WELDED STEEL

STORAGE TANKS
By :
AHMED OMER HASSAN
 TANKS:

 Broadly, the storage tanks can be divided into two basic types:
 Atmospheric storage
 Atmospheric storage is a term applied to tanks operating at or near
atmospheric pressure. This type of tank is used to hold liquid which will not
vaporize at ambient temperature. Tanks used in this category are primarily
the open top, fixed roof (cone & dome) and floating roof.
 Pressurized storage
 Pressurized storage applies to those vessels which are designed to withstand
pressure sufficient to keep the liquid stored, from vaporizing. High vapor
pressure hydrocarbons such as propane, butane are the types of products
requiring pressurized storage vessels .
Vertical tanks:

Open top tanks:

 This type of tank has no roof and shall be used for storing city water,
 fire water and cooling
 Fixed roof tanks:
 These types of tanks can be divided into:
 - Cone roof
 - Dome roof
 Fixed roof tanks:
 Fixed roof tanks are well known type of storage tanks, constructed over 100
years ago and provided mostly with self supporting roof structures either
cone or dome type roofs. For large diameter fixed roof tanks column
supported roof are used, for example in North America. Fixed roof tanks are
designed as atmospheric , low , or high pressure tanks
Fixed roof tanks with floating covers :

In a fixed roof tank a floating cover can be installed to give a further

reduction of vapor losses. These tanks are fitted with breather vents
either at the top course of the shell plate or on the roof edge.
 2.1.4 Floating roof tank
 This type of tanks are designed to work at atmospheric pressure. The
 diameter of a floating roof tanks shall at least be equal to its height to
 enable the use of a normal rolling ladder for access to the roof.
 Typical Products stored are: Crude oil, Gasoline and Gasoline
 components, Solvents……
Special features

 Since most liquids can spill, ev aporate, or seep through even the smallest
opening, special consideration must made for their safe and secure handling.
This usually involves building a bunding, or containment dike, around the tank,
so that any leakage may be safely contained.
 Some storage tanks need a floating roof in addition to or in lieu of the fixed roof
and structure. This floating roof rises and falls with the liquid level inside the tank,
thereby decreasing the vapor space above the liquid level. Floating roofs are
considered a safety requirement as well as a pollution prevention measure for
many industries including petroleum refining.
 In the United States, metal tanks in contact with soil and
containing petroleum products must be protected from corrosion to prevent
escape of the product into the environment. The most effective and common
corrosion control techniques for steel in contact with soil is cathodic protection.
Tank failures:

 There have been numerous catastrophic failures of storage tanks, one of the most
notorious being that which occurred at Boston Massachusetts USA on January 14, 1919.
The large tank had only been filled eight times when it failed, and resulting wave
of molasses killed 21 people in the vicinity. The Boston molasses disaster was caused by
poor design and construction, with a wall too thin to bear repeated loads from the
contents. The tank had not been tested before use by filling with water, and was also
poorly riveted. The owner of the tank, United States Industrial Alcohol Company, paid out
$300,000 (nearly $4 million in 2012 ) in compensation to the victims or their relatives.
 There have been many other accidents caused by tanks since then, often caused by
faulty welding or by sub-standard steel. New inventions have at least fixed some of the
more common issues around the tanks' seal.[7][8] However, storage tanks also present
another problem, surprisingly, when empty. If they have been used to hold oil or oil
products such as gasoline, the atmosphere in the tanks may be highly explosive as the
space fills with hydrocarbons. If new welding operations are started, then sparks can easily
ignite the contents, with disastrous results for the welders. The problem is similar to that of
empty bunkers on tanker ships, which are now required to use an inert gas blanket to
prevent explosive atmospheres building up from residues.
American Petroleum Initiate (API) :

www.api.org/
 the American Petroleum Institute (API) is the only national trade association
that represents all aspects of America’s oil and natural gas industry
 API 650 Welded Tanks for Oil Storage
 API 620 Design and Construction of Large, Welded, Low-Pressure Storage
Tanks
API 650 REQUIREMENT
Welding:
 The minimum size of fillet welds shall be as follows: On plates 5 mm (3/16 in.)
thick, the weld shall be a full fillet weld, and on plates more than 5 mm
(3/16 in.) thick, the weld thickness shall not be less than one-third the
thickness of the thinner plate at the joint and shall be at least 5 mm.
 Single-welded lap joints are permissible only on bottom plates and roof
plates
 Lap-welded joints, as tack-welded, shall be lapped at least five times the
nominal thickness of the thinner plate joined; however, with double-welded
lap joints, the lap need not exceed 50 mm (2 in.), and with single-welded
lap joints, the lap need not exceed 25 mm
Typical Joints:

Vertical Shell Joints:

