Page 260 - Fluid mechanics, heat transfer, and mass transfer

DIMENSIONLESS NUMBERS, TEMPERATURE MEASUREMENT, AND CONDUCTION HEAT TRANSFER
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. What is the relationship between dew point temperature solution. An overfitted system will result in reduced
of ambient air and insulation thickness? energy costs. It also restores existing deteriorated
& It is crucial that sufficient insulation is added so that insulation to its former thermal values, reduces sys-
the outer temperature of the insulation remains above tem downtime, and improves plant safety by properly
the dew point temperature. At the dew point temper- maintaining surface temperatures.
ature, moisture in the air will condense onto the & Even though pipes and equipment are at high tem-
insulation and damages it. peratures, it is impossible to completely dry out
. Why the surface of the insulating material is treated to permeable insulation once it becomes wet. The fuel
give smooth and impervious surface finish, blocking the loads required to maintain the operating temperature
pores inside the insulation? of steam systems with wet permeable insulation can
double and, in some cases, triple.
& The surface is made impervious to prevent ingress of
moisture, which lowers effectiveness of insulation, & Overfitting pipes and equipment with cellular glass
acting as vapor barrier as explained earlier. insulation with a minimal amount of cellular glass,
the insulation can become self-drying by creating an
& Leaked flammable liquids into the insulation catch
interface temperature that is high enough to vaporize
fire due to self-heating, and continued oxygen supply
the existing moisture in the permeable insulation.
feeds such fires. Self-heating occurs as oxidation of
leaked flammable liquids into the insulation releases & A typical overfit system consists of a 3–5 cm layer of
heat that accumulates inside the insulation as the closed cell glass insulation, the thickness of which is
surrounding insulation prevents heat dissipation, determined by the resultant interface temperature
raising temperatures to self-ignition values of the between the existing insulation and the new overfit
leaked fuel, causing insulation fires, as explained system. This interface temperature must be kept at or

before. above 100 C to drive out the moisture in the existing
insulation. The vaporized moisture is forced to
& The impervious surface prevents oxygen supply to
escape through the interface and out of the system
the interior of the insulation and kills insulation fires
through the joints of the cellular insulation.
that are precursors to major fire and explosion
hazards in process industry. & After the system is overfitted with cellular glass, it
has an outer layer of impermeable insulation that
. What are the effects of water ingress into insulation?
eliminates the penetration of moisture (in both liquid
& Water ingress reduces ability of the material to
and vapor forms) into the permeable insulation.
insulate.
. What are the precautions required while applying min-
& Just 4% moisture by volume can reduce thermal
eral wool insulation to process equipment?
efficiency by 70%, since water has a thermal con-
& Mineral wool insulation, when comes in contact with
ductivity of up to 20 times greater than most insu-
skin, gives rise to irritation and skin rashes. Skilled
lation materials.
labor using protective clothing and gloves are
& The risk of plant corrosion increases.
required.

& If the plant is operating at temperatures below 0 C,
& The wool insulation requires protective sheathing in
then water vapor may pass through the insulation and
the form of metal sheets wrapped over it to prevent
condense and freeze. This may cause mechanical
peeling off due to wind or contact with moving
breakage of the insulation, corrosion of the plant, and
objects.
total breakdown of the insulation system or the
& Alternative to wrapping, magnesia insulation in the
equipment itself.
form of paste may be applied to cover surface pores in
. What are the solutions to handle wet insulation?
order to prevent entry of air into it in the event of
& If the insulation is wetted by leaked flammable
lagging fires.
liquids, it should be replaced by new insulation.
. What are the common maintenance snags with respect
& If the insulation gets wet due to moisture ingress,
to insulation to process equipment and piping in
replacement may not be the best option as it involves
industry?
high costs—material costs, labor costs, and disposal
& During maintenance work in a plant, commonly
costs of defective insulation due to its environmental
observed snags include the following:
pollution effects.
➢ Damage to wrappings and insulation.
& Applying new insulation directly over the existing
➢ During valve/fitting maintenance, omissions in
insulation, known as overfitting. Overfitting is a good
restoring back molded insulation blocks in place.
option, because it is a cost-effective and immediate