Hydrocarbon Processing - October 2021 - 73

Heat Transfer
in operation due to coil/connecting coil header and tubesheet
relative thermal movements-and the effect in the ligaments.
For case study purposes, a FEA model was conducted with
a contact area between the tubes and the tubesheet holes at
10°, 20°, 30° and 60°, with four different models. This phenomenon
is captured in the FEA by connecting the tube center
point and the tube hole nodes in the vicinity of the contact
area by using multi-point constraint elements. Cases 1-4 in
TABLE 2 show the contact area of 10°, 20°, 30° and 60° between
the tube hole and the tubes. As the contact area increases, the
overstressed area near the ligament also increases.
A case study was also performed on the tube's resting position
in the tube hole, with a contact angle of 10°. The study
was performed with tube rest locations at 0°, 45°, 90°, 135°
and 180°. Cases 5-8 in TABLE 2 show the various tube contact
points in the tube hole and the stress distribution during thermal
expansion for various contact areas. From the stress distribution
pattern, as the contact area increases, stresses in the
ligaments increased, as well.
Stress distribution during thermal expansion for various
contact angles is also shown in TABLE 2. From the stress distribution
pattern, the overstressed region in the tubesheet will
vary in line with the contacting location of the tube. If the contact
takes place in the ligament and not at 0° or 180°, overstress
is higher.
New design. From the detailed FEA and case study, a new
tubesheet design was completed (FIG. 7). The following design
improvements were incorporated:
* The shape of the tubesheet's holes were changed from
circular to slotted to allow the free movement of the
tubesheet and coil terminal headers during thermal
expansion; therefore, the tubes will not exert lateral
forces on the tubesheet's web ligaments.
* In the bottom tube row, the ligaments between the
tube holes in the tubesheet experience a very high
temperature gradient. To avoid this, the tubesheet
flange height below the bottom tube row and above
the top row were increased.
* In the original installation, the tubesheet support
location was above the first tube row. Considering
thermal expansion, to ensure that the tube rests
within the tubesheet during operation, the support
location was shifted to below the first tube row
bottom flange.
* The materials composition of the tubesheet was
changed to 25Cr-35Ni-Nb (A297 HP Mod) since
it has high strength at elevated temperatures.
The new design was simulated for the following load conditions:
1. Temperature load
2. Dead weight
3. Friction load.
During thermal expansion, the tube's position in the tube hole
did not exert force on the ligaments due to the tubesheet hole's
slotted shape change, which allowed for free movement of the
tubesheet and coil headers/tubes. Therefore, an analysis was
not performed for this load.
FIG. 8 shows temperature distribution in the tubesheet and
in the bottom tube row ligaments. In the new design, thermal
FIG. 8. Stress distribution in the new design.
gradient across the ligament is lower vs. the original design.
Stresses in the tubesheet are within allowable limits, as well.
Takeaways. The probable causes of failure in cast intermediate
tubesheets within a CCR heater were investigated.It was found
that insufficient space for coil thermal expansion/displacement
in the tube holes and a large temperature gradient in the bottom
row ligaments were the major causes of tubesheet web and
flange cracks, which led to the tubesheet failures.
A new design of the tubesheet was proposed to overcome
the limitations in the initial design. The new design was simulated
by FEA and found suitable for the intended design and
loading conditions.
The new tubesheet design was implemented in one of the
CCR heater installations as part of a heater revamp. All convection
bottom coil bank cast tubesheets were replaced. The
CCR heater, with the new cast tubesheet design, has operated
efficiently for the past 2 yr, with no cracks.
MAHESH KULKARNI is Manager-Mechanical in Technical
Expertise and Discipline Engineering at BASF. He earned a
mechanical engineering degree from Pune University in 1999
and an ME degree in mechanical-heat power engineering in
2002 from Government College of Engineering, Karad, Shivaji
University. He has more than 21 yr of experience and is a specialist
in FEA in the processing industries. Mr. Kulkarni has experience in
various types of static equipment in chemical and petrochemical projects. He has
also worked as a FEA expert, project engineer and root cause analysis and
troubleshooting expert in refining projects.
M. SUBRAHMANYAM is Manager, Furnace Engineering at
Technip Energies India. He earned a mechanical engineering
degree from Sri Venkateswara University, Tirupati and an MS
degree in applied mechanics from IIT, Madras. He has more
than 26 yr of EPC experience and is a specialist in process fired
heaters, including reformers and crackers, which are widely used
in petroleum refineries and petrochemical and fertilizer plants.
Mr. Subrahmanyam has experience in the design and engineering of heaters and
has also been involved in various failure analysis studies and troubleshooting
services for furnaces and furnace components.
Hydrocarbon Processing | OCTOBER 2021 73
Hydrocarbon Processing - October 2021

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