TYPES AND DESIGN OF STEEL CHIMNEY
The design of steel chimney can be done as two types:
- Self-supporting steel chimneys
- Guyed steel chimneys.
Self-supporting steel chimneys: When the lateral forces (wind or seismic forces) are transmitted to the foundation by the cantilever action of the chimney, then the chimney is known as self-supporting chimney. The self-supporting chimney together with the foundation remains stable under all working conditions without any additional support. A self-supporting chimney is shown in Fig. The self-supporting chimneys are made upto 10 m diameter and from 50 m to 100m in height.
Guyed steel chimneys: In high steel chimneys, the mild steel wire ropes or guys are attached to transmit the lateral forces. Such steel chimneys are known as guyed steel chimneys. In guyed steel chimneys, all the externally applied loads (wind, seismic force, etc.) are not totally carried by the chimney shell. These attached guys or stays do share these applied loads. These guys or stays ensure the stability of the guyed steel chimney. These steel chimneys may be provided with one, two or three sets of guys. In each set of guys, three or four or sometimes six wires are attached to the collars. When one set of guy is used, then the guys are attached to a collar at one-third or one-fourth of the height from the top. When- more than one set of guys are used, then these are used at various heights.
A particular type of steel chimney is selected depending on the advantage and disadvantages with reference to economy. A choice between self-supporting and guyed steel chimney is made by considering some of the important factors.
Number of units, type of equipment and the type of fuel to be used are considered. In case the chimney is to be used for boilers, the surface area, output efficiency, draft requirements etc. are taken into account. The mode of operation of the equipment shall also be considered.
The temperature of the flue gases before entering the chimney and its likely variation, are studied. The type of lining is decided knowing the composition of the flue gases. The specific weight, the quantity of dust and data about the aggressiveness of the flue gases must be known. The local statutory regulations, relating to height, dispersion of ash, provision for earthing aviation warning lamp, health etc. are the factors which should be considered for selecting a type of steel chimney. The mode of erection is also considered.
STEEL -PLATES FOR CHIMNEY
The width of steel plates required for the steel chimney varies from 0.9 m to 2.5 m. The steel plates of 1.50m width are most commonly used. The thickness of steel plates should not be less than 6 mm. The thickness of steel plates in the two upper sections of the chimney should not be less than 8 mm to resist more corrosion likely at the top of chimney. The thickness of steel plate in the flared portion should not be less than the thickness at the lowest section of the cylindrical portion. The steel plates are available in thickness of 5, 6, 8, 10, 12, 14, 16, 18, 20, 22, 25, 28, 32, 36, 40, 45, 50, 56 and 63 mm.
For the ease in construction, the upper diameter of plates forming the side of chimney is kept less than the lower diameter. Each course fits telescopic over the lower course.
Fig 1 Self supporting chimney
The breech opening is also known as flue opening, The flue opening is provided for the entrance of flue gases. The flue gases come from furnaces of the boilers.
A breech opening is provided in the steel chimney as shown in Fig. The area of breech opening is kept about 20 percent larger than the internal cross-sectional area of the chimney. The maximum width of the breech opening should not be greater than two-thirds of the diameter. In, order to compensate the removed material. The reinforcement should be provided all around the breech opening. The vertical reinforcement provided should be 20 percent larger than the material removed in the ratio of diameter to the long chord perpendicular to the face of the opening. The horizontal reinforcement provided at the top and bottom of the opening is kept equal to the vertical reinforcement. The reinforcing material provides sufficient vertical stiffness. In order to transfer distribute the stress into the steel of the chimney, the reinforcing material should be extended above and below the opening. In the self-supporting steel chimney the breech opening is kept well above the flared base, so that it does not extend into the flared base.
The steel chimneys may have one breech opening, two breech openings in the same direction two breech openings at right angles and three breech openings as shown in Fig. The number of flue-openings maybe one, two, three or four depending on the requirement. It is suggested that a maximum of two flue-openings may be provided at one level so that the chimney remains enough to resist the applied forces at the plane of the openings. However, it is possible to provide three openings in one plane. This is done only when the number of flue openings is three only. The width of opening does not exceed one third of the diameter of the chimney at that plane.
A clear out door as shown by dotted lines in Fig. 1 is provided preferably on the opposite side of the breech opening near the base. The minimum size of Cleanout door shall be 500 mm x 800 mm clear. The cleanout doors are also properly reinforced.
