Hydrocarbon Processing - April 2022 - 39

Heat Transfer
tube and tubesheet. Light expansion of
tubes in tube holes does not, in any capacity,
provide mechanical strength to
overall TTSJ capacity except by
centering the tubes in the tube
holes and helping to reduce crevices
(large gaps) between the tube
hole and the tube outside diameter
(OD). Mock-up tests for such
a TTSJ do not include expansion
related parameters (torque, tube
wall thinning, etc.), and the joint
is purely qualified considering the
weld for load carrying capacity.
ASME Code Part UHX calculations
for tubesheets require the
length of tube expansion in the calculations
to be considered in those equations.
The description of ℓtx in nomenclature
of UHX-5.1 emphasises various factors
to be considered for expansion length,
which is clearly shown in this excerpt
from ASME Code:1
ℓtx = expanded length of tube in
tubesheet (0 ≤ ℓtx ≤ h) [see Figure
UHX-11.3-1, sketch (b)]. An
expanded tube-to-tubesheet joint
is produced by applying pressure
inside the tube such that contact is
established between the tube and
tubesheet. In selecting an appropriate
value of expanded length, the
designer shall consider the degree
of initial expansion, differences in
thermal expansion, or other factors
that could result in loosening of the
tubes within the tubesheet.
If the length of light expansion is considered
in the strength calculation of the
tubesheet, the tubesheet calculation will
provide significantly reduced required
thickness, as the equations are written so
that the expansion length will add to the
strength of the tubesheet; however, the
equations do not differentiate between
light and heavy expansion
heavy expansion will aid in strengthening
the TTSJ.
The industry uses different terminology
for expansion, one being light
expansion, or light-rolled and expanded
with or without groves. Light expansion
is considered when tube thinning is
1%-3%; mechanically, this aids the tube
OD to contact the tubesheet hole inside
diameter (ID).
Conversely, tube expansion carried
out for strength purpose has a minimum
wall thinning of 5%-depending on the
material of the tube and tubesheet. Often,
such expansion is provided with
recommended. This type of inaccuracy
will lead to a non-performing body flange
and is likely to jeopardize the exchanger.
While the tube-to-tubesheet joint, girth flange
and expansion bellow are the most critical
components in an STHE, often the least attention is
paid to these parts in the design and review process.
These components must be specified, designed and
reviewed thoroughly to avoid loss of integrity.
grooves in the tubesheet where tube
material will flow during the expansion
process. It is the engineer's choice to take
advantage of tube expansion in the tube
hole. As discussed earlier, light expansion
does not contribute to the TTSJ
capacity. If the TTSJ is lightly expanded
tubes, the expansion length of the tubes
should not be considered in the calculations
(as its effect towards strengthening
the tubesheet will be nearly " zero " ). FIG. 2
shows different types of commonly used
tube-to-tubesheet joints in the industry.
The authors noted tubesheet calculations
that were using the lightly expanded
length of tubes in the equations and arriving
at thinner tubesheets to save material
thickness and cost, jeopardizing the
strength and long-term reliability of the
TTSJ. In those examples, the tubesheets
were calculated 20 mm thinner than
the actual required thickness computed
without considering the lightly expanded
length of tubes in the calculations.
and only
Incorrect flange finish. Kammprofile,
camprofile or " grooved " gaskets are made
of a grooved, solid metal core with soft
coating of suitable non-metallic sealing
layer(s) [graphite, polytetrafluoroethylene
(PTFE), etc.] on each side. According
to API Standard 660, the gasket
contact surface required for a kammprofile
gasket is 125 micro-in.-250 microin.
However, the authors noted that for
some heat exchangers, a 63 micro-in. surface
roughness finish was used for gasket
faces. A flange gasket surface roughness
of 63 micro-in. is appropriate for solid
metal gaskets, while a kammprofile gasket
uses solid metal (core) but has a soft
covering on the top and bottom faces;
therefore, a higher surface roughness is
Incorrect gasket factor " m " and
seating stress " Y " for a kammprofile
gasket. The value of " m " and " Y " for a
kammprofile gasket varies from manufacturer
to manufacturer. To perform the
mechanical design of a girth flange using a
kammprofile gasket, the correct gasket factor
" m " and seating stress " Y " values must
be obtained from the manufacturer to ensure
the production gasket specifications
match the values used in the design of the
girth flange for production. In one case,
the mechanical design of a girth flange
with a kammprofile gasket was performed
using " m " and " Y " values from internet
sources; when the actual gasket manufacturer's
certificate was received, the " m "
and " Y " values were larger compared to
those used in the design and construction
of the girth flange. The larger " m " and " Y "
values per the gasket manufacturer's certificate
overstressed the flange, causing
the already-manufactured girth flange to
fail ASME Code requirements.
Shell-side vent and drain nozzle for
vertical
fixed tubesheet heat exchanger.
All chambers of shell-and-tube
heat exchangers should be capable of being
vented and drained independent of
process piping. A fixed tubesheet heat exchanger
is often provided with vent and
drain connections. For a horizontal exchanger,
there are many location choices
for the vent and drain nozzles; however,
for a vertical fixed tubesheet exchanger-
if provided on the shell-there is always
some dead space left out without complete
venting and draining. The preferred
method is to provide vent and drain on
the tubesheet rather than on the shell:
this allows complete venting and draining
of shell-side fluid. Tubesheets are drilled
Hydrocarbon Processing | APRIL 2022 39
Hydrocarbon Processing - April 2022

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