Page 260 - Pressure Vessel Design Manual

238 Pressure Vessel Design Manual

0 The two most important effects on refractory linings are
General Refractory Notes creep and shrinkage.
0 Optimum anchor spacing is 1.5-3 times the thickness of
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0 Once the hot spots have occurred, there is obviously a heat the lining.
leak path to the vessel wall. The subsequent heating of the 0 Optimum anchor depth is approximately two-thirds of the
shell locally also affects the anchors. Since the anchors are lining thickness.
made of stainless steel, they grow more than the shell and
therefore relax their grip on the refractory. This in turn
~
allows the gap between the shell and the refractory to Notation
grow.
0 Refractory failures are categorized as either tension or
compression failures. These failures can result from bend- Shell Properties
ing or pure tensiodcompression loads. In a tension failure
the crack is initiated and grows. A "cold joint" is the pre- D = shell ID, in.
ferred fix for a tension failure. D, = shell OD, in.
I:
0 A compression failure will tend to pull the lining away E, = modulus of elasticity, shell, si
from the wall. A flexible joint with ceramic fiber is a I, = moment of inertia, shell, in.
good solution of this type of failure. K, = thermal conductivity, shell, Btu/in.-hr-ft2-"F
0 During operation, the hot face is in compression, varying t, =thickness, shell, in.
through the thickness to tension against the steel shell. W, = specific density, steel, pcf
This is caused by thermal expansion of the material and a, = thermal coefficient of expansion, shell, in./in./"F
thermal gradient forces developed internally.
0 During the cooling cycle, the hot face will be in tension. If Refractory Properties
the cooling cycle is io0 rapid or the anchoring too rigid,
then the tensile stress of the material becomes critical in DL = refractory OD, in.
resisting cracking. dL = refractory ID, in.
0 Due to low tensile strength, cracking occurs at early stages EL = modulus of elasticity, refractory, psi
of load cycles, which ultimately results in load redistribu- F, = allowable compressive stress, refractory, psi
tion. IL = moment of inertia, refractory, in. 4
0 Temperature loading, such as heat-up, cool-down, and KL = thermal conductivity, refractory, Btu/in.-hr-ft'-
holding periods at lower temperatures, results in stress "F
cycling. STS, STL =irreversible shrinkage of lining @ temperatures
0 Refractory properties are nonlinear. Ts, TL
0 Compressive strength is practically independent of tem- tL = thickness, refractory, in.
perature, whereas tensile strength is highly dependent on WL = specific density of refractory, pcf
temperature. aL = thermal coefficient of expansion, refractory,
0 Refractory material undergoes a permanent change in in./in./" F
volume due to both loss of moisture during the dryout pL = Poisson's ratio, refractory
cycle as well as a change in the chemical structure. The
effects of moisture loss as well as chemical metamorphosis General
are irreversible.
0 During initial heating, the steel shell has a tendency to E, = modulus of elasticity of composite section, psi
pull away from the refractory. The cooler the shell, the hi, h, = film coefficients, inside or outside, Btu/ft2-hr/"F
less the impact on the refractory. The cooler shell tends to P = internal pressure, psig
hold the refractory in compression longer. Q =heat loss through wall, Btu/ft2-hr
The use of hoIding periods during the heat-up and cool- T, =temperature, outside ambient, "F
down cycles results in relaxation of compressive stresses T, = temperature, outside ambient during construc-
due to creep. However, this same creep may introduce tion, "F
cracks once the lining is cooled off. TL =temperature, refractory, mean, OF
TL~ =temperature, lining, inside, OF