-
Lecture 15C.1: Design of Tanks for the
Storage of Oil and Water OBJECTIVE/SCOPE:
The lecture describes the basic principles used in the design of
tanks for the storage of oil or water. It covers the design of
vertical cylindrical tanks, and reference is made to the British
Standard BS 2654 [1] and to the American Petroleum Industry
Standard API650 [2].
PREREQUISITES
None.
RELATED LECTURES
Lecture 8.6: Introduction to Shell Structures
Lecture 8.8: Design of Unstiffened Cylinders
SUMMARY
Welded cylindrical tanks are commonly used to store oil products
or water.
The principal structural element of these tanks is a vertical
steel cylinder, or shell, which is made by welding together a
series of rectangular plates and which restrains the hydrostatic
pressures by hoop tension forces. The tank is normally provided
with a flat steel plated bottom which sits on a prepared
foundation, and with a fixed roof attached to the top of the shell
wall.
This lecture explains the design basis for the structural
elements of cylindrical tanks and illustrates the arrangements and
the key details involved.
1. DESIGN OF WELDED CYLINDRICAL TANKS
1.1 General
Oil and oil products are most commonly stored in cylindrical
steel tanks at atmospheric pressure or at low pressure. The tanks
are flat bottomed and are provided with a roof which is of conical
or domed shape.
Water is also sometimes stored in cylindrical steel tanks. When
used to store potable water they are of a size suitable to act as a
service reservoir for a local community; they
-
have a roof to prevent contamination of the water. Cylindrical
tanks are also used in sewage treatment works for settlement and
holding tanks; they are usually without a roof.
The sizes of cylindrical tanks range from a modest 3m diameter
up to about 100m diameter, and up to 25m in height. They consist of
three principal structural elements - bottom, shell and roof.
For petroleum storage, the bottom is formed of steel sheets,
laid on a prepared base. Some tanks for water storage use a
reinforced concrete slab as the base of the tank, instead of steel
sheets.
The shell, or cylindrical wall, is made up of steel sheets and
is largely unstiffened.
The roof of the tank is usually fixed to the top of the shell,
though floating roofs are provided in some circumstances. A fixed
roof may be self supporting or partially supported through membrane
action, though generally the roof plate is supported on radial
beams or trusses.
1.2 Design Standards
Clearly, common standards are generally applicable whether a
tank holds oil or water, though it is the petroleum industry which
has been responsible for the development of many of the design
procedures and standards.
The two standards applied most widely are British Standard BS
2654 [1] and the American Petroleum Institute Standard API 650 [2].
These two Standards have much in common, although there are some
significant differences (see Appendix A). Other standards, American
and European, are not applied much outside their respective
countries.
This lecture will generally follow the requirements of BS 2654
[1]. This standard is both a design code and a construction
specification. The design code is based on allowable stress
principles, not on a limit state basis.
1.3 Design Pressure and Temperature
Tanks designed for storage at nominally atmospheric pressure
must be suitable for modest internal vacuum (negative pressure).
Tanks may also be designed to work at relatively small positive
internal pressures (up to 56 mbar (5,6 kN/m2), according to
BS2654.
Non-refrigerated tanks are designed for a minimum metal
temperature which is based on the lowest ambient air temperature
(typically, ambient plus 10oC) or the lowest temperature of the
contents, whichever is the lower. No maximum service temperature is
normally specified.
-
1.4 Material
Tanks are usually manufactured from plain carbon steel plate
(traditionally referred to as mild steel) of grades S235 or S275
(to EN 10 025 [3]), or equivalent. Such material is readily
weldable. The use of higher strength grades of low alloy steel
(e.g. Grade S355) is less common, though its use is developing.
Notch ductility at the lowest service temperature is obtained
for thicker materials (> 13 mm) by specifying minimum
requirements for impact tests. This is normally achieved by
specifying an appropriate sub-grade to EN 10 025 [3].
Internally, oil tanks are normally unpainted. Water tanks may be
given a coating (provided it is suitably inert, where the water is
potable), or may be given cathodic protection. Externally, tanks
are normally protected. Where any steel is used uncoated, an
allowance must be made in the design for loss of thickness due to
corrosion.
2. DESIGN LOADING A tank is designed for the most severe
combination of the various possible loadings.
2.1 Dead Load
The dead load is that due to the weight of all the parts of the
tank.
2.2 Superimposed Load
A minimum superimposed load of 1,2 kN/m2 (over the horizontal
projected area) is applied to the roof of the tank. This load is
commonly known as the 'snow load', but in fact represents, as well
as a nominal snow load, any other imposed loads, such as
maintenance equipment, which might be applied to the roof, and it
includes the internal vacuum load. It is therefore applicable even
in locations where snow is not experienced.
Non-pressure tanks are often fitted with valves which do not
open until the vacuum reaches a value of 2,5 mbar, to contain
vapour losses. By the time a valve is fully open, a vacuum of 5
mbar (0,5 kN/m2) may have developed. Even without valves a tank
should be designed for a vacuum of 5 mbar, to cater for
differential pressure under wind loads. In pressure tanks the
valves may be set to 6 mbar vacuum, in which case a pressure
difference of 8,5 mbar (0,85 kN/m2) may develop.
Actual predicted snow load or other superimposed load, plus
appropriate vacuum pressure, should be used when it is greater than
the specified minimum.
-
2.3 Contents
The weight and hydrostatic pressure of the contents, up to the
full capacity of the tank, should be applied. Full capacity is
usually determined by an overflow near the top of the tank; for a
tank without any overflow, the contents should be taken to fill the
tank to the top of the shell.
For oil and oil products, the relative density of the contents
is less than 1.0, but tanks for such liquids are normally tested by
filling with water. A density of 1000 kg/m3 should therefore be
taken as a minimum.
2.4 Wind Loads
Wind loads are determined on the basis of a design wind speed.
Maximum wind speed depends on the area in which the tank is to be
built; typically a value of 45 m/s is taken as the design wind
speed, representing the maximum 3-second gust speed which is
exceeded, on average, only once every 50 years.
2.5 Seismic Loads
In some areas, a tank must be designed to withstand seismic
loads. Whilst some guidance is given in BS 2654 [1] and API650 [2]
on the design of the tank, specialised knowledge should be applied
in determining seismic loads.
