filters have internal elements that may be cleaned, rather than replaced. The type of filter consists of individual steel housings secured to a central frame with a filter cartridge fitting into each of the housings. Each cartridge is fabricated in one of the following manners:
1. A series of annular discs are placed over an X shaped center spindle and held in longitudinal compression by threaded rods
2. a series of long cylindrical elements called “candles” are spring loaded between two end plates.
These filters are fitted in duplex, with one filter element in use and the other being cleaned or in standby. Oil enters through one of two flanges at the central frame and is directed to one of the filter housings. The oil passes from the outside to the inside of the filter elements and to the outlet flange. The elements in service are selected by means of a change over lever fitted to the central frame. A pressure differential indicator is fitted between the inlet and outlet flanges to show when the element is becoming clogged.
To clean the filter, the elements must be removed and disassembled then washed in a solvent solution. They are then reassembled and replaced in the housing or held in “spare”. The disadvantage of this type of filter is that it may become damaged from repeated cleaning, thus losing its capacity to effectively remove contaminants from the oil.
- Coalescing Filters
These filters remove both solid and water contaminants. This type of filter is mainly used on fuel systems as the high viscosity of lube oil makes separation difficult. A coalescing filter is made up of two parts, the coalescing bowl and the cartridge filter element.
Fuel enters the filter and is directed down through the center tube. Fuel flows pas a non-return ball valve and into the turbine. The turbine is composed of two plastic discs that impart rotation and swirl to the fluid. Any contaminants and water droplets are separated, due to their higher specific gravity, and settle in the bottom of the bowl. Fuel now travels upwards and any remaining water droplets collect on the cone and bowl surfaces. As more water droplets are separated from the fuel, the droplets join together (coalesce) and form bigger droplets which eventually flow downward into the bowl. The fuel is now filtered by the cartridge and exits at the outlet port.
Periodically the bowl of the filter will be drained of all debris and water. At regularly scheduled intervals, the filter element will be replaced.
- Automatic Full flow Lube oil Filters
Fitted so that there can be extended operating hours between cleaning and inspection of the filters. This type of filter is installed in the lube oil circuit directly after the lube oil pump and uses pressure from the circuit to operate.
The cast steel filter housing is composed of three different sections:
1. The lower section supports the whole filter unit and contains a mesh basket. Four feet secure the filter to the ship’s structure.
2. Mounted above this is the full flow chamber. Inside are the filter elements and the distributor. The filter elements are a series of discs that are stacked one on top of the other and held in tension by the means of threaded rods. Each filter element consists of an upper half and a lower half. The halves are fabricated from aluminum and are fitted with stainless steel fine mesh screens. Ribs are cast into the halves so that they divide the element into 12 individual segments. The distributor is fabricated of cast steel, is machined to fit inside the filter elements, and is supported on the bottom by a spigot cast into the lower section.
3. Mounted to the top of the housing is the diversion chamber, which contains the back-flush filter elements and the hydraulic motor. A cock is fitted for draining the oil and debris that collects in the chamber.
Lubricating oil at system pressure is supplied from the lubricating oil pump and enters the filter at the lower section. The basket strainer removes large particles of dirt and debris that may otherwise clog the fine mesh of the filter element. Oil then passes up through the center of the filter and out through the filter elements. The elements are fabricated in such a way that when stacked together, the oil passes from the upper and lower sides of the individual elements and out through the small space between them. Filtered oil now passed through the outlet flange and on to the main engine. There is a pressure differential from the inlet to the outlet of around 0.3 bar and an alarm is fitted if the differential exceeds 0.8 bar.
A certain quantity of clean filtered oil is removed from the full flow chamber and piped to the hydraulic motor attached to the top of the diversion chamber.
The hydraulic motor is directly connected to the distributor. The distributor sits in the center of the filter elements and is designed to permit the back flushing (and cleaning) of one of the 12 segments of the filter elements at a time. The distributor is fabricated with a U-shaped indentation. Radially drilled holes in the bottom of this indentation connect to a centrally drilled passageway in the distributor. When the indentation passes by one of the 12 segments, it blocks off the flow of inlet oil to that segment. Oil from the filtered side of the element now flows backwards through the segment, removing all of the contaminants that have accumulated. The contaminants flow into the trench, through the radially drilled holes to the center passage way and up into the diversion chamber.
In the diversion chamber, the oil passes through a small stack of filter elements, where the contaminants are removed and then on to the main engine sump. The discs in the diversion chamber must be periodically cleaned. This is done by manually back flushing the discs while the engine is running. The drain valve is opened and pressurized filtered oil is back flushed through the chamber by means of a connection on the bottom of the chamber. Dirty oil and contaminants from this chamber are sent to the waste lube oil tank.
- Centrifugal oil filters
Small filters that are used for bypass filtering of engine lubricating oil. The centrifugal oil filter is mounted after the lube oil pump so that it can be supplied with hot high pressure lubricating oil. A small amount of oil is diverted from the system and sent to the filter for cleaning, after which it returns to the crankcase.
High pressure hot oil enters the filter at the side and travels up through a hollow center spindle. A baffle, fitted to the center of the spindle, evenly distributes the oil in the filter chamber, which is rotating at a speed in excess of 6000 RPM. The centrifugal force generated in the chamber is in excess of 2000 G’s and any contaminants in the oil are forced to the outside wall of the bowl. Clean oil leaves the chamber through a screen fitted to the center spindle and passed down into the drive chamber. Oil exits the drive chamber through 2 circumferential jets. The force of the oil being expelled through the jets is what gives the bowl the rotational speed required for operation. Oil then drains back to the sump by gravity, or in some cases, is forced back by a compressed air float chamber located in the base of the unit.
Periodically, the filter will be disassembled for maintenance. The bowl is opened up and the oil contaminants, which at this point have usually built up into a thick film on the bowl, are removed.
