Filters and Strainers
Filtration is the process of removal of solid particles from a fluid by passing them through a porous medium. Different filtration techniques are available to deal with various types and sizes of foreign materials in the fluid to be filtered. All the systems on board ships such as fuel oil system, lube oil system, hydraulic oil system, feed water system, bilge system, etc. are fitted with line filters for removal of foreign matters. As all these systems are entirely different from one another, design and construction of the filter unit is also varies accordingly.
Size of Particles Filtered (Microns)
0.001 - 0.03
0.3 - 5
Micro Particulate Filtration
1 - 25
Macro Particulate Filtration
A wide range of filter elements are available for different purposes. It depends on the suitability of the filter material for its use in the filtering medium, nature and size of the particle to be separated, method of filtration, etc.
Screen Cylinders Elements
Screen elements are available in a variety of designs, which depends on filter body type, application and desired degree of filtration. Following configurations of screen elements are commonly used.
- Moulded Plastic Construction with Polyester Screen.
- Moulded Plastic Screen Construction with Stainless Steel Screen.
- Perforated Stainless Steel Screens for coarse filtration.
- Perforated Stainless Steel Screens with inserted Stainless Steel Polyester Sleeve(s) for medium and fine filtration.
- Wedgewire screens, which can filter particles of 800-200 micron size. They are used mainly in automatic filters. Here a very strong element, yet simple wiring designs allows perfect cleaning of self-cleaning operated filters.
- Weave Wire Screens, which can filter particles of 200-25 micron size. Consist of different weaving types inside a wedgewire construction. They are normally used for automatic filters.
Grooved Discs Elements
This type of elements are formed of numerous thin plastic discs stacked on a telescopic core. Both sides of the plastic discs are grooved, these grooves when piled up cross each other and all discs are tightened together. Most important advantage of these filter elements is that they can effectively retain organic matters.
Membrane elements may be natural or synthetic fibrous woolen felt or paper.
Magnetic filter elements retain metal particles so that the source of these particles can be investigated while cleaning and inspection. Main engine lube oil system uses magnetic elements in their line filters.
Filters and Strainers
Filters and strainers are devices designed to prevent the passage of unwanted solids into or further along a system. A strainer is usually a coarse filter to remove the larger contaminating particles.
The simplest strainer consists of a box with a removable lid in which a flat perforated plate is inserted such that the fluid must pass through the perforations. Such strainers will be found close to sea water suction valves and immediately before bilge valves. In the latter applications they are known as strum boxes. Perforation sizes vary according to duty and manufacture but are invariably in the range 3-12 mm.
Perforation plates must be inspected when checking strainers, since they can be eroded or corroded with time. In case of gap formation at the top or bottom of the perforation plate or poor condition of the plate is observed, it must be replaced as soon as possible. Care must be taken while assembling the filter cover. Check that gasket is properly sealing. Insufficient sealing of suction side filter cover results in pump loosing suction, especially in bilge pumps.