Mechanical Design and Analysis of Steel Stack by Varying its Height with Constant Diameter

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 04 Issue: 09 | Sep -2017 www.irjet.net p-ISSN: 2395-0072
© 2017, IRJET | Impact Factor value: 5.181 | ISO 9001:2008 Certified Journal | Page 239
MECHANICAL DESIGN AND ANALYSIS OF STEEL STACK BY VARYING ITS
HEIGHT WITH CONSTANT DIAMETER
Kalagouda R Patil1, Dr. B S Manjunath2
1Department of Mechanical Engineering (M.Tech in Design Engineering)
KLE Dr. M S Sheshgiri College of Engineering and Technology Udyambag, Belagavi, Karnataka, India 590008
2Department of Mechanical Engineering (Professor)
KLE Dr. M S Sheshgiri College of Engineering and Technology Udyambag, Belagavi, Karnataka, India 590008
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Abstract - Chimneys are tall and slender structures which
are used to discharge waste/flue gases at higher elevation
with sufficient exit velocity such that the gases and suspended
solids(ash) are dispersed in to the atmosphere over a defined
spread such that their concentration , on reaching the ground
is within acceptable limits specified by pollution control
regulatory authorities. This project summarizes the analysis
and design concepts of chimneysasperIndiancodesprovisions
incorporation was also made through finite element analysis.
Chimney models are designed on the basis of constant
diameter with change in height taken into consideration.
These models are analyzed by finite element software STAAD
Pro, ANSYS, emphasis also placed on effect of geometric
limitations on the design aspects in designing chimney. The
main objective of this study the design and constructional
aspects of steel stack (with particular reference tosteelplant)
adhering to the guidelines given in internationally accepted
standards/codes. Therefore, objective of this study was to take
a practical case study and carry out design calculations by
using the rules of codes viz., IS: 6533 part 1 and 2, IS: 875
part-3, IS: 1893 part1 and 4. Further to get full insight
into the design of the steel stacks a complete 3-D finite
element analysis was carried out by using ANSYS software.
Results have been summarized and conclusions have been
drawn.
Key Words: Finite Element SoftwareSTAADPro,ANSYS,
IS: 6533 part-1 and part-2, IS: 875 part-3, IS: 1893
part-1 and part-4.
1. INTRODUCTION
Stacks as we know them today are tall slender structures
which fulfill an important function. They had a humble
beginning as household vents and over the years, as vent
grew larger and taller they came to be known as
chimneys/stacks. Steel stacks are ideally suited for process
work where a short heat up period and low thermal
capacity are required where as it encourages acid
condensation and corrosion hence smutting and reductionin
the life of stack. A chimney which was scientifically designed
to take cognizance of gas temperature and velocity, corrosion
aspects etc was called a stack. By usage the term stack has
gained popularity and today it also signifies a chimney.
2. STATEMENT OF PROBLEM, OBJECTIVES AND
CONSTRUCTION MATERIALS
Design analysis of 1.9m ID and varying heights of 30, 40, 50
and 60m height steel stacks to be used in “Salem” steel plant.
Table -2.1: Input data for steel stack design from Salem
steel plant
Location
Salem Industrial Area
(Tamilnadu)
Stack size 1900mm ID, 30m Height
Type Self Supporting
Basic wind speed 47m/s
Terrain category
3 (Industrial suburb with avg.
height above 10m)
Average Height of
surrounding buildings
10m
Suburb distance 5 Km from city edge
Gas handled Flue gas
Gas flow rate 153000 m³/hr @ 250 °C
Gas Temperature 350 °C (max)
Ambient Temperature 41.6 °C (max)
Material Carbon steel(IS: 2062 Gr. B)
Corrosion allowance 2mm
2.1: OBJECTIVES
To study the design and constructional aspects of steel stack
(with particular reference to steel plant) adhering to the
guidelines given in internationallyacceptedstandards/codes.
Therefore, objective of this study was to take a practical case
study and carry out design calculations by using the rules of
codes viz., IS:6533 part 1 and 2, IS:875 part-3, IS:1893 part1
and 4. Further to get full insight into the design of the
steel stacks a complete 3-D finite element analysis was
carried out by using ANSYS software. Results have been
summarized and conclusions have been drawn.

