6.6 Oil strainers and filters
The oil strainers are coarse wire-mesh basket type assemblies which are designed to catch large foreign material from the oil drains into the main oil tank, thus ensuring that material reaching the centrifugal pump suction will not cause pump damage. They can be lifted out of the tank to remove and inspect any debris collected.
Two main systems of filtration of the lubricating oil are used:
(a) Cartridge filters fitted into the oil cooler casing which are designed to filter 13 litres/s of hot oil per cartridge, to a nominal particle size of 10 microns. Each cooler can have up to four disposable cartridges giving a total filtration capacity of around 52 litres/s, with two coolers required for 100% duty.
(b) Duplex filters fitted after the oil coolers. These can be either multiple disposable filters, with facilities for on-load replacement, or a duplex plate filter which can be cleaned on-load. The plate-type filter consists of two compartments, each with five filter assemblies. Each assembly consists of a series of plates and spacers, which give the required degree of filtration of 75 micron. Each compartment normally passes 50% of the oil; facilities exist to pass 100% flow through one compartment in order to allow replacement, maintenance or inspection of one compartment whilst the unit is on-load. Facilities are provided to enable a filter to be primed and pressurised after service, before putting a filter compartment back into service. The filter normally provides a capacity of around 106 litres/s under a pressure drop of 0.3 bar with both compartments in service. Normal filter cleaning is carried out automatically when a rising pressure differential reaches 0.45 bar across the filter. Ro¬tation of each plate filter assembly then occurs against a scraper bar; the debris off each filter falls into a sump at the bottom of the filter housing and is cleaned out on a regular basis. A typical arrange¬ment of a plate-type filter is shown on Fig 2.61.
Where a plate-type filter is used in the main oil supply to the bearings, additional filtration is required for the oil supply to:
- The main thrust and surge bearings
- The shaft turning gear, jacking oil pump, gearing and clutch
- The main oil pump, thrust bearing and cooling sprays
The oil for these duties is taken from the filtered oil supply to the bearings and passed through a duplex plate-type filter system similar to that described above and shown on Fig 2.61. This additional filter has only four plate filter assemblies in each compartment and has only manual filter cleaning. The filter provides a capacity of 16 litres/s under a pressure drop of 0.2 bar with both compartments in service.