Civil Engineering Reference
In-Depth Information
The chimneys releasing hot and highly aggressive gases have an inside lining of gunite,
low-alloy steel or stainless steel. Two-halved chimneys consist of two steel pipes of
different diameters concentrically located with thermal insulation between the pipes. The
largest group of the chimneys both constructed and under construe-tion are the single
detached chimneys. However, their durability is questionable because of high failure
frequency due to faults, construction and design defects, or improper operational use.
Although the material deficiences are rarely mentioned as the chief reasons for failures
or defects, it should be noted that the design and construction shortcomings are often to
blame. The selection of the correct steel grade and type, electrodes and welding
techniques, screw grades, kind of insulating material in field collars etc. is important.
Many a time changes were introduced arbitrarily: the grade steel was changed to an the
unalloyed kind, heat-resisting steel was used instead of acid-resistant steel, auxiliary
materials or welding techniques did not comply with the project's requirements. Another
controversial issue is the materials used for the surface anti-corrosion insulation.
However, though the letter this issue is important, it is not the primary concern relevant
to this topic.
Severe and extensive corrosive damage in steel chimney shafts can be the ground for
the judgement failure is principally caused by steel corrosion. Medium losses in the plate
thickness of the single-shell steel chimneys used in the local and industrial heating plants,
calculated from 140 examined chimneys, amounted to 0.3 mm per annum in the middle
segments, 0.4-0.5 mm per annum in the upper segments, and up to 0.65 mm per annum
in joints and vicinity of fasteners areas [93]. In some steel chimney shafts inspected,
greater or lesser corrosive damage was detected, the greater damage occurring close to
shaft collars and the supporting ring in the chimneys stabilized by tripods. Some
chimneys were extensively damaged by corrosion of plates and shaft segment joints;
numerous perforations, cracks and local deformations, etc. were plainly visible.
Corrosive damage to chimney shafts did not vary at collar joints or the height above
the gas inflow to the chimney. Variable degrees of damage may be the result of the
prevailing wind direction, and the lowered temperature of the surface of the steel plate in
the windward direction. The region subject to most corrosive damage are pipe segments
that are below the dew point, because there the steam and acids in the fumes will
condense on the inner surface of the chimney shaft. Otherwise the corrosive damage to
chimney shafts tend to occur at random and therefore in the detailed analyses of damaged
chimneys and their capacity they should be treated accordingly.
Improper manufacture and erection of steel chimneys may also accelerate corrosion
damage (e.g. the faulty field welds on shaft segments or tight deformed collars, changing
the specified thickness or grade of steel, faulty application of the specified surface
protection). Mistakes and negligence during construction and erection are the most
frequent reasons, cited by experts for the unsatisfactory technical condition of some steel
chimneys.
Corrosion of steel chimneys is a function of the operating conditions, which are often
adverse the basic design. Particularly hazardous can be alterations to chimney operational
techniques that lead, among other things, to changes in temperature and pollutants in
exhaust gases, and frequent operational stoppages. Current environmental protection
regulations require the fitting of fume desulphurization devices, which result in lowering
temperature of the exhaust gas and increasing their humidity through the desulphurization
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