3. BOTTOM DESIGN For petroleum storage tanks, steel bottom
plates are specified, laid and fully supported on a prepared
foundation.
The steel plates are directly supported on a bitumen-sand layer
on top of a foundation, usually of compacted fill or, if the
subsoil is weak, possibly a reinforced concrete raft. A typical
foundation pad is shown in Figure 1 and a detailed description of
the formation of this example is given in Appendix A of BS 2654
[1].
-
The bottom is made up of a number of rectangular plates,
surrounded by a set of shaped plates, called sketch plates, to give
a circular shape, as shown in Figure 2. The plates slightly overlap
each other and are pressed locally at the corners where three
plates meet (see Figure 3). Lapped and fillet welded joints are
preferred to butt welded joints (which must be welded onto a
backing strip below the joint) because they are easier and cheaper
to make.
-
For larger tanks (over 12,5 m diameter, according to BS 2654) a
ring of annular plates is provided around the group of rectangular
plates. The radial joints between the annular plates are butt
welded, rather than lapped, because of the ring stiffening which
the plates provide to the bottom of the shell. A typical
arrangement is shown in Figure 4.
-
The shell sits on the sketch or annular plates, just inside the
perimeter and is fillet welded to them (see Figure 5).
-
The bottom plates act principally as a seal to the tank. The
only load they carry, apart from local stiffening to the bottom of
the shell, is the pressure from the contents, which is then
transmitted directly to the base. Stress calculations are not
normally required for the base, though BS 2654 sets out minimum
thicknesses of plate depending on the size of the tank.
Water tanks may also have a steel bottom. In some circumstances
a reinforced concrete slab is specified instead. There are no
standard details for the connection between a shell and a concrete
slab, though a simple arrangement of an angle welded to the bottom
edge of the shell and bolted to the slab will usually suffice.
-
4. SHELL DESIGN
4.1 Circumferential Stresses
Vertical cylinder tanks carry the hydrostatic pressures by
simple hoop tension. No circumferential stiffening is needed for
this action. The circumferential tension in the shell will vary
directly, in a vertical direction, according to the head of fluid
at any given level. For a uniform shell thickness, the calculation
of stresses is therefore straightforward. At a water depth H, the
stress is given by:
where D is the diameter of the tank
t is the thickness of the plate
is the density of the fluid g is the gravity constant
For practical reasons, it is necessary to build up the shell
from a number of fairly small rectangular pieces of plate, butt
welded together. Each piece will be cylindrically curved and it is
convenient to build up the shell in a number of rings, or courses,
one on top of the other. This technique provides, at least for
deeper tanks, a convenient opportunity to use thicker plates in the
lower rings and thinner plates in the upper rings.
The lowest course of plates is fully welded to the bottom plate
of the tank providing radial restraint to the bottom edge of the
plate. Similarly, the bottom edge of any course which sits on top
of a thicker course is somewhat restrained because the thicker
plate is stiffer. The effect of this on the hoop stresses is
illustrated in Figure 6.
-
Consequently, because of these restraints, an empirical
adjustment is introduced into the design rules which effectively
requires that any course is simply designed for the pressure 300mm
above the bottom edge of the course, rather than the greater
pressure at the bottom edge. (This is known as the 'one foot rule'
in API 650 [2].)
When the load due to internal pressure is taken into account and
an allowance for corrosion loss is introduced, the resulting design
equation is of the form in BS 2654:
where t is the calculated minimum thickness (mm)
w is the maximum density of the fluid (kg/l)
H is the height of fluid above the bottom of the course being
designed (m)
S is the allowable design stress (N/mm2)
p is the design pressure (pressure tanks only) (mbar)
c is the corrosion allowance (mm)
The allowable design stress in tension in the shell is generally
taken to be a suitable fraction of the material yield stress. BS
2654 defines it as two-thirds of the yield stress, thus giving an
overall factor of 1,5 on the plastic strength of the plate. API650
also uses
-
two-thirds of the yield stress, but additionally limits the
design stress to a smaller fraction of the ultimate strength; for
higher strength steels, this is slightly more restrictive. Further,
API650 allows a slightly higher stress during the hydrostatic test
than the allowable design stress for service conditions when the
relative density is less than 1,0.
Each course is made of a number of plates, butt welded along the
vertical join between the plates. Each course is butt welded to the
course below along a circumferential line. Good weld procedures can
minimise the distortions or deviations from the ideal flat or
curved line of the surface across the weld, but some imperfection
is inevitable, especially with thin material. Consequently the
rules call for the vertical seams to be staggered from one course
to the next - at least one third of the length of the individual
plates, if possible.
Holes in the shell for inlet/outlet nozzles or access manholes
cause a local increase in circumferential stresses. This increase
is catered for by requiring the provision of reinforcing plates.
These plates may take the form of a circular doubling plate welded
around the hole or of an inset piece of thicker plate. The nozzle
provides some stiffening to the edge of the hole; it may also be
made of sufficient size that shell reinforcement can be
omitted.
4.2 Axial Stresses in the Shell
The cylindrical shell has to carry its weight, and the weight of
the roof which it supports, as an axial stress. In addition, wind
loading on the tank contributes tensile axial stress on one side of
the tank and compressive stress on the other.
A thin-walled cylinder under a sufficient axial load will of
course buckle locally, or wrinkle. The critical value of this
stress, for a perfect cylinder, can be obtained from classical
theory and, for steel, has the value:
In practice, imperfect shells buckle at a much lower stress; an
allowable stress level of as little as a tenth of the above might
be more appropriate. However, in normal service the axial stresses
in shells suitable to carry the circumferential loads for the size
of tank used for oil and water storage are much smaller than even
this level of stress. The calculation of axial stress is therefore
not even called for in codes, such as BS 2654 and API650, for the
service conditions.
But under seismic conditions, larger stresses result because of
the large overturning moment when the tank is full. In that case
the axial stresses must be calculated. Axial stress due to
overturning moment, M, is given simply by the expression:
a = 4M/tD2
-
In BS 2654 the axial stress under seismic conditions is limited
to 0.20Et/R, which is considered a reasonable value when the
cylinder is also under internal hydrostatic pressure. API650 uses a
similar value, provided that the internal pressure exceeds a value
which depends on the tank size.
