MIL-HDBK-1005/9A
pumping rate can be used for initial skimming operations, and as
the oil layer decreases in thickness, the pumping rate can be
reduced to raise the weir and minimize the potential for water
carryover. Average pumping rates of 6 to 7 gpm (22.7 to 26.5
L/min) are suggested, with the maximum recommended rate at 10
gpm (37.9 L/min). Since they operate in contact with and are
partially immersed in the oily waste, floating weir-type
skimmers should be constructed of plastic or stainless steel.
Figure 17 illustrates a floating weir skimmer installation.
3.10.2.6 Sample Taps. Provide sample taps along one wall of
the LET, near the discharge end, spaced at 1-foot maximum depth
intervals. For accessibility, they should be adjacent to and
follow the incline of the tank top access stairway. Each tap
should consist of a piece of 1-inch (25-mm) diameter PVC pipe,
with length equal to the wall thickness plus 6 inches (152 mm),
and with a 1-inch (25-mm) diameter PVC ball valve at the
sampling end outside the tank. The through wall pipes should be
set when the tank wall is formed and poured. The exterior
surface of the pipe should be roughened prior to setting in the
forms to ensure a good bond with the concrete and a grout ring
should be provided at the midwall point as a precautionary
obstruction to leakage along the pipe surface through the wall.
3.10.2.7 Water Supply. Provide a water supply with minimum
delivery of 15 gpm (56.8 L/min) at 40 psig (276 kPa) around the
top of the tank. At least two discharge points with hose bibb
outlet controls should be provided on tanks less than 100 feet
(30.5 m) long. For tanks longer than 100 feet, three or four
outlet points should be provided. In areas subject to extremely
cold weather, freezeless hydrants should be provided in lieu of
hose bibbs. The water supply will be used for tank cleaning,
foam control, and general housekeeping; therefore, potable
quality is not essential. If a source of nonpotable, service
water is available at a facility, it should be used for the LET
supply. If it is necessary to tap the LET supply from the
potable water system, a suitable backflow preventer must be
installed in the LET supply line immediately downstream from the
tapping point.
3.10.2.8 Corrosion Protection. Apply protective coatings to
both the interior and exterior wall surfaces to ensure long-term
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