STEEL STACKS DESIGN GUIDE This guide is prepared by myself Khaled Sayed on November 2015 due to lack of information about this topic, this is as simple guidance line for steel stack design based on best practice industrial design, the effort is dedicated this work to my friend Jin from Sinoma-CDI China who passed on 13th of November 2015
INTRODUCTION This standard document covers terminology, loading, materials, structural design, construction, inspection, maintenance and painting of both self-supporting and some information about guyed stack form, the sequence are based on the Asme-Sts-1-2010 and some information are extracted from the Indian standards, in my search this was found to be a grey area between the mechanical engineer and the civil, very few books and standers cover this topic.
STACKS Due to the particular nature of stacks and their susceptibility to failures due to wind and seismic-induced vibrations, along with corrosion and erosion, the design process is a complex one. Additionally, recent regulations by the Environmental Protection Agency concerning emissions have placed a strong emphasis on the mechanical design of stack.
TYPES OF SUPPORTS There are many types of stack 1. Free standing 2. Multi flue stacks 3. Base support and braced 4. Base support and guyed There are also different types of supports vertical and lateral, or braced. Vertical supports may be above ground. Examples of this kind of support would be a stack supported on a steel frame within a structural tower or a stack supported on a floor or on top of a building.
STACKS SUPPORTED BY OTHER STRUCTURES. Stacks may be laterally supported by other structures such as towers and adjacent buildings. No credit for shielding provided by the bracing building shall be considered when computing design wind. The bracing assembly should allow vertical movement due to thermal expansion.
Stacks may also be vertically supported by other structures. For proper analysis, structural interaction between the stack and its supporting structure should be considered.
ADVANTAGES OF VERTICALLY SUPPORTED AND BRACED STACKS. Stacks supported above ground usually have the option of receiving exhaust duct attachment from below, as well as from the sides. A braced stack will; require a smaller foundation as compared to a free; standing stack with the same height since some of the wind load will be transferred to the adjacent bracing: structure. Due to the same load transfer, a braced stack 'also has fewer shell stresses as compared to a free-standing stack, therefore requiring thinner shell or smaller diameter. For multiplatform and tall stacks, sometimes access to the platform can be provided by catwalks from the adjacent building rather than a ladder from ground level. In the case of the tower-supported stacks, the tower also has the advantage of providing an easy and safe framework for staircase and test platforms.
DIMENSION OF STEEL STACK Dimension of steel stack are cylindrical in shape the high depends on several factors such as gas velocity, temperature. The chimney should be at least 5 meter taller than the surrounding area of 150 meter radius. The diameter of the stack can be calculated from the given formula however the diameter shall be so chosen that the velocity will not exceed, under any cases 30m/sec. The optimum range of velocity may be taken as 15 to 20 m/Sec. The height of chimney depends upon the description requirement of the flue gases into the atmosphere
THERM MAL EXP PANSION N Differenttial expansio on between components c of a stack shhould be careefully studieed in areas too include (a) Between externall and internaal shells of a dual wall orr multi-flue sstack c and d ladder attacchment bracckets (b) At tesst platform, catwalk, (c) At roo of flashing and a counter flashing f (d) At staack tops and d truncated co one (e) Betweeen stack shells and exteernal insulatiion (f) At weeld joints bettween dissim milar metals
MATEERIAL CO ONSIDER RATION Corrosion n allowances shall be co onsidered (ty ypically to .)) for all typees of steel. B Base plate shaall be 1) Carbon steels confforming to th he AS1M A 36,A 283, oor A 529 Speecifications. (2) High--strength, low w steels con nforming to the t AS1M A 242, A 5722 alloy, or A 588 Specificaations. (3) Stainlless steels co onforming to o the AS1M A 666 Speccification. (4) Stainlless chromiu um-nickel steeel clad platte conforminng to AS1M A 264 and nnickel-base aalloy clad steell conforming g to ASTM A 265 may be b considereed for use as shell plate.
