The pressure oil filter housing contains a wire-wound or mesh, Paper or felt elements and incorporates a by-pass valve. The filter housing can be drained independently of the main oil system. This is done through a drain valve in the housing base. When drained, the filter can be removed for examination, servicing, or replacement, as necessary, without disturbing the rest of the system. Typical pressure filters are illustrated in figure.
Filters are usually fitted with an impending by-pass indicator. This is usually a red pop out indicator which will pop out and stay out it the differential pressure across the filter element exceeds a predetermined value. This value will be less than the by-pass valve value, to allow the filter to be replaced before the filter goes into by-pass. The pop out usually has a thermal lock on it, which prevents the pop out extending when the oil is cold and thick.