Filter Types

The two filter types most often seen in the oil industry are the bed type (usually a bed of diatomaceous earth) and the disposable cartridge type; a washable cartridge type is also available, but less frequently used. The diatomaceous earth filter consists of a rack or frame which supports the beds of filter medium (see Figure 1).

Figure 1 Cartridge Filter Unit

Figure 1 Cartridge Filter Unit

This type of filter works by formation of a filter cake on the surface of the filter bed. Because the filter medium is lightly packed, it can erode or crack and must be replenished periodically. These filters are generally used as pre-filters in series with a cartridge-type filter canister.

The most common cartridge filters are disposable, usually formed by continuous wrapping of a fibrous material around a hollow perforated core. The filters can be designed to have a very positive cut point in terms of solids sizes removed. These filters are rated and described in different terms and it is essential to be familiar with the terms used. A filter may be rated as either a nominal or absolute filter for a certain particle size. A nominal rating means the filter will remove 90% of the particles greater than the rated size. It will remove some smaller particles, but will also leave in suspension some particles larger than the nominal rating. An absolute rating means the filter will remove essentially all particles larger than the rated size. Obviously, the filters with an absolute rating tend to be more expensive than the nominal filters. An example of a cartridge filter set up can be seen in Figure 2.

Usually, for the purpose of filtering clear brines, a diatomaceous earth pre-filter is used, followed by a cartridge type filtering system. The pre-filter will remove particles down to an average of a few microns and is able to handle high flow rates and heavy solids loading. This type of filter medium must be replenished periodically and requires an experienced operator to be present. The cartridge type filter system is fairly low maintenance and easily replaced when dirty. It is recommended that the filters be replaced when the differential pressure across the filters is in excess of 40 psi.

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