Figure 9-40.—Inlet line filter.
Figure 9-41.—Inlet strainer.
the various micron sizes with mesh and standard
sieve sizes.
A simple screen or a wire strainer is rated for
filtering fineness by a MESH number or its near
equivalent, STANDARD SIEVE number. The
higher the mesh or sieve number, the finer the
screen.
When a filter is specified as so many microns,
it usually refers to the filter’s NOMINAL rating.
A filter nominally rated at 10 microns, for
example, would trap most particles 10 microns in
size or larger. The filter’s ABSOLUTE rating,
however, would be a somewhat higher size,
perhaps 25 microns. The absolute rating is the size
of the largest opening or pore in the filter.
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.
FILTER/STRAINER LOCATION
There are three general areas in a system for
locating a filter: the inlet line, the pressure line,
or a return line. Both filters and strainers are
available for inlet lines. Filters are normally
used in other lines.
Inlet Filters and Strainers
Figure 9-40 shows the location of an inlet line
filter. An inlet line filter is usually a relatively
coarse mesh filter. A fine mesh filter (unless it is
very large) creates more pressure drop than can
be tolerated in an inlet line.
Figure 9-41 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 about
150 microns in size.
Pressure Line Filters
A number of filters are designed for installa-
tion right in the pressure line (fig. 9-42) and
can trap much smaller particles than inlet line
Figure 9-42.—Pressure line filter.
Figure 9-43.—Return line filter.
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