Introduction:
Heavy duty stacks are smoke or vent stack more than 75 feet high, and in which the cross sectional area of the flue more than 500 square inches. The guidance provided by UL for protecting the Heavy-Duty Stacks in the application guide is as follows.
Challenges in Protection:
- Since stacks are very high structures, they are more prone to lightning strikes. The chances of side flashes to the stacks are also high. Hence care should be taken on designing the proper lightning protection system and installation should be made as per the design to provide protection to the structure and the people within the zone.
- In addition to the height of the stacks, the one more important factor that has to be considered while designing the protection sys em of stack is material selection. Since the stack acts as vent for flue gases, the materials used on the top of stack (air-terminal) are more prone to corrosion. Hence the material should possess good corrosion resistance property.
Corrosion Zone:
The top 25 feet of a stack is generally the high corrosion zone and hence a minimum of 1/16 inch coating of lead should be provided for terminals, mounting brackets and conductors. The components used in the upper 25 feet shall be made of materials like copper, copper alloy. bronze or stainless steel. Aluminium components are prohibited in this installation.
Designing:
All components shall be designed by considering Class Il protection. The materials of air terminals on stacks shall be solid copper, copper alloy, stainless steel or monel metal
Air terminal:
- The height of air terminal shall be not less than 18 inches and not more than 30 inches The diameter shall be not less than 5/8 inch.
- The installation shall be evenly distributed around the top of a round stack at intervais not to exceed 8 feet.
- If the stack is square, the location shall be not more than 2 feet from corners and spaced not more than 8 feet apart.
Down conductor:
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Two numbers of down conductors should be provided for one air terminal.
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If more than one air terminal is installed then all the terminals shall be connected together and there shall be at least 2 down conductors on opposite sides of the stack.
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The natural components of the structure can also be used as a down conductor of the lightning protection system.
Reinforcing steel in concrete stacks shall be electrically continuous and shall be bonded to the lightning protection system at its upper and lower ends at down-lead locations
Earthing System:
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The grounding electrode shall be a rod of not less than % inch in diameter and made of copper-clad steel, solid copper, or stainless steel. The rod shall extend vertically not less than 10 feet into the earth and below the frost line where possible.
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The bare conductor can also be directly buried from each down conductor in the form of a radial.
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The length of the radial conductor must be at least 12 feet and these conductors must be buried at least 18inch in depth. Any aluminium or aluminium alloy product shall not come into direct contact with earth.
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A ground ring shall be used at a depth of 18 inches under the earth.
Conclusion:
Stacks above 75ft in height require some special consideration in both design and material used for the system. Materials with good corrosion resistance property should be preferred for the corrosion zone. Aluminium components shall not be used in the corrosion zone and shall not be in direct contact with earth. Aluminium down conductors shall be connected to copper conductors using bimetallic connectors to avoid galvanic corrosion.
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