Tubes and Pigtails
Steam
methane/naphtha reformer heater tubes and pigtails are susceptible to
creep and stress rupture due to high thermal and mechanical stresses
and high operating temperatures. Failures generally occur due to
stress rupture at the hottest, most highly stressed portion of the
tube. The hottest areas are normally near the bottom for down-flow
systems or top of the tube for up-flow system, since the temperature
of the gas inside the tubes rises during reaction by about 500°F
(260°C), from about 900°F (482°C) to about 1400°F (760°C). If
flame from burners or from combustion products deflected off walls
and impinges upon the tube, stress rupture can occur in the hottest
parts of the tube.
Steam
methane/naphtha reformer heater tubes can fail by creep rupture that
is different from most other heater tubes. The tubes have a thick
wall with a large thermal gradient across it such that there are
significant thermal stresses in the region between the ID and
mid-wall. These thermal stresses are high enough to promote creep
initiating where the combination of stress and temperatures are above
a threshold and propagating to the inner diameter. Finally, the
cracks propagate to the outer diameter resulting in failure.
Minimizing
mechanical stresses from thermal growth are critical to pigtail and
tube reliability. Steam methane/naphtha reformer heaters
have an elaborate support and hanger system designed to allow the
tubes to grow in service and to reduce the stress on the pigtails and
headers. If the support system is not functioning as designed, it can
produce high stresses on the pigtails and tubes to the extent of
promoting creep rupture. Without adequate support, tubes can bow in
service, further increasing stresses. Bowed tubes have higher stress
levels at their bends than do straight tubes. Bending stresses are
induced on pigtails from tube bowing, tube movement, sagging of the
pigtail under its own weight, and thermal expansion of a pigtail
loop. The pigtails are susceptible to thermal fatigue, if the
movement is cyclic because of swings in operation or numerous
start-ups and shutdowns.
Some cast tube materials may embrittle after
exposure to high temperatures. Weld materials that embrittle during
postweld cooling have high residual stresses. Weld material with a
carbon-silicon ratio that does not match that of the base metal
fissures easily during welding. Any microfissures not detected during
fabrication can propagate during subsequent heating, thermal cycles,
or continual high stresses from bowing or localized heating. Welding
flux must be removed from tube welds. Grit blasting is recommended
for flux removal. Flux of lime with fluorides is corrosive if the
combustion gases are reducing (because of very little excess air) and
sulfur is present.
Outlet Headers
The cast alloy headers, like those fabricated
from HK material, have a history of cracking near junctions because
of embrittlement due to carbide precipitation and sigma formation.
Other areas of concern include inlets, outlets, laterals, tees and
elbows. These headers are horizontal and do not float freely. The
embrittlement that occurs does not allow any restraint of the thermal
growth and results in high stresses with resultant cracking. Because
of the embrittlement, welding repairs are difficult unless the
surfaces are annealed or buttered with a ductile weld material before
welding. Proprietary cast materials have been developed to avoid
embrittlement and their use in outlet headers has been satisfactory.
Wrought alloy headers, like Alloy 800H,
operating at temperatures near 1400°F (760°C) have also had a good
service history. They maintain ductility and can yield, by creep or
stress relaxation, to reduce localized stresses. As in any high
temperature design, however, stresses must be kept low, particularly
at supports and at openings in the headers.
Headers fabricated from carbon steel or low
Cr-Mo require internal refractory to keep metal temperatures low
enough to have an adequate design stress and to resist
high-temperature hydrogen attack. Because the base metal is not
resistant to hydrogen at high temperatures, the refractory must be
sound to preserve its insulating properties. Refractory used in
hydrogen and carbon monoxide service should have low iron and silicon
content to avoid the possibility of hydrogen or carbon monoxide
reacting with components of the refractory and the degradation of the
refractory's essential properties. Start-up and shutdown procedures
must minimize wetting of the refractory, partly to avoid destroying
the insulating refractory and partly to avoid carbonic acid corrosion
of the steel.
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