DESIGN OF SELF SUPPORTED STEEL CHIMNEY

DESIGN OF STEEL CHIMNEY

DESIGN OF STEEL CHIMNEY Chimney- Those are structures made of steel plates bent into cylindrical shape and are meant to escape and disperse the flue gasses to such a height that gasses do not contaminate surrounding atmosphere in content with inhabitant. Steel chimneys are also known as steel stacks. Height of chimneys are kept to provide sufficient draft for escaping flue gasses. DRAFT-It is defined as the difference between absolute gas pressure at any point in the doct steel chimney and ambient atmospheric pressure. At base Steel chimneys are made in to cylindrical shape lower portion widened or flared in order to provide a large base and greater stability. Widened section of chimney reduces the unit stress at base.

Base are two types (a) Conical base -It is used widely (b) Bell mount type-Fabrication cost is high and have no advantages over conical base..

*Recommended height of flared portion =1/3 x height of chimney. *Bottom diameter = 5/4diameter of shaft

TYPES OF STEEL CHIMNEY 1-Self supported chimney 2-Guyed chimney

1-Self supported chimneyWhen lateral forces due to wind or earthquake are transmitted to foundation by cantilever action of chimney, it is called self supported chimney .Chimney together with foundation remains stable under all working conditions without any additional support. Those are made up to 10 m diameter and height between 50 to 100 m.

2-Guyed chimney If mild steel wire ropes or guys are attached at different heights to transmit the lateral forces , such chimneys are called Guyed chimney. In this case all externally applied forces due to wind or earthquake are carried by both chimney shell and guys. Choice depends upon Nos of units, types of equipments and fuel.

Before designing following point must be noted

1-Temperature of flue gasses should be known. 2- Types of lining is decided knowing the composition of flue gas. 3-Specific weight, dust quantity, aggressiveness of gas should be known. 4-Local statutory regulations regarding Height, dispersion of ash ,provision of, aviation warning lamp health, and made of err action etc

DESIGN OF SELF SUPPORTING CHIMNEY 1 Steel plate- Thickness of steel plates to be used is obtained from stress calculation and deflection consideration. It shall be increased by permitting allowing corrosion during whole design life and aggressiveness of flue gas. 2 -Width of plates- Width of plates to be used varies from 0.9 m to 2.5 m and 1.5 m is common. 3-Thickness-(a) Thickness should not be less than 6mm even when copper bearing steel is used. (b) In two upper sections thickness should not be less than 8 mm (c) In flared portion thickness should not be less than thickness at lowest portion of cylindrical part (d) Available thickness of plates are 5 ,6, 8. 10, 12, 14, 16, 18, 20, 22, 25, 28, 32, 36, 40 ,45, 50, 56, and 63 mm 4- Painting-Heat resisting paints like Graphite paint or Carbon paint are painted to both inside and out side of chimney.

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5)Corrosion Allowance Non copper bearing steel. 10 years design life- 2 mm------------Light corrosion 3 mm-------------Normal corrosion 5 mm-------------Exceptional corrosion 20 years design life--- 4 mm------------Light corrosion 5 mm-------------Normal corrosion 8mm-------------Exceptional corrosion (6) Riveted connection(a) Upper part Lap joint both I n horizontal and vertical joint (b) cylindrical and flared part---- do( c) Lower part- --------- Butt joint in sections with heavy plates both in horizontal and vertical joints Thickness of plate in mm Diameter of rivet in mm 6 16 8-10 12-16 16 18 22 24 Types of joint Single riveted lap joint Double riveted lap joint Triple riveted lap joint Butt joint

(7) Pitch (a) Not greater than 10 x Thickness of plate (b) Not less than 2.5 x nominal diameter of rivet (8) Rivet Holes(a) Thickness of plate t less than 16 mm-----by punching (b) Thickness of plate is 18 to 22 mm-----Sub punched to dia. 5mm less than nominal dia then drilled to gross dia.

( c) Thickness greater than 22 mm------ Drilled

LINING OF CHIMNEY Usually chimneys are lined to 1-Protect the chimneys from heat 2-Act as a protective cover against corrosion. 3-Maintain the temperature of flue gas . MATERIALS OF LINING 1 -Fire bricks ,2-Common bricks ,3-Solid grade diatomaceous brick, 4-Acid resistance brick ,5-Moler concrete ,6-Refractory concrete ,7-Gunitting (cement and sand admixture), 8-Vitrobestos (capable of withstanding temperature up to 2000o F.

DEPTH OF LINING 1-Starts below the breach opening to a height where gas does not damage the chimney shell. 2-Height of lining should not be less than 10 x diameter of chimney or1/3 height of chimney above breach opening. 3-Self supporting chimneys are lined fully. 4- When independent lining is provided thickness should be 200mm for lower half 100 mm for upper half of chimney. LINING SUPPORT Linings are supported over steel angles placed at 2 to 5m height riveted to inside of chimney. 1-Up to 50 m height 125mm thick lining (100mm +25 mm thick backing insulator) are provided with angle 100 x 75 x10 or 12 , 100 mm leg being horizontal. 2-When 175 mm thick lining is to be provided (150+25mm backing) the angle 150 x75 x10 or 12 is used with 150 mm leg horizontal.

CAPPING Capping is provided at top to protect the upper part of chimney bas well as lining. Capping consists of cast iron plate bolted with non corrosive bolts. BREACH OPENING OR FLUE OPENING Those are openings provided for entrance of flue gas. 1-Area of breach opening is kept about 20% larger than cross sectional area of chimney 2- Maximum width should not be greater than 2/3 x diameter of chimney. 3-Reinforcement should be provided all around the opening to compensate the material removed. REINFORCEMENT Reinforcement should be provided all around the breach opening to compensate the material removed 4-Vertical reinforcement 20% metal removed x d/l where l=long chord of opening perpendicular to opening 5- Vertical reinforcement =Horizontal reinforcement both at top and bottom. 6- At one level maximum two openings can be provided. 7-Width 1/3 xd d=Diameter

CLEAN OUT DOOR A door is provided at bottom of size 500 mm x 800 mm in order to remove ash and for inspection. LADDER Provided for painting and go up for inspection. LIGHTENING CONDUCTOR Provided to protect the chimney from lightening.