hydraulic filter with a contamination indicator.
Figure 9-12 shows a full-flow bypass-type
hydraulic filter without a contamination indicator.
A filter bypass indicator provides a positive
indication, when activated, that fluid is bypassing
the filter element by flowing through the bypass
relief valve. This indicator should not be confused
with the pop-up differential pressure indicator
previously discussed which simply monitors the
pressure across the element. With the bypass
indicator, a similar pop-up button is often used
to signal that maintenance is needed. However,
the bypass indicators further signal that, as a
result of the high differential pressures across the
element, an internal bypass relief valve has lifted
and some of the fluid is bypassing the element.
Identification of the type of installed indicator
can be obtained from filter manifold drawings or
related equipment manuals. Both a fluid bypass
indicator and a differential pressure indicator or
gauge may be installed on the same filter
As with differential pressure indicators, bypass
relief indicators can be activated by pressure
surges, such as may develop during cold starts or
rapid system pressurization. On some relief
indicators, the pop-up button, or whatever signal
device is used, will return to a normal position
when the surge passes and pressure is reduced.
Other relief indicators may continue to indicate
a bypass condition until they are manually reset.
Figure 9-12.Full-flow bypass-type hydraulic filter.
Before corrective action is taken based on
indicator readings, the bypass condition should
be verified at normal operating temperature and
flow conditions by attempting to reset the
This type of filter operates on the venturi
principle. (See glossary.) As the fluid passes
through the venturi throat a drop in pressure is
created at the narrowest point. See figure 9-13.
A portion of the fluid flowing toward and away
from the throat of the venturi flows through the
passages into the body of the filter. A fluid
passage connects the hollow core of the filter with
the throat of the venturi. Thus, the low-pressure
area at the throat of the venturi causes the fluid
under pressure in the body of the filter to flow
through the filter element, through the hollow
core, into the low-pressure area, and then return
to the system. Although only a portion of the fluid
is filtered during each cycle, constant recirculation
through the system will eventually cause all the
fluid to pass through the filter element.
Figure 9-13.Proportional-flow filter.