Miscellaneous Mechanical Components
DOE-HDBK-1018/2-93
FILTERS AND STRAINERS
One characteristic of the precoating process is that a very even layer of filter medium
(approximately 1/8 inch thick) is deposited on the septums. This occurs because the circulating
slurry follows the path of least resistance. When the coating at one point reaches a certain
thickness, the slurry takes the fibers to another point, and this process continues until precoating
is complete.
Because water pressure holds the filter in place, flow must be maintained through the
recirculating loop to keep the medium from falling off. This is called a holding flow. As the
inlet and outlet valves are opened for normal usage, called service flow, the holding flow is
gradually cut off.
Backwashing Precoat Filters
After a filter has been precoated, it is put into service and kept on line until the pressure
differential indicates that the filter medium is becoming plugged. When this occurs, the old filter
medium is removed and the filter is precoated again. Filters are usually installed in pairs, so that
one filter can remain in service while the other is undergoing the filter backwashing and
precoating process.
Since water pressure helps to hold the filter medium against the septums, some of the old filter
medium will fall off as soon as this pressure is removed. Backwashing is used to remove the
filter medium that does not fall off. Backwashing is usually done in one of two ways. With
some filters, demineralized water is pumped backwards through the center of the septums, and
the filter medium coating is knocked off by the water as it comes out through the septums.
Most filters use a multi-step backwashing procedure. First, the inlet valve and the outlet valve
are closed, and the drain valve and the top vent are opened to allow the water to drain. Then
the drain valve and the vent are closed, and the inlet water valve is opened to raise the water
level. The filter is equipped with a special high-domed top to trap and compress air. When the
water inlet valve is closed and the drain valve is opened quickly, the compressed air forces water
down through the center of the septums. This water knocks the filter medium off of the
septums.
With both types of backwashing, the filter medium coating that is removed is sluiced out through
a drain line to a filter sludge tank, where it is stored for further processing. The filter is then
precoated again and put back into service.
With precoat filters, the type and quantity of filter medium is critical. If too little material or
too coarse a material is used, some of the finely divided crud in the water may get into the
openings of the septums. When the filter is backwashed, this crud is usually not removed. It
continues to build up during subsequent use of the filter until the septums become so plugged
that they have to be replaced.
Rev. 0
ME-05
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