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About Dampers : MATERIALS OF CONSTRUCTION
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Materials of Construction
Dampers can be exposed to an infinite variety of operating conditions and therefore considerable attention must be given to the selection of materials. This is especially true of dampers utilized in SO2 Scrubber systems, which represents about as severe environment as any piece of equipment should encounter. However, don’t forget that we have no control over the system chemistry (and it will change, no matter the specification says) and therefore we cannot and will not guarantee materials of construction.
The architect/engineer system supplier or the user must do the selection of materials.
Usually the worst conditions are encountered at the SO2 Scrubbed Outlet and typically used materials include the following”
Most specifications received recently are calling for Inconel 625
as the base material for the frames and blades. This is one of the
most expensive materials but has excellent resistance to corrosion.
Therefore, common grades of carbon steel (A-36) or corten are
used for the damper frame and blade.
Power plant dampers have certain features in common that influence decisions on materials. Dampers are made of thin sheets and have large areas of metal exposed to unfavorable environment inside and often outside the duct. Temperatures and the make up of gas and entrained liquids vary through wide limits. Deposits can promote corrosion, too. Finally, high gas speeds and heavy particle loading will increase erosion.
The first classification of conditions influencing material choice can be made according to temperature. For frames and blades (both louver and guillotine) up to 300F, A36 carbon steel plate is often selected. Some manufacturers and users will go with this steel to 600F if other conditions are favorable. In one case, on gas turbine exhausts, a swinging blade damper of A36 may work well on 900F service.
For moderately high temperatures in coal burning units, where resistance to both temperature and scaling is sought, A242 or A588 low alloy steels are common. These steels can resist 900F in some industrial services.
For the high temperatures encountered in advanced heat recovery work, 316 and 309 stainless steels are resistant to around 1500F. Above that, inconel alloys can withstand 1800F in damper service.
If corrosion is a danger, then carbon steel is undesirable for even frame components. Steels like A242 and A588 are favored for cases where there is mild corrosion in flue gas ducts, especially those where temperatures remain above dew point for long periods of operation.
Results have been disappointing around scrubbers and near the stack, where temperatures are close to dew point and where wet solid deposits promote the corrosive effects of scrubber chemicals. Stainless steels, such as 316L and 317L, have stood up in some applications but have been questioned for others. Use of Inconel 625 and Incoloy 825 is increasing here, along with Hastelloy G and C grades.
If chloride content of vapor and droplets is low, an alloy like 316L can be effective. At high chloride levels, with sulfuric acid and various corrosive and synergistic agencies present, higher grade and more expensive alloys are best. Not all highly resistant alloys are available in the thickness, size or heat treatment conditions desired, and this can constrain design.
For seals, where thin flexible strips are common, Inconel 625 and Hastelloy C-276 give good resistance. More flexible but less corrosion resistant is type 316 stainless steel. Nonmetallic seals are all or partly made of Teflon, Viton, Asbestos, and other materials that have good chemical resistance but often indifferent temperature resistance.
Erosion and abrasion are factors in flue gas service. Design measures are important here, but thickness allowances help, too, going up to 1/16 in. for blades of louver dampers.
Chain drives, with their many components and close clearances, are susceptible to corrosion. External design measures rather than specially resistant metals are common. The attachment of the chain to the guillotine blade is an especially corrosion prone area.
Surface protection is an answer to some damper corrosion problems. Thin protective films, such as vinylesters, are favored over very thick layers, which can flake off during vibration and sudden flexing of duct and blade metal. Guillotine blades, which scrape past seals in opening and closing, cannot rely on paint or coatings. Hard facing of seal seats and blade edges has increased erosion and corrosion resistance in some cases.
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