Hydrocarbon Processing - May 2021 - 65

Heat Transfer
in adjacent double-tubesheets
induce bending and shear forces
on tubes and tubesheet ligaments,
their concentricity is ensured
with this Go gauge.
* Tubesheet ligament tolerances
should be strictly ensured, as per
TEMA Table RCB 7.22 or RCB
7.22M.1 With double-tubesheet
constructions, these tolerances
can be made tighter based on
the manufacturer's capabilities
and confidence.
Careful selection of the type of tubeto-tubesheet joint and the sequence
of welding and expansion within the
tubesheet are vital. Displaced holes and
ligament distortions make it very difficult
to produce tight expanded joints. The
outer tubesheet joints can be made tight
by welding. However, the challenge remains at the inner tubesheet, where joints
can only be made by the process of expansion, as there is no access for welding.
In general, tube end rolling (expansion) within the tubesheet should always follow the welding of the tube-totubesheet joint. This is due to:
* Tube expansion (rolling) before
welding may leave lubricant from
the tube expander in the tube holes.
Other fabrication impurities can
also accumulate at the tube ends.
Satisfactory welds are rarely possible
without extreme cleanliness.
* During tube expansion and before
welding, the expander pushes
tubes against the inside surface of
the tubesheet in the tube holes,
creating uneven gaps between the
outer periphery of the tube and
tube hole within the tubesheet.
Successful welding with an uneven
weld gap is very difficult.
* Tube-to-tubesheet joint welding
after expansion creates uneven tube
movement within the tubesheet
due to tube thermal expansion. This
leads to non-uniform tube tightness
with the tubesheet surface within
the tube holes, which was already
achieved by rolling operation.
* Tube-to-tubesheet joint welding
after expansion will trap the
welding gases in the space between
the outer tube surface and the
tubesheet hole.
During tube expansion, the expanded
portion should never extend beyond the

shell-side face of the tubesheet since the
removal of such a tube is extremely difficult. Additionally, tube expansion for the
inner tubesheets should happen before
welding to the outer tubesheets.2
Consequently, the correct sequences
of assembly and testing are very important while fabricating the doubletubesheet construction, particularly in a
fixed tubesheet-such as TEMA L, M,
N and outside-packed floating heads (Ptype rear heads)-where the number of
tubesheets become four considering a
double-tubesheet arrangement. In such
cases, insertion of tubes through all four
tubesheets becomes critical and presents
many challenges within the facility. Utube, double-tubesheet constructions are
relatively easy in assembly.

FABRICATION AND
ASSEMBLY SEQUENCES
Fabrication and assembly sequences
are presented here for a fixed doubletubesheet heat exchanger.
Method 1:

* In the case of small-diameter shells,
the tubesheet/baffle/tie rod/spacer
skeleton should be built outside the
shell, considering the inaccessible
shell inside area. The same can be
made inside the shell (in the case of
a larger-diameter shell) where the
operator can enter inside and work.
* The first bundle skeleton should be
made with a tie-rod end tubesheet
pair in place, as well as spacers and
clamping, as discussed previously
(FIG. 6A).
* The skeleton is then inserted into
the main shell. The non-tie rod end
tubesheet pair (along with spacers

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FIG. 4. A tube-to-tubesheet (tube side) joint.

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FIG. 5. A tube-to-tubesheet (shell side) joint.
Hydrocarbon Processing | MAY 2021

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Hydrocarbon Processing - May 2021

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