FILTERS AND STRAINERS
DOE-HDBK-1018/2-93
Miscellaneous Mechanical Components
Another type of cartridge filter is the wafer, or disk filter. In this filter, disks are stacked to
form a cartridge and placed down over a central perforated pipe. Each disk is typically 1/8 inch
to 1/4 inch thick and made of cellulose or asbestos fibers.
Liquid that enters the disk filter moves up around the outside of the stack of disks, is forced
between the disks, travels through the perforations in the central pipe, and then leaves the filter.
The filtering action takes place as the liquid is forced between the disks.
As with the smaller cartridges, if a disk filter is used to filter radioactive water, it may be very
radioactive when it is removed, and must be handled very carefully. One way to remove a disk
filter is by means of a crane, which lifts the filter out of its housing and moves it to a shielded
container. The disposal problem is one of the major disadvantages of cartridge and disk-
cartridge filters.
Precoat Filters
A precoat filter eliminates the problem of physically handling radioactive materials, because the
filter material (called the medium) can be installed and removed remotely. Inside the filter
housing is a bundle of septums (vertical tubes, on which the filter medium is deposited). The
septums in some filters are approximately 1 inch in diameter and 3 feet long and are usually
made of perforated or porous metal (normally stainless steel). There may be several hundred
of these septums in a filter. Septums in other filters are approximately 3 inches in diameter and
3 feet long and are made of porous stone or porous ceramic material. There are usually less
than 100 of these larger septums in a filter.
The filtering medium fibers may be finely divided diatomite, perlite, asbestos, or cellulose.
Diatomite, the least expensive medium, is used to filter liquid waste that will be discharged from
the plant. Cellulose is generally used for processing water that will be returned to a reactor,
because diatomite can allow silica leaching.
When a precoat filter is in use, water that enters the filter vessel passes through the filter
medium that is deposited on the septums and then leaves through the outlet. Before the filter
can be placed into operation, however, the filter medium must be installed; that is, the filter must
be precoated.
The first step in precoating the filter is to close the inlet and outlet valves to the filter. The filter
medium used is mixed with demineralized water in an external mixing tank to form a slurry,
which is pumped through the filter. Some of the filter medium deposits on the septums and is
held there by the pressure of water on the outside of the septums. At the beginning of the
precoating process, some of the fibers of the filter medium pass through the septums, either
because they are smaller than the openings or because they pass through lengthwise. Thus, there
is still some filter medium in the water as it leaves the filter, so the slurry is recirculated again
and again until the water is clear. Clear water indicates that all of the filter medium is deposited
on the septums, and the filter is precoated.
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