Vertical shell joints shall be butt joint
 Vertical joints in adjacent shell courses shall not be aligned, but shall be
offset from each other a minimum distance of 5t, where t is the plate
thickness of the thicker course at the point of offset.
Typical Horizontal Shell Joints
 Three-plate laps in tank bottoms shall be at least 300 mm (12 in.) from each
other, from the tank shell, and from joints between annular plates and the
bottom.
 A three-plate lap is created where three plates come together and all
plates are joined to one another by lap welds. A location where a pair of
bottom plates are lap-welded to each other and are lapped onto an
annular plate constitutes a three-plate lap, but lapping a single bottom
plate onto a buttwelded annular plate splice does not constitute a three-
plate lap weld since the two annular plates are not joined together by a
lap weld. These lap joint connections to the butt-weld annular plate are
illustrated.
Lap-Welded Bottom Joints :
But -Welded Bottom Joints

 Butt-welded bottom plates shall have their parallel edges prepared for butt
welding with either square or V grooves. Butt-welds shall be made using an
appropriate weld joint configuration that yields a complete penetration
weld. Typical permissible bottom butt-welds without a backing strip are the
same as those shown previous.
 Shell-to-Bottom Fillet Welds
 a) For bottom and annular plates with a nominal thickness 13 mm (1/2 in.),
and less, the attachment between the bottom edge of the lowest course
shell plate and the bottom plate shall be a continuous fillet weld laid on
each side of the shell plate. The size of each weld shall not be more than 13
mm (1/2 in.) and shall not be less than the nominal thickness of the thinner
of the two plates joined (that is, the shell plate or the bottom plate
immediately under the shell) or less than the following values:
 For annular plates with a nominal thickness greater than 13 mm (1/2 in.), the
attachment welds shall be sized so that either the legs of the fillet welds or
the groove depth plus the leg of the fillet for a combined weld is of a size
equal to the annular-plate thickness (see Figure 5.3c), but shall not exceed
the shell plate thickness.
 Roof and Top-Angle Joints
 Roof plates shall, as a minimum, be welded on the top side with a
continuous full-fillet weld on all seams. Butt weld are also permitted
 tank shells shall be supplied with top angles of not less than the following
size:
 For tanks with a diameter less than or equal to 9 m (30 ft) and a supported
cone roof , the top edge of the shell may be flanged in lieu of installing a
top angle. The bend radius and the width of the flanged edge shall
conform to the details below. This construction may be used for any tank
with a self-supporting roof if the total cross-sectional area of the junction
fulfills the stated area requirements for the construction of the top angle.
No additional member, such as an angle or a bar, shall be added to the
flanged roof to- shell detail
Defect in welds:
lack of side-wall fusion
lack of root fusion
Annular Bottom Plates:

 When the bottom shell course is designed using the allowable stress for
materials in Group IV, IVA, V, or VI, butt-welded annular bottom plates shall
be used (see 5.1.5.6).
 Annular bottom plates shall have a radial width that provides at least 600
mm (24 in.) between the inside of the shell and any lap-welded joint in the
remainder of the bottom. Annular bottom plate projection outside the shell
shall meet the requirements of 5.4.2. A greater radial width of annular plate
is required when calculated as follows:
 The thickness of the annular bottom plates shall not be less than the greater
thickness determined using Table 5.1a and Table 5.1b for product design
(plus any specified corrosion allowance) or for hydrostatic test design.
 Table 5.1a and Table 5.1b are applicable for effective product height of H
× G ≤ 23 m (75 ft). Beyond this height an elastic analysis must be made to
determine the annular plate thickness.
 In lieu of annular plates, the entire bottom may be butt-welded provided
that the requirements for annular plate thickness, welding, materials, and
inspection are met for the annular distance specified in 5.5.2.
Shell Design:

 Two methods for designing shell thickness:

 Calculation of Thickness by the 1-Foot Method
 Calculation of Thickness by the Variable-Design-Point Method
Calculation of Thickness by the 1-Foot Method:

The 1-foot method calculates the thicknesses required at design points 0.3 m (1 ft.)
above the bottom of each shell course.
Annex A permits only this design method. This method shall not be used for tanks
larger than 61 m (200 ft) in diameter.
 Allowable Stress
 5.6.2.1 The maximum allowable product design stress, Sd, shall be as shown
in Table 5.2a and Table 5.2b. The corroded plate thicknesses shall be used
in the calculation.
 The design stress basis, Sd, shall be either two-thirds the yield strength or
two-fifths the tensile strength, whichever is less.
 5.6.2.2 The maximum allowable hydrostatic test stress, St, shall be as shown
in Table 5.2a and Table 5.2b.
 The nominal plate thicknesses shall be used in the calculation. The
hydrostatic test basis shall be either three-fourths the yield strength or three-
sevenths the tensile strength, whichever is less
The required shell thickness shall be the greater of the design shell thickness, including any
corrosion allowance, or the hydrostatic test shell thickness, but the shell thickness shall not be
less than the following
Roof:

