Contents

INTRODUCTION TO STORAGE TANKS ........................................................................................................ 1

1. STORAGE TANKS .......................................................................................................................................... 1

2. TYPES OF STORAGE TANKS ............................................................................................................................ 1

3. CONTAINMENT BASIN ................................................................................................................................... 2

4. FIXED-ROOF TANK ....................................................................................................................................... 3

5. EXTERNAL FLOATING ROOF TANK .................................................................................................................. 3

6. INTERNAL FLOATING ROOF TANK .................................................................................................................. 4

7. DOMED EXTERNAL FLOATING ROOF TANK ...................................................................................................... 5

8. HORIZONTAL TANK ...................................................................................................................................... 5

9. PRESSURE TANK .......................................................................................................................................... 5

10. VARIABLE VAPOR SPACE TANK .................................................................................................................. 6

11. LNG STORAGE TANK................................................................................................................................ 6

Introduction to Storage Tanks

1. Storage Tanks

Storage tanks containing organic liquids, non organic liquids, vapors and

can be found in many industries. Most storage tanks are designed and built

to the American Petroleum Institute API-650 specification.

These tanks can have different sizes, ranging from 2 to 60 m diameter or

more. They are generally installed inside containment basins in order to

contain spills in case of rupture of the tank.

Industries where storage tanks can be found are: petroleum producing and

refining, petrochemical and chemical manufacturing,bulk storage and

transfer operations, other industries consuming or producing liquids and

vapors.

2. Types of storage tanks

Basically there are eight types of tanks used to store liquids:

1. Fixed-roof tanks

2. External floating roof tanks

3. Internal floating roof tanks

4. Domed external floating roof tanks

5. Horizontal tanks

6. Pressure tanks

7. Variable vapor space tanks

8. LNG (Liquefied Natural Gas) tanks

The first four tank types are cylindrical in shape with the axis oriented

perpendicular to the sub grade. These tanks are almost exclusively above

ground. Horizontal tanks can be used above and below ground. Pressure

tanks often are horizontally oriented and spherically shaped to maintain

structural integrity at high pressures. They are located above ground.

Variable vapor space tanks can be cylindrical or spherical in shape.

3. Containment Basin

A containment basin of a product should be built around the tanks are

made of brick or concrete and the lining should be impervious to liquid

stored to prevent spills that can cause fire, property damage or

contaminate the environment. The minimum capacity of the basin volume

should be equal to the capacity of the largest tank plus 10% of the sum of

the capacities of others. To prevent a spill or other emergency the walls of

the containment basin must be resistant to the product and must be able

to withstand considerable pressure. The drain Valve, which should be

incorporated into the outer side of the containment basin, must be closed

to prevent possible contamination to the environment.

4. Fixed-Roof Tank

Of currently used tank designs, the fixed-roof tank is the least expensive to

construct and is generally considered the minimum acceptable equipment

for storing liquids. A typical fixed-roof tank consists of a cylindrical steel

shell with a cone- or dome-shaped roof that is permanently affixed to the

tank shell. Storage tanks are usually fully welded and designed for both

liquid and vapor tight, while older tanks are often have a riveted or bolted

construction and are not vapor tight.

Lifting and handling of a stainless steel storage tank (large image).

A breather Valve (pressure-vacuum Valve), which is commonly installed on

many fixed-roof tanks, allows the tank to operate at a slight internal

pressure or vacuum. This Valve prevents the release of vapors during only

very small changes in temperature, barometric pressure, or liquid level, the

emissions from a fixed-roof tank can be appreciable.

Additionally, gauge hatches/sample wells, float gauges, and roof manholes

provide accessibility to these tanks and also serve as potential sources of

volatile emissions.

5. External Floating Roof Tank

A typical external floating roof tank consists of an open-topped cylindrical

steel shell equipped with a roof that floats on the surface of the stored

liquid, rising and falling with the liquid level. The floating roof is comprised

of a deck, fittings, and rim seal system. Floating roof decks are constructed

of welded steel plates and are of three general types: pan, pontoon, and

double deck. Although numerous pan-type decks are currently in use, the

present trend is toward pontoon and double-deck type floating roofs.

Manufacturers supply various versions of these basic types of floating

decks, which are tailored to emphasize particular features, such as full

liquid contact, load-carrying capacity, roof stability, or pontoon

arrangement. The liquid surface is covered by the floating deck, except in

the small annular space between the deck and the shell; the deck may

contact the liquid or float directly above the surface on pontoons.

External floating roof tanks are equipped with a rim seal system, which is

attached to the roof perimeter and contacts the tank wall. The rim seal

system slides against the tank wall as the roof is raised and lowered. The

floating deck is also equipped with fittings that penetrate the deck and

serve operational functions. The external floating roof design is such that

evaporative losses from the stored liquid are limited to losses from the rim

seal system and deck fittings (standing storage loss) and any exposed

liquid on the tank walls (withdrawal loss).

