L12 special 2 - ct.upt.ro · %rwwrp sodwhv vkrxog eh ods zhoghg ru exww zhoghg 7kh vshflilhg...

Tanks, pipelines, and silos

Florea Dinu

Lecture 12: 27/02/2017

European Erasmus Mundus Master Course

Sustainable Constructions under Natural Hazards and Catastrophic Events

520121-1-2011-1-CZ-ERA MUNDUS-EMMC



L12 - 2C08 Tanks and pipelines

Tanks

2

• Tanks are commonly used to store oil products or water, stiffened as well as unstiffened.

• 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.

• According to shape: cylindrical vertical, cylindrical horizontal, spherical, rectangular, other.

• According to internal pressure: low-pressure (up to 20 mbar = 2kPa), high-pressure.

Types of cylindrical tanks Horizontally placed cylinders Spherical tank



L12 - 2C08 Tanks and pipelines 3

Storage tank, dome roof

Storage tank, conical roof

Fuel storage tank, capacity approx 2,000,000 liters, fixed roof and internal floating roof

Irrigation Water Storage Tank




Analysis and design of tanks (EN 1993-4-2)

3 consequence (reliability) Classes: Class 1, 2, 3Class 1- Simple structures for agriculture or tanks containing water.- Membrane theory may be used, with simple formulas for boundary disturbance and asymmetric loading.

Class 2- Medium size tanks with flammable or water-polluting liquids in urban areas - Axisymmetric actions and support:

- Membrane theory may be used to determine the primary stresses, with bending theory elastic expressions to describe all local effects.

- A validated numerical analysis may be used (for instance, finite element shell analysis). - Loading condition is not axisymmetric: a validated numerical analysis should be used, except under some conditions (see EN 1993-4-2)




Analysis and design of tanks (EN 1993-4-2)

Class 3: Tanks storing liquids or liquefied gases with toxic or explosive potential and large size tanks with flammable or water-polluting liquids in urban areas. Emergency loadings should be taken into account for these structures where necessary. The internal forces and moments should be determined using a validated analysis (for instance, finite element shell analysis)

Wall design- Global and local stability, static equilibrium- strength of the structure and joints- cyclic plasticity- fatigue- SLS (deflections and vibrations)




ActionsLiquid induced loads: During operation, the load due to the contents should be the weight of the product to be stored from maximum design liquid level to empty.Internal pressure loads: During operation, the internal pressure load should be the load due to the specified minimum and maximum values of the internal pressure.Thermally induced loads: Stresses resulting from restraint of thermal expansion may be ignored if the number of load cycles due to thermal expansion is such that there is no risk of fatigue failure or cyclic plastic failure.Dead loads: The dead loads on the tank should be considered as those resulting from the weight of all component parts of the tank and all components permanently attached to the tank.Insulation loads: The insulation loads should be those resulting from the weight of the insulation.Distributed live load, Concentrated live loadSnow: The loads should be taken from EN 1991-1-3.Wind: The loads should be taken from EN 1991-1-4.Suction due to inadequate venting: The loads should be taken from EN 1991-1-4.Seismic loadings: The loads should be taken from EN 1998-4, which also sets out the requirements for seismic design.Loads resulting from connections: Loads resulting from pipes, valves and other items connected to the tank and loads resulting from settlement of independent item supports relative to the tank foundation should be taken into account.Loads resulting from uneven settlement: Settlement loads should be taken into account where uneven settlement can be expected during the lifetime of the tank.Emergency loadings: The loads should be specified for the specific situation and can include loadings from events such as external blast, impact, adjacent external fire, explosion, leakage of inner tank, roll over, overfill of inner tank.




Wind Loads

The loads should be taken from EN 1991-1-4. In addition, the following pressure coefficients may be used for circular cylindrical tanks, see figure:a) internal pressure of open top tanks and open top catch basin: cp = -0,6. b) internal pressure of vented tanks with small openings: cp = -0,4. c) where there is a catch basin, the external pressure on the tank shell may be assumed to reduce linearly with height. Due to their temporary character, reduced wind loads may be used for erection situations according to EN 1991-1-4.

Tank with catch basin Tank without catch basin

Transformation of typical wind external pressure load distribution

(for simplified design)




Design of shell - simplified relations

Required thickness of cylindrical tank web:

where design loading by liquid and overpressure, pd is:

design overpressureabove liquid level

unit weight

For spherical tanks: (one half in comparison to the above)

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 the figure

Variation of stress in shell wall




• The design of the bottom plate should take corrosion into account.• Bottom plates should be lap welded or butt welded.• The specified thickness of the bottom plates should not be less than specified in

table below.