 a) A supported cone roof is a roof formed to approximately the surface of a

right cone that is supported principally either by rafters on girders and
columns or by rafters on trusses with or without columns.
 b) A self-supporting cone roof is a roof formed to approximately the surface
of a right cone that is supported only at its periphery.
 c) A self-supporting dome roof is a roof formed to approximately a
spherical surface that is supported only at its periphery.
 Roof Plate Thickness: Roof plates shall have a nominal thickness of not less
than 5 mm increased thickness may be required for supported cone roofs
(see 5.10.4.4).
 Any required corrosion allowance for the plates of self-supporting roofs shall
be added to the calculated thickness unless otherwise specified by the
Purchaser.
 Any corrosion allowance for the plates of supported roofs shall be added to
the greater of the calculated thickness or the minimum thickness or [5 mm
(3/16 in.)
 5.10.5 Self-Supporting Cone Roofs
 the nominal thickness of the roof plates shall not be less than 4.8 mm (3/16
in.).
 5.10.5.1 Self-supporting cone roofs shall conform to the following
requirements:
 θ ≤ 37 degrees (slope = 9:12)
 θ ≥ 9.5 degrees (slope = 2:12)
where
D is the nominal diameter of the tank, in meters;
T is the greater of load combinations 5.2.2 (e)(1) and (e)(2) with
balanced snow load Sb, in kPa;
U is the greater of load combinations 5.2.2 (e)(1) and (e)(2) with
unbalanced snow load Su, in kPa;
θ is the angle of cone elements to the horizontal, in degrees;
CA is the corrosion allowance.
 5.10.5.2 The participating area at the roof-to-shell joint shall be determined
using Figure F.2 and the nominal material thickness less any corrosion
allowance shall equal or exceed the following:

where
p is the greater of load combinations 5.2.2 (e)(1) and (e)(2);
D is the nominal diameter of the tank shell;
θ is the angle of cone elements to the horizontal;
Fa equals (0.6 Fy), the least allowable tensile stress for the materials
in the roof-to-shell joint;
Fy is the Least Yield Strength of roof-to-shell joint material at
maximum design temperature
Wind Load on Tanks (Overturning Stability)
 Unanchored Tanks
 Unanchored tanks shall meet the requirements of 5.11.2.1 or 5.11.2.2. See
Figure 5.27.
 5.11.2.1 Unanchored tanks shall satisfy all of the following uplift criteria:
 1) 0.6Mw + MPi < MDL /1.5 + MDLR
 2) Mw + Fp(MPi) < (MDL + MF)/2 + MDLR
 3) Mws + Fp (MPi) < MDL /1.5 + MDLR

 where
 FP is the pressure combination factor, see 5.2.2;
 MPi is the moment about the shell-to-bottom joint from design internal pressure;
 Mw is the overturning moment about the shell-to-bottom joint from horizontal
plus vertical wind pressure;
 MDL is the moment about the shell-to-bottom joint from the nominal weight of
the shell;
 MF is the moment about the shell-to-bottom joint from liquid weight;
 MDLR is the moment about the shell-to-bottom joint from the nominal weight of
the roof plate plus any attached structural;
 MWS is the overturning moment about the shell-to-bottom joint from horizontal
wind pressure.
 5.11.2.2 Unanchored tanks with supported cone roofs meeting the
requirements of 5.10.4 shall satisfy the following criteria:
 Mws + Fp (MPi ) < MDL /1.5 + MDLR
 5.11.2.3 The liquid weight (wL) is the weight of a band of liquid at the shell
using a specific gravity of 0.7 and a height of one-half the design liquid
height H. wL shall be the lesser of 140.8 HD for SI Units (0.90 HD for USC units)
or the following“:
 where
 Fby is the minimum specified yield stress of the bottom plate under the shell, in
MPa (lbf/in.2);
 H is the design liquid height, in meters (ft);
 D is the tank diameter, in meters (ft);
 tb is the required corroded thickness of the bottom plate under the shell, in mm
(inches), that is used to resist
 wind overturning. The bottom plate shall have the following restrictions:
 1) The corroded thickness, tb, used to calculate wL shall not exceed the first
shell course corroded thickness less
 any shell corrosion allowance.
 5.11.4 Sliding Friction
 Unless otherwise required, tanks that may be subject to sliding due to wind
shall use a maximum allowable sliding friction of 0.40 multiplied by the force
against the tank bottom.
Shell opening :
Shell Manhole
Roof Manholes:
Venting requirement :

