Author: Design of chimneys
More and more chimneys are required to carry vertically and discharge to the atmosphere gaseous products of combustion, chemical waste gases, exhaust air, etc.
There are several types of steel chimneys:
- self-supporting chimney
- guyed chimney
- stayed chimney (on a supporting column)
- bracketed chimney (on a nearby building).
For high temperatures, the yield stress and the Young's modulus of steel are modified.
The permanent load includes the weight of all permanent parts, i.e. shell, fittings, linings, flues, and insulation and any permanent equipment. In many cases, it is necessary to consider the carry over of ash or dust. Ash and dust may adhere to the interior surface of the structural shell or liner and cause an additional load. The basic wind speed, corresponding to the chimney site, is defined as the mean hourly speed at 10m above ground level in open country which occurs once every 50 years. Although steel chimneys are normally replaced after a shorter period than 50 years, the basic recurrence period is taken as 50 years and the factor of safety is determined according to a design life period (20 years for instance).
Forces due to vortex shedding cause a response of the chimney perpendicular to the wind direction. Important amplitudes occur when the shedding frequency coincides with a structural frequency.
Cross-wind vibrations usually can be reduced by aerodynamic stabilizers. The useful effect of three helical vanes has been proved; the radial width of the vanes is 10% of the diameter, the pitch of vanes is 5d and the vanes are fitted over the upper third of the height of the chimney. The extra wind drag must be considered. If there is no object causing interference to the wind flow within an effective distance, cross-wind actions on a fitted chimney can be neglected. In other cases, e.g. where there are nearby chimneys, the fitting of aerodynamical stabilizers remains beneficial but cannot be calculated.
The earthquake stress on a steel chimney is usually less than the wind loading stress. Normal steel chimneys can generally resist earthquake with an intensity of up to Mercalli scale 10 without serious damage.
Stresses are high when a single unlined chimney carries gases from several sources at different temperature or when a single side entry source introduces gases at high temperature. In addition, the resulting differential steel temperature introduces secondary thermal stresses. Typically restraint occurs in bracketed, stayed or guyed chimneys.
The degree of chemical load is in relation with the number of operating hours (n) when the temperature of the surface in contact with flue gases is below the estimated acid dew point. The fatigue check ascertains that the loading due to vortex shedding will not result in initiation and propagation of cracks in the steel material.
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