Hydrocarbon Processing - October 2021 - 69

Heat Transfer
M. KULKARNI, BASF, Mumbai, India; and
M. SUBRAHMANYAM, Technip Energies, Delhi, India
CCR heater convection section bottom
tubesheet design improvements
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
CCR process licensors, fired heater designers/suppliers and end
users/customers in the petroleum refining industry.
Process fired heater configuration. A process fired heater
(i.e., furnace) is a heat transfer-type of equipment that plays an
integral role in the petroleum refining, petrochemicals, fertilizers
and chemicals industries. It consists of a furnace box, heat
transfer coils, coil supports and a flue gas passage. FIG. 1 shows
the configuration of a typical CCR fired heater. FIG. 2 shows
a typical convection coil bottom tubesheet geometric model
used for FEA. Fuel gas, fuel oil, or oil and gas is fired in the
heater through burners, and the generated heat is transferred to
the fluid circulating through coils placed near heater walls and
in the flue gas passage.
Though this study covers typical CCR fired heater convection
bottom tubesheets, the results and outcome can be extended
to other types of fired heaters/furnace configurations.
The point of concern for cast tubesheets used in convection
bottom coils is tubesheet exposure to very high flue gas temperatures
and large thermal gradients present from the coil side
(e.g., steam service or other service coils).
Conventional tubesheet design. A typical conventional
intermediate tubesheet used in a CCR heater convection bottom
section is shown in FIG. 2. The tubesheet is manufactured
by a static casting process. In this study, the tubesheet geomFIG.
1. Typical CCR fired heater configuration.
Hydrocarbon Processing | OCTOBER 2021 69
Hydrocarbon Processing - October 2021

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