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.
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.