Good Design of Tanks l2

8/10/2019 Good Design of Tanks l2

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DESIGN OF STEEL TANKS

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 ordomed shape. The sizes of cylindrical tanks range from a modest 3m diameter up to about 100mdiameter, and up to 25m in height. They consist of three principal structural elements - bottom, shelland roof.

For petroleum storage, the bottom is formed of steel sheets, laid on a prepared base. Some tanks forwater 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 membraneaction, though generally the roof plate is supported on radial beams or trusses.

The two standards applied most widely are British Standard BS 2654 [1] and the AmericanPetroleum Institute Standard API 650 [2]. These two Standards have much in common, althoughthere are some significant differences. BS 2654 standard is both a design code and a constructionspecification. The design code is based on allowable stress principles, not on a limit state basis.

The Technical Committee CEN/TC 265 Site built metallic tanks for the storage of liquids, thesecretariat of which is held by BSI has prepared prEN 14015:2003: Specification for the design andmanufacture of site built, vertical, cylindrical, flat-bottomed, above ground, welded, steel tanks forthe storage of liquids at ambient temperature and above. This European Standard reflects the currentpractice within the oil, petrochemical, chemical, food and general bulk liquid storage industry, bothEuropean and world-wide. The practice is based on the theory of design stresses or allowablestresses.

There is a parallel pre-standard, ENV 1993-4-2 Tanks. It is based on the Limit State Theory (LST),which is being used more and more by the structure steel and reinforced concrete industry.Experience in designing steel storage tanks to LST is limited, and there is little information onwhich to base the values for load factors, load combinations and serviceability. When sufficientexperience has been gained in designing tanks to, and credible values become available for loadfactors, etc., it is envisaged that there may be a gradual move towards the use of LST for the designof tanks covered by this European Standard.

Tanks are usually manufactured from plain carbon steel plate (traditionally referred to as mild steel)of grades S235 or S275 (to EN 10 025), or equivalent. Such material is readily weldable. The use ofhigher strength grades of low alloy steel (e.g. Grade S355) is less common, though its use isdeveloping.

Notch ductility ( ) at the lowest service temperature is obtained for thickermaterials (> 13 mm) by specifying minimum requirements for impact tests. This is normallyachieved by specifying an appropriate sub-grade to EN 10 025.


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2. DESIGN LOADS AND FEATURES OF SERVICE

A tank is designed for the most severe combination of the various possible loadings . Tanksdesigned 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.Dead load: The dead load is that due to the weight of all the parts of the tank.

Superimposed load: A minimum superimposed load of 1,2 kN/m 2 (over the horizontal projectedarea) is applied to the roof of the tank. This load is commonly known as the 'snow load', but in factrepresents, as well as a nominal snow load, any other imposed loads, such as maintenanceequipment, which might be applied to the roof, and it includes the internal vacuum load. It istherefore 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 (0,5kN/m 2) may have developed. Even without valves a tank should be designed for a vacuum of 5mbar, to cater for differential pressure under wind loads. In pressure tanks the valves may be set to 6mbar vacuum, in which case a pressure difference of 8,5 mbar (0,85 kN/m 2) may develop.

Actual predicted snow load or other superimposed load, plus appropriate vacuum pressure, shouldbe used when it is greater than the specified minimum.

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 ofthe tank; for a tank without any overflow, the contents should be taken to fill the tank to the top ofthe shell.

For oil and oil products, the relative density of the contents is less than 1.0, but tanks for suchliquids are normally tested by filling with water (hydro-test). A density of 1000 kg/m 3 shouldtherefore be taken as a minimum.

Wind effects: Wind loads are determined on the basis of a design wind speed. Maximum windspeed depends on the area in which the tank is to be built; typically a value of 45 m/s is taken as thedesign wind speed, representing the maximum 3-second gust speed which is exceeded, on average,only once every 50 years ( this provision is from BS only; other codes may give other values ).

Seismic action: In some areas, a tank must be designed to withstand seismic loads. Whilst someguidance is given in BS 2654 and API650 on the design of the tank, specialised knowledge shouldbe applied in determining seismic loads.