Although axial stresses do not need to be calculated for service
conditions, the tank does have to be checked for uplift when it is
empty and subject to wind loading. If necessary, anchorages must be
provided; a typical example is shown in Figure 7.
4.3 Primary Wind Girders
A tank with a fixed roof is considered to be adequately
restrained in its cylindrical shape by the roof; no additional
stiffening is needed at the top of the shell, except possibly as
part of an effective compression ring (see Section 5.2).
At the top of an open tank (or one with a floating roof),
circumferential stiffening is needed to maintain the roundness of
the tank when it is subject to wind load. This stiffening is
particularly necessary when the tank is empty.
-
The calculation of the stability of stiffened tanks is a complex
matter. Fortunately, investigations into the subject have led to an
empirical formula, based on work by De Wit, which is easily applied
in design. In BS 2654 this formula is expressed as a required
minimum section modulus given by:
Z = 0,058 D2 H
where Z is the (elastic) section modulus (cm3) of the effective
section of the ring girder, including a width of shell plate acting
with the added stiffener
D is the tank diameter (m)
H is the height of the tank (m)
The formula presumes a design wind speed of 45 m/s. For other
wind speeds it may be modified by multiplying by the ratio of the
basic wind pressure at the design speed to that at 45 m/s, i.e. by
(V/45)2.
Wind girders are usually formed by welding an angle or a channel
around the top edge of the shell. Examples are shown in Figure 8.
Note that continuous fillet welds should always be used on the
upper edge of the connection, to avoid a corrosion trap.
-
It is recognised that application of the above formula to tanks
over 60 m diameter leads to unnecessarily large wind girders; the
code allows the size to be limited to that needed for a 60 m
tank.
Primary wind girders are normally external to the tank.
Settlement tanks usually require a gutter around the inside edge of
the tank, into which the water spills and passes to the outlet.
Although this detail is not covered in the code, a suitable gutter
detail can participate as a primary wind girder, provided it is
relatively close to the top of the tank. In that event a kerb angle
is also required at the free edge; the arrangement of a low ring
girder and a kerb angle is covered by the design rules.
4.4 Secondary Wind Girders
Although the primary wind girder or the roof will stabilise the
tank over its full height, local buckling can occur in empty tall
tanks between the top of the tank and its base. To prevent this
local buckling, secondary wind girders are introduced at intervals
in the height of the tank. The determination of the number and
position of these secondary wind girders is dealt with in BS 2654
(but not in API 650).
The procedure is based on determining the length of tube for
which, with the ends held circular, the elastic critical buckling
will occur at a given uniform external pressure. Such buckling
would also occur in a longer tube which is restrained at intervals
equal to that length.
The critical stress for a length of tube, l, of radius R and
thickness t, is given in Roark [4] by the formula:
Using values of E and for steel, rearranging and simplifying,
this reduces approximately to the expression in the code:
where D is the diameter of the shell (m)
Hp is the maximum permitted spacing of rings (m)
(equivalent to critical length, l)
tmin is the thickness of the shell plate (mm)
-
Vw is the design wind speed (m/s)
va is the vacuum (mbar)
However, tank shells in practice are made up of courses, and the
thickness of the plating increases from the top to the bottom.
Fortunately, this non-uniform situation can be converted into an
equivalent uniform situation by noting that the critical length l
(or maximum spacing Hp) is proportional to t5/2. Taking the
thinnest plate (the top course) as reference (tmin), courses of
height h and thickness t can be converted to an equivalent height
of a tube of the thin plate which has the same effective
slenderness by applying the correction:
where t is the thickness of each course in turn
He is the equivalent height of each course at a thickness of
tmin
The equivalent heights of all the courses are added to give the
total equivalent height (length of tube) and divided by the
critical length Hp to determine the minimum number of intervals and
thus the number of intermediate rings. The positions of the
intermediate rings, which are equally spaced on the equivalent
tube, must be established by converting positions on the tube back
to positions on the tank, by the reverse of the above
procedure.
The whole process is illustrated by an example in BS 2654.
The stiffening is achieved by welding an angle to the surface of
the shell plate in the same manner as for the primary wind girder.
Minimum sizes for this angle are given in the code [1].
5. FIXED ROOF DESIGN
5.1 General
Fixed roofs of cylindrical tanks are formed of steel plate and
are of either conical or domed (spherically curved) configuration.
The steel plates can be entirely self supporting (by 'membrane'
action), or they may rest on top of some form of support
structure.
Membrane roofs are more difficult to erect - they require some
temporary support during placing and welding - and are usually
found only on smaller tanks.
Permanent support steelwork for the roof plate may either span
the complete diameter of the tank or may in turn be supported on
columns inside the tank. The use of a single
-
central column is particularly effective in relatively small
tanks (15-20 m diameter), for example.
The main members of the support steelwork are, naturally, radial
to the tank. They can be simple rolled beam sections or, for larger
tanks, they can be fabricated trusses.
Roof plates are usually lapped and fillet welded to one another.
For low pressure tanks, they do not need to be welded to any
structure which supports them, but they must normally be welded to
the top of the shell.
5.2 Membrane Roofs
In a membrane roof, the forces from dead and imposed loads are
resisted by compressive radial stresses. The net upward forces from
internal pressure minus dead load are resisted by tensile radial
stresses.
Conical roofs usually have a slope of 1:5. Spherical roofs
usually have a radius of curvature between 0,8 and 1,5 times the
diameter of the tank.
Limitations on buckling under radial compression are expressed
in BS2654 as:
where R1 is the radius of curvature of the roof (m)
Pe is the external loading plus self weight (kN/m2)
E is Young's modulus (N/mm2)
tr is the roof plate thickness (mm)
For conical roofs, R1 is taken as the radius of the shell
divided by the sine of the angle between the roof and the
horizontal, i.e. R1 = R/sin . Using a value of Pe = 1,7 kN/m2, i.e.
