API TR 942-B:2017 pdf free download.Material, Fabrication, and Repair Considerations for Austenitic Alloys Subject to Embrittlement and Cracking in High Temperature 565 °C to 760 °C (1050 °F to 1400 °F) Refinery Services
3.2.2.3 Carburization Typical operating conditions in FCCU regenerators cause low rates of carburization of the stainless steel cyclones and other internal components. Resid FCCUs regenerator internals, with their corresponding higher temperatures, have an increased susceptibility to carburization, as well as faster sigma formation. One resid FCCU using Type 304H stainless steel at 730 °C to 760 °C (1350 °F to 1400 °F) reported a cyclone life of only 5 to 6 years due to the development of deep carburized layers [3]. Afterburn in regenerators not only raises combustion temperatures, but because it involves the combustion of CO, can cause severely carburizing atmospheres. FCCU internal components can also suffer sensitization [4]. Sensitization can be aggravated by excess dissolved carbon due to carburization and can make the material susceptible to polythionic acid stress corrosion cracking (PASCC).
A polythionic acid can be formed when sulfide scales formed at high temperatures on the surface of a stainless steel are cooled and exposed to moisture. The polythionic acid can cause intergranular SCC on sensitized stainless steel which is referred to as “PASCC”. This mechanism is covered more fully in NACE SP0170-2012 [78]. Some refiners reduce the potential for PASCC by minimizing moisture when the vessel is opened during shutdowns, while others use a neutralizing soda ash washing.
3.2.2.4 Creep Long-term creep cracking can occur in the regenerator internal support structures. The shortening of creep life can be impacted by sigma formation, particularly in the welds. Short-term creep/stress rupture is usually caused by afterburning episodes that result in local hot spots. Localized heating promotes high thermal stresses and weakens the material, increasing the potential for cracking. During afterburning, stresses can be high due to non-uniform temperature gradients inside the regenerator, resulting in distortion of cyclones, cyclone support structure, and regenerator plenums, and particularly at gas outlets. Detection of creep cracks is usually performed by visual inspection or by liquid penetrant testing. To determine the extent of creep damaged material, field metallography and replication techniques can be used prior to deciding the amount of material to repair or replace.
3.2.2.5 Stress Relaxation Cracking Stress relaxation cracking (SRC) is usually associated with fabrication heat treatments or short-term thermal in- service failures due to low creep ductility, but has not been widely reported in FCCUs. For Type 347 stainless steel and Alloy 800 series components with wall thicknesses greater than 12 mm ( 1 / 2 in.) or for fillet weld joints, fabrication, and in particular repair welding procedures, should consider measures such as stress relief to avoid or minimize this damage mechanism. Experience has shown that SRC is unlikely with 304H stainless steel. Refer to Table 3 and Table 4 for susceptibility to SRC for different materials.
3.2.3 Component Specific Considerations
3.2.3.1 Regenerator Cyclones Regenerator cyclones are commonly fabricated from Type 304H stainless steel; however, other 300 series stainless steels have been used. Primary embrittlement is due to sigma phase formation, although carburization can also occur. In-service cyclone cracking failures have not been reported, however, there have been cases of cyclones dropping while the regenerator was being shut down. Failures during shut down are often due to the fact that sigma phase is reasonably tough at high temperature, but typically loses toughness when cooled below about 260 °C (500 °F) (more details are given in Section 4).
Based on comments captured in the minutes of meetings of NACE Committee TEG 205X (formerly T-8) on Refining Industry Corrosion, regenerator cyclones are seldom replaced due to sigma phase content alone, but occasionally are replaced when they become too brittle to repair efficiently. It is more common to see cyclones replaced in order to increase capacity or improve efficiency. Decisions to repair or replace them should be judged case-by-case. If repairs are very difficult during a given shutdown, it may be more cost effective to build new cyclones to be installed during the subsequent shutdown.API TR 942-B pdf download.API TR 942-B:2017 pdf free download
