This document provides an introduction and overview of different types of storage tanks. It discusses 8 main types of storage tanks: fixed-roof tanks, external floating roof tanks, internal floating roof tanks, domed external floating roof tanks, horizontal tanks, pressure tanks, variable vapor space tanks, and LNG storage tanks. Each tank type is designed for storing different types of liquids and gases and has distinct features such as floating roofs, fixed roofs, insulation, and sizes that can range from meters to over 100 meters in diameter. The document also discusses containment basins, which storage tanks are often placed inside of to contain spills.

1. 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

2. 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.

3. 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.

4. 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.

5. 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

6. 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..