1,2 kN/m2, superimposed load plus 0,5kN/m2 for dead load,
(equivalent to about 6 mm plate thickness) and the E value for
steel, gives:
tr = 0,36 R1
A similar expression is given in API650, expressed in imperial
units and for a loading of 45lb/ft2 (= 2,2 kN/m2).
For tensile forces, stresses are limited to:
-
(for spherical roofs)
(for conical roofs)
where is the joint efficiency factor S is the allowable design
stress (in N/mm2)
p is the internal pressure (in mbar)
Although lapped and double fillet welded joints are acceptable,
they have a joint efficiency factor of only 0,5; butt welded joints
have a factor of 1,0.
For downward loads, the radial compression is complemented by
ring tension.
For upward loads, i.e. under internal pressure, the radial
tension has to be complemented by a circumferential compression.
This compression can only be provided by the junction section
between roof and shell. This is expressed as a requirement for a
minimum area of the effective section, as shown in Figure 9:
-
where Sc is the allowable compressive stress (in N/mm2)
R is the radius of the tank (in m)
is the slope of the roof at roof-shell connection The allowable
compressive stress for this region is taken to be 120 N/mm2 in
BS2654 [1].
5.3 Supported Roofs
Radial members supporting the roof plate permit the plate
thickness to be kept to a minimum. They greatly facilitate the
construction of the roof.
Radial beams are arranged such that the span of the plate
between them is kept down to a minimum of about 2 m. This limit
allows the use of 5 mm plate for the roof. The plate simply lies on
the beams and is not connected to them.
-
Supported roofs are most commonly of conical shape, although
spherical roofs can be used if the radial beams are curved.
The roof support structure can either be self supporting or be
supported on internal columns. Typical arrangements are shown in
section in Figures 10 and 11. Self supporting roofs are essential
when there is an internal floating cover.
-
When columns are used to support the roof, the slope may be as
low as 1:16. When the roof is self supporting it may be more
economic to use a steeper roof.
-
Not all radial members continue to the centre of the tank. Those
that do may be considered as main support beams; the secondary
radial members may be considered as rafters - they are supported at
their inner ends on ring beams between the main support members.
Where internal columns are used they will be beneath the main
support members. Typical plan arrangements are shown in Figure
11.
The main support members need to be restrained at intervals to
stabilise them against lateral-torsional buckling. Cross bracing is
provided in selected bays.
In API650 it is permitted to assume that friction between the
roof plate and the beam is adequate to restrain the compression
flange of the secondary rafter beams, provided that they are not
too deep; such restraint cannot be assumed for the main beams,
however.
The main support members may be subject to bending and axial
load. Where they are designed for axial thrust, the central ring
must be designed as a compression ring; the top of the shell must
be designed for the hoop forces associated with the axial forces in
the support members.
Design of beams and support columns may generally follow
conventional building code rules, though it must be noted that both
BS 2654 and API650 are allowable stress codes. In the British code
reference is therefore made to BS449 [5], rather than to a limit
state code.
The shell/roof junction zone must be designed for compression,
in the same way as described above for membrane roofs.
5.4 Venting
Venting has to be provided to cater for movement of the contents
into and out of the tank and for temperature change of the air in
the tank. Venting can be provided by pressure relief valves or by
open vents.
For storage of petroleum products, emergency pressure relief has
to be provided to cater for heating due to an external fire.
Pressure relief can be achieved either by additional emergency
venting or by designing the roof to shell joint as frangible (this
means, principally, that the size of the fillet weld between the
roof and the shell is limited in size - a limit of 5 mm is
typical).
6. DESIGN OF FLOATING ROOFS AND COVERS
6.1 Use of Floating Roofs and Covers
As mentioned in Section 5.4, tanks need to be vented to cater
for the expansion and contraction of the air. In petroleum tanks,
the free space above the contents contains an air/vapour mixture.
When the mixture expands in the heat of the day, venting expels
some of this vapour. At night, when the temperature drops, fresh
air is drawn in and more
-
of the contents evaporates to saturate the air. The continued
breathing can result in substantial evaporation losses. Measures
are needed to minimise these losses; floating roofs and covers are
commonly used for this purpose.
6.2 Floating Roofs
A floating roof is sometimes provided instead of a fixed roof.
The shell is then effectively open at the top and is designed
accordingly.
During service, a floating roof is completely supported on the
liquid and must therefore be sufficiently buoyant; buoyancy is
achieved by providing liquid-tight compartments in one of two forms
of roof - pontoon type and double deck type.
A pontoon roof has an annular compartment, divided by bulkheads,
and a central single skin diaphragm. The central diaphragm may need
to be stiffened by radial beams.
A double deck roof is effectively a complete set of compartments
over the whole diameter of the tank; two circular skins are joined
to circumferential plates and bulkheads to form a disk or
piston.
Both types of roof must remain buoyant even if some compartments
are punctured (typically two compartments). The central deck of a
pontoon roof should also be presumed to be punctured for this
design condition.
Because the roof is open to the environment, it catches rain,
which must be drained off. Drainage is achieved by a system on the
roof which connects to flexible pipework inside the tank and thence
through the shell or bottom plates to a discharge. The design is
required to ensure that the roof continues to float in the event of
a block in the drainage system which results in a surcharge of
water on the roof (usually 250 mm of water).
When the tank is emptied, the roof cannot normally be allowed to
fall to the bottom of the tank, because there is internal pipework;
the roof is therefore fitted with legs which keep it clear of the
bottom. At this stage the roof must be able to carry a superimposed
load (1,2 kN/m2) plus any accumulated rainwater.
For maintenance of the drainage system and for access to nozzles
through the roof for various purposes, maintenance personnel need
access from the top of the shell to the roof whatever the level of
contents in the tank. Access is usually achieved by a movable
ladder or stairway, pinned to the shell and resting on the roof.
For maintenance of the tank when it is empty, an access manhole
must be provided through the roof.
A typical arrangement of a pontoon type roof is shown in Figure
12.
-
6.3 Floating Covers
Where a cover to the contents is provided inside a fixed roof
tank, to reduce evaporation or ingress of contaminants (e.g. water
or sand), a much lighter cover or screen can be provided.
Such a cover is likely to be manufactured from lighter materials
than steel, though a shallow steel pan can sometimes be provided.
