Page 259 - Pressure Vessel Design Manual

Special Designs 237

PROCEDURE 4-7

DESIGN OF VESSELS WITH REFRACTORY LININGS [ 13-16]

The circular cross sections of vessels and stacks provide an
ideal shape for supporting and sustaining refractory linings, Allowable Refractory Stresses
from a stress standpoint. There are a variety of stresses
developed in the lining itself as well as stresses induced in There is no code or standard that dictates the allowable
the steel containment shell. Compressive stresses are devel- stresses for refractory materials. Refractory suppliers do not
oped in the lining and are a natural result of the temperature have established criteria for acceptable stress levels. In addi-
gradient. These compressive stresses help to keep the lining tion, there is very limited experimental information on the
in position during operation. This compressive condition is behavior of refractory materials under multiaxial stress
desirable, but it must not be so high as to damage the lining. states.
Several idealized assumptions have been made to simplify One criterion that has been used is a factor of safety of 2,
the calculation procedure. based on the minimum specified crush strength of the mate-
rial at temperature for the allowable compressive stress. The
corresponding allowable tensile stress is 40% of the modulus
1. Steady-state conditions exist.
2. Stress-strain relationships are purely elastic. of rupture at 1000°F.
3. Shrinkage varies linearly with temperature.
4. Thermal conductivity and elastic moduli are uniform
throughout the lining. Refractory Failures and Potential Causes of
5. Circumferential stresses are greater than longitudinal Hot Spots
stresses in cylindrical vessels and therefore are the only
ones calculated here. The following are some potential causes of refractory fail-
ure, cracking, and subsequent hot spots.
The hot face is in compression during operation and heat- 0 Refractory spalling: Spalling can be caused by excessive
up cycles and is in tension during cool-down cycles. The moisture in the material during heating, by too rapid heat-
tension and compressive loads vary across the cross section up or cool-down cycles, by too high a thermal gradient
of the lining during heating and cooling phases. The mean across the lining due to improper design, either too
will not necessarily result in compression during operation thick a lining or too low a thermal conductivity. This
but may be tension or neutral. The hot-face stress should case leads to excessive hot-face compression.
always he compressive and is the maximum compressive
stress in the lining. If it is not compressive, it can be made 0 Poor refractory installation.
so either by increasing the thickness of the lining or by 0 Poor refractory material.
choosing a refractory with a higher thermal conductivity. 0 Excessive deflection or flexing of the steel shell due to
Excessive compressive stresses will cause spalling. pressure, surge, or thermal stresses.
The cold face is under tensile stress. This stress often 0 Differential expansion.
exceeds the allowable tensile stress of the material, and
cracks must develop to compensate for the excessive tensile 0 Excessive thermal gradient.
stress. The tensile stress is always maximum at the cold face. 0 Upsets or excursions leading to rapid heating or cooling
Upon cooling of the vessel, the irreversible shrinkage wiIl rates. These should be limited to about 100"Fhr.
cause cracks to propagate through the lining. The shrinkage 0 Upsets or excursions leading to temperatures near or
of the hot face amounts to about 0.001 in./in. crack width at exceeding the maximum service temperature.
the surface would vary from 0.01 to 0.03 in. These cracks will 0 Poor design details.
close early in the reheat cycle and will remain closed under 0 Poor refractory selection.
compression at operating temperatures.
Monolithic refractories creep under compressive stress. At 0 Improper curing or dry-out rates.
stresses much less than the crush strength, the creep rate 0 Poor field joints.
diminishes with time and approaches zero. Creep occurs 0 Temperature differential.
under nominally constant stress. When strain instead of 0 Incorrect anchorage system.
stress is held constant, the stress relaxes by the same
mechanism that causes creep. Creep rate increases at 0 Vibration.
lower temperatures and drops off with temperature. 0 Anchor failure.