6. Internal Floating Roof Tank

That tanks has both, a permanent fixed roof and a floating roof inside.

There are two basic types of internal floating roof tanks:

tanks in which the fixed roof is supported by vertical columns within

the tank

tanks with a selfsupporting fixed roof and no internal support

columns

The fixed roof is not necessarily free of openings but does span the entire

open plan area of the vessel. Fixed roof tanks that have been retrofitted to

employ an internal floating roof are typically of the first type, while external

floating roof tanks that have been converted to an internal floating roof

tank typically have a self-supporting roof.

Tanks initially constructed with both a fixed roof and an internal floating

roof may be of either type. An internal floating roof tank has both a

permanently affixed roof and a roof that floats inside the tank on the liquid

surface (contact deck) or is supported on pontoons several inches above

the liquid surface (noncontact deck). The internal floating roof rises and

falls with the liquid level.

7. Domed External Floating Roof Tank

Domed external floating roof tanks have the heavier type of deck used in

external floating roof tanks as well as a fixed roof at the top of the shell

like internal floating roof tanks. Domed external floating roof tanks usually

result from retrofitting an external floating roof tank with a fixed roof.

As with the internal floating roof tanks, the function of the fixed roof is not

to act as a vapor barrier, but to block the wind. The type of fixed roof most

commonly used is a self supporting aluminum dome roof, which is of

bolted construction. Like the internal floating roof tanks, these tanks are

freely vented by circulation vents at the top of the fixed roof. The deck

fittings and rim seals, however, are basically identical to those on external

floating roof tanks.

8. Horizontal Tank

Horizontal tanks are constructed for both above-ground and underground

service. Horizontal tanks are usually constructed of steel, steel with a

fiberglass overlay, or fiberglass-reinforced polyester. Horizontal tanks are

generally small storage tanks.

Horizontal tanks are constructed such that the length of the tank is not

greater than six times the diameter to ensure structural integrity.

Horizontal tanks are usually equipped with pressure-vacuum vents, gauge

hatches and sample wells, and manholes to provide accessibility to these

tanks. In addition, underground tanks may be cathodically protected to

prevent corrosion of the tank shell. Cathodic protection is accomplished by

placing sacrificial anodes in the tank that are connected to an impressed

current system or by using galvanic anodes in the tank. However, internal

cathodic protection is no longer widely used in the petroleum industry, due

to corrosion inhibitors that are now found in most refined petroleum

products.

9. Pressure Tank

Go to Spherical and Cylindrical Pressure Vessels

10. Variable Vapor Space Tank

Variable vapor space tanks are equipped with expandable vapor reservoirs

to accommodate vapor volume fluctuations attributable to temperature and

barometric pressure changes. Although variable vapor space tanks are

sometimes used independently, they are normally connected to the vapor

spaces of one or more fixed roof tanks. The two most common types of

variable vapor space tanks are lifter roof tanks and flexible diaphragm

tanks. Lifter roof tanks have a telescoping roof that fits loosely around the

outside of the main tank wall. The space between the roof and the wall is

closed by either a wet seal, which is a trough filled with liquid, or a dry

seal, which uses a flexible coated fabric. Flexible diaphragm tanks use

flexible membranes to provide expandable volume. They may be either

separate gasholder units or integral units mounted atop fixed roof tanks.

Variable vapor space tank losses occur during tank filling when vapor is

displaced by liquid. Loss of vapor occurs only when the tank's vapor

storage capacity is exceeded.

11. LNG Storage Tank

A liquefied natural gas storage tank or LNG storage tank is a specialized

type of storage tank used for the storage of Liquefied Natural Gas. LNG

storage tanks can be found in ground, above ground or in LNG carriers.

The common characteristic of LNG Storage tanks is the ability to store LNG

at the very low temperature of -162°C. LNG storage tanks have double

containers, where the inner contains LNG and the outer container contains

insulation materials. The most common tank type is the full containment

tank. Tanks are roughly 55 m (180 ft) high and 75 m in diameter.

In LNG storage tanks if LNG vapors are not released, the pressure and

temperature within the tank will continue to rise. LNG is a cryogen, and is

kept in its liquid state at very low temperatures. The temperature within

the tank will remain constant if the pressure is kept constant by allowing

the boil off gas to escape from the tank. This is known as auto-

refrigeration. The world's largest above-ground tank (Delivered in 2000) is

the 180 million liters full containment type for Osaka Gas Co., Ltd. The

world's largest tank (Delivered in 2001) is the 200 million liters Membrane

type for Toho Gas Co., Ltd..