INTRODUCTION
Tank cleaning is a task that should not be approached lightly as there are many hazards involved in the
process. These hazards are lead, hydrogen sulfide, and explosive vapors, just to mention a few. If you find
yourself in a position to supervise the cleaning of storage tanks, you must be thoroughly familiar with these
hazards.
PART A - DESIGN AND USAGE OF PETROLEUM STORAGE TANKS
The following factors that must be considered when designing and using petroleum storage tanks:
Losses result from filling, breathing, and seepage.
Filling loss: As an atmospheric storage tank is filled, the vapor above the product will be forced out of the
tank which results in a loss of product.
Breathing loss: An atmospheric storage tank used for static storage will lose product as vapors are
forced out of the tank due to the expansion of the product as it heats up during the day. Conversely, as
the product cools and contracts, the air will be drawn in.
Seepage loss: Due to small ruptures in the tanks.
Security against fire hazards:
Escaping vapor: Vapors, being heavier than air, will tend to seek low spots and will present hazardous
conditions. This is particularly dangerous during filling operations.
Leaks: Leaks are always a hazard and must be corrected as soon as observed.
Real estate: The proximity of storage tanks to one another and other facilities must be taken into
consideration when surveying potential hazards. Firewalls, dikes and berms, must be constructed in
order to minimize some of the risks involved with bulk petroleum storage and distribution.
PART B - TYPES OF TANKS
Keeping in mind the problems we have just discussed, let us classify and discuss the various types of tanks
according to the job they will do so that we can employ them to best solve the problem.
Fixed Roof Tanks.
Bolted steel cone roof. This is the type used extensively by the military. It comes in various sizes and is
classified as semipermanent and can be moved. This tank cannot withstand pressures of more than 1 to
3 ounces (oz.) per cubic in. and a vacuum of no more than 1/2 oz. cubic inch. For this reason, the tanks
have free vents and have a high vapor loss.
Welded steel cone roof. This type of tank is the most commonly used tank for permanent installations.
Because of the construction, it requires skilled personnel. This type of tank will withstand pressures from
3 to 8 oz. cubic inch in a vacuum of 1/2 oz. per cubic inch and is equipped with pressure vacuum vents.
For this reason, the welded cone roof tank is better suited for the storage of high volatile products than
the bolted steel tank.
Floating Roof Tanks. The floating roof "floats" on the surface of the product and virtually eliminates
breathing and filling loss. There are three types of floating roofs which we will discuss.
Pan type. A large, floating pan, slightly smaller in diameter than the tank shell. A system of flexible
"shoes" closes the space between the edge of the roof and the tank shell.
Pontoon type. System of closed compartments or "pontoons" to increase floating stability and simplify
the structure.
Double deck. Two separate decks over entire back surface. Provides insulation from the sun's rays and
cuts down on loss of product from evaporation.
All tanks with floating roofs have a wind girder around the top edge to stiffen the shell when the roof is down.
Many commercial firms and Air Force bases employ these tanks for gasoline and JP4. (Best adapted where
tanks are filled and tied frequently.) They are somewhat of a maintenance problem because of the sealing
material used between the tank shell and the roof.
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QM 5096
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