Steel chimneys are structures with tall, slender and tapering with circular cross sections. They are ideally suited for process work where a low thermal capacity required and short heat-up period. This article presents different types of steel chimneys are their designs.
- Types of Steel Chimney Structure
- Design of Steel Chimney Structure
- Forces Acting on Steel Chimney
- Bending Moment
- Permissible Stresses
Types of Steel Chimney Structure
Types of steel chimney structure fallen under two main category which include:
- Self-supporting steel chimneys
- Guyed steel chimneys.
1. 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.
The self-supporting chimneys are made up to 10 m diameter and from 50 m to 100m in height.
2. 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 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.
Design of Steel Chimney Structure
Selection of Steel Chimney
Factors considered for selection of steel chimney type are as follows:
- Cost effectiveness
- Number of units, equipment type, and fuel type to be used.
- If chimney used for boilers, consider surface area, output efficiency, draft requirements.
- Equipment operation mode
- flue gas temperature before entering the chimney and its variation.
- Specific weight, dust quantity, and data about flue gas aggressiveness.
- local statutory regulations relating to height, ash dispersion, provision for earthing aviation warning lamp, and health.
- Erection mode
- Select lining type based on composition of the flue gases.
Steel Plates for Chimney
- Required width of steel plates ranges between 0.9m-2.5m (1.50m most common).
- Minimum steel plates thickness is 6 mm.
- Thickness of steel plates in the two upper sections of the chimney is at least 8 mm to resist more corrosion 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.
- Available steel plates thickness are 5, 6, 8, 10, 12, 14, 16, 18, 20, 22, 25, 28, 32, 36, 40, 45, 50, 56 and 63 mm.
- Breech opening is also known as flue opening.
- Provided for the entrance of flue gases. These gases come from furnaces of the boilers.
- 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 is two-thirds of the diameter.
- Reinforcement should be provided all around the breech opening to compensate for removed material.
- 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.
- reinforcing material should be extended above and below the opening in order to transfer and distribute the stress into the steel of the chimney.
- 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. 3.
- The width of opening does not exceed one third of the diameter of the chimney at that plane.
Base Plate of Steel Chimney
structural steel, cast iron, or cast steel can be used as base plate for steel chimney. It is desired to employ structural steel base plate compared with other types.
The width of steel plate should be adequate to transmit the compressive stresses to the foundation.
Thickness of base plate should be such that the bending stress and shearing stress in the base plate do not exceed the allowable bending and shearing stresses.
Forces Acting on Steel Chimney
1. Self-weight of Steel Chimney
The self-weight of steel chimney, Ws acts vertically and expressed as:
d: chimney diameter, m
t: steel plate thickness, m
h: Height of steel chimney above the section XX in meters
79 kN/m2 is Unit weight of steel
The compressive stress in the steel plates at the section due to the self weight of chimney is equal to the 0.079 tome chimney section height.
2. Weight of 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.
(20 kN/m3 ) is Unit weight of brick lining in equation 2.
3. Wind Pressure
The wind pressure acts horizontally and depends on the shape, width, height, 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 up to 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.
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
d’: Outer diameter of the chimney
4. Seismic Forces
The seismic forces act horizontally on a structure. The following load combinations for calculations of stress at any point of steel chimney are considered:
- Dead load + Wind load+ Temperature effect
- 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.
Bending Moment 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 of steel chimney, the wind force is assumed to act at the middle height above the section.
The bending moment due to wind at a section, h metres below the top:
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 expressed as:
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.
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 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.