Major cracking problems have occurred during fabrication of thick wall stainless steel (SS) piping for hydroprocessing units, primarily during heat treatment. This has resulted in costly repairs and schedule delays. These heat treatments are specified for some units to improve sensitization resistance and to reduce residual stresses, which provides the potential benefit of improved resistance to stress corrosion cracking (SCC). However, piping which has not been heat treated has been performing successfully for many years in numerous units. This paper provides some case histories of fabrication cracking and discusses the cracking mechanisms, suggests guidelines for when and when not to heat treat, and gives recommendations for minimizing the risk of cracking when heat treatment is required.
Hydroprocessing units in oil refineries upgrade hydrocarbon feedstocks by removing undesirable constituents and/or by converting heavier feeds into more valuable lighter products. The feedstocks range from naphtha to vacuum residuum. The reactions occur under a hydrogen-rich environment at moderately high temperatures and high pressure, in the presence of catalysts.
Heavy wall stainless steel pipes, such as Type 321 or Type 347, are widely used in hydroprocessing units. Figure 1 shows a simplified process flow diagram of a single stage hydrocracking reactor section. Applications include furnace tubes, reactor clad/overlay, reactor inlet and effluent lines and heat exchangers. In the process scheme shown in Figure 1, the recycle hydrogen is added to the feed primarily in the feed/effluent exchanger train. The highest temperature piping in these units is the reactor effluent piping. While most units have reactor effluent temperatures of less than 425°C (800°F) in some units the effluent temperature can be up to 455°C (850°F). Another common unit design is shown in Figure 2. In this case, the recycle hydrogen is heated separately in a heater and is added to the feed just upstream of the reactor. The outlet piping from these heaters is often in the 480-565°C (900- 1050°F) range. This operating temperature presents some unique concerns and challenges for the stainless piping as discussed later in the paper.
Principal corrosives in the high temperature (>290°C or >550°F) sections of hydroprocessing units are H2S, H2 and possibly naphthenic acid. Chlorides and polythionic acid are also concerns during turnarounds. Except for possibly naphthenic acids under certain conditions, Type 321 and 347 can resist such corrosive environments with proper fabrication and some shutdown protection measures. For resistance to H2S and H2, most equipment and piping in the reactor sections for applications 290°C (550°F) and above are made of or clad with these stainless steels. Heavy wall reactors and exchanger shells are typically made of stainless overlaid chrome-moly steels which are more economical than solid stainless steel construction.