2.2: CONSTRUCTION MATERIALS
1. Stack shell – Carbon steel –IS: 2062 Gr. B (weldable grade)
2. Bolting material – 8.8 Gr. (IS: 1363/1364)
3. Platforms and ladders – St –42 (structural steel IS: 2062)
3. ASSUMPTIONS MADE DURING DESIGN
Following are the assumptions made during the design of
steel stack.
1) The wind pressure varies with the height. It is zero at
the ground and increase as the height increases. For the
purpose of design it is assumed the wind pressure is
uniform throughout the height of the structure.
2) For the purpose of calculations, it is assumed that the
static wind load (projected area multiplied by wind
pressure) is acting at the center of pressure.
3) In calculating the allowable stresses both tensile and
bending, the Joint efficiency for butt welds is assumed to
be 0.85.
4) The base of the stack is perfectly rigid and the effect of
gussets and stool plate on the deflection and the stresses
in the stack is not considered. This is applicable only for
manual calculations.
5) There are no additional lateral movements from the
duct transferred to the stack and Suitable arrangement
has to be provided to absorb this movement from the
duct.
6) Since the D/t ratio (mean diameter of stack to
thickness of plate) is greater than 10, the structure is a
thin cylinder and shell (plate) element can be used for
finite element modeling.
7) Earthquake causes impulsive ground motions, which
are complex and irregular in character, changing in
period and amplitude each lasting for a small duration.
Therefore, resonance of the type as visualized under
steady state sinusoidal excitations will not occur, as it
would need time to build up such amplitudes.
8) Earthquake is not likely to occur simultaneously with
maximum wind or maximum flood or maximum sea
waves.
4. METHODOLOGY
1) The stack is a cantilever system subjected to wind and
earthquake loads. In order to determine the wind force
acting on the stack, the stack is divided into number
of zones. The zoning of the stack depends on the
maximum plate width available fora particular thickness
and capacity of the lifting equipment at the time of
erecting of the stack. Normally the height of each zone is
between 2.5m to 5m and typically 4m if the stack is to be
welded at the site of location. In the current design
calculations, the height of zone has been fixed at5mafter
consultations with the client as well as fabricator (Refer
layout drawing A-1 of Annex F).
2) The bending moment at the end of each zone is
determined to find the required section modulus and
hence the thickness.
3) The deflection at the top of the stack is calculated and
checked against the allowable limits. In case the
deflection at the top is more, and then the section
modulus at the base of the stack is increased till the
deflection is within limits. If, however,itisnotpossible to
contain the deflection in spite of increasing the section
modulus, the stack is guided at 2 or more locations.
4) The natural frequency is determined to check if the
dynamic effect of the wind needs to be taken care during
the design stage.
5) The check for resonance is carried out.
5. DESIGN MANUAL CALCULATIONS
5.1: DESIGN PARAMETRS
1. Deflection in mm
2. Compressive Stress in MPa
3. Natural Frequency in Hz
 Deflection is given by the equation:
……………………………5.1
Where,
X=Deflection in mm.
W=Total force acting on each zone.
L=Length of stack in mm.
a=Distance from base to the point of force acting.
E=Young’s modulus= 2.1×105MPa.
I=Moment of inertia in mm4.
……………………………………5.2
Deflection is calculated by
using equation 5.1 for each zone and adding all X=187.49mm
In practice since the stack is tapered and thickness also
varies along the height, taking the coefficient for the
variation from Roark’s formula for stress-strain,
X=187.49×0.4248
X=79.64mm
 Natural frequency is given by the equation:
……………………………..5.3
Where,
f=frequency in Hz.
m= mass of the zone including the lining or covering in kg.
x=Deflection of same zone due the force equal to gravity
acting on its mass centre with the base fixed and top free, in

