the polymer. Although unusual, it is possiblef or absorbed monomers
to polymerized.
Several steps can be takent or educe absorption. For example, thermal
insulation of the substrate will reduce the temperature gradient across
the vessel,t hereby,p reventing condensation and subsequent expansion of
the absorbedfluids. This also reduces the rate and magnitude of temperature
changes, keeping blisters to aminimum. The use of operating procedures or
devices that limit the ratio of process pressurer eductions or temperature
increases will provide additionalp rotection.
6.3.3 Environmental Stress Cracking
Stress cracks developw henatough polymer is stressed for an extended
period of time underl oads that are smallr elative to the polymer’s yield
point.C racking willo ccur with little elongation of the material. The higher
the molecular weight of the polymer, the less likelihood of environmental
stress cracking, other things beinge qual. Molecular weight is af unction
of length of individual chainst hat make up the polymer. Longerc hain
polymers tend to crystallize less than polymers of lower molecular weight or
shorter chains,and they also have ag reater load-bearing capacity.
Crystallinity is an important factor affecting stress corrosion cracking.
The less the crystallization that takes place, the less the likelihood of stress
cracking. Unfortunately,the lower the crystallinity,the greater the likelihood
of permeation.
Resistance to stress cracking can be reduced by the absorption of substances
that chemically resemble the polymer and will plasticize it. In addition,the
mechanical strength willa lso be reduced. Halogenated chemicals, particularly those consisting of small molecules containing fluorine or chlorine, are
especially likely to be similar to the fluoropolymers and should be testedfor
their effect.
The presence of contaminantsi nafluid may act as an accelerator.F or
example, polypropylene can safelyh andle sulfuric or hydrochloric acids,
but irono rc opper contamination in concentrated. sulfuric or hydrochloric
acids can result in the stress cracking of polypropylene.
6.4C auses of Lining Failure
Linings,i fp roperly selected, installed, and maintained, and if the vessel
has been properly designed, fabricated, and prepared to accept the lining,
promise manyu seful years of service. However,o no ccasion, there have
beenl iningf ailures that canb ea ttributed to oneo rm oreo ft he
following causes.
Sheet Linings
293
CAT8247—CHAPTER 6—6/10/2006—12:28—SRIDHAR—XML MODEL B–pp. 287–402
to polymerized.
Several steps can be takent or educe absorption. For example, thermal
insulation of the substrate will reduce the temperature gradient across
the vessel,t hereby,p reventing condensation and subsequent expansion of
the absorbedfluids. This also reduces the rate and magnitude of temperature
changes, keeping blisters to aminimum. The use of operating procedures or
devices that limit the ratio of process pressurer eductions or temperature
increases will provide additionalp rotection.
6.3.3 Environmental Stress Cracking
Stress cracks developw henatough polymer is stressed for an extended
period of time underl oads that are smallr elative to the polymer’s yield
point.C racking willo ccur with little elongation of the material. The higher
the molecular weight of the polymer, the less likelihood of environmental
stress cracking, other things beinge qual. Molecular weight is af unction
of length of individual chainst hat make up the polymer. Longerc hain
polymers tend to crystallize less than polymers of lower molecular weight or
shorter chains,and they also have ag reater load-bearing capacity.
Crystallinity is an important factor affecting stress corrosion cracking.
The less the crystallization that takes place, the less the likelihood of stress
cracking. Unfortunately,the lower the crystallinity,the greater the likelihood
of permeation.
Resistance to stress cracking can be reduced by the absorption of substances
that chemically resemble the polymer and will plasticize it. In addition,the
mechanical strength willa lso be reduced. Halogenated chemicals, particularly those consisting of small molecules containing fluorine or chlorine, are
especially likely to be similar to the fluoropolymers and should be testedfor
their effect.
The presence of contaminantsi nafluid may act as an accelerator.F or
example, polypropylene can safelyh andle sulfuric or hydrochloric acids,
but irono rc opper contamination in concentrated. sulfuric or hydrochloric
acids can result in the stress cracking of polypropylene.
6.4C auses of Lining Failure
Linings,i fp roperly selected, installed, and maintained, and if the vessel
has been properly designed, fabricated, and prepared to accept the lining,
promise manyu seful years of service. However,o no ccasion, there have
beenl iningf ailures that canb ea ttributed to oneo rm oreo ft he
following causes.
Sheet Linings
293
CAT8247—CHAPTER 6—6/10/2006—12:28—SRIDHAR—XML MODEL B–pp. 287–402