The cover does not need to be provided with access ladders, nor to
be designed for surcharge. It does have to be designed to be
supported at low level when the tank is empty and to carry a small
live load in that condition.
Detailed recommendations for the design of internal floating
covers are given in Appendix E of BS 2654 [1].
-
7. MANHOLES, NOZZLES AND OPENINGS
7.1 Manholes
Access is required inside fixed roof tanks for maintenance and
inspection purposes. Such access can be provided through the roof
or through the shell wall. Manholes through the roof have the
advantage that they are always accessible, even when the tank is
full. Access through the shell wall is more convenient for cleaning
out (some access holes are D-shaped and flush with the bottom for
cleaning out purposes).
A manhole through a roof should be at least 500 mm diameter.
Stiffening arrangements
around the hole in the roof plate, and the type of cover, depend
on the design of the roof. Access to the roof manhole must be
provided by ladders, with suitable handrails and walkways on the
roof.
A manhole through the shell wall should be at least 600 mm
diameter and is normally positioned just above the bottom of the
tank. A typical detail is shown in section in Figure 13. Further
details of this example, and details of clean-out openings, are
given in BS2654 [1].
-
Clearly, the cutting of an opening in the shell interferes with
the structural action of the shell. The loss of section of shell
plate is compensated by providing additional cross-section area
equal to 75% of that lost. The area must be provided within a
circular region around the hole, though the actual reinforcement
should extend beyond that region. Reinforcement can be provided in
one of three ways:
(i) a reinforcing plate welded onto the shell plate (similar to
the section in Figure 13)
(ii) an insert of thicker plate locally (in which the manhole is
cut)
(iii) a thicker shell plate than that required for that course
of the shell
7.2 Nozzles
As well as manholes for access and cleaning out, nozzles are
required through the shell roof and bottom for inlet, outlet, and
drainage pipes, and for vents in the roof. They are normally made
by welding a cylindrical section of plate into a circular hole in
the structural plate. For small nozzles, no reinforcement is
necessary, the extra material is considered sufficient. Larger
holes must be reinforced in the same way as manholes. An example of
a roof nozzle detail is shown in Figure 14.
8. CONCLUDING SUMMARY
-
Oil and oil products are most commonly stored in cylindrical
steel tanks at atmospheric pressure or at low pressure. Water is
also sometimes stored in cylindrical steel tanks.
The two design standards applied most widely to the design of
welded cylindrical tanks are BS2654 and API 650.
Tanks are usually manufactured from plain carbon steel plate. It
is readily weldable.
A tank is designed for the most severe combination of the
various possible loadings.
For petroleum storage tanks, steel bottom plates are specified,
laid and fully supported on a prepared foundation. Water tanks may
also have a steel bottom or a reinforced concrete slab may be
specified.
Vertical cylindrical tanks carry the hydrostatic pressure by
simple hoop tension. The cylindrical shell has to carry both its
own weight and the weight of the supported roof by axial stresses.
Wind loading on the tank influences the axial stress.
For open tanks, primary wind girders are required to maintain
the roundness of the tank when it is subject to wind load.
Secondary wind girders are needed in tall tanks.
Roofs may be fixed or floating. A cover to the contents of a
fixed roof tank may be provided to reduce evaporation or ingress of
contaminants.
Manholes are provided for access and nozzles allow inlet, outlet
and drainage, and venting of the space under the roof.
9. REFERENCES [1] BS 2654: 1984, Specification for manufacture
of vertical steel welded storage tanks with butt-welded shells for
the petroleum industry, British Standards Institution, London.
[2] API 650, Welded Steel Tanks for Oil Storage, 8th Edition,
November 1988, API.
[3] BS EN 10025, 1990, Hot Rolled Products of Non-alloy
Structural Steels and their Technical Delivery Conditions, British
Standards Institution, London.
[4] Young, W. C., Roark's Formulas for Stress and Strain, McGraw
Hill, 1989.
[5] BS 449: Part 2: 1969, Specification for the Use of
Structural Steel in Building, British Standards Institution,
London.
Appendix A Differences between BS 2654 and API 650
The following are the principal differences between the British
Standard, BS 2654 [1] and the American Petroleum Institute
Standard, API650 [2]:
-
(a) API 650 specifies different allowable stresses for service
and water testing. BS 2654 specifies an allowable stress for water
testing only, which will allow oils with any specific gravity up to
1 to be stored in the tank.
(b) The allowable design stresses of BS 2654 are based on
guaranteed minimum yield strength whereas the design stresses of
API 650 are based on the guaranteed minimum ultimate tensile
strength.
(c) BS 2654 specifies more stringent requirements for the
weldability of the shell plates.
(d) The notch ductility requirements of BS 2654 are based on the
results of a great number of wide plate tests. This system
considers a steel acceptable if, for the required thickness, the
test plate does not fail at test temperature before it has yielded
at least 0,5%. This system gives the same safety factor for all
thicknesses.
In API 650 a fixed value and test temperature is given for the
impact tests for all thicknesses. As the tendency to brittle
fracture increases with increasing plate thickness it means that
API 650 in fact allows a lower safety factor for large tanks than
for smaller ones.
(e) The steels specified by API 650 guarantee their notch
ductility by chemical analysis but without guaranteed impact
values. BS 2654 requires guaranteed impact values where
necessary.
(f) BS 2654 gives a clearer picture of how to determine the size
and location of secondary wind girders.
Previous | Next | Contents
Plant Layout - Storage Tanks
Table of Contents
1. Tankage Grouping 2. Classification of Crude Oil and Its
Derivatives 3. Tankage Layout 4. Pump Areas 5. Fire Protection
-
6. Road and Rail Loading Facilities
1. Tankage Grouping Tankage area will be subdivided into various
groups determined by the contents of the tanks and the relative
shape and area of the plot available, access and fire fighting must
also be considered. See below table API tank size for layout
purposes.
2. Classification of Crude Oil and Its Derivatives Crude oil and
its derivatives are potentially hazardous materials. The degree of
the hazard is determined essentially by volatility and flash
point.