CORROSSION ALLOW WANCE
GUY WIRES, CABLES, OR FITTINGS (a) Guy wires w and caables typicallly may be off one or moree of the folloowing materrials, and consideraation should d be given to the initial sttretch of the material: (1) Alum minum-coated d steel wire strand confo orming to thee ASTM A 4474 Specificcation (2) Zinc--coated (galv vanized) steeel wire strand d conforminng to the AS11M A 475 annd A 586 Specificaations (3) Zinc--coated (galv vanized) steeel wire rope conforming to the ASTM M A 603 Speecification (4) Stainlless steel wire strand con nforming to the AS1M A 368 Specifiication.
ANCHOR BOLTS, WASHERS, AND NUTS Anchor bolts may be of threaded bolt and stud stock normally used as connectors, or of round stock of structural material that may be threaded. They are typically one of the following specifications: (1) Carbon steel threaded fasteners conforming to the AS1M A 307 Specification (2) Carbon steel bolts for general applications conforming to the AS1M A 449 Specification (3) Alloy steel bolts, studs, and threaded fasteners conforming to the AS1M A 354 Specification (4) Alloy steel bolts and studs with enhanced impact properties conforming to the AS1M A 687 Specification (5) Carbon steel conforming to the ASTM A 36 Specification
BOLTS, WASHERS, AND NUTS (a) Unless otherwise specified, carbon and high strength steel bolts conforming to the ASTM A 307, A 325, or A 449 Specifications will be utilized. (b) High-strength alloy steel bolts may be required and these should conform to the ASTM A 354 or A 490 Specifications.
WELDING
LININGS (a)
Linings for the interior of steel stacks may be required to provide resistance to corrosive gases, vapors, or condensates; to provide resistance to heat; and to maintain stack surface temperatures for the prevent of condensate corrosion.
STRUCTURAL DESIGN DEAD LOAD The dead load shall consist of the weight of steel stack, coatings, internal liner, insulation, and cladding, and all permanent accessories such as ladders, platforms, and gas sampling equipment. The applied weight of the refractory material shall be used to calculate dead load stresses LIVE LOAD
The minimum live load of shall be included for platforms and walkways, an estimate of 2.5kN/m2 is a good estimate. This load need not be considered for wind or earthquake combinations. A horizontal load of 0.5 kN/m acting on the handrail capping piece to the outside or inside must be assumed for measuring the handrails
WIND LOAD
THERMAL LOADS According to the Indian standards Maximum permissible stresses as obtained shall be corrected for the most adverse temperature conditions to which the member or part may reasonably be expected to be exposed by multiplying with the appropriate temperature coefficient Kt given in below the expected temperature of steel components shall not be allowed to exceed 400°C. For temperatures exceeding 400°C the effects of temperature creep should be considered to avoid creep rupture Temperature, °C
0-200
250
300
Kt 1.0 0.75 0.67 NOTE — Intermediate values shall be linearly Interpolated. .
350
400
0.6
0.5
THE FREQUEN NCY CAL LCULATION To avoid ovallity of thee shell t/d mu ust be greater than 0.004wheere t is the thicckness and d the diameter. The natural frequency y, first mode, for f a chimney y of varying diameter or thickness, shall be calc culated by div viding the chim mney into a number of convenient zones z as give en in
Large vortex-induced vibrations perpendicular to the wind direction may occur when the vortex shedding frequency coincides with a natural frequency f of the chimney. This occurs at a mean wind velocity “V” equal to the critical wind velocity “Vcr” determined by V_Vcr_f · d / St
WIND EXCITED OSCILLATIONS Chimneys are subject to oscillation due to wind action. This following explains the a very simple procedures to include the effects of wind excited oscillations as enumerated and suggests alternative procedures for making an appropriate increase in the design wind loading and indicates when strengthening or the incorporation of devices for suppressing von Karman type of oscillations is advisable has been found that chimneys of circular cross section oscillate strongly across wind than along wind. It is, therefore, reasonable to continue with the current practice which implies that along wind.
