Hydrocarbon Processing - May 2021 - 64

Heat Transfer
construction, shown in FIG. 3. In all
three types of constructions, care should
be taken while designing the tube-totubesheet joint. Primarily, the tube-to-

ly transfer all mechanical and thermal axial
loads between the tubesheets.
In general, various types of stresses
originated in the construction include:

Leakage can typically occur through the tube-totubesheet joint. Careful selection of the type of
tube-to-tubesheet joint and the sequence of welding
and expansion within the tubesheet are vital.
tubesheet joint on the tube side must
be strength-welded with light expansion
(FIG. 4) to eliminate the possibility of a
leakage of the tube-side fluid through
the tube-to-tubesheet joint. On the other
side, the tube-to-tubesheet joint on the
shell side must be grooved with a minimum of two grooves and expanded to
the full length, as shown in FIG 5. This
grooved expansion joint must be selected
considering the fact that on the tube-totubesheet joint of the shell side, welding
is practically impossible.
Design. In connected (FIG. 3B) and integral (FIG. 3A) double-tubesheets, axial load
distribution is accomplished by an interconnecting element/shroud shell (in the
case of a connected double-tubesheet) or
an integral portion of the tubesheet (in the
case of integral double-tubesheet). In the
integral double-tubesheet construction, an
interconnecting element is so rigid that it
distributes thermal and mechanical radial
loads between the tubesheets and prevents
the individual radial growth of tubesheets.
For both constructions, tubes can mutual-

FIG. 2. With a drastic difference in mean metal
temperature of the shell side and tube side
and different metallurgy used for a shell-side
and tube-side tubesheet, a shroud may be
provided with an expansion bellow.

* Differential pressure stresses due to
a difference in operating pressures
between tube-side fluids and shellside fluids
* Axial stresses due to tension
or compression of the tubes;
differential thermal expansion
between shell and tubes is another
parameter that induces axial stresses
* Shear stresses induced due to
differential thermal expansion
between the tubes and tubesheet
in radial direction
* Thermal and pressure stresses
induced due to upset conditions.
Interconnecting elements and tubes
between tubesheets of the connected
double-tubesheet must be designed according to these parameters:
* Interconnecting element-Radial
shear stress at the junctions due to
differential thermal expansion of the
tubesheets
* Interconnecting element-The
combined stresses due to bending
and axial tension induced due to
differential thermal expansion of
tubesheets and thermal expansion of
tubes, respectively
* Tubes-Axial tensile or
compressive/buckling stresses
acting due to operating pressure and
thermal expansion.
Fabrication and testing. Tube-totubesheet leak tightness is directly affected by how the double-tubesheet ex-

FIG. 3. Integral (A), connected (B) and separate (C) double-tubesheet type constructions.

MAY 2021 | HydrocarbonProcessing.com

changers are manufactured. High-quality
tubesheets, baffles and tube supports are
produced by drilling holes-either individually or in a stack-with the help
of computerized numerically controlled (CNC) machines. The
CNC machines ensure that holes
in tubesheets, baffles and supports
plates are concentric and precise
enough to allow them to be easily occupied by tubes. If tubesheets
and baffles/support plates are
stacked and drilled on conventional
radial drilling machines, the drill
can drift as it penetrates the stack.
During assembly, hole-to-hole positions may also be displaced if tubesheet
main center lines are not maintained
congruently. Additionally, major difficulties may also be created if tubesheets
are not kept parallel with each other.
For these reasons, it is important for a
purchaser to review the manufacturer's
equipment/tools and techniques used
for drilling and assembly.
Guidelines for manufacturers to ensure proper assembly include:
* Tube-side and shell-side faces of
a tubesheet should be machined
flat and perpendicular to tube
(and bolt) holes. Adjacent faces of
tubesheets should also be machined
in similar fashion from just outside
of outside tube limit (OTL) to the
tubesheet periphery.
* A suitable number of spacers (made
from pipe, rod or plate) should be
prepared and preciously machined
to the specified gap distance
between the tubesheets.
* The spacers must be placed
equally on the periphery between
the pair of tubesheets. These
aligned tubesheet pairs should be
clamped and kept in place until all
tubing, tube-to-tubesheet joining,
and tubesheet-to-shell/channel
assembly have been completed.
* A Go gauge machined from a rod
that is slightly longer than the
distance between the outer faces of
the tubesheets should be prepared.
The Go gauge ensures free entry
of tubes in the tube holes of both
tubesheets. Before tubing the
assembly, randomly check each
quadrant of the tubesheet layout
to ensure that the gauge is entering
freely. Since non-concentric holes

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