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Catalyst Reduction Reactor

During a routine inspection cracks were found in some of the welds of the load bearing structure of an 8 meter diameter, 36 meter high catalyst reduction reactor located at a petrochemical manufacturing plant.  The potentially disastrous effects of a structural collapse prompted the request for an evaluation of the problem and audit of the design with the goal of redesigning or repairing the vessel structure.

An analysis was conducted to evaluate the magnitude and resulting stress distributions that the vessel experienced during one normal operating cycle consisting of startup, production and shutdown, and the associated fatigue effects.  The first step towards setting up the transient analysis required the transient thermal loading cycles and the dynamic loading due to turbulence of the chemical reaction to be defined.  A transient thermal analysis could then be conducted to calculate the changing temperature distribution, the results of which were verified by placing a number of thermocouples on the vessel during operation.  From the transient temperature field the stress distributions over the cycle could be calculated and combined with those resulting from the dynamic excitation of the chemical process, producing a complete stress cycle from startup to shutdown.  These results were used to calculate the fatigue effects of one cycle and then extrapolated to determine the fatigue life of the vessel support, all evaluated against ASME pressure vessel codes.  Our results were in agreement with the physical observations made on the vessel and thus validated.

We determined that the cause of the problem was excessive stress due to overconstraining of the structure, ultimately resulting in fatigue cracking.  The recommendation made was the complete removal of the cracked welds to allow the structure to flex unconstrained under cyclic thermal loading.  The remaining structural welds were intact and more than capable of carrying the vessel loads with sufficient safety margin.

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	Complete model of catalyst reduction reactor incorporating a mixture of solid and shell elements.
	Half solid model of catalyst reduction reactor showing thermal distribution during transient analysis