Page 259 - Fluid mechanics, heat transfer, and mass transfer

CONDUCTION HEAT TRANSFER 239

& On vertical runs of pipe where occasional leaks can and vapor breathers for hot service. Each type may or
develop at flanges, protective tight fitting caps should may not be self-weatherproofing.
be installed to divert any fluid leakage outside the . What is the common vapor barrier wrapping in thermal
insulation. insulations?
& Valve stems should be installed horizontally or in a & Aluminium foil wrapping is commonly used. Typical
downward position so that any stem leakage does not thickness of aluminium used ranges from 0.2 to
enter the insulation. 0.4 mm. In case of low-temperature insulations, such
& Applying impervious protecting layer over the cel- a wrapping provides reflecting surface to reduce heat
lular insulation to prevent air ingress into the insu- leak into the system. End sections of the insulation
lation. Weather barriers, vapor barriers, rigid and soft and where insulation abuts supports the vapor barrier
jackets, and a multitude of coatings exist for all types should be taped by aluminium foil to all inserts. Any
of applications. stickpins protruding through the foil facing should be
& The possibility of leakage through joints and fittings securely taped with a minimum of 100 mm wide.
is characteristic of most organic fluids unless the . What are the reasons for wrapping insulated surfaces
fittings are extremely tight. System design should with metallic sheets?
minimize the number of connections in the piping & To protect insulation from peeling off.
layout. The best way to prevent piping leakage is to & To prevent heat ingress into insulated cold systems
weld all connections. Use of threaded fittings should
with reflective wrapping sheets.
be strongly discouraged due to their tendency to leak.
& To prevent moisture ingress.
Where access is necessary, raised face flanges with
& To provide stagnant air film between insulation and
weld neck joints or equivalent raised face flanges
wrapper to increase effectiveness, that is, resistance
should be used.
for heat flow.
. What are the applications of compressible insulation?
& To prevent crumbling of insulation by erosion by
& Compressible insulation is used for filling voids and
wind, contact by plant personnel that can also be a
closing gaps in insulation, which allows expansion,
health hazard to them and fire exposure?
contraction, or movement of rigid insulation.
. “For high fire risk plants, stainless steel shielding is
. Name materials used for insulation of electrical
recommended to protect the insulation even though
systems.
stainless steel is an expensive material compared to
& PVC, rubber, and other polymer and ceramic
normally used shielding material.” When such a con-
composites.
sideration does become attractive?
. What is a vapor barrier over insulation? Why is it
& For high fire risk areas, the finished insulation system
necessary to use it?
should not get dislodged when subjected to the fire
& Vapor barrier is a coating or metal shield applied over
water stream used for fire fighting, either by hand
the surface of the insulation, especially when ambi- lines or monitor nozzles. Most insulation systems
ent temperatures are below 0 C, to prevent ingress of used in fire protection are metal with stainless steel

water into the pores of the insulation. jackets and bands, which meet these criteria.
& Particular attention should be paid to ensure integrity & Stainless steel shield not only provides a vapor
and continuity of the vapor barrier. Care should be barrier to the insulation but also protects insulation
taken at termination points, where any exposed in- from crumbling on fire exposure, taking the role of
sulation edges should be covered properly. fireproofing material, thus protecting the structural
& Metal cladding applied over insulation in hot service integrity of piping/equipment from collapse.
should have drain holes located at the lowest point to . “In cold insulation system design, vapor barriers are
allow any entrapped water to escape. extremely important compared to hot insulation.”
& Where a liquid-applied (mastic) vapor barrier is Comment.
used (e.g., on cryogenic processes), joints should & Usually, the cost of removing heat (heat gain) by
be filled with a suitable sealant to effectively refrigeration is greater than that of producing process
isolate each section and hence localize water (and heat (heat loss) by heat-generating equipment.
water vapor) ingress in case of any mechanical
& Therefore, the heat gain in cold processes must be
damage.
kept to a minimum. The rule of thumb is to provide
& It should be noted that liquid-applied finishes are
sufficient insulation to maintain heat gain of
divided into water vapor barriers for cold applications 25–30 W/m to the cold process.
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