40 microns. Figure 6-22 shows the relationship of
the various micron sizes with mesh and standard
There are three general areas in a system for
A simple screen or a wire strainer is rated for
locating a filter; the inlet line, the pressure line,
filtering "fineness" by a MESH number or its
or a return line. Both filters and strainers are
near equivalent, STANDARD SIEVE number.
available for inlet lines. Filters are normally
The higher the mesh or sieve number, the finer
used in other lines.
When a filter is specified as so many microns,
it usually refers to the filter's NOMINAL rating.
Inlet Filters and Strainers
A filter nominally rated at 10 microns, for
example, would trap most particles 10 microns in
Figure 6-23 shows the location of an inlet
size or larger. The filter's ABSOLUTE rating,
line filter. An inlet line filter is usually
however, would be a somewhat higher size,
a relatively coarse mesh filter. A fine mesh
perhaps 25 microns. The absolute rating is the size
filter (unless it is very large) creates more
of the largest opening or pore in the filter.
pressure drop than can be tolerated in an
Absolute rating is an important factor only when
it is mandatory that no particles above a given
size be allowed to circulate in the system.
Figure 6-24 shows a typical strainer of the type
installed on pump inlet lines inside a reservoir.
It is relatively coarse as filters go, being
constructed of fine mesh wire. A 100-mesh
strainer protects the pump from particles above
about 150 microns in size.
Pressure Line Filters
A number of filters are designed for
installation right in the pressure line (fig. 6-25)
and can trap much smaller particles than inlet
line filters. Such a filter might be used where
system components, such as valves, are less
Figure 6-23.--Inlet line filter.
dirt-tolerant than the pump. The filter thus
fluid as it leaves the pump. Pressure line
filters must be able to withstand the operating
pressure of the system.
Figure 6-24.--Inlet strainer.
Figure 6-25.--Pressure line filter.