Bottom design

Minimum nominal bottom plate thickness

Typical tank foundation




Typical bottom layout for tanks up to and including 12.5m diameter

Cross joints in bottom plates where three thicknesses occur

Typical bottom layout for tanks over 12.5 diameter

Joints in bottom plates bellow shell plates



L12 - 2C08 Tanks and pipelines 11Typical tank anchorage detail

Tank anchorage • Tanks are usually not equipped with anchoring devices.

• Anchoring devices should be provided for fixed roof tanks, if any of the following conditions can cause the cylindrical shell wall and the bottom plate close to it to lift off its foundations:

a) Uplift of an empty tank due to internal design pressure counteracted by the effective corroded weight of roof, shell and permanent attachmentsb) Uplift due to internal design pressure in combination with wind loading counteracted by the effective corroded weight of roof, shell and permanent attachments plus the effective weight of the product always present in the tank c) Uplift of an empty tank due to wind loading counteracted by the effective corroded weight of roof, shell and permanent attachments; d) Uplift of an empty tank due to external liquid caused by flooding. In such cases it is necessary to consider the effects upon the tank bottom, tank shell etc. as well as the anchorage design. e) Uplift of filled tank due to seismic action




Fixed roof design• 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

• In a membrane roof, the forces from dead and imposed loads are resisted by compressive radial stresses.

• 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.

Supported Roofs

• Radial members supporting the roof plate permit the plate thickness to be kept to a minimum.

• 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. Self supporting roofs are essential when there is an internal floating cover.



L12 - 2C08 Tanks and pipelines 13Self-supporting fixed roofs

Alternative support systems for roofs




Edge ring at the shell to roof junction

• The force in the effective edge ring (area where the roof is connected to the shell) should be verified using:

in which:

where:Aeff is the effective area of the edge ring indicated in

figure

α is the slope of the roof to the horizontal at the junction;

pv,Ed is the maximum vertical component of the design distributed load including the dead weight of the supporting structure (downward positive).

• The bending moments in the ring should be considered if rafter is located at a distance to the edge that exceeds 3,25m.

At the connection of the rafter At half span between the rafters




Seismic design (EN 1998-4)Seismic motion induces two main effects in tanks:- The rocking motion, accompanied

by an uplifting of the rim of the annular plate or the bottom plate, is induced by the overturning moment due to the horizontal inertia force. In this case, particular attention should be paid to the design of the bottom corner of the tank

- On the other hand the rocking motion in the liquefied gas storage tank caused by the overturning moment induces pulling forces in anchor straps or anchor bolts in place of uplifting the annular plate. In this case, the stretch of the anchor straps or anchor bolts which are provided at the bottom course of the tank, should be the subject of careful design.

Pulled out of anchor bolt

Cut off of anchor bolt

Pulled out of earthing wire

Elephant foot bulge type buckling of tank wall Diamond pattern buckling of

tank wallTank damage mode at Hanshin-Awaji

Earthquake




Pipelines• Design is done according to EN 1993-4-2: Eurocode 3 - Design of steel

structures - Part 4-3: Pipelines• Fundamental requirements

• The pipeline shall be designed and constructed in such a way that: • With acceptable probability, it will remain fit for the use for which it is

required, having due regard to its intended life and its cost;• With appropriate degrees of reliability, it will sustain all actions and

other influences likely to occur during the execution and use and have adequate durability in relation to maintenance costs;

• It will not be damaged by events like explosions, impact or consequences of human errors, to an extend disproportionate to the original cause.

• The potential damage of pipelines shall be limited or avoided by appropriate choice of one or more of the following:

• Avoiding, eliminating or reducing the hazards which the structure is to sustain.

• Selecting a structural form that has low sensitivity to the hazards considered.




Actions to be considered in design

The following actions should be considered, where appropriate: - Internal pressure;- External pressure;- Self weight of the pipeline;- Self weight of the contents of the pipeline- Soil loads;- Traffic loads;- Temperature variations;- Construction loads;- Imposed deformation: due to differential settlements, mining subsidence and landslides;- Earthquake loads (reference should be made to Eurocode 8).




Pipelines for dusty material Pipeline bridge - general view Pipeline - detail

Oil pipelines




• Silos are used by a wide range of industries to store bulk solids in quantities ranging from a few tones to hundreds or thousands of tones.

• The term silo includes all forms of particulate solids storage structure, that might otherwise be referred to as a bin, hopper, grain tank, or bunker.