3. BOTTOM DESIGN

For petroleum storage tanks, steel bottom plates are specified, laid and fully supported on a preparedfoundation. The steel plates are directly supported on a bitumen-sand layer on top of a foundationpad, usually made of compacted fill. If the subsoil is weak, the foundation pad is replaced by a


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reinforced concrete raft. For larger volume of contents (larger tank diameter), additional RCfoundation ring is recommended to reliably support the bottom immediately below the shell-to-bottom joint. Such a RC foundation ring is especially useful when the tank shell needs to beanchored.

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.

FIG. 2

For volume V 5000 m3

constant thickness of bottom plates t b = 5 mm is typically used.

For larger tanks (over 12,5 m diameter, according to BS 2654) a ring of annular plates is providedaround 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.


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FIG. 4

The bottom act as a seal to the tank. The only load the bottom plates carry, apart from localstiffening to the lowest part of the shell, is the pressure from the contents, which is then transmitteddirectly to the base. Stress calculations are not normally required, though BS 2654 sets out minimumthicknesses of plate depending on the size of the tank. In Bulgarian practice, t b = 6 mm is typicallyused, while the annular plates are thicker and their width and thickness are subject to further designchecks.

FIG. 5


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4. SHELL DESIGN

For practical reasons, it is necessary to build up the shell from a number of fairly small rectangularpieces of plate, butt welded together. Each piece will be cylindrically curved and it is convenient tobuild 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 thinnerplates in the upper rings. 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 lineof 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 .

Circumferential Stresses

Vertical cylindrical tanks carry the hydrostatic pressures by simple hoop tension. Thecircumferential 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 thereforestraightforward. At a fluid depth H, the hoop stress is given by:

wrx t r p=2 ,

where 2 Dr = is the cylinder wall radius, and


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n fai fprx ph p 0 += is the sum of the hydrostatic pressure at the specified depth hi and designpositive internal pressure.

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 athicker course is somewhat restrained because the thicker plate is stiffer. Consequently, because ofthese restraints, an empirical adjustment is introduced into the design rules, which effectivelyrequires that any course is simply designed for the pressure 300mm above the bottom edge of thecourse, rather than the greater pressure at the bottom edge. ( This is known as the 'one foot rule' in

API 650 )

Axial Stresses in the Shell

The cylindrical shell has to carry its self-weight, and the weight of the roof which it supports, as anaxial stress. In addition, wind loading on the tank contributes tensile axial stress on one side of the

tank and compressive stress on the other.

The shell-to-bottom joint introduces additional bending stresses which arise do to the abrupt changeof shell shape and its statically indeterminate support conditions (no outward expansion of the tankbody under hydrostatic pressure is possible due to the high in-plane stiffness of the steel base(bottom). A possible approximate solution for obtaining the resulting internal moment and shear isillustrated on the figure below.

N2 hoop tension (kN/m)


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Although axial stresses do not need to be calculated for service conditions, the tank does have to bechecked for uplift when it is empty and subject to wind loading. If necessary, anchorages must beprovided; a typical example is shown in Figure 7. Anchorage is recommended for tanks situated inseismic areas as well.

Major design checks for the tank wall:

Strength verification for the tensile hoop stress 2 (full tank condition) Stability check for combined action of compressive hoop and meridional stresses 1

(empty tank condition) Strength check for the local overstressing of the shell-to-bottom joint

In general, the basic analysis and design principles given in Lecture 1 for thin shells of revolutionapply for the tank wall as well. As mentioned in Lecture 1, the ECCS procedure for stability checkinvolves the so-called "knock-down" factor , which accounts for the detrimental effect ofimperfections, residual stresses and edge disturbances. Each critical stress (in circumferential ormeridional direction) shall be calculated separately.

For the conditions of purely axially loaded cylinder shell with fixed base, the critical stress can beestimated as a function of the equivalent slenderness , see the graph below:


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The knock-down factor is explicitly included in the calculation of the shell slenderness.

5. 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 effectivecompression ring.

