guide to storage tanks for oil & water

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.

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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.