Critical considerations for retrofit designing a platformer reactor’s partial bypass piping in a NHT plant

A. Kittur, Saudi Aramco, Dhahran, Saudi Arabia

The naphtha hydrotreater/continuous catalyst regeneration (NHT/CCR) platformer produces reformate to be utilized as a blending stock in the gasoline pool (FIG. 1). In the platformer, low-octane naphtha feed is converted to high aromatic reformate, while producing hydrogen on platinum/alumina catalysts. It employs a suitable catalyst and hydrogen to remove organic sulfur and nitrogen components that are detrimental to CCR platformer catalysts.

The catalyst is deactivated by coke deposition during the reaction. Deactivated spent catalyst is continuously replaced with regenerated catalyst. The reactor inlet temperature is designed for 548.89°C (1,020°F). Operational experience has shown that, when the unit operates above 40,000 bpd, the second reactor’s center pipe slowly plugs from the top down, and the reactor’s ΔP increases slowly over time. Eventually, the reactor’s ΔP increases to the point where feed maldistribution causes pinning of the catalysts in the reactor. A 5-yr run length at 40,000 bpd is achievable; however, a 5-yr run at 46,000 bpd or more was not possible without a shutdown to clean and maintain the center pipe.

A plan was needed to operate at a higher throughput of 50,000 bpd without causing an excessive ΔP buildup in the second reactor. Based on a process revamp study by the licensor to evaluate unit operation at 50,000 bpd and to limit high-pressure buildup across the second reactor, a 20-in. partial bypass (FIG. 2) of feed around the second reactor was required.

Problem statement. A detailed examination of the challenges associated with the mechanical design, materials, installation and field testing of a new 20-in. partial bypass line across the second reactor inlet line was required. As the scope would need to consider the existing brownfield environment, the following aspects would require special attention:

1. The reactor’s inlet temperature was designed for 548.89°C (1,020°F), resulting in the reactor vessels and their associated 36-in. vertical inlet headers undergoing a differential operating thermal expansion growth of nearly 4 in.—at the tie-in location for the 20-in. bypass—in the vertically upward direction. This aspect would require immense piping flexibility with minimum restraining supports that would impede thermal movement.
2. The new 20-in. reactor bypass line would be installed at a substantial elevation within the plant, which would be significantly impacted by the anticipated external wind loading. The abrupt branch of the 20-in. bypass line would also be required to accommodate the internal fluid flow-based dynamic loading. The dynamic loading consideration would essentially require that the new high-temperature bypass header be sufficiently rigid (not flexible) to ensure no potential vibration due to low-frequency excitations. This was contrary to the requirements in point (a) above; hence, maintaining a delicate design balance was required to comply with both point (a) and this aspect (b).
3. The branch joint of the 20-in. bypass pipe with the 36-in. header would require special attention in terms of checking the local stresses in detail based on both the static loads in point (a) as well as the dynamic loads mentioned in point (b). This would be in addition to the traditional design based on pressure temperature rating, which would normally consider the overall piping’s sustained (SUS), expansion (EXP) and operational (OPE) stresses.
4. The low-alloy 1.25CR-0.5Mo (1.25 Chromium–0.5 molybdenum) piping material required special attention with regard to procurement, fabrication and welding, inspection and testing.
5. Integrity testing of the completed installation by means of a pressure test to fulfill governing code requirements required careful examination, especially since the scope of piping involved isolating the reactor that was filled with catalyst.

Global piping stress and flexibility analysis. Using commercially available software^a, a static piping stress and flexibility analysis of the existing piping configurations (based on as-built conditions) was completed in accordance with ASME B31.3 requirements, using the beam elements with 6° of freedom. FIG. 3 illustrates the 20-in. #300-rated bypass piping that connected each of the two 36-in. pipe headers to and from the second reactor. The displacements and rotations of a global stress model of the overall piping stress and flexibility of the entire 36-in. reactor inlet/outlet lines of the second reactor were applied as “boundary conditions.”

In addition, all loading scenarios related to the ASME B31.3 code were accounted for, based on the environmental loads. The analysis was completed to evaluate SUS, OPE and EXP stress categories, and these were found to be well within ASME B31.3’s allowable limits. The piping support design was established based on the minimum thermal expansion restraint provided by a constant spring hanger (CSH) that could accommodate the 4-in. thermal travel and maintain the expansion stresses within the code’s allowable limits, while simultaneously supporting the 20-in. bypass piping and maintaining the sustaining stresses well within the allowable limits. The CSH piping support is illustrated in FIG. 4.

Further to the static analysis, a modal (dynamic) analysis was also obtained, utilizing commercial software^a, to compute the low-frequency flexural modes (the first 20 modes of natural frequencies) of the main pipework. As the design based on the static analysis was found to be overly flexible, minor support location adjustments were made. After multiple successive iterations, the most optimum design of the support locations was established, and these extremely low fundamental natural frequencies were eliminated from the overly flexible piping system.