The Institute of Petroleum has specified the following
classes:
Class 0 Liquified petroleum gases (LPG)
Class I Liquids which have flash points below 21 oC
Class II (1) Liquids which have flash points from 21 oC upto and
including 55 oC handled, below flash point
Class II (2) Liquids which have flash points from 21 oC upto and
including 55 oC handled, at or above flash point
Class III (1) Liquids which have flash points above 55 oC upto
and including 100 oC handled, below flash point
Class III (2) Liquids which have flash points above 55 oC upto
and including 100 oC handled, above flash point
Unclassified. Liquids with flash points above 100 oC
-
For further information see IP refinery safety code part 3.
3. Tankage Layout 3.1 General The layout of tanks, as distinct
from their spacing, should always take into consideration the
accessibility needed for fire-fighting and the potential value of a
storage tank farm in providing a buffer area between process plant
and public roads, houses, etc. , for environmental reasons.
The location of tankage relative to process units must be such
as to ensure maximum safety from possible incidents.
Primarily requirements for the layout of refinery tanks farms
are summarised as follows.
1. Inter tank spacings and separation distances between tank and
boundary line and tank and other facilities are of fundamental
importance. (See 3.2) .
2. Suitable roadways should be provided for approach to tank
sites by mobile fire fighting equipment and personnel.
3. The fire water system should be laid out to provide adequate
fire protection to all parts of the storage area and the transfer
facilities.
4. Bunding and draining of the area surrounding the tanks should
be such that a spillage from any tank can be controlled to minimise
subsequent damage to the tank and its contents. They should also
minimise the possibility of other tanks being involved.
5. Tank farms should preferably not be located at higher levels
than process units in the same catchment area.
6. Storage tanks holding flammable liquids should be installed
in such a way that any spill will not flow towards a process area
or any other source of ignition.
3.2. Spacing of Tanks for LPG Stocks of Class 0
Factor Recommendations for LPG
-
1. Between LPG pressure storage tanks
One quarter of the sum of the diameters of the two adjacent
tanks.
2. To Class I, II, or III product tanks.
15 M from the top of the surrounding Class I, II or III product
tanks.
3. To low pressure refrigerated LPG tanks.
One diameter of the largest low pressure refrigerated storage
tanks but not less than 30 M.
4. To building containing flammable material e.g. filling shed,
storage building.
15 M
5. To boundary or any fixed source of ignition.
Related to water capacity of tank as follows :
Capacity Up to 135 cu.M Over 135 to 565 cu.M Over 565 cu.M
Distance 15 M 24 M 30 M
The distance given in the above table are minimum
recommendations for aboveground tanks and refer to the horizontal
distance in plan between the nearest point on the storage tank and
a specified feature, e.g. an adjacent storage tank, building,
boundary. The distances are for both spherical and cylindrical
tanks.
3.3 Bunding and Grouping of LPG Tanks The provision of bunds
around LPG pressure storage tanks is not normally justified.
Separation kerbs, low to avoid gas traps, maximum 600 mm high,
may be located to prevent spillage reaching important areas, e.g.
pump manifold area, pipe track.
Ground under tanks should be graded to levels which ensure that
any spillage has a preferential flow away from the tank.
Pits and depressions, other than those which have been provided
as catchment areas, should be avoided to prevent the forming of gas
pockets.
Pressure storage tanks for LPG should not be located within the
bunded enclosures of Class I, II or III product tankage or of low
pressure refrigerated LPG tankage.
-
The layout and grouping of tanks, as distinct from spacing,
should receive careful consideration with the view of accessibility
for fire fighting and the avoidance of spillage from one tank
flowing towards the other tank or towards a nearby important
area.
3.4 Spacing of Tanks for Low Pressure Refrigerated LPG Storage
Class 0
Factor Recommendations for Low Pressure Refrigerated LPG
Storage
1. Between refrigerated LPG storage tanks
One half of the sum of the diameters of the two adjacent
tanks.
2. To Class I, II, or III product tanks.
One diameter of the largest refrigerated storage tank but not
less than 30 M.
3. To pressure storage tanks. One diameter of the largest
refrigerated storage tank but not less than 30 M.
4. To process units, office building, work-shop, laboratory,
warehouse, boundary, or any fixed source of ignition.
45 M
The distance given in the above table are minimum
recommendations and refer to the horizontal distance in plan
between the nearest point on the storage tank and a specified
feature, e.g. an adjacent storage tank, building, boundary.
3.5. Bund or Impounded Basin for Refrigerated LPG Storage A bund
should be provided around all low pressure tanks containing
refrigerated LPG. The tank should be completely surrounded by the
bund, unless the topography of the area is such, either naturally
or by construction, that spills can be directed quickly and safely,
by gravity drainage and diversion walls if required, to a
depression or impounding basin located within the boundary of the
plant.
Bunds should be designed to be of sufficient strength to
withstand the pressure to
-
which they would be subjected if the volume within the bunded
enclosure were filled with water. The area within the bund,
depression, or impounding basis should be isolated from any outside
drainage system by a valve, normally closed unless the area is
being drained of water under controlled conditions.
Where only one tank is within the bund, the capacity of the
bunded enclosure, including the capacity of any depression or
impounding basis, should be 75 per cent of the tank capacity. Where
more than one tank is within the main enclosure, intermediate bunds
should be provided, so as to give an enclosure around each tank of
50 per cent of the capacity of that tank, and the minimum effective
capacity of the main enclosure, including any depression or
impounding basin, should be 100 per cent of the capacity of the
largest tank, after allowing for the volume of the enclosure
occupied by the remaining tanks. It is desirable for the required
capacity to be provided with bunds not exceeding an average height
of 6 foot as measured from the outside ground level.
The area within the bund should be graded to levels which ensure
that any spillage has a preferential flow away from the tank.
No tankage other than low pressure tankage for refrigerated LPG
should be within the bund. The layout and grouping of tanks, as
distinct from spacing, should receive careful consideration with
the view of accessibility for fire fighting.
3.6 Piping Installation and Flexibility Liquid and vapour
pipelines should have adequate flexibility to accommodate any
settlement of tanks or other equipment, thermal expansion or other
stresses that may occur in the pipe work system.
Precaution must be taken to prevent drain or sample valves
freezing in the open position. The flow diagram will indicate the
type of double valving to be installed, with a minimum distance
between the valves of 1 meter. Do not allow liquid traps in vent
lines.
