MIL-HDBK-1005/9A
requires little maintenance, can operate continuously and
unattended, provides a high recovery efficiency, and can be
variably positioned for optimum coverage of LET area.
a) Flexible Tube Skimmer. This unit provides
excellent removal rates of all types of waste oils, greases, and
floating sludges, and it minimizes the removal of water with the
waste oil. The unit consists of a flexible, polyurethane
collector tube which is long enough to enable 16 feet (4.9 m) of
tubing to be in contact with the water surface at all times.
Floating oil, grease, and sludge adhere to the surface of the
tube and are thus skimmed from the surface. The tube is
circulated through a drive unit where scrapers clean the surface
and divert waste oil to storage. A typical installation
features the drive unit mounted at the end of a beam that is
cantilevered from a mounting post at the side of a tank or
basin. The unit should be mounted near the discharge end of the
LET. For tanks ≤ 20 feet (6.1 m) wide, one unit may be
sufficient. Tanks over 20 feet wide should be provided with two
units mounted on opposite sides of the tank. Operating 24
hours/day, these units can remove up to 240 gpd (908 L/d) for
light oils, 600 gpd (2,271 L/d) for medium oils, and 1,440 gpd
(5,450 L/d) for heavy oils. For cold climate installations, the
drive head/skimming assembly should be enclosed, insulated, and
heated. Figure 16 illustrates a flexible tube skimmer
installation.
b) Floating Weir Skimmer. This skimmer type uses an
adjustable weir to set the overflow depth below the oil layer
surface. These units are best applied to installations where
separated oil is allowed to accumulate for a number of days and
is then skimmed in a single operation. This differs from the
continuous or daily intermittent operating scheme for the tube
or mop type skimmer. There is a greater potential for skimming
significant amounts of water with the weir-type device. The
floating, weir-type skimmer is usually connected to a pump with
a flexible hose. This enables an operator to manually move and
position the skimmer for optimum interception of pockets of
accumulated oil, if the layer is not continuous or is disturbed
by skimming turbulence. In some units, varying the pumping rate
will change the submergence level of the weir. Thus, a higher
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