Once the piping design based on global stress and related flexibility was established, it was followed by a local stress analysis based on the following methodology. For each of the three identified load cases (SUS, OPE and EXP stress categories), the three sets of nodal displacements (translations and rotations) for the corresponding solid finite element model (FEM)—two nodes for the 36-in. header, and one node each for the 20-in. branch pipe—were extracted from the static stress results obtained from this commercial software’s analysis. These sets of nodal displacements, which would be applied as the boundary conditions for each of the corresponding three nodes of the solid model (two nodes for the 36-in. header, and one node each for the 20-in. branch pipe), are shown in FIG. 5. The “local FEM” using the 3D solid model is detailed in the following section.

Local stress analysis. The stress intensification factors (SIFs) built into the commercial software’s program account for the stress intensification, based on the configuration of the branch connection joint between the 36-in. inlet header and the 20-in. bypass pipe in the global pipe stress and flexibility analysis. Although the ASME B31.3 code for process piping provides the reinforcement requirement calculations for the branch joints between cross piping runs, it does not provide an additional framework to investigate the local stresses at the branch connection in depth, if required. Using commercially available software^b in accordance with ASME Section VIII Division 2 principles, a FEM-based stress simulation was completed, since it was one of the most effective ways to evaluate this aspect.

The FEM with continuum elements provided the total stress distribution for evaluation, and the simulated stresses were compared with stress limits that are defined in the ASME Section VIII Division 2 requirements. The 20-in. and 36-in. pipe materials were of material specification ASTM A335 Gr-P11, and these properties were utilized for the maximum allowable acceptable limits of induced stress. The code stress checks were found to be well within ASME B31.3’s allowable limits, as well.

Low-alloy 1.25Cr-0.5Mo piping material requirements. For the field welds to be completed on the 36-in. reactor inlet header, the old/existing pipe wall at the tie-in locations was thoroughly checked with ultrasonic testing (UT) scans to ensure sufficient thickness and the absence of laminations or parent metal defects prior to cutting. After the cutout of the hole for the branch welds of the connecting 20-in. weldolet that was to be welded at the tie-in location was completed with grinding and edge preparation, a dye penetrant test (DPT) was conducted on the beveled edge of the hole in the 36-in. pipe to detect the presence of any surface defects. Additional requirements applicable to 1.25Cr-0.5Mo materials for services with operating temperatures over 440°C (825°F) included:

1. Charpy V-notch impact testing for all pressure-retaining longitudinally fusion-welded pipe, components and welds. Impact tests were required in accordance with ASME B31.3 requirements, except that there were no exemptions from impact testing, and the test temperature was lower than the design minimum metal temperature of –18°C. The test specimens were supplied in the same heat-treated condition as the new pipe.
2. The product analysis content of tin and phosphorous for pressure-retaining components and welding consumables was required to be less than 0.015% and 0.012%, respectively. The percentage limits were weight percent.

The low-alloy 1.25Cr-0.5Mo piping material required special attention regarding procurement, fabrication and welding, inspection and testing. All welding was required to be completed with welding and welders qualified in accordance with ASME B31.3 and ASME Section IX. This welding was followed by a required post-weld heat treatment (PWHT) as per code requirements. A thorough visual assessment of the welds was completed, followed with a surface flaw-detection, non-destructive examination (NDE) method of DPT.

Finally, the weld-through-wall quality was ensured by completing a radiography test (RT) of the weld. In general, RT was completed twice for all welds—once before the PWHT, and after the PWHT to ensure the absence of any delayed cracking associated with the material. The 36-in. x 20-in. branch welds were checked to confirm the absence of any through-thickness weld-related defects by completing a true volumetric NDE utilizing UT shear wave.

Integrity testing requirements. To ensure the integrity of the fabricated 20-in. bypass header piping, a shop hydrostatic pressure test was performed at the full code requirement corresponding to the ASME B16.5 standard for #300-rated piping. The hydrostatic pressure testing was performed after the completion of the PWHT. For 1.25Cr-0.5Mo piping, the minimum metal temperature during pressure testing was ensured to be at least 17°C (63°F) above the brittle-ductile transition temperature of all pressure-retaining components at the time of pressure testing, but not less than 15°C (59°F). Due to operational limitations posed by the brownfield environment, it was not possible to subject the new piping to the final field hydrotest due to the service conditions involving reactor isolations and catalyst contamination. A pneumatic pressure test was completed per the requirements of the original equipment manufacturer’s piping design—nitrogen was used for the new 20-in. bypass, along with the connected 36-in. reactor inlet lines.

Takeaway. This article has provided helpful insights and guidelines on the critical aspects that should be considered in a retrofit design of low-alloy 1.25Cr-0.5Mo piping material employed in fabricating and installing the platformer reactor’s partial bypass piping in a NHT plant. HP

NOTES

^a Hexagon CAESAR II software

^b Ansys software

Aslam Kittur is a mechanical engineer with expertise in troubleshooting integrity issues related to refinery static equipment. He now works in the technical services division of the global manufacturing administration area of Saudi Aramco’s downstream segment. Kittur has more than 23 yr of experience in the energy industry and specializes in performing stress analysis to understand the complex failure mechanisms of critical equipment. His focus is primarily on establishing root causes for repetitive failures and developing solutions to ensure reliable design for any given operating condition. Kittur earned an MS degree in solid mechanics and employs FEM-based structural and thermal simulations that are often required for a Level 3 API 579 fitness-for-service evaluation. Prior to joining Saudi Aramco, he was involved in a similar capacity for 10 yr in Canadian oil sands facilities.