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About Dampers : DAMPER SYSTEM CHARACTERISTICS
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System Characteristics Which Define The Design Of Dampers:
Flow Velocity And Pressure Loss
THE GAS FLOW
Gas streams in the boiler backend are very different from the simple and tranquil flow of air in ventilation ducts. In the first place, the turbulence and vortices of the flow are pronounced. Thorough combustion requires thorough mixing of fuel and air, and every boiler manufacturer takes pains to do this.
Once the products of combustion are created, they move forward through line after line of tubes, around baffles, past sharp turns, in and out of fans, and through dampers. All this is at high speed, so the countless vortices and flow irregularities set up a rapidly pulsing flow, varying over duct sections and along the duct. Buffering and vibration inducement occur at all metal along the ducts.
The gas itself, although principally nitrogen and carbon dioxide, can have many other constituents, especially in coal burning units. Oxides of sulfur and nitrogen, along with water, are customary. Ash from coal combustion is a solid that varies widely in composition, with minerals, metal oxides, and other metal compounds always present. Oil fuel can put several corrosive metal compounds into the ash.
Various fuel conditioners add to flue gas makeup, too. Steam from leaking tubes or soot blowers is another source of trouble for downstream equipment.
Corrosion is not the sole effect of gas stream particles, of course. Because of the high flow velocities, erosion is also a danger. In addition, the complex nature of the gas contaminants and the variants in temperature make difficult the prediction of whether deposits will adhere to duct metal or not. If a deposit does occur, subsequent wide swings in temperature, velocity, and composition of gas, liquids, and solids can lead to severe attack.
One example for the section between boiler and scrubber: a leaking tube and a slow shutdown can result in High water vapor content in the duct gas. Cooling of the duct during cold weather will then produce condensation, which will saturate most of the existing deposits in the duct. The concentration of chemicals in the water will have a wide range of values, so that at some time before the situation is cleared up, the deposits will have maximum corrosive effect.
In the scrubber section of a coal fired unit, conditions are even worse. High concentrations of chemicals and heavy splashing and spraying mean that adjacent ductwork and dampers often receive thick and frequently fewer coatings of potentially corrosive deposits.
With scrubber sprays and sparges, no matter how carefully the piping and adjacent structures are located and supported, there is high chance that either a pipe will partly block or twist, or a rack will fall down, in such a way as to divert a jet of what is literally liquid wallboard and crumbled sidewalk onto nearby duct and dampers. With the air, water vapor, and flyash coming along in the flue gas, there is a good possibility that deposits will become more adherent or more corrosive as time passes.
For dampers, erosion by flyash particles can take several destructive forms. For example, a narrow crevice, such as a minor leak at a seal, will let through a stream of ash particles that will steadily cut the seal walls and worsen the leak. Often the closed damper will have full pressure differential across it, causing high velocity of gas leakage.
Wear of shafts in packing bearings is another costly effect of particles. A control damper shaft turns constantly on the hard particles. In the duct interior, where small eddies remain near metal surfaces, the constantly whirling flyash can cut away metal.
Heavy deposits of flyash can occur not only on the duct bottoms and behind steps in ductwork, but also inside frames and other spaces with narrow passages connecting the spaces with the gas flow. Gas entering a space at a high velocity slows down in the space, and the ash drops out to the floor. Sudden turns in gas flow through confined spaces in damper frames also separate ash. Ash deposits in interior spaces can be alternately wetted and dried, like the deposits in the ducts and on damper surfaces, with undesired concretion or corrosion.
For dampers, corrosion of low alloys can be uniform scaling or other surface roughening, which is very bad for seals and sealing effect. With high alloys, pitting and stress corrosion may be the first serious result of long term corrosion.
Damper environments range from clean, dry locations inside the plant to exposure outside in areas where rain, cold, heat, flu gas, and scrubber chemicals wreck dampers in a few months.
Temperature and its rate of change are two important factors in assessing the environment of a damper. Although the maximum temperature is not usually a threat to metal strength, except for gas turbine dampers, the stress relieving effect of gas temperatures can be a problem over the long run.
Stress relief effects in dampers and ductwork are controversial, and are not completely understood, but it is safe to say that the operating and excursion temperatures in much of backend ducting will relieve locked-in stress over damper life. This includes stress in the original metal and in the welds, along with some of the stress put in by bending and twisting components for assembly fit up. The highest the locked-in stress, and the higher the fraction of metal volume under internal stress, the greater the chance that uneven stress relief will lead to permanent shape change.
In many components of dampers, in service warping means nothing as far as damper effectiveness is concerned. The warping can be fatal, however, if it causes leaking or jamming. Consider louver dampers first.
Blades with thin plates, bolted or welded together, are the basic moving component of a louver damper. If plate flattening operations, either at the steel mill or the damper manufacturer's shop, have created internal stresses, service temperatures will relieve them. Bolted joints may give a little more than welded joints, but it would be hard to predict just what the shape of either a welded or a bolted blade will be after a few thousand hours at 440°F. Slowly changing temperatures, uniform over the entire blade surface, could prevent warping. Non uniform temperature conditions or rapid temperature changes could be another matter.
For many seals on louver dampers, a combinations of blade deflection under gas pressure and blade warping in service will cause leakage. This is worth considering in evaluating seal design.
Outside ducting exposed to rain and aggressive atmosphere will corrode seriously in a matter of months. The effect on dampers is selective, with concentrated attack on some very sensitive locations. For example, rainwater, in which gases like SO2 and HCl dissolve, will often run down into frames and collect at the bottom of enclosed spaces, where insidious corrosion will occur, rotting away plates and support pieces. Penetration into chains and drive linkage is another danger.
In some dampers, where surface protection is inadequate, exposure to the normal environment near scrubbers can cause such heavy rusting that damper life is in serious questions.
Available space for the damper must be part of an evaluation. Retrofit of glue gas treatment equipment means fitting ducting and dampers into close quarters, both along the ducting and a right angles to the ducting. Guillotines may have to go in with unfavorable draws and at poor locations. With a new unit, a duct layout done early in design may assume certain types and locations for dampers. Later changes for any reason can run into troubles similar to those found in retrofits.
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