Steel Stacks Desing Guide

S

TEEL STACKS DESIGN GUIDE

This guide is prepared by myself Khaled Sayed on November2015 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}

I

NTRODUCTION

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.

S

TACKS

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.

T

YPES OF

S

UPPORTS

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.

S

TACKS

S

UPPORTED BY

O

THER

S

TRUCTURES

.

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.

A

DVANTAGES OF

V

ERTICALLY

S

UPPORTED AND

B

RACED

S

TACKS

.

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.

D

IMENSION 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

T

HERM

Different include (a) Betw (b) At tes (c) At roo (d) At sta (e) Betwe (f) At we

M

ATE

Corrosion 1) Carbo (2) High-Specifica (3) Stainl (4) Stainl clad steel

C

ORROS

G

UY

W

(a) Guy w considera (1) Alum (2) Zinc-Specifica (3) Zinc-(4) Stainl AS1M A

MAL EXP

tial expansio een external st platform, c of flashing a ack tops and een stack sh eld joints bet

ERIAL CO

n allowance n steels conf -strength, low ations. less steels co less chromiu l conforming SION ALLOW

W

IRES

,

C

wires and ca ation should minum-coated -coated (galv ations -coated (galv less steel wi A 368 Specifi

PANSION

on between c l and interna catwalk, and and counter f d truncated co

ells and exte tween dissim

ONSIDER

s shall be co forming to th w steels con onforming to um-nickel ste g to ASTM A WANCE

C

ABLES

ables typicall d be given to d steel wire vanized) stee vanized) stee re strand con ication.

N

components al shells of a d ladder attac flashing one ernal insulati milar metals

RATION

onsidered (ty he AS1M A nforming to t o the AS1M

eel clad plat A 265 may b

,

OR

F

IT

ly may be of the initial st strand confo el wire strand el wire rope nforming to of a stack sh dual wall or chment brac ion ypically to .) 36,A 283, o the AS1M A A 666 Spec te conformin be considere

TINGS

f one or more tretch of the orming to the d conformin conforming the hould be care r multi-flue s ckets

) for all type or A 529 Spe A 242, A 572 cification. ng to AS1M ed for use as e of the follo material: e ASTM A 4 ng to the AS1 to the ASTM efully studie stack es of steel. B ecifications. 2 alloy, or A A 264 and n shell plate. owing mater 474 Specific 1M A 475 an M A 603 Spe ed in areas to

Base plate sha 588 nickel-base a rials, and cation nd A 586 ecification o all be alloy

A

NCHOR

B

OLTS

,

W

ASHERS

,

AND

N

UTS

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

B

OLTS

,

W

ASHERS

,

AND

N

UTS

(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.

W

ELDING

L

ININGS

(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.

S

TRUCTURAL DESIGN

D

EAD 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

T

HERMAL 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

0-200 250 300 350 400

K t 1.0 0.75 0.67 0.6 0.5 NOTE — Intermediate values shall be linearly Interpolated.

T

HE

F

To avoid 0.004whe frequency thickness number o

F

REQUEN

ovallity of the ere t is the thic y, first mode, f , shall be calc f convenient z

NCY CAL

e shell t/d mu ckness and d for a chimney culated by div zones as give

LCULATI

ust be greater the diameter. y of varying d viding the chim

en in

ON

than The natural iameter or mney into a

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

WINDEXCITEDOSCILLATIONS

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

N

UMBER OF

S

TRESS

C

HANGES

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.

E

XISTING

S

UPPRESSION

M

ECHANISMS

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.

A

LLOWABLE

S

TRESSES

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.

A

LLOWABLE 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.

C

ASE

1

LONGITUDINAL

C

OMPRESSION

.

The longitudinal compressive stress in cylindrical stacks and liners (P / A) shall not exceed the allowable limit, Sci

C

ASE

2

LONGITUDINAL

C

OMPRESSION AND

B

ENDING

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 used a qu

C

IRCUM .The size (a) T d

C

ONSTR Consider wind and CIRCUM The circu spaced at

M

INIMU owing table i uick guidanc MFERENTIAL e of stiffener The stiffener etermined b RUCTION LO ration shall b d seismic loa MFERENTIAL umferential t distance, ls UM PLATE T is a guidance ce L

C

OMPRES s shall satisf and plate se by the follow OADS. be given in th ads that may

STRESS.

stress in th s, shall be de

THICKNESS

e for the stre

SSION

I

N

S

T fy the follow ection shall h wing equation he design fo reasonably he shell due t etermined us ess according TIFFENERS

wing three req have a mome n: r applied con be expected to external w ing g to the IS65 quirements ent of inertia nstruction lo d to occur du wind pressur 533 for fy 25 a equal to or oads in comb uring constru e pz between 50Mpa can b greater than bination with uction n stiffeners be n that h

A

CCORDING TO THE

A

SME

W

HILE IN THE

I

NDIAN

S

TANDARDS

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.

F

ATIGUE

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.

O

PENINGS

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

C

OMMON 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.

I

NSPECTION

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

B

ASE 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.