Roxul Industrial Insulation Process - Manual (Page 79)

1.7 Insulation of flue gas ducts
1.7.1 Installation of the insulation systems for flue gas ducts
Observe the following when pinning the insulation:
With insulation thicknesses ≤ 5" (120 mm), use
8GA (6AWG) pins with a minimum diameter of
0.162" (4 mm).
With insulation thicknesses ranging from 5 1/2"
to 10" (130 to 240 mm), use 6GA (4AWG) pins
with a minimum diameter of 0.2043" (5 mm).
With insulation thicknesses ≥ 10" (240 mm) use
4GA (3AWG) pins with a minimum diameter of
1/4" (6 mm).
If the cladding rests directly on the insulation
without a gap between the two, the pins must
be 3/8" (10 mm) shorter than the insulation
thickness.
Fasten each insulation layer with clips.
With wired mats, all the lengthwise and crosswise
joints must be sewn or wired together, or joined
with six mat hooks per meter. If the insulation is
assembled in multiple layers, the joints of the
individual insulation layers must be staggered.
To reduce convection in the insulation, fitting
barriers is recommended, for example made from
steel, at intervals of 16 to 26 feet (5 to 8 m) when
working on large vertical surfaces. The barrier
must be effective across the entire section of
insulation up to the cladding.
ProRox ® insulation is recommended insulation for
round flue gas ducts, where temperatures are
below 570 °F (300 °C). These are mounted directly
onto the flue gas duct and are fastened with
straps. A fastening with welding pins and spring
clips is generally not required in this instance.
Insulation of reinforcement elements
Large flue gas ducts are fitted with reinforcement
profiles to stabilize the duct. These can consist of
double T-girders, hollow sections or reinforcing
ribs and form potential thermal bridges. This may
cause the following problems:
The thermal bridges cause an increased heat
flow and lead to a temperature decrease on the
inside wall of the ducts.
Temperature variations between the inner and
exterior lead to stress in the profiles. If the
tensile forces become too great, this can lead to
deformations and breaking of the welding.
Preventing temperature drops on
the inside wall
To prevent a drop in temperature on the inside wall
in the area of reinforcement profiles, they must
always be insulated. The insulation thickness
required depends on factors such as the size and
geometry of the profiles, the temperature level
and rate of flow within the flue gas duct and the
operating method. Complex calculations may be
required to determine the insulation thickness.
These are usually established by the plant
manu facturer, who is aware of the installation
parameters. When starting up the installation, a
brief drop in temperature below the dew point of
the flue gas is unavoidable on the inside wall of
the duct.
Reduction of stress due to temperature
in the reinforcement profiles
The operating method of the installation
influences the problem of stress in the
reinforcement profiles caused by temperature.
Less critical is the steady operation, where the
flue gas temperature does not change with the
passage of time. Generally, stresses due to
temperature are not critical if the implementation
principles outlined in the AGI guideline Q101 are
observed:
The insulation thickness across the
reinforcement elements should be of the same
thickness as the insulation on the flue gas duct.
In the case of ducts with reinforcing ribs up to a
height of 4" (100 mm), the thickness of the
insulation layer across the ribs must measure
at least one third of the insulation thickness
required for the duct.
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