NUMBER OF STRESS CHANGES The number of stress changes must be determined for the respective critical wind speed in order to verify the fatigue limit. For example, if several critical wind speeds arise in the case of offset stacks, the respective critical loads with the accompanying number of stress changes can be combined into a single group. The number of stress changes is proportional to the service life. The number N of stress changes can be determined according to equation (A.30) for a service life of 50 years, assuming a wind frequency distribution according to Weibull.
EXISTING SUPPRESSION MECHANISMS The dynamic control of stacks is a complex problem. Many methods already exist to suppress vibrations, including helical strakes, shrouds, and variation of structural parameters such as wall thickness and diameter. Figure 1-1 shows various suppression devices which are designed to alter the flow field around the cylinder to prevent periodic vortex shedding.
HELICAL STRAKES. A three-start set of curved-plate helical strakes 120 deg apart on the stack circumference may be attached to the outer surface of the stack with the strake plate approximately perpendicular to the stack surface at all points. The pitch of the helix should be five times the aerodynamic diameter and the strake should project ~o diam. from the aerodynamic diameter. Strakes of adequate structural thickness should be provided on the top ~ of the stack height. Each strake is to be aerodynamically continuous except at specific locations where cuts may be necessary to clear ring stiffeners or other attachments. The maximum gap allowed between the stack shell and helical strake shall be equal to 0.1 x strake width. The presence of strakes significantly increases the drag forces and a drag force coefficient of 1.4 used in conjunction with the outside diameter (including insulation and lagging) of the stack is recommended. Segments of flat vertical strakes at helical locations are not acceptable methods for disrupting vortices.
ALLOWABLE STRESSES An increase in allowable shell stresses due to wind or seismic loads shall not be allowed, the following equation must be satisfied this means the shell must be thin.
ALLOWABLE DEFLECTION The maximum deflection at the top of the steel chimney produced by the wind load without taking into account the dynamic factors, calculated as acting on the circular cross section shall not be greater than h/200. Where ‘h’ is the unsupported height of the chimney, while for the EN 1993-3-2 it provides h/50 which seems too large displacement.
CASE 1 LONGITUDINAL COMPRESSION. The longitudinal compressive stress in cylindrical stacks and liners (P / A) shall not exceed the allowable limit, Sci
CASE 2 LONGITUDINAL COMPRESSION AND BENDING The combined longitudinal compressive and bending stress in cylindrical stacks and liners shall not exceed the allowable stress, Sbl' the details are as shown in the example below
The follo owing table is i a guidancee for the streess accordingg to the IS65533 for fy 2550Mpa can bbe used a qu uick guidancce
CIRCUM MFERENTIAL L COMPRES SSION IN STIFFENERS T .The sizee of stiffeners shall satisffy the follow wing three reqquirements (a) The T stiffener and plate seection shall have h a momeent of inertiaa equal to or greater thann that determined by b the follow wing equation n:
CONSTR RUCTION LO OADS. Considerration shall be b given in th he design for applied connstruction looads in combbination withh wind and d seismic loaads that may reasonably be expectedd to occur duuring construuction
CIRCUM MFERENTIAL STRESS. The circu umferential stress in th he shell due to t external w wind pressure pz betweenn stiffeners spaced att distance, lss, shall be deetermined using
MINIMU UM PLATE THICKNESS T
ACCORDING TO THE ASME
WHILE IN THE INDIAN STANDARDS Thickness of the structural chimney shell in single or multiple shell constructions, shall be the calculated thickness obtained from stress and deflection considerations plus the corrosion allowance, but shall not be less than 6.0 mm nor less than 1/500 of the outside diameter of the chimney at the considered height in my opinion these values seems more practical also consider that to avoid ovality it should be 1/250.
