Chemical Engineering March 2017 - 77

Engineering Practice
Designing Atmospheric Storage Tanks
Insights into the basics of process design of atmospheric storage tanks and an
example of how to prepare a process datasheet are presented here
Prasanna Kenkre
Jacobs Engineering India
S
torage
tanks
are
widely
used in the petroleum refining
and petrochemical
sectors to store a variety of
liquids, from crude petroleum to finished
product (Figure 1). This article
presents the basic process of designing
atmospheric storage tanks
(ASTs), as well as a discussion about
preparing a process datasheet. An
example is used to illustrate the
points made.
When to opt for ASTs
In simple terms, storage tanks that
are freely vented to the atmosphere
are known as (aboveground) atmospheric
storage tanks (ASTs). They
have a vertical cylindrical configuration
and can be easily identified by
the open vent nozzle or " gooseneck "
vent pipe on the tank roof.
ASTs may be shop-welded or fieldwelded
and are customarily fabricated
from structural quality carbon
steel, such as A-36 or A-283 Gr.C.
The vertical cylindrical shape and
relatively flat bottom helps to keep
costs low.
ASTs store low-vapor-pressure
fluids that do not pose any environmental,
hazard or product-contamination
issues, so they can be freely
vented to the atmosphere. However,
when storing certain fluids, such
as when vapors of the stored liquid
are flammable or when oxidation
of liquid may form hazardous compounds,
it is undesirable to have
the tank vapor space freely vented.
In such cases, inert gas blanketing
of the vapor space may be used.
Tanks with inert-gas blanketing are
also often included in this category.
A blanketing system is normally designed
so that it operates at slightly
higher than atmospheric pressure,
therefore preventing outside air from
FIGURE 1. Storage tanks are a common sight at petroleum refineries and petrochemical plants
entering the vessel.
Typically, ASTs are considered to
have an operating pressure ranging
from 0 to 0.5 psig. Tanks designed
to operate at pressures between 0.5
and 15 psig are termed as low-pressure
storage tanks. Designs above 15
psig are treated as pressure vessels.
Tank roof types
There are two basic types of vertical-tank
roof designs - fixed or
floating roof.
Fixed roof. In this design, the tank
roof is welded with the shell and the
roof remains static.
Floating roof (internal or external).
In this design, the tank roof floats
on the liquid surface and rises and
falls with changes in liquid level. The
internal floating-roof tank (IFRT) has
a permanent fixed roof with a floating
roof inside while the external
floating-roof tank (EFRT) consists
of an open-topped cylindrical shell
with a roof that floats on the liquid.
An IFRT is used where heavy accumulation
of snow or rainwater, or
lightning is expected and may affect
the roof buoyancy of an EFRT.
In an IFRT, tank vapor space located
above the floating roof and below
the fixed roof includes circulation
vents to allow natural ventilation of
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MARCH 2017
the vapor space, which reduces the
accumulation of product vapors and
possible formation of a combustible
mixture. In some cases, the natural
ventilation is avoided and the vent
is either sent for treatment (for example,
to a scrubbing tower) or to a
vapor-recovery system (for example,
a benzene-vapor-recovery system).
As a rule of thumb, fixed-roof
tanks are used to store liquids with
true vapor pressures (TVP) of less
than 10 kPa(a) (TVP is the absolute
pressure when the vapor is in equilibrium
with liquid at a constant temperature).
Floating roofs are limited to
storing liquids with a maximum TVP
of 75 kPa(a). For liquids with flash
point (the lowest temperature, corrected
to a barometric pressure of
101.3kPa(a), at which application of
a flame test causes the vapor of the
test portion to ignite under the specified
conditions of the test) below
37.8°C, excessive loss of volatile
liquids occurs from the use of openvented
fixed-roof tanks. Hence,
floating roofs are mostly used for liquids
with flash points below 37.8°C.
Codes for tank design
The American Petroleum Institute
(API; Washington, D.C.; www.api.
org) has developed a series of atmo77

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Chemical Engineering March 2017

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