3.7 Spacing of Tank for Petroleum Stocks of Classes I, II and
III (2) .
Factor Type of Tank Roof
Recommended Minimum Distance
1. Within a group of small tanks
Fixed or Floating
Determined solely by construction / maintenance
-
operational convenience 2. Between a group of small tanks or
other larger tanks.
Fixed or Floating
10 M minimum, otherwise determined by the size of the larger
tanks (see 3 below)
3. Between adjacent individual tanks (other than small
tanks).
(a)Fixed Half the diameter of the larger tank, but not than 10 M
and need not be more than 15 M.
(b)Fixed
0.3 times the diameter of the larger tank, but not less than 10
M and need not be more than 15 M. (In the case of crude oil tankage
this 15 M option does not apply)
4. Between a tank and the top of the inside of the wall of its
compound
Fixed or Floating
Distance equal to not less than half the height of the tank.
(Access around the tank at compound grade level must be
maintained)
5. Between any tank in a group of tanks and the inside top of
the adjacent compound wall.
Fixed or Floating
6. Between a tank and a public boundary fence.
Fixed or Floating Not less than 30 M
7. Between the top of the inside of the wall of a tank compound
and a public boundary fence or to any fixed ignition source.
- Not less than 15 M
8. Between a tank and the battery limit of a process plant.
Fixed or Floating Not less than 30 M
9. Between the top of the inside wall of a tank compound and the
battery limit of a process plant
- Not less than 15 M
The table above gives a guidance on the minimum tank spacing for
Class I, II and III (2) storage facilities, the following points
should be noted.
1. Tanks of diameter up to 10 M are classed as small tanks 2.
Small tanks may be sited together in groups, no group having an
aggregate
-
capacity of more than 8000 m3. Such a group may be regarded as
one tank. 3. Where future changes of service of a storage tank are
anticipated the layout
and spacing should be designed for the most stringent case. 4.
For reasons of fire fighting access there should be no more than
two rows of
tanks between adjacent access roads. 5. Fixed roof with internal
floating covers should be treated for spacing purposes
as fixed roof tanks. 6. Where fixed roof and floating roof tanks
are adjacent, spacing should be on
the basis of the tank(s) with the most stringent conditions. 7.
Where tanks are erected on compressible soils, the distance between
adjacent
tanks should be sufficient to avoid excessive distortion. This
can be caused by additional settlements of the ground where the
stressed soil zone of one tank overlaps that of the adjacent
tank.
8. For Class III (1) and unclassified petroleum stocks, spacing
of tanks is governed only by constructional and operational
convenience. However, the spacing of Class III (1) tankage from
Class I, II or III (2) tankage is governed by the requirements for
the latter.
9. For typical tank installation, illustrating how the spacing
guides are interpreted see below figures. For details of a typical
vertical tank foundation see below figures.
3.8 Tank Farm Piping and Layout Pipelines connected to tanks
should be designed so that stresses imposed are within the tank
design limits. The settlement of the tank and the outward movement
of the shell under the full hydrostatic pressure should be taken
into account. The first pipe support from the tank should be
located at a sufficient distance to prevent damage both to the line
and to tank connections. Consideration may be given to installing
spring supports near to tank connection for large bore
pipework.
As large diameter tanks have a tendency to settle on their
foundations, provision must be made in the suction and filling
piping to take care of tank settlement. This may require the use of
expansion joints, victaulic couplings, or a lap joint flange
installed as shown in see below figure.
The following note must be added to all piping drawings
containing storage tanks:
All piping must be disconnected from tank during hydrostatic
test of storage tank
The number of pipelines in tank compounds should be kept to a
minimum. They should be routed in the shortest practicable way to
the main pipe tracks located outside the tank compounds, with
adequate allowance for expansion.
-
Flexibility in piping systems may be provided through the use of
bends, loops or offsets. Where space is a problem suitable
expansion joints of the bellows type may be considered for
installation in accordance with manufacturers design specifications
and guides. These expansion joints should be used only in services
where the fluid properties are such that plugging of the bellows
cannot occur. They should be anchored and guided, should not be
subjected to torsional loads, and should be capable of ready
inspection.
Tank farm piping should preferably be run above ground on
concrete or steel supports. Ground beneath piping should be so
graded as to prevent the accumulation of surface water or product
leakage. Manifolds should be located outside the tank bunds.
Piping should pass over earth bund walls, however, if this is
impossible, a suitable pipe sleeve will be provided to allow for
expansion and possible movement of the lines. The annular space
should be properly sealed. Likewise lines passing through concrete
bund walls will be provided with pipe sleeves.
Pedestrian walkways should be provided to give operational
access over ground level pipelines.
Pipelines should be protected against uneven ground settlement
where they pass under roadways, railways or other points subject to
moving loads.
Buried pipelines should be protected externally by corrosion
preventing materials, or by cathodic means.
Routes of buried pipelines should be adequately marked above
ground and recorded.
Pipe racks carried across paths or roads should have adequate
clearance from grade. Adequate access stairways or ladders and
operating platforms should be provided to facilitate operation and
maintenance at tanks. Tanks may be interconnected at roof level by
bridge platforming.
All nozzles, including drains on a tank shell, should have block
valves adjacent to the tank shell or as close as practicable.
3.9 Tank Bund Compound Capacities Above ground tanks for Class
I, II (1), II (2) and III (2) petroleum liquids should be
completely surrounded by a wall or walls. Alternatively, it is
acceptable to arrange that spillage or a major leak from any tank
are directed quickly and safely by gravity to a depression or
impounding basis at a convenient location.
The distance between the edge of the impounding basin and the
nearest tank or the
-
inside top of the nearest bund wall should be a minimum of 30 M.
The distance between the edge of the basin and road fence battery
limit of a process plant should not be less than 15 M.
The height of the bund wall as measured from outside ground
level should be sufficient to afford protection for personnel when
engaged in fire fighting and the wall should be located so that a
reasonably close approach can be made to a tank fire to allow use
of mobile fire fighting equipment. Access roads over bund walls
into very large compounds are helpful in certain fire
situations.
