Filtration is a process of collecting or separating solid particles from a fluid by passing through a filtration medium where targeted particles are retained. This is done by using physical barriers like screen mesh, membranes or strainers.
There are two process of filtration (i.e. two methods):
Surface filtration: In this method, particles are collected on the surface of filter medium.
Deep-bed filtration: In this method, particles are collected inside the filter medium when particles pass through it.
Note: As solid particles accumulate in filter medium either in deep-bed filter or surface filter, drop in pressure increases continuously. And when this pressure drop attains a certain permissible level, the filter must be cleaned or replaced.
Factors for determining filtration
Properties of impurities: size, shape, charge, density
Properties of liquids: density, viscosity, corrosiveness
Temperature of fluids
Note: Rate of filtration is directly proportional to the surface area of filter medium. And rate of filtration is proportional to pressure drop.
Filtration & Centrifugal separation
Filter separates the impurities on the basis of size. Centrifugal separator separates the impurities on the basis of density difference between fluids and impurities.
Filtration is economical and can be used in various ways and system. Centrifugal separation can’t be used for other than fluids.
Importance of filter is increased when we talk about ship’s machinery where we have to deal with fluids like Lube oil, Fuel oil, Sea water, Bilges etc. in economical piping system. We can’t afford very high quality (already purified) fuel oil & lube oil for machinery as it costs very high, so we have to purify as much as possible at ship only to minimize machinery problems.
Types of filters at ship
On the basis of media (fluid), filters can be categorized into 4 types:
Hydraulic (Oil) filter
Water (SW or FW) filter
Oil filters include filters for Lube oil, Fuel oil & Hydraulic oil. These oil are quintessential for ships. These filters are most important for efficient operation of ship’s machinery. Different types of oil filters are used for different purposes.
Fine mesh screen filter: It’s most commonly used filter in FO & LO system. Cylinder oil, ME/AE fuel oil & lube oil and Boiler fuel oil system generally includes it.
Magnetic filter: Actually, filtering elements are magnetic in nature, that’s why called magnetic filter. Magnetic elements are surrounded by basket type screen for enhanced filtering. Magnetic elements catch ferrous materials by attracting them. This type of filter is mostly used in Lube oil system because chances of ferrous metal particles is higher in lube oil. Thus it provides extra protection for engines and gearboxes where ferrous particles are more likely to be present.
Auto-back wash filter: It can be used for LO system as well as FO system also. In this filter, dirty oil is passed through a set of filtering elements which are divided into chambers.
Centrifugal filter: Its working principle is centrifugal force action with density of substances. Higher density impurities are separated from lower density fluid. This type of filter is commonly used in LO system of auxiliary engine.
Note: Some filters are manually cleaned type, in which filter is attached with a shaft and handle. It’s cleaned manually with the help of handle lever that’s why called manual clean filter.
Sea water and Fresh water system on-board ship are equipped with filters (strainers) to catch the solid impurities. Generally, Sea water system has more filters than Fresh water.
Sea chest filter: Actually it’s a strainer but we are used to call it as filter. This filter is fitted in main sea water suction line and also connected with MGPS (Marine Growth Prevention System). Sea water system consists of 2 sea suctions i.e. High sea chest & Low sea chest and these are like gate for sea water supply in Engine Room through pipelines. Sea chest filter cleaning is another important skill.
Fire line suction filter: It has medium screen mesh size for filtering sea water in main fire line.
Bilge line filter: It has generally large screen mesh size or holes in cylindrical strainer. It is fitted in suction side of bilge line for filtering mud, rags and dirt.
Fresh water filter: In fresh water system, filters are fitted in suction side. It has small size screen mesh.
Filter used for filtering moisture, compressed air or supply air on-board ship is generally used in control system, turbocharger, IG system & compressors. It is used for supplying good quality air for enhanced efficiency of the machinery.
