Cicind Model Code For Steel Chimneys Pdf 23: How to Design and Construct Steel Chimneys According to

FOREWORDWhen it was formed in 1973, the Comit InternationaldesChemines Industrielles (CICIND) adopted as a major goaltheharmonisation of national codes for the design ofindustrialchimneys. As a means to this end, a subcommittee wasappointed in1981, charged with drafting a proposal for a model codefor steelchimneys which reflected the current state-of-the-art andaconsensus of views, internationally. This document was publishedin1988, with Commentaries being published the following year.Since1988, the science and technology of chimneys has advancedand in1995, CICIND appointed a committee to revise the ModelCode,recognising current best international practice and knowledge.Thecommittee comprises:

Cicind Model Code For Steel Chimneys Pdf 23

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Chimneys are required to carry vertically and discharge totheatmosphere, gaseous products of combustion, chemical wastegases, orexhaust air or for the combustion (aring off) ofindustrial waste gases.This Model Code contains guide-lines whichreect the current stateof art in the design and construction ofsteel chimneys. Nevertheless,the design, fabrication and erectionof steel chimneys require athorough knowledge of these structures,the properties of thematerials used, the actions occurring upon thestructure and therecognised rules of the relevant technologies. Thedesign of steelchimneys should therefore only be entrusted toappropriatelyqualied and experienced engineers. The constructionand erectionshould be carried out by rms competent in this class ofwork. At alltimes the work should be under the direction ofappropriatelyqualied supervisors. CICIND will continue to try toimprove the understanding of thebehaviour of chimneys. Furtherrevisions of this model code willtherefore be published from timeto time.

This Model Code is accompanied by extensive appendicesandcommentaries. The appendices provide information whichthecommittee believes will be of use to a steel chimney designer,eventhough its inclusion in a chimney design code could not bejustied.The commentaries have the following objectives:a)Justication of the regulations of the model code.b) Simplication ofthe use of the model code.c) Understanding of the meaning of theregulations of the

One of the main objectives of any code governing construction isthecreation of a model which resembles as far as possible, therealsituation. The model should be sufficiently safe, simple andtrue. Itis very rarely that simplicity and truth are compatible, soa modelmust be used which provides an optimum compromise betweentruth,simplicity, safety and economy.While the judgements ofsufficiently true and sufficiently simpleare subjective,sufficiently safe is capable of rational judgement.This codeinterprets sufficiently safe in terms of the social andeconomicconsequences of failure. It does this by comparing theprobabilitiesof failure for given safety factors during its design lifewith thefailure probabilities required to satisfy accepted socialandeconomic criteria. This leads to the development of safetyfactorswhich ensure that a chimney will have a probability offailure duringits design lifetime between 103 and 104, dependingupon itsreliability category.CICIND has departed from generallyaccepted principles ofsteelwork design and construction only whenthis was required by thephilosophy outlined above or by specicchimney requirements.

This model code relates to the structural design andconstruction ofsteel chimneys of circular cross-section, with aminimum height of15m, with or without linings, and to the designand application oflinings to such chimneys where required. It alsorelates to chimneyswith a height less than 15m and a slendernessratio more than 16. Themodel code does not deal with architecturalor thermal aspects ofsteel chimneys nor with their foundations,except insofar as theyaffect the chimneys structural design. Themodel code does not dealwith those aspects of the design andconstruction of steelwork,refractories and insulation which are notpeculiar to chimneys.

The model code is valid for all steel chimneys of circularcross-section. However, the design rules have been formulated forselfsupporting chimneys taller than 15m. For otherchimneyssimplication may be acceptable.Additional information isgiven in the Appendices andCommentaries.

Chimneys shall be designed to avoid movements across thewinddirection sufficient to cause failure or fatigue damage or toalarmbystanders.The code contains means of estimating the amplitudeof movementand consequent stress range due to crosswind loading.Limiting stressranges are given for various weld classications anddesign lives. Inaddition to a material safety factor 1.1, appliedto fatigue category, amodelling factor of 1.4 shall be applied tothe Miner Number derivedin fatigue calculations for temperatures upto 200C and 1.5 fortemperatures between 200C and 400C.To avoidalarming personnel, the maximum permitted amplitude ofoscillationsdue to cross-wind effects or aerodynamic interferenceshall beagreed between the owner and designer. This limit will begovernedby the prominence and visibility of the chimney and thefrequencywith which maximum amplitudes can be expected.Guidance is given inCommentary 3.

The materials generally used for steel chimneys are describedbelow.Special steels can be used providing that they are preciselyspeciedand that their characteristics, such as yield stress,tensile strength,ductility and weldability, enable the Model Codeto be put intoapplication. In zones where bearing elements aresubjected to tensionas a result of external loads or in zones ofthree-dimensional stress,the ductility requirements, in addition tothe minimum strengthvalues, shall be considered.

