hydraulic filter with a contamination indicator.Figure 9-12 shows a full-flow bypass-typehydraulic filter without a contamination indicator.A filter bypass indicator provides a positiveindication, when activated, that fluid is bypassingthe filter element by flowing through the bypassrelief valve. This indicator should not be confusedwith the pop-up differential pressure indicatorpreviously discussed which simply monitors thepressure across the element. With the bypassindicator, a similar pop-up button is often usedto signal that maintenance is needed. However,the bypass indicators further signal that, as aresult of the high differential pressures across theelement, an internal bypass relief valve has liftedand some of the fluid is bypassing the element.Identification of the type of installed indicatorcan be obtained from filter manifold drawings orrelated equipment manuals. Both a fluid bypassindicator and a differential pressure indicator orgauge may be installed on the same filterassembly.As with differential pressure indicators, bypassrelief indicators can be activated by pressuresurges, such as may develop during cold starts orrapid system pressurization. On some reliefindicators, the pop-up button, or whatever signaldevice is used, will return to a normal positionwhen the surge passes and pressure is reduced.Other relief indicators may continue to indicatea bypass condition until they are manually reset.Figure 9-12.—Full-flow bypass-type hydraulic filter.Before corrective action is taken based onindicator readings, the bypass condition shouldbe verified at normal operating temperature andflow conditions by attempting to reset theindicator.Proportional-Flow FilterThis type of filter operates on the venturiprinciple. (See glossary.) As the fluid passesthrough the venturi throat a drop in pressure iscreated at the narrowest point. See figure 9-13.A portion of the fluid flowing toward and awayfrom the throat of the venturi flows through thepassages into the body of the filter. A fluidpassage connects the hollow core of the filter withthe throat of the venturi. Thus, the low-pressurearea at the throat of the venturi causes the fluidunder pressure in the body of the filter to flowthrough the filter element, through the hollowcore, into the low-pressure area, and then returnto the system. Although only a portion of the fluidis filtered during each cycle, constant recirculationthrough the system will eventually cause all thefluid to pass through the filter element.Figure 9-13.Proportional-flow filter.9-10
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