The full-flow filter provides a positive filtering action. However, it offers resistance to flow, particularly when the filter element becomes dirty. Hydraulic fluid enters the fil ter through the inlet port in the body and flows around the filter element inside the filter bowl. Filtering takes place as the fluid passes through the filtering element and into the hollow core, leaving the dirt and impurities on the outside of the filter element.
The filtered fluid then flows from the hollow core through the outlet port and into the system. Figure 5-5 illustrates a typical full-flow filter. Some full-flow filters are equipped with a contamination indicator. These indicators, also known as differential pressure indicators, are available in three types: gauge, mechanical pop-up, and electrical. As contaminating particles collect on the filter element, the differential pressure across the element increases. In some installations using gauges as indicators, the differential pressure must be obtained by subtracting the readings of two gauges located somewhere along the filter inlet and outlet piping. For pop-up indicators, when the increase in pressure reaches a specific valve, an indicator (usually on the filter head) pops out, signifying that the filter must be cleaned or replaced. A low-temperature lockout feature is installed in most pop-up types of contamination indicators to elimi nate the possibility of false indications due to cold weather.
Filter elements used in filters that have a contamination indicator are not normally removed or replaced until the indicator is actuated. This decreases the possibility of contaminated fluid bypassing the filter element and contaminating the entire system. This type of filter will minimize the necessity for flushing the entire system and lessen the possibility of failure of pumps and other components in the system.
A bypass relief valve is installed in some filters. The bypass relief valve allows the fluid to bypass the filter element and pass directly through the outlet port in the event that the filter element becomes clogged. These filters may or may not be equipped with a contamination indicator. Figure 5-6 shows a full-flow, bypass-type hydraulic filter. This example includes a bypass indicator. Figure 5-7 shows a similar filter with out the indicator.
A filter bypass indicator provides a positive indication, when activated, that the fluid is bypassing the filter element. This indicator should not be confused with the pop-up differential pressure indicator.