 CAUSES OF OVERPRESSURE OR VACUUM:

 General
 "When the possible causes of overpressure or vacuum in a tank are being
determined, the following circumstances must be considered:
 a. Liquid movement into or out of the tank.
 b. Tank breathing due to weather changes (e.g., pressure and temperature
changes).
 c. Fire exposure.
 d. Other circumstances resulting from equipment failures and operating
error
FOUNDATION (Annex B ):
Typical Foundation Types:

 Earth Foundations Without a Ring wall:

 When an engineering evaluation of subsurface conditions that is based on
experience and/or exploratory work has shown that the subgrade has
adequate bearing capacity and that settlements will be acceptable,
satisfactory foundations may be constructed from earth materials.
 The performance requirements for earth foundations are identical to those
for more extensive foundations. Specifically, an earth foundation should
accomplish the following:
 a) provide a stable plane for the support of the tank;
 b) limit overall settlement of the tank grade to values compatible with the
allowances used in the design of the connecting piping;
 c) provide adequate drainage;
 d) not settle excessively at the perimeter due to the weight of the shell wall.
 Earth Foundations With a Concrete Ring wall:
 tanks and tanks with heavy or tall shells and/or self-supported roofs impose
a substantial load on the foundation under the shell. This is particularly
important with regard to shell distortion in floating-roof tanks.
 When there is some doubt whether a foundation will be able to carry the
shell load directly, a concrete ring wall foundation should be used.
A foundation with a concrete ring wall has the following advantages:
 a) It provides better distribution of the concentrated load of the shell to
produce a more nearly uniform soil loading under the tank.
 b) It provides a level, solid starting plane for construction of the shell.
 It provides a better means of leveling the tank grade, and it is capable of
preserving its contour during construction.
 d) It retains the fill under the tank bottom and prevents loss of material as a
result of erosion.
 e) It minimizes moisture under the tank.
 A disadvantage of concrete ring walls is that they may not smoothly conform to
differential settlements. This disadvantage may lead to high bending stresses in
the bottom plates adjacent to the ring wall.
 When a concrete ring wall is designed, it shall be proportioned so that the
allowable soil bearing is not exceeded. The ring wall shall not be less than
300 mm (12 in.) thick. The centerline diameter of the ring wall should equal
the nominal diameter of the tank; however, the ring wall centerline may
vary if required to facilitate the placement of anchor bolts or to satisfy soil
bearing limits for seismic loads or excessive uplift forces.
 The depth of the bottom of the ring wall, as a minimum if founded on soil,
shall be located 0.6 m (2 ft) below the lowest adjacent finish grade.
 A ring wall should be reinforced against temperature changes and
shrinkage and reinforced to resist the lateral pressure of the confined fill with
its surcharge from product loads. ACI 318 is recommended for design stress
values, material specifications, and rebar development and cover. The
following items concerning a ring wall shall be considered.
 a) The ring wall shall be reinforced to resist the direct hoop tension resulting
from the lateral earth pressure on the ring wall's inside face. Unless
substantiated by proper geotechnical analysis, the lateral earth pressure
shall be assumed to be at least 50 % of the vertical pressure due to fluid
and soil weight.
 The total hoop steel area required to resist the loads noted above shall not
be less than the area required for temperature changes and shrinkage.
 The hoop steel area required for temperature changes and shrinkage is
0.0025 times the vertical cross-sectional area of the ring wall

 For ring walls, the vertical steel area required for temperature changes and
shrinkage is 0.0015 times the horizontal cross-sectional area of the ring wall
or the minimum reinforcement for walls called for in ACI 318, Chapter 14.
Additional vertical steel may be required for uplift or torsional resistance. If
the ring foundation is wider than its depth, the design shall consider its
behavior as an annular slab with flexure in the radial direction. Temperature
and shrinkage reinforcement shall meet the ACI 318 provisions for slabs.
(See ACI 318, Chapter 7.)
 When the ring wall width exceeds 460 mm (18 in.), using a footing beneath
the wall should be considered. Footings may also be useful for resistance to
uplift forces.
 Structural backfill within and adjacent to concrete ring walls and around
requires close field control to maintain settlement tolerances. Backfill should
be granular material compacted to the density and compacting as
specified in the foundation construction specifications. For other backfill
materials, sufficient tests shall be conducted to verify that the material has
adequate strength and will undergo minimal settlement.
 Slab Foundations
When the soil bearing loads must be distributed over an area larger than the
tank area or when it is specified by the owner, a reinforced concrete slab shall
be used. Piles beneath the slab may be required for proper tank support.
 The structural design of the slab, whether on grade or on piles, shall properly
account for all loads imposed upon the slab by the tank.
 The reinforcement requirements and the design details of construction shall
be in accordance with ACI 318