• They can be constructed of steel or reinforced concrete and may discharge by gravity flow or by mechanical means.

• Steel bins range from heavily stiffened flat plate structures to efficient unstiffened shell structures.

• They can be supported on columns, load bearing skirts, or they may be hung from floors.• Flat bottom bins are usually supported directly on foundations.

Silos

Terminology used in silo structures




Flat Bottom Silos

Used for long-term storage of large quantities of grain, seeds and granular products Are used for the storage

and subsequent delivery of bulk products

Storage of grains (cereals, seeds, legumes, industrial products and other products) that require special storage conditions

Hopper Silos

Truck load silos




Basis of designClassification of two parameters, the size and the type of operation into consequence classes

Consequence Class Design situations

Consequence Class 3 Ground supported silos or silos supported on a complete skirt extending to the ground with capacity in excess of W3a tonnes Discretely supported silos with capacity in excess of W3b tonnesSilos with capacity in excess of W3c tonnes in which any of the following design situations occur:a.eccentric dischargeb.local patch loadingc.unsymmetrical filling

Consequence Class 2 All silos covered by this Standard and not placed in another class

Consequence Class l Silos with capacity between W1a tonnes† and W1b tonnes

† Silos with capacity less than W1a tonnes are not covered by this standard.

Class boundary Recommended value (tonnes)

W3a 5000

W3b 1000

W3c 200

W1b 100

W1a 10

Recommended values for class boundaries

Consequence classes depending on size and operation



L12 - 2C08 Tanks and pipelines

• Actions on silos

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Another actions:

Coefficient of active earth pressure:

Temperature variationThermal contraction of a bin wall is restrained by the stored material. The magnitude of the resulting increase in lateral pressure depends upon the temperature drop, the difference between the temperature coefficients of the wall and the stored material, the occurrence of temperature changes, the stiffness of the stored material and the stiffness of the bin wall.ConsolidationConsolidation of the stored material may occur due to release of air causing particles to compact (a particular problem with powders), physical instability caused by changes in surface moisture and temperature, chemical instability caused by chemical changes at the face of the particles, or vibration of the bin contents. The accurate determination of wall pressures requires a knowledge of the variation with depth of bulk density and the angle of internal friction.




Moisture ContentAn increase in the moisture content of the stored material can increase cohesive forces or form links between the particles of water soluble substances. The angle of wall friction for pressure calculations should be determined using both the driest and wettest material likely to be encountered. Increased moisture can result in swelling of the stored solid and should be considered in design.SegregationFor stored material with a wide range of density, size and shape, the particles tend to segregate. The greater the height of free fall on filling, the greater the segregation. Segregation may create areas of dense material. More seriously, coarse particles may flow to one side of the bin while fine cohesive particles remain on the opposite side. An eccentric flow channel may occur, leading to unsymmetrical loads on the wall. The concentration of fine particles may also lead to flow blockages.

Segregation patterns due to different mechanisms




DegradationA solid may degrade on filling. Particles may be broken or reduced in size due to impact, agitation and attrition. This problem is particularly relevant in bins for the storage of silage where material degradation may result in a changing pressure field which tends to hydrostatic.CorrosionStored material may attack the storage structure chemically, affecting the angle of wall friction and wall flexibility. Corrosion depends on the chemical characteristics of the stored material and also the moisture content. Typically, the design wall thickness may be increased to allow for corrosion and the increase depends upon the design life of the bin.AbrasionLarge granular particles such as mineral ores can wear the wall surface resulting in problems similar to those described for corrosion. A lining may be provided to the structural wall, but care should be taken to ensure that wall deformation does not cause damage to the lining. The linings are usually manufactured from materials such as stainless steel or polypropylene.Impact PressuresThe charging of large rocks can lead to high impact pressures. Unless there is sufficient material to cushion the impact, special protection must be given to the hopper walls. The collapse of natural arches which may form within the stored material and hold up flow, can also lead to severe impact pressures. In this case, a preventative solution is required at the geometric design stage.




Rapid Filling and DischargeThe rapid discharge of bulk solids having relatively low permeability to gasses can induce negative air pressures (internal suction) in the bin. Rapid filling can lead to greater consolidation, and the effects are discussed above.PowdersThe rapid filling of powders can aerate the material and lead to a temporary decrease in bulk density, cohesiveness, internal friction and wall friction. In an extreme case, the pressure from an aerated stored material can be hydrostatic.Wind LoadingDesign against wind loads is especially critical during bin construction.Dust ExplosionsBins storing materials may explode should either be designed to resist the explosion or should have sufficient pressure relief area.Differential SettlementsLarge settlements often occur as bins are filled, particularly the first time. The effects of differential settlement of groups of bins should be considered. Differential settlements may lead to buckling failure of membrane steel bins.Seismic ActionsRules for seismic design are given in Eurocode 8, part 4.Mechanical Discharge EquipmentMechanical discharge equipment can lead to unsymmetrical pressure distributions even when it is considered to withdraw the stored material uniformly.Roof LoadsBin roofs impose an outward thrust and axial compression on bin walls and should be considered during wall design. The design of bin roofs is beyond the scope of this lecture.




