Cone Roof With Floating Pan. Floating roof tanks present a problem in cold climates because of the
accumulation of ice and snow. Because of this problem, floating roof tanks can be modified and a fixed roof
installed. This procedure employs the advantages of the fixed roof and the floating roof.
Construction. Underground tanks are coated with a coal tar coating to prevent corrosion. They may
have a concrete supporting wall around the exterior surface.
Vapor loss. Underground tanks also come equipped with deepwall pumps and pressure vacuum vents.
Because of the stabilized temperature (mid 50s), underground storage tanks have very little vapor loss.
Leakage. Leaks in underground storage tanks are very hard to detect and may show up in water tables
after the leak has occurred.
Cut and Cover Tanks. Cut and cover tanks are constructed by scooping out a hole or depression in the
earth, setting a steel storage tank in the cut, and covering it with earth so that the only part of the structure
above ground would be a small shack or manhole and vent to permit access to the tank and allow excessive
vapors to escape.
PART C - TANK ACCESSORIES
Pressure-vacuum vent. The pressure and vacuum vent (breather valve) (Figure 1-1) is an automatic
device designed to reduce evaporation loss and to help relieve excessive pressures or vacuums built up
during operations or as a result of atmospheric temperature variations. The vent consists of a pressure
section and a vacuum section. Each section opens according to a predetermined setting. To ensure
efficient operation, vents should be inspected monthly.
Flame arrester. Flame arresters (Figure 1-2) are used to prevent flashback into a tank in case of fire.
The flame arrester is usually placed between the roof and the vent. One type of flame arrester consists
of a tight coil of fine wire mesh enclosed in a heavy casing which is flanged to the roof.
Manometer. Manometers are installed on the roofs of floating roof tanks to facilitate gaging. They are
used to measure the difference between the level of product in the tank and the level of product in the
gaging well, and to subsequently arrive at an accurate gage.
Gage hatch. Self-explanatory. Individuals involved in gaging should be careful not to drop articles down
the gaging hatch in as much as this has a bad effect on the product and also might block valves and
Manhole (or clean out door). Opening designed to permit entry for such purposes as cleaning the tank
and making repairs.
Grounding. Some tanks are inherently grounded by their contact with the ground, and grounding wires
are not required.