When metal temperatures are expected to exceed 400C, stainlessoralloy steels should be used.Ordinary stainless steels (includinghigh molybdenum stainless steel)have poor corrosion resistance inthe presence of condensingsulphuric or other acids in the range ofconcentrations andtemperatures normally found within chimneys.These materials aretherefore not recommended in chimneys burningfuels containingsulphur under conditions of medium or high chemicalload, seeparagraph 7.6.3.When metal temperatures and condensatesulphuric acidconcentrations are expected to be less than 65C and5% respectively,the corrosion rates of high molybdenum stainlesssteels, such asASTM Type 316L, are acceptable. Such conditions canbe expectedon the external surface at the top (over a height ofabout 3 diameters)of any chimney handling high sulphur uegases.(Note: the conditions downstream of a ue gas scrubber orthepresence of chlorides in the condensate will radically increasethecorrosion rate, possibly rendering these stainless steelsunsuitable forthese applications.) Ordinary stainless steels arenot suitable for use in contact with uegases containing alkalis.Incases where it is not possible to avoid high chemical load ontheinternal face of the structural shell, see paragraph 7.6.3, theuse of aprotective coating may be considered (see lit[19]).Alternatively, asteel liner or liners, possibly of titanium or highnickel alloy, is apossible solution. See section 10 on SteelLiners.Low copper alloy steels have good resistance toatmosphericcorrosion, except in a marine environment or otherenvironmentwhere chlorides are present. These steels also show somecorrosionimprovement over carbon steel when in contact with uegases whereacid condensation of SO2/SO3 (not of HCL condensation)isintermittent only (e.g. during shutdowns of a stack inintermittentservice, its metal temperature being normally aboveacid dew point).

When a chimney stands alone, its cross-wind vibrations canusuallybe reduced by aerodynamic stabilizers. The useful effect ofthreecontinuous helical vanes has been proved on many steelchimneys.The radial width of the vanes must be 10% of the diameter.The pitchof the vanes should be 5 D. The vanes must be tted over atleast theupper 1/3 of the height. The extra wind drag due to thevanes must beconsidered (see 7.2.3.2.3).Aerodynamic stabiliserswill not reduce the wind interference effectsof nearby chimneys orstructures.

Most such dampers are mounted near the top of thechimney.Because of their prole and small size, the associatedincrease inwind drag is minimised. The use of damping devices,therefore, hasbeen proved to be benecial in the design of steelchimneys and theycan be safely retro-tted without incurringsignicant increase inwind drag loads.

The stress due to wind loading on a steel chimney is usuallymorethan the earthquake stress and, consequently, normal steelchimneyscan resist earthquakes with an intensity of up to modiedMercalliscale 10 without serious damage. However, in cases where aheavymass (e.g. a water tank or a heavy lining) is tted to theupper portionof the chimney, a special investigation must be made(tanks areoutside the scope of the model code). Guyed chimneys mustalso besubject to special investigation.

The resistance of steel chimneys to external explosions is veryhigh.If such explosions can occur in the direct vicinity suchthatstrengthening for this reason is required, it is outside thescope of thismodel code.

In the case of bare steel chimneys, having neither an internalliner norexternal insulation, the metal temperature can be assumedto be aboutmidway between ambient air temperature and that of theue gas overthe range of ue gas velocities between 5m/s and 15m/s.For ue gasvelocities faster than 15m/s or for steel stacks equippedwith either a lineror external insulation, heat transfercalculations shall be made todetermine the maximum metaltemperature of the structural shell. Thesecalculations shall assumestill air and highest anticipated air temperature.

dynamic control)The carrying capacity check shall prove that theforces resulting fromthe working loads multiplied by the loadfactors do not exceed theresistance of the shell. The check shouldcomprise both the strength andstability proof. The calculationsshall be carried out for the corrodedthickness of the steel(without corrosion allowance). The serviceabilityshall be checkedunder working loads without load factors.A fatigue check shall becarried out if movement due to vortexshedding is expected (see7.2.4).For unstiffened chimneys with a ratio of L/R 50 (where Lheightof chimney and R radius), stresses may be safelycalculatedassuming beam theory, exural stresses being addedvectorially toovalling stresses. For unstiffened chimneys (i.e.chimneys withoutstiffening rings or substantial anged joints)having L/R 50, shelltheory or nite element modelling should beused, consideringexural and ovalling stresses simultaneously. Thiswill lead toreduction in compression stress at the chimney base orimmediatelyabove changes in chimney diameter, but will increasecompressionstresses elsewhere. Similarly, this will lead toincreases in tensilestresses at the base and immediately above achange in chimneydiameter, which will be important in deriving bolttensions.The increase in tensile stress in these regions may beapproximatedby the expression:- 2ff7e9595c