Filters remove particles from liquids and gases, decreasing wear and tear on downstream systems and increasing operating stability and dependability. Filters should have a high level of cleaning efficiency, create minimal resistance, have a high impurity pick-up capacity, and be simple to clean. Filters eliminates solids in the flowing media undergoing filtration, almost completely above the filtration limit and with diminished efficiency below the filtration limit. Particles with diameters greater than the mesh size of the filter fineness can be caught and separated, whilst smaller particles gather on the filter’s exit side. These have weak stability, and when subjected to force (for example, in a bearing’s lubricating wedge), they readily dissolve into their original particle sizes, inflicting less harm.

They might be straining filters, gap filters, or membrane filters.

I’m not going to describe how they’re manufactured, how they work or what they’re composed of since you’ve already learned that in college. Instead, I’d want to discuss their significance and the issues that might arise as a result of their failure.

They are, as you may have guessed, both manually and automatically operated filters.

Manual filters are fairly basic, and they are utilized in ship operations, namely in ventilation and refrigeration installations, as well as as an indication filter in lubricating oil and fuel oil systems. They are often stacked in parallel of two or more filters, with one filter inactive in each case. A switching handle switches the input and output from active to passive filtering at the same time (lever or wheel). Prior to switching on, the passive filter should be ventilated.

Automatic filters are a group of filters that self-clean the filter surface when a certain differential pressure is reached. All cleaning activities are program-controlled and performed automatically.

Differential pressure is used to monitor the operation of the filter. Cleaning is required with increased differential pressure in the case of manual filters, whilst cleaning occurs automatically when the predetermined differential pressure is reached in the case of automated filters.
The cleaning interval indicates the quantity of pollutants in the liquid to be filtered, and a shortening of the period indicates a continually growing degree of filter soiling. If the time gap between two subsequent cleaning processes is shortened under the same operating circumstances, the filter must be manually cleaned to eliminate clinging contaminants. An increase in the time gap between two subsequent cleaning procedures, on the other hand, indicates filter dysfunction.

Differential pressure indicator malfunction can be a typical failure of the filtering system. If the differential pressure indicator is not properly working, the system or operator cannot monitor the condition of the filter regarding impurities accumulation and can’t asses its condition, until a downstream low pressure alarm is triggered or an emergency shutdown takes place. Sometimes the differential pressure indicator can be repaired if all necessary parts are available onboard.

In case of residue accumulation there will be a rise in the differential pressure between the filter inlet and outlet and as the differential pressure increases, the downstream system is starved of oil and pressure will drop under a certain preset low limit which will trigger an emergency shutdown of the engine.

Filter’s surfaces are cleaned by first removing impurities with gas oil, then mechanically cleaning. An efficient and easy way to clean the filter’s surface is by using an ultrasonic method because high frequency vibrations of an exciter are transferred to the filter cloth through a liquid in ultrasonic baths, which will easily remove solid impurities such as oil breakdown products. The vibrations loosen the filth, which is then rinsed away by the liquid. After filters have been rinsed they are air blown from inside out using a special made multi-nozzle pipe tool.

Strongly adhering residues that, like a coat of paint, wrap the individual woven threads of the filter mesh will produce lacquering, which will increase the filter fineness that will lead to an increment of the differential pressure. Lacquering cannot be removed by back-flushing in most cases and requires filters to be removed and mechanically cleaned and sometimes there were situations when filter candles need to be burned out using a flame gas torch in order to melt down the sticky residues. I have met situations when the filter candles were beyond cleaning due hard and strongly adhering lacquering and need to be discarded.

Filter mesh damage or rupture lead to a volumetric flow that permit unfiltered fuel or oil to pass through the filter. Moreover, residues that have already been separated can pass through the rupture and will posses a serious danger to the downstream system and can cause serious damage to the engine components. There were situations when the engine bearings were completely destroyed due oil filter damage, when wire mesh goes inside the oil system with the flow. Moreover, with the new common rail system, when everything is controlled by control or servo oil, the oil cleanliness is of the outmost importance as it will lead to serious damage of the engine control valves and system. Similarly with fuel oil filters, when the residues went inside system and destroyed the fuel injection pumps or fuel injector valves. So, when a damage filter is detected it must be immediately replaced and discarded.

The viscosity of the filtered fuel or oil increases as the temperature decreases. In the absence of adequate preheating, the viscosity might rise to the point that passing through the filter is restricted or obstructed. The differential pressure builds to the point where, in the case of automated filters, continual automatic cleaning takes place. The downstream system’s required operating pressure may not be met, resulting in an engine emergency shutdown.

Another typical failure is represented by gasification which takes place when the gas solubility of a liquid decreases as the temperature raises and the pressure falls in the filter. In this case, gas accumulates inside filter casing and is vented or passes into the system, where it can lead to an operating pressure drop resulting in an engine emergency shutdown.

So, in conclusion the proper functioning and integrity of filtering equipment is of the outmost importance for the proper functioning of the lube oil and fuel oil systems onboard vessels. Under no circumstances, damaged equipment should be kept in use and all necessary spare parts must be always available onboard. If for example spares are not available, the slot should not be kept empty and another type of filter or filtering equipment should be made temporary available. Sometimes in critical situations, the filter candles are swapped from lube oil to fuel oil system and vice versa. This must be a temporary measure and should be fixed as soon as possible. You need to keep in mind that the mesh sizes are different for these filters and prior installation need to check if the correct size is installed. This can be found marked on the filter element as per below:

On most of the vessels, especially on fuel oil system, the filtering equipment is build in stages, from fuel transfer pumps having coarse basket type filters to engine final duplex fine filters, all having different mesh sizes and arrangements. Fuel transfer pumps and fuel feed pumps are usually equipped with magnetic basket type filters, used to restrict bigger size ferrous particles.

Needless to say that proper care and maintenance must be carried out according with manufacturer’s instruction manual and/or company’s plan maintenance system. Failure to do so can lead to extended damage of the equipment and vessel’s engines.

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