Type 347H SS was specified for the tubulars in a Refinery Lube Oil Vacuum Flasher Heater. The heater was designed for tube metal temperatures of 565°C ( 1050°F), and the tubes were specified to be stabilized annealed in order to resist polythionic acid stress corrosion cracking (PTA-SCC). Testing was performed to determine if resistance to PTA-SCC had been compromised during fabrication of the heater. The test protocol was based on ASTM A-262 Practice C, the 65% nitric acid (Huey) test. All return bends failed the tests, while most of the straight tubes sections passed the tests. The columbium carbide phase was present in very large particles instead of being evenly dispersed as fine particles throughout the matrix. This compromised the effectiveness of the stabilizing anneal and reduced the resistance to PTA-SCC.
A Lube Oil Vacuum Flasher unit was designed to process a high sulfur pitch into lube oil products and residue. The unit receives the pitch from an atmospheric distillation column and heats the pitch under deep vacuum conditions to separate the different lube oil components. At the heart of the unit is the Vacuum Flasher heater. The heater was designed with a tube metal temperature of 565°C (1050?F). Tube materials were specified as Type 347H SS stainless steel in order to prevent sulfidation and resist potential polythionic acid stress corrosion cracking (PTA-SCC). Tubulars were purchased to appropriate ASTM specifications, with a supplemental requirement to stabilize anneal at 899°C (1650?F) for four hours.
In the current business climate many refinery heaters are ordered on a turnkey basis. The owner/operator works with an engineering company who specifies and purchases a heater from a heater supplier. The heater supplier in turn orders the tubes from a distributor or directly from a manufacturer of the tubulars. Conveying very detailed materials information directly to the manufacturer can be difficult with these types of business relations. Such is the case with relaying detailed specifications meant to avoid sensitization and PTA-SCC of the tubulars in this heater.
Polythionic Stress Corrosion Cracking
Austenitic stainless steels and other austenitic alloys may become sensitized, a condition that makes the alloy susceptible to rapid intergranular corrosion, as a result of adverse thermal treatment which causes precipitation of chromium carbides at the grain boundaries. This depletes the matrix adjacent to the grain boundary of the chromium necessary to maintain the overall corrosion resistance of the alloy. Sensitization may occur during weld fabrication, hot processing or operation in the sensitizing range from approximately 426?C (8OO?F) to 816°C (1500?F). The sensitization range varies somewhat for different alloys. In order to prevent sensitization during processing of the material, all of the commonly used ASTM specifications for the various product forms of austenitic stainless steel and other austenitic alloys require a solution anneal heat treatment after manufacture. Chemically stabilized grades may also be procured with a stabilizing heat treatment which further prevents sensitization and increases resistance to PTA-SCC. The stabilizing heat treatment may be invoked at the time of purchase (per Supplemen