meters.
g=Rate of gravitational acceleration=9.81m/s².
Table-5.1: Showing the weight of each zone
Table-5.2: Showing x and mx2 to calculate frequency
f=3.09Hz
 Compressive stress is given by equation:
The stress at the base of the stack is determined by;
……………………………………………………………5.4
Where,
σ= compressive stress in MPa.
M= Bending moment in N-mm
Z= Section modulus in mm3.
Compressive stress=50.35MPa
6. VALIDATIONSOF MANUAL RESULTS BY FEA
USING ANSYS
6.1: ANALYSIS PROCEDURE
The manual calculations are compared with the analysis
results to validate the assumptions made and later the
complete model with all the main features like, stool
plate, base plate, gussets, platform, breach opening and
manhole opening are made to simulate the actual
construction. The platform supporting structure is made
out of beam element (Beam – 188). The stack is fixed at
anchored bolt points. Now in this case the base of stack is
fixed rigidly with all DOF fixed. The figure 7.2 shows the
meshed model and boundary conditions. The steps involved
in analysis are as follows
1) Create layout drawings using AutoCAD.
2) Create actual model in Catia/ Ansys.
3) Mesh the modal, specify the material properties,
and apply loads and boundary conditions.
4) Extract the required results, Deflection, stresses,
Natural frequency etc
6.2: LOADING
Following loading where considered
Total wind load = 228989N
Total weight = 11332.9N
Imposed load on platforms = 2.943x10-3N/mm2
6.3: BONDARY CONDITIONS
The base of stack was fixed with all degrees of freedom and
resultant force as calculated earlier was applied atthe center
of each zone. For further analysis shell is welded to base
plate of given thickness, anchor bolts of calculated size are
used to fix at the base. The same effect was transmitted to
FEM model, by fixing at the determined bolt locations
with all degree of freedom fixed.
FIG: 6.1 Shows the boundary conditions for the analysis of
the stack with base fixed rigidly with all DOF fixed.
zone
Thickness
t
in mm
OD
in
mm
ID
in
mm
Area
in
mm²
Volume
(v)
in mm³
Mass
ρ×v
in Kg
Weight
in N
1 25 1950 1900 151189 755945732 5941 58288.4
2 20 1940 1900 120637 603185750 4741 46509.5
3 16 1032 1900 96308 481543321 3785 37130
4 12 1924 1900 72080 360403509 2832 27789
5 10 1920 1900 60004 300022098 2358 23134
6 8 1916 1900 47953 239766351 1884 18482
Total 21453 211332
Zone
Mass
in kg
Deflection
in mm (x)
x2 mx mx2
1 5941 3.84 14.7 22816.2 87614
2 4741 11.50 132.3 54521.8 62000121
3 3784 19.16 367.1 72519.2 1389468
4 2832 26.83 719.8 76003.2 2039166
5 2358 34.50 1190.2 81356.8 28066811
6 1884 42.16 1777.4 79453.4 3349740
Total 386670 10299803

6.4: ANALYSIS RESULTS
The results from ANSYS are shown below and comparison
with the manual calculations is summarized in table below.
1. Deflection in mm at the top of stack
FIG: 6.2 Deflection at the top of the stack for analysis with
base fixed rigidly.
2. Compressive stress in MPa at the base of stack
FIG: 6.3 Compressive stress at the base of the stack for
analysis with base fixed rigidly.
3. Von misses stress in MPa at the base of the stack
FIG: 6.4 Von misses stress at the base of the stack for
analysis with base fixed rigidly.
6.5: RESULIT SUMMERY
Table-6.1: Tabulation of results to compare manual
results with FEA results.
Parameters
Manual
calculation
Analyzing
using
ANSYS
Percentage
of variation
w.r.t to
ANSYS
Deflection in
mm
79.69 73.18 8.16%
Compressive
stress in
MPa
52.63 50.35 4.33%
Natural
frequency in
Hz
3.06 3.18 -4%
Note: The manual calculations assume that the base of
stack is rigid i.e. the elastic rigidity of bolts and
foundation is neglected. The same has been simulated in
Ansys by fixing all DOF at the base of the stack.
CONCLUSION:
From the above table it is clear that the manual calculations
are within the range of ±10% hence we can conclude that
the mathematical model and assumptions made during the
design process are correct.

REFRENCES:
[1] IS: 875 (Part 3–1987), Indian standard, Code of
practice for design of loads (other than earthquakes)
for buildings and structures, part 3(wind loads).Bureau
of India Standards, New Delhi.
[2] IS: 6533 (part1): 1989 I nd ian standard, Design and
construction steel chimneys - code of practice
(Mechanical Aspects), Bureau of Indian standards, New
Delhi.
[3] IS: 6533 (PART-2):1989 Indian standard, Code of
practice of design and construction steel chimneys
(structural Aspect), Bureau of Indian standards, New
Delhi.
[4] IS: 1893 (part4–2005), Criteria for earthquake
resistant design of structures including stack- like
structures (Earthquake resistant) , Bureau of Indian
standards, New Delhi.
[5] Ramchandra and Virendra G. Design of steel
structures-2, Standard book house Delhi, 7
th
Edition
1991.
[6] Warren C Young, Roarks Formulas for stress and
strain, McGraw-Hill Book Company.

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