29-09-2012, 11:07 AM
Model Code for Steel Chimneys
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INTRODUCTION
General
Chimneys are required to carry vertically and discharge to the
atmosphere, gaseous products of combustion, chemical waste gases, or
exhaust air or for the combustion (flaring off) of industrial waste gases.
This Model Code contains guide-lines which reflect the current state
of art in the design and construction of steel chimneys. Nevertheless,
the design, fabrication and erection of steel chimneys require a
thorough knowledge of these structures, the properties of the
materials used, the actions occurring upon the structure and the
recognised rules of the relevant technologies. The design of steel
chimneys should therefore only be entrusted to appropriately
qualified and experienced engineers. The construction and erection
should be carried out by firms competent in this class of work. At all
times the work should be under the direction of appropriately
qualified supervisors.
CICIND will continue to try to improve the understanding of the
behaviour of chimneys. Further revisions of this model code will
therefore be published from time to time.
SCOPE
This model code relates to the structural design and construction of
steel chimneys of circular cross-section, with a minimum height of
15m, with or without linings, and to the design and application of
linings to such chimneys where required. It also relates to chimneys
with a height less than 15m and a slenderness ratio more than 16. The
model code does not deal with architectural or thermal aspects of
steel chimneys nor with their foundations, except insofar as they
affect the chimney’s structural design. The model code does not deal
with those aspects of the design and construction of steelwork,
refractories and insulation which are not peculiar to chimneys.
Cross-wind Effects (Vortex shedding)
Chimneys shall be designed to avoid movements across the wind
direction sufficient to cause failure or fatigue damage or to alarm
bystanders.
The code contains means of estimating the amplitude of movement
and consequent stress range due to crosswind loading. Limiting stress
ranges are given for various weld classifications and design lives. In
addition to a material safety factor 1.1, applied to fatigue category, a
modelling factor of 1.4 shall be applied to the Miner Number derived
in fatigue calculations for temperatures up to 200°C and 1.5 for
temperatures between 200°C and 400°C.
To avoid alarming personnel, the maximum permitted amplitude of
oscillations due to cross-wind effects or aerodynamic interference
shall be agreed between the owner and designer. This limit will be
governed by the prominence and visibility of the chimney and the
frequency with which maximum amplitudes can be expected.
Guidance is given in Commentary 3.
Stainless and alloy steels
When metal temperatures are expected to exceed 400°C, stainless or
alloy steels should be used.
Ordinary stainless steels (including high molybdenum stainless steel)
have poor corrosion resistance in the presence of condensing
sulphuric or other acids in the range of concentrations and
temperatures normally found within chimneys. These materials are
therefore not recommended in chimneys burning fuels containing
sulphur under conditions of “medium” or “high” chemical load, see
paragraph 7.6.3.
When metal temperatures and condensate sulphuric acid
concentrations are expected to be less than 65°C and 5% respectively,
the corrosion rates of high molybdenum stainless steels, such as
ASTM Type 316L, are acceptable. Such conditions can be expected
on the external surface at the top (over a height of about 3 diameters)
of any chimney handling high sulphur flue gases.
(Note: the conditions downstream of a flue gas scrubber or the
presence of chlorides in the condensate will radically increase the
corrosion rate, possibly rendering these stainless steels unsuitable for
these applications.)
Basic wind speed
The determination of the effective wind pressure is based on the basic
wind speed.
The basic wind speed Vb, appropriate to the location where the
chimney is to be erected, is defined as follows: It is the mean hourly
speed, measured 10m above ground level in open flat country,
without obstructions, at the chimney location, which occurs on
average once every 50 years.
The value of the basic wind must be taken from meteorological
measurement. An indication of values of the basic wind speeds for
various countries may be obtained from the Commentary No.3.
Where the terrain of the location of the chimney is hilly or built-up,
measurements for the determination of Vb should be taken as near as
possible at a place which is flat and open. However, in some very
hilly areas, where flat ground is rare, Vb is sometimes measured at the
chimney location and includes the “Topographical factor”.