• The design of bins and silos to store bulk solids involves bulk material, geometric, and structural considerations.- Bulk material considerations are important because the frictional and cohesive properties of bulk solids vary from one solid to another, and these properties affect material behavior considerably.- When considering the geometric design of a silo, potential problems include arching across an outlet, ratholing through the material, and the flow pattern during discharge.- Established design procedures include selection of the optimum hopper angles and minimum outlet dimensions. The ideal discharge mode is one where, at steady state, all material flows without obstruction. This is referred to as mass flow. The discharge mode where only some of the material flows is called funnel flow.

Silo design

Funnel flow Mass flow




Graphical method for the determination of flow pattern




Analysis and design of silos (EN 1993-4-1)

Design checks for:• global stability and static equilibrium,• strength of the structure and joints,• stability (global and local – formulas given in Eurocode),• cyclic plasticity,• fatigue,• SLS (deflections and vibrations)

• Design allowance for corrosion and abrasion min. 2 mm is recommended!

• Simplified rules for circular silos in Consequence Class 1 can be used:• The following simplified action combinations may be considered for

silos in Consequence Class 1:• Filling• Discharge• Wind when empty• Filling, combined with wind• A simplified treatment of wind loading is permitted.

for Class 1 may be ignored




Cylindrical silos (shell):




Failure of silos

The major causes of silo failures are due to shortcomings in one or more of four categories:

• Failure due to design• Failure due to construction• Failure due to usage• Failure due to maintenance.

Result of mass flow developing in a silo designed structurally for funnel flow




Buckling of unsupported wall above a sweep arm unloader

Failure of a grain silo Failure due to design

• The designer must first establish the material's flow properties and design criteria, including load combinations, load paths, primary and secondary effects on structural elements, and the relative flexibility of the elements.

1999 Kocaeli, Turkey earthquake




• In the construction phase, there are two main problems that can cause potential failures:• The more common of these is poor workmanship. Faulty construction, such

as using the wrong materials and uneven foundation settlement are two examples of such a problem. Uneven settlement is rare but when it does occur, the consequences can be catastrophic since usually the center of gravity of the mass is well above the ground.

• The other cause of construction problems is the introduction of badly chosen, or even unauthorized, changes during construction in order to expedite the work or reduce costs.

Failure due to construction errors

Failure due to usage

• Problems can arise when the flow properties of the material change, the structure changes because of wear, or an explosive condition arises. If a different bulk material is placed in a silo than the one for which the silo was designed, obstructions such as arches and ratholes may form, and the flow pattern and loads may be completely different than expected.




Arching Ratholing

Flow problems experienced in an improperly designed

Damage to upper part of silo due to flow problems

Burst silo had fallen onto adjacent silo on left causing collateral damage (most probably due to improper emptying process)




• Maintenance of a silo comes in the owner's or user's domain, and must not be neglected. Two types of maintenance work are required:

• The first is the regular preventative work, such as the periodic inspection and repair of the walls and/or liner used to promote flow, protect the structure, or both. Loss of a liner may be unavoidable with an abrasive or corrosive product, yet maintaining a liner in proper working condition is necessary if the silo is to operate as designed.

• The second area of maintenance involves looking for signs of distress (e.g., cracks, wall distortion, tilting of the structure) and reacting to them. If evidence of a problem appears, expert help should be immediately asked.

Failure due to improper maintenance




This lecture was prepared for the 1st Edition of SUSCOS(2012/14) by Prof. Josef Macháček (CTU) and Michal

Jandera, PhD. (CTU).

Adaptations brought by Florea Dinu, PhD (UPT)

The SUSCOS powerpoints are covered by copyright and are for the exclusive use by the SUSCOS teachers in the framework of this Erasmus

Mundus Master. They may be improved by the various teachers throughout the different editions.

[email protected]

http://ct.upt.ro/suscos

L12 special 2 - ct.upt.ro · %rwwrp sodwhv vkrxog eh ods zhoghg ru exww zhoghg 7kh vshflilhg...

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