Control air filter: It is used for efficient working of pneumatic system. It filters or removes moisture from compressed air which is generally carried away through control air compressor.
Turbocharger filter: It is used in compressor (blower) side which in-takes fresh air. It is made of copper mesh and prevent carbon, dirt & particles from taking inside the blower.
Compressor suction filter: It is fitted for taking fresh pure air inside the compressor and prevents damage of parts due to foreign materials.
Note: Inert Gas System has also air filter which is generally called as demister filter. It removes moisture from the gas.
Some special filters are nothing but filters used or designed for special purpose.
OWS filter: Oily water separator has filters in both stages of separation i.e. in 1st stage, it has simple felt filter and in 2nd stage, it has coalescer filter, which is costly.
Cartridge filter: It is constructed like a cartridge which can be easily replaced or cleaned by removing cartridge. It is used for portable machinery like generator and compressor of small size. It is generally of duplex type.
Duplex filter implies that filters are in pair, so that when maintenance is required, one filter can be removed by changing over. This system is generally used for all filters at ship for maintenance point of view.
Filter removal & cleaning (LO or FO filter cleaning procedure)
Observe the sufficient pressure drop for change-over or routine maintenance.
Ensure that which is running filter and which is stand-by (clean filter). For checking you can open purging cock slightly, if oil is continuously coming out that means it’s a running filter.
Change the filter by turning the handle to opposite side in one go if by-pass option is there, if not then slowly-slowly change-over. During change-over, open the equalizing valve, if any.
After changing, wait for few minutes because oil is hot.
Now, open purging cock and open the drain cock slightly and drain some oil for dropping the oil level in filter casing. (Always purge before opening filter)
Loose bolts in cross direction and open the cover.
Remove filter element and keep it in a clean bucket. During removal of filter always try to hold few seconds in casing so that maximum oil will drain in casing only.
Dismantle candles, if there and put filter element in diesel or thinner for cleaning purpose and wait for 15 minutes at least.
Now, blow-through the filter elements (candles) with compressed air (service air) and box back the filter. Always take care of O-ring or gasket of filter cover, it should be in proper condition.
Sea chest filter cleaning procedure
Changeover to the standby sea chest filter and ensure all systems running normally.
Shut the inlet and outlet valves of the sea chest filter to be cleaned. Lock and tag them as closed so that nobody opens it accidentally.
Open the vent valve and release any residual pressure as well as check whether the inlet and outlet valves are holding. When no further water or air escapes from the vent valve, it signifies that the valves are holding.
Drain the sea water from the sea chest through the drain valve at the bottom.
Open the top cover of the filter and remove the mesh filter.
Clean the mesh filter thoroughly.
Place back the mesh filter and make sure to align any locking notches on the filter body.
Clean the sealing surface on the filter body and filter cover.
Replace sealing gasket if need be and place back the filter cover carefully.
Tighten the cover and shut the drain valve.
Keep the vent valve crack open and then open the inlet valve slowly.
Purge the filter and check for leakages around the filter body.
Shut the vent valve after purging and then open the outlet valve
Bilge line & Fire line filter cleaning procedure
These filters are not of duplex type. So we have to isolate the system for cleaning filters.
First of all stop respective pump and breaker-off with proper tag (Men at work).
Close the suction & discharge valve and valve before filter also.
Now, purge the filter by opening purging cock and slack the bolts.
Open cover and remove filter (generally these are bucket type strainer).
Use pressure jet of water and compressed air for cleaning and box back properly.
Mesh number (Sieve number)
Mesh is nothing but the measurement of particle size of material. Mesh number indicates the number of openings per linear inch. 10 mesh means 10 holes in 1 inch in a line. Although it’s not a precise measurement of particle size as it can vary for different wire materials. It’s a USA standard of categorizing particles and thus filter’s quality is also determined by its mesh number.
Higher mesh number = Finer particle size
(For filtration: particle size must be greater that mesh screen size).