FATIGUE Aerodynamic methods disturb the formation of vortices on the sides of the stack and limit the source of vibration Helical Strakes. A three-start set of curved-plate helical strakes 120 deg apart on the stack circumference may be attached to the outer surface of the stack with the strake plate approximately perpendicular to the stack surface at all points. The pitch of the helix should be five times the aerodynamic diameter and the strake should project ~o diam. from the aerodynamic diameter. Strakes of adequate structural thickness should be provided on the top ~ of the stack height. Each strake is to be aerodynamically continuous except at specific locations where cuts may be necessary to clear ring stiffeners or other attachments. The maximum gap allowed between the stack shell and helical strake shall be equal to 0.1 x strake width.
OPENINGS Openings have to be strengthened to prevent local reduction of Strength Resistance against fatigue and instability The strength of the cross-section with openings is the same as the strength of an undisturbed section if the section modulus is the same. This equality of section moduli is sufficient to fulfill the first condition of strength Across section with an opening is sensitive to the effects of buckling. This is due to the stiffness of the weakened cross-section being reduced by the possibility of the shell bending in or out at the edges of the opening. To prevent this the reinforcement stiffeners
have to be placed normal to the shell {see Figures C5.2 & C5.3) and concentrated along the edge of the opening However, sudden ending of of the reinforcement above and below the opening can cause stress concentrations. These can treble stresses locally and lead to fatigue damage such as local cracks. To avoid this, in the case of openings with width greater than 40% of the chimney diameter locally, the vertical stiffeners should connect at each end with a horizontal stiffener extending around the full circumference (see fig. C5.2).When the width of opening is less than 40% of the chimney’s diameter locally, it is not necessary to provide a horizontal stiffener extending around the full circumference and a more local arrangement may be used (see fig.. C5.3). Vertical reinforcement should be continued above and below the opening to a point where the added stress is unimportant. The code deems that continuing the reinforcement beyond horizontal stiffeners above and below the opening a distance at least 0.5 times the width of the opening will suffice. If the vertical height of the opening is more than twice its horizontal width, a stability check is needed. Guidance on such checks is given in the chapter on bending of plates under lateral loads in “Plates and shells”, by Timoshenko. When the duty of the chimney requires flue gas inlets whose width exceeds two-thirds of the structural shell’s diameter, a possible solution would be to provide a large number of small circular openings, giving a total area equivalent to that required. Reinforcement could then be threaded between the small holes and around the whole group, as require
COMMON PROBLEMS a) Atmospheric corrosion and weathering on exterior surface (b) Corrosion due to acid condensation in flue gases on internal surfaces (c) Fly ash or particulate collection at the base, false bottom, or roof cap of the stack (d) Moisture condensate at the base of the stack (e) Acid/moisture infiltration of insulation (f) Deformation due to thermal or other loading (g) Corrosion of anchor bolts (h) Fatigue cracks (i) Loss or deterioration of insulation, coating, or linings (j) Loosening of anchor bolts.
INSPECTION For early detection of the commonly occurring problems, it is recommended that the stack be inspected periodically to enable the user of the stack to take appropriate measures to counteract such problems.
FOUNDATION DESIGN The foundation for stacks shall be designed for all cases of loading, any foundation movement or rotation will cause partial or total collapse so foundation must be carefully designed, the foundation must be of size and shape that the load on the soil below will not exceed the maximum load which it will fully support also no allowance for tension by any means below foundation since earth have no strength whatever in tension. The connection of the shell to the concrete foundation or to the supporting structure should resist the overturning moment, normal force and shear force developed at the shell base and transmitted to the foundation
BASE PLATE DESIGN Either a full raft or an annular raft can be provided. The latter .has the , advantage that because of a higher uniform soil pressure under dead loads, it minimizes possible gradual tilting of a foundation laid on cohesive soil when the structure is subjected to lateral loads from a predominant wind direction
REFERENCES
ASME STS COMMITTEE “Steel Stacks - Asme-Sts-1-2006” “Foundation design hand book” Hydrocarbon processing. Gulf publishing company 1968 “German standards DIN4133” “BSI (2011) BS EN 13084-1:2007” - Free-standing industrial chimneys - Part 1: General requirements.