Separate walls around each tank are not necessary, but the total
capacity of the tanks in one bunded area should be restricted to
the following maximum figures:
Single tanks No restriction
Groups of floating roof tanks 120,000 m3
Groups of fixed roof tanks 60,000 m3
Crude tanks Not more than two tanks of greater individual
capacity than 60,000 m3
The figures for b. and c. may be exceeded for groups of not more
than three tanks, where there can be no risk of pollution or hazard
to the public.
Intermediate walls of lesser height than the main bund walls may
be provided to divide tankage into groups of a convenient size so
as to contain small spillages and act as fire breaks.
Buried, semiburied or mounded tanks need not be enclosed by a
bund wall except when they are located in ground higher than the
surrounding terrain. However, consideration should be given to the
provision of small bund walls around associated tank valves.
The net capacity of the tank compound should generally be
equivalent to the capacity of the largest tank in the compound.
However, a reduction of this capacity of 75% will provide
reasonable protection against spillage and may be adopted where
conditions are suitable (e.g. where there can be no risk of
pollution or hazard to the public). The net capacity of a tank
compound should be calculated by deducting from the total capacity
a. the volume of all tanks, other than the largest, below the level
of the top the compound wall and b. the volume of all intermediate
walls.
A low wall which need not be more than 0.5 m high, should be
constructed for Class III (1) and unclassified petroleum product
tankage where conditions are such than
-
any spillage or leakage could escape from the installation and
cause damage to third party property drainage systems, rivers or
waterways.
Where there is a possibility that tanks storing these products
may be in the future required for Class I, II (1) or III (2), then
the compound walls should be suitable for this potential
situation.
4. Pump Areas Pumps will be located outside bund areas. The
vessels practice is to group the pumps into bays. Keep the suction
lines as short as practical. The discharge piping will run on low
level tracks to the process or loading areas. These tracks will
usually pass under roads in culverts, but may pass over on a pipe
bridge. Long pipe runs may require expansion loops to provide
flexibility. Consult with stress section.
5. Fire Protection For storage areas the major fire fighting
effort will be provided by mobile equipment laying down large
blankets of foam and/or applying large volumes of water for cooling
purposes.
It is essential to provide a good system of all weather roads to
facilitate the transfer of fire protection materials and equipment
to the scene of the fire. These roads must be of adequate width
and, wherever possible, with no deadends.
It is important in the siting of tanks, bund walls and access
roads that the tanks can be protected by cooling water or foam
appliances situated outside the compound walls. Account must be
taken of the height of the tank and the possible need to cool the
roof or project foam on to a tank.
Dry risers for foam may be provided to the top of storage tanks
with their connections adjacent to access roads, fixed monitors may
also be employed. The flow diagram will define the system to be
employed.
6. Road and Rail Loading Facilities Road and rail loading
facilities are usually associated with storage area. The safe
-
location of these in relation to storage tanks is laid down in
section 3.7.
The road or railcar will be filled from a loading island, the
supply lines will be either routed underground, or on an overhead
pipe bridge. Check for clearances.
Below figures show such installations.
It has become common practice to provide a vapour collection
system for the safe removal of vapours during the loading process.
This system would employ unloading arms which are connected to a
collection system and piped to a vent stack at a safe location.
When laying out a loading area consideration must be given to
the number of vehicles or rail cars per hour to be loaded. A
suitable movement pattern must be established for incoming and
outgoing vehicles or railcars. Weigh bridges will be required, the
system of moving rail cars must be defined, building housing,
operation offices and facilities for drives etc. , must be
provided.
Figures:
API TANK SIZE - FOR LAYOUT PURPOSE
Based on API650
Capacity Approximately Diameter Height
US Barrels CU Meters Meters Meters
500 75 4.6 4.9
1.000 150 6.4 4.9
1.500 225 6.4 7.3
2.000 300 7.6 7.3
3.000 450 9.2 7.3
4.000 600 9.2 9.3
5.000 750 9.2 12.2
6.000 900 9.2 14.6
7.000 1050 12.2 9.9
-
9.000 1350 12.2 12.2
10.000 1500 12.8 12.2
12.000 1800 12.8 14.6
15.000 2250 14.6 14.6
20.000 3000 18.3 12.2
30.000 4500 22.3 12.2
40.000 6000 26.0 12.2
50.000 7500 27.5 14.6
90.000 12000 36.6 12.2
100.000 15000 41.0 12.2
120.000 18000 41.0 14.6
140.000 21000 49.8 12.2
180.000 27000 54.9 12.2
200.000 30000 54.9 14.6
300.000 45000 61.0 17.0
450.000 60000 73.2 17.0
600.000 90000 91.5 14.6
800.000 100000 105.0 14.6
-
Figure 1. TANKS A, B, C ARE FIXED OR FLOATING ROOF SMALL TANKS
(LESS THAN 10 m. DIAMETER) WITH A TOTAL CAPACITY OF LESS THAN 8000
m3; NO INTER-TANK SPACING REQUIREMENTS OTHER THAN FOR CONSTRUCTION
/ OPERATION / MAINTENANCE CONVENIENCE. TANKS D1 & D2 ARE TANKS
WITH DIAMETERS GREATER THAN 10 m., & WITH DIAMETER OF D2
GREATER THAN D1. Inter-tank spacings between small and larger
tanks.
-
Tank and compund wall distances from typical features. Figure
2.
FLOATING ROOF TANKS OF DIAMETER D1 D2 D3 GREATER THAN 10 m.
WITHIN THE SAME COMPUND. D1 GREATER THAN D2 & D2 GREATER THAN
D3.
-
Figure 3.
Inter-tank spacing for floating roof tanks (greater than 10 m
diameter).
FIXED & FLOATING ROOF TANKS WITHIN THE SAME COMPOUND. D1
GREATER THAN D2, D2 EQUAL TO D3. Figure 4.
-
Inter-tank spacings for fixed and floating roof tanks (greater
than 10 m diameter) Lap joint Flange Detail for Tank Settlement
Figure 5.
-
Foundation for vertical tank Based on BS2654 Figure 6.
< Prev Page | Up | Top | Next Page >
'reliability is yet to become the most important characterstic
of modern companies'
home | site | privacy | legal 2001 - 2007 Red-Bag