FORCES ACTING ON STEEL CHIMNEY
The various forces acting on the self-supporting steel chimney are as follows:
- Self-weight of the steel chimney
- Weight of lining
- Wind pressure
- Seismic forces,
1. Self-weight of the chimney.
The self-weight of steel chimney, Ws acts vertically.
Consider a horizontal section XX as shown in Fig1. The thickness of steel plates of chimney above the section XX, may be assumed constant. The self-weight of chimney is given by
Where ?= Unit weight of steel = 79 kN/m3
d = Diameter of chimney in meters
t = Thickness of steel plates in meters
h = Height of steel chimney above the section XX in meters
The compressive stress in the steel plates at the section XX due to the self weight of chimney is, given by
2. Weight lining.
The weight of the lining in the steel chimney WL, also acts vertically. The thickness of brick lining may be assumed as 100 mm.
The weight of brick lining,
?1=Unit weight of brick lining = 20 kN/m3
The compressive stress in the steel plates at the section XX due to the weight of lining
3. Wind pressure.
The wind pressure acts horizontally. The wind pressure acting on a structure depends on the shape of the structure, the width of the structure, the height of the structure, the location of the structure, and the climatic condition. The wind pressure per unit area increases with the height of the structure above the ground level. In order to simplify the· design, the steel chimney is divided into number of segments of equal height. Each segment may be kept equal upto 10 m. The intensity of wind pressure in throughout the area of each segment may be assumed as uniform. The intensity of wind pressure corresponding to the mid-height of each segment may be noted from IS: 875-1984. The wind pressure on the flared portion may be found by using average diameter. The wind pressure is assumed to act at the mid-height of each segment and as also in the flared portion. It has also been practice to take uniform wind pressure over the full height of chimney. The wind pressure
P = k*Pl*(d’*h1) (projected area of chimney)
k = Shape factor. It accounts for the shape of the structure; the shape factor for cylindrical portion is 0.7.
PI = Intensity of wind pressure
P =0.7(PI* d’* h1)kN
where, d’ = Outer diameter of the chimney.
In addition to the overturning effect due to wind pressure, the wind has also aerodynamic effect. The aerodynamic effect of wind has, not been taken into consideration for the design of steel chimney.
4. Seismic forces
The seismic forces also act horizontally. The seismic forces act on a structure, when the structures are located in the seismic areas.
The following load combinations for calculations of stress at any point of steel chimney are considered :
(i) Dead load + Wind load+ Temperature effect
(ii) Dead load + Earthquake (seismic) load + Temperature effect
The worst combination out of the effect due to seismic (earthquake) forces and wind effect is only taken into consideration. Only one effect is considered for the design of the structure out of these two forces.
ON SELF-SUPPORTING STEEL CHIMNEY
The wind force acts as uniformly distributed load on the self-supporting steel chimney. For the purpose of determining bending moment at any section XX Fig. 1, the wind force is assumed to act at the middle height above the section. The bending moment due to wind at section XX, h metres below the top,
Mw = (p x h/2) where P = Total wind force
Stress on the extreme fibre of steel chimney due to wind is equal.
BENDING STRESS ON STEEL CHIMNEY DUE TO WIND
The bending stress, fw at the extreme fibre of steel chimney due to overturning moment Mw is
IS: 6533-1971 ‘Code of Practice for ‘Design and Construction of Steel Chimney’ recommends the value of section modulus of steel chimney ring with no breech opening as below.
Z =0.77 d2*t
The windward side of steel chimney is subjected to tensile stress due to the combined effect of the wind and weight of steel chimney. The leeward side of steel chimney is subjected to compressive stress due to the combined effect of the wind, weight of steel chimney and the weight of lining. On the compressive side the efficiency of the joint depends on the strength of rivet in shear and in bearing and does not depend on the tensile strength of plate. The efficiency of joint on compression side is 100 percent. The efficiency of joint on the tension side is 70 percent.
In order to prevent the flattening of the steel plates on the tension or windward side, and buckling of the steel plates on compression or leeward side, the permissible stress in compression on gross-sectional area is adopted less than the permissible stress in tension on the net sectional area. The permissible stresses in steel chimney in axial tension, shear and bearing shall be adopted as specified in IS: 800-1984. The allowable stresses in axial compression and in bending from the table of IS 6533.