FCC’s are complex units – and the design of their transfer lines has some unique considerations.
While many of the lines are refractory lined to permit construction with carbon steel piping, some sections are hot walled, at temperatures well into the creep range, for the purpose of balancing thermal expansion or, in the case of piping between the final catalyst separations device and flue gas expanders, to prevent dislodged chunks of refractory from being drawn into and damaging the expander. Below are some critical considerations:
- The stiffness of the piping and resultant loads on equipment are affected by the presence of internal refractory. The composite action of the steel pipe and refractory needs to be considered. Note that this is not simply including the refractory as a monolithic element, since the refractory will have shrinkage cracks. There is a paper by T Chadda on an approach that can be used.
- When a piping system has both hot walled (unlined) and cold walled (lined) piping, a condition called elastic follow-up can occur. In this condition the thermal displacement of the system gets concentrated in time into the hot walled portion and can result in creep cracking. In design of such systems, one conservative approach is to limit the combined stresses including thermal displacement induced stresses to the basic code allowable stress (i.e. that contained in B31.3 Appendix A). Other less conservative approaches can also be taken. Another blog will be published next month with a more detailed description of elastic follow-up.
- Transient relative displacements during startup and shut down can be a consideration. An example actual case was a refractory lined reactor vessel and an unlined reactor overhead line. During startup the piping heated up more quickly than the reactor causing the spring supports to bottom out and put a large load on the reactor overhead line nozzle during the startup process, and then come back into the spring operating ranges as the reactor vessel wall temperature increased to its normal operating condition. In another example, a hot walled inlet line to an expander heated unevenly across the diameter, bowed significantly, and placed excessive loads on the expander.
- Surge loads in the transfer lines results in dynamic loading which needs to be considered.
- Transfer lines use expansion joints to accommodate thermal expansion as loops are not an option due to process considerations. The actual behavior of such joints needs to be properly considered in the analysis.
- In some designs, there are relatively stagnant dense phase catalyst zones on the bottom of sloped lines with dilute phase catalyst above. This has occurred for example in some interconnecting piping between regenerators and combustors. This causes a temperature gradient around the pipe causing the pipe to curl and introduce additional displacements into expansion joints.
- Hot walled piping requires the use of specific design details to achieve reliability. For example, pad reinforcement typically results in cracking around the pad fillet welds over time. So integral reinforcement is used.
- Flue gas expanders have large, hot walled piping attached to them, but have low allowable load limits. Very detailed design is required to assure that loads, including consideration of friction (sliding and hinge pins), does not put loads on the expander that will adversely impact performance.
- Proper details need to be followed for small bore connections to the transfer lines. Sometimes they are not sufficiently braced and are vulnerable to fatigue failure due to vibration. At other times they are attached to structure and movements of the transfer line can cause failure of the branch.
- Proper details are required in the cold to hot wall transition at vessel nozzles, for hot wall piping such as catalyst withdrawal lines.
- The pressure drop in the expander bypass line needs to be properly checked and designed to avoid the generation of very high sound power levels that can cause acoustic fatigue failures in the piping. The sound causes the pipe walls to vibrate at hundreds of hertz frequencies leading to rapid fatigue failures at discontinuities.
- We have seen cold-wall piping insulated for personnel protection. This is a safety issue as refractory failures are then not readily detectable, and can lead to creep and graphitization issues in the carbon steel pipe. Mesh cages are a more appropriate solution for personnel protection.
- We have also encountered a surprising number of construction errors in recent FCC revamps as well including at least two with incorrectly installed expansion joints. In one, an expansion joint with pantographic linkages was deliberately installed rotated 90 degrees from its proper installation, preventing it from working properly and, in another case, a hinged expansion joint was installed rotated one bolt hole from proper alignment also preventing it from functioning properly.
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