CCR heater convection section bottom tubesheet design improvements

摘要

Continuous catalytic reforming(CCR)is a general reforming process used in the petroleum refining industry.The process fired heater plays an important role in this process.Fired heaters consist of heat transfer,convection and radiation coils.The radiant section is typically used for process heating,while the convection section is typically used for heat recovery and steam generation.Convection coils have horizontal tubes that are supported on refractory-lined end tubesheets(plated construction)and cast intermediate tubesheets.In the typical installation of a CCR heater convection bottom section,multiple cast intermediate tubesheets are used to support coil banks.The bottom tubesheet that supports steam generation and/or steam superheating coils is exposed to high-temperature flue gases coming from the radiant section.Simultaneously,the tubesheet is cooled by the heat transfer tubes through which boiler feed/steam generation/steam super heating takes place.Therefore,the CCR convection bottom tubesheets are exposed to severe thermal gradients(flue gas side-to-process side temperature variations).Cast intermediate tubesheet flanges and web ligaments(material between tube holes)experienced loading(e.g.,thermal expansion)due to high temperatures,frictional force between the tubes and the tube holes,and the dead weight of the tubes.In one of the fired heater installations,the cast intermediate tubesheet in the CCR heater developed cracks in the web ligaments and flanges of the bottom tube row,leading to failure of the tubesheet and frequent replacements.In this article,finite element analysis(FEA)root cause analysis of the failed tubesheet and the countermeasures adopted in an improved new tubesheet design are explained in detail.The findings are beneficial to CCRprocess licensors,fired heater designers/suppliers and end users/customers in the petroleum refining industry.