At the top of an open tank (or one with a floating roof), circumferential stiffening is needed tomaintain the roundness of the tank when it is subject to wind load. This stiffening is particularlynecessary when the tank is empty.


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The calculation of the stability of stiffened tanks is a complex matter. Fortunately, investigationsinto the subject have led to an empirical formula, based on work by De Wit, which is easily appliedin design. In BS 2654 this formula is expressed as a required minimum section modulus given by:

Z = 0,058 D 2 H

where Z is the (elastic) section modulus (cm3) of the effective section of the ring girder, including awidth of shell plate acting together with the added stiffener. D is the tank diameter (m) and H is theheight of the tank (m).

The formula presumes a design wind speed of 45 m/s (corresponds to wind pressure = 1,24 kN/m 2).For other wind speeds it may be modified by multiplying by the ratio of the basic wind pressure atthe design speed V to that at 45 m/s, i.e. by (V/45) 2. It is recognised that application of the aboveformula to tanks over 60 m diameter leads to unnecessarily large wind girders; the code allows thesize to be limited to that needed for a 60 m dia. tank.

Wind girders are usually formed by welding an angle, a channel or other curved member around thetop edge of the shell. Examples are shown in Figure 8. Note that continuous fillet welds shouldalways be used on the upper edge of the connection, to avoid a corrosion trap.

FIG. 8 Details for primary wind girder

6. FIXED ROOF DESIGN

General


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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 ormay in turn be supported on columns inside the tank. The use of a single central column isparticularly 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 simplerolled beam sections or, for larger tanks, they can be fabricated trusses.

Supported roofs

Radial members supporting the roof plate permit the plate thickness to be kept to a minimum. Theygreatly facilitate the construction of the roof. Radial beams are arranged such that the span of theplate between them is kept down to a minimum of about 2 m. This limit allows the use of 5 mmplate for the roof.

Supported roofs are most commonly of conical shape, although spherical roofs can be used if theradial beams are curved. The roof support structure can either be self-supporting or be supported oninternal columns. Typical arrangements are shown in section in Figures 10 and 11. Self-supportingroofs are essential when there is an internal floating cover.


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Not all radial members continue to the centre of the tank. Those that do may be considered as mainsupport beams; the secondary radial members may be considered as rafters - they are supported attheir inner ends on ring beams between the main support members. Where internal columns are usedthey 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.

The main support members of the spherical dome are subject to bending and axial load. Where theyare designed for axial thrust, the central ring must be designed as a compression ring; the top of theshell must be designed for the hoop forces associated with the axial forces in the support members.

For storage of petroleum products, emergency pressure relief has to be provided to cater for heatingdue to an external fire. Pressure relief can be achieved either by additional emergency venting or bydesigning 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). ( : , , - .)


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7. USE OF FLOATING ROOFS AND COVERS

As mentioned above, tanks need to be vented to cater for the expansion and contraction of the air. Inpetroleum tanks, the free space above the contents contains an air/vapour mixture. When the mixtureexpands 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 continuedbreathing can result in substantial evaporation losses. Measures are needed to minimise theselosses; floating roofs and covers are commonly used for this purpose .

During service, a floating roof is completely supported on the liquid and must therefore besufficiently buoyant; buoyancy is achieved by providing liquid-tight compartments in one of twoforms of roof - pontoon type and double deck type.

A pontoon roof has an annular compartment, divided by bulkheads, and a central single skindiaphragm. The central diaphragm may need to be stiffened by radial beams. A double deck roof iseffectively 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 mustremain buoyant even if some compartments are punctured (typically two compartments). The centraldeck 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 isachieved by a system on the roof which connects to flexible pipework inside the tank and thencethrough the shell or bottom plates to a discharge. The design is required to ensure that the roofcontinues to float in the event of a block in the drainage system which results in a surcharge of wateron 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 thebottom. At this stage the roof must be able to carry a superimposed load (1,2 kN/m2) plus anyaccumulated rainwater.

A typical arrangement of a pontoon type roof is shown in Figure 12.


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Good Design of Tanks l2