Line temperature | 700 °C |
Design temperature | 755 °C |
Design pressure | 4.2 barg |
Pipe material | A 312 type 304H |
Allowable stresses | ASME B31.3 Appendix A |
Expansion loop | 4 inch, 6,000 mm deep |
Creep-range service on the catalyst withdrawal line
A waste catalyst line runs from the regenerator to the waste catalyst cooler. At a line temperature of 700 °C the pipe material is in its creep range.
Creep-range operation sets the terms of the design. For a high-temperature system of this kind, the failure mechanism of concern is excessive pipe strain due to creep, and long-term strain is the result of long-term, sustained loads. The code therefore requires the sustained stress, the stress due to weight and internal pressure, to stay within the creep value for the applicable material. That creep stress value at temperatures above 700 °C for type 304H materials is relatively low, and the low material allowable stress imposes relatively strong requirements with respect to supporting. At the same time, the large thermal line expansion in combination with the thermal vessel growth requires pipe system flexibility. Both requirements apply to the line simultaneously.
In the existing system, line flexibility is generated by a number of expansion joints. In the proposed modification, the expansion joint in the horizontal line is replaced by an expansion loop. The analysis had to establish whether the pipe and nozzle stresses of the new configuration would stay within allowable limits.
Modelling the line together with the connected vessels
The pipe line was modelled with the regenerator, the waste catalyst hopper and the waste catalyst cooler included for the purpose of model closure. The regenerator and the catalyst hopper are fabricated from carbon steel; the catalyst cooler is made from stainless steel. The allowable stresses for the pipe material correspond to the values indicated in ASME B31.3 Appendix A. Finned pipe was modelled in accordance with pipe class 6325X, material 304H, with 100 mm insulation used to model the finned material. Operating temperature and operating pressure were taken from the client sketch. All tee connections in the expansion loops are welding tees.
Two options, similar to each other but different in supporting, were carried through the analysis. The loop size is 4 inches in both, in accordance with client information, and the loop is 6,000 mm deep to accommodate thermal expansion. In the first option the expansion loop is located in the vertical plane; in the second the expansion loop is horizontal.
Converging the support arrangement through iterative analysis
The support arrangement, meaning both support locations and support functions, was designed in a large number of iterative analyses for both configurations. The objective was to keep the sustained and expansion stresses within allowable limits. Given the low creep value, the support arrangement was designed particularly to accommodate the weight loads of the piping system. The allowable stress range for thermal expansion is much larger, and the stresses resulting from thermal expansion are self-limiting; there the failure mechanism of interest is low-cycle fatigue, and a proper selection of the loop size has taken care of that problem.
Modifications common to both options
- The expansion loop itself, 6,000 mm deep.
- The lower of the two expansion joints in the vertical line can be removed and replaced by pipe.
- Spring hangers are changed, with the setting of one existing hanger on the vertical line changed to 2,900 N, from 2,400 N.
- One tee connection is changed into an 8 inch by 6 inch tee with 6 inch flanges and a reducer.
- The wall thickness in one spool is increased to 10 mm, one of a limited number of pipe sections where the pipe wall thickness was increased to limit sustained stress levels.
In addition, a pipe stop in the x-direction is introduced in the tee at the tie-in of the MAB air stream; this pipe stop is required to force expansion into the loop.
Where the two options differ
| Vertical loop | Horizontal loop |
Loop orientation | Vertical plane | Horizontal |
Supporting principle | Completely supported from springs | Mixture of spring supports and rigid supports |
Spring hangers | Six nodes, alongside the two existing hangers on the vertical line | Six nodes, alongside the same two existing hangers |
Rigid supports | None | Two, near the bottom of the loop |
Lateral restraint for wind loads | Six nodes, z-direction | Four nodes |
Pipe and nozzle stresses | Within allowable limits | Within allowable limits |
Spring sizes for the horizontal option are different from the springs for the vertical loop, and some support locations are different between the two options. The existing hangers on the vertical line are supported from platforms connected to the catalyst cooler, and these platforms are assumed to grow with the vessel expansion; otherwise those hangers would need a large working displacement and would have to be redesigned.
Two feasible support arrangements
The calculation results demonstrate that with the proposed support arrangement the pipe and nozzle stresses for both options are within allowable limits. Stress levels, support loads and hanger sizing are reported separately for the vertical loop and for the horizontal loop. The analysis results demonstrate that the indicated support arrangements for both the vertical and the horizontal loop variant result in a feasible solution for the withdrawal line redesign.
What the work shows for creep-range piping
Where the allowable stress is set by the creep value of the material, the support arrangement is the controlling part of the design. |
Loop size takes care of the expansion side, where the stresses are self-limiting and the mechanism of interest is low-cycle fatigue. The weight side is settled by iteration on the locations and functions of springs, rigid supports and lateral restraints, supported by an increase in wall thickness at a limited number of pipe sections to limit sustained stress levels and by a pipe stop at the branch tie-in that forces the expansion into the loop. On that basis the expansion joint in the horizontal line can be replaced by an expansion loop, with pipe and nozzle stresses remaining within allowable limits, on a model in which the connected vessels are included so that their thermal growth is carried in the analysis.