Hydrocarbon Processing - April 2022 - 38

Heat Transfer
ing MAWP of the chamber. The MAWP
of the chamber is the lowest MAWP of all
the parts that make up the chamber. Due
to the complexity of expansion bellow
calculations, the computer program may
not be able to model the equipment inclusive
of the expansion bellow to determine
the shell-side MAWP. When any part of
an exchanger is designed using an FEA,
the pressure used in the FEA should be
considered for MAWP calculation; otherwise,
a higher pressure may get stamped
on the nameplate and ASME forms.
A similar situation may arise for
tube-side, single-pass, TEMA-type AES
exchangers that are provided with tubeside
internal expansion bellows. The bellows
are designed considering shell-side
and tube-side design conditions separately-it
is possible that the internal
bellows are not verified to shell-side and
tube-side MAWPs.
Design of common elements. Regarding
design pressure for common elements,
such as tubes, tubesheet and floating head
assembly, when a full vacuum exists on
the shell-side or tube-side (or both sides),
vacuum pressure on one side should be
added to the design pressure of the opposite
side to arrive at the final design pressure
for the applicable side, according to
UG-21 of the ASME Code.1
The authors have encountered many
heat exchangers in which the floating
head cover and flange, and tubes were
designed without considering the full
vacuum on the shell side.
Coefficient of thermal expansion
(TE). The coefficient of thermal expansion
(TE) is required to verify the
shell-side differential thermal expansion
stresses. The TE values in Tables TE-1
to TE-5 of ASME Section II-D4
provide
the values of TE coefficients for temperatures
down to 20°C (68°F).
When evaluating the need for the
expansion bellow for heat exchangers in
low-temperature and cryogenic service,
the TE at the mean metal temperature
(MMT) of the materials should be used.
It is obvious that at lower temperatures,
materials will experience contraction
rather than expansion. A common understanding
among code users is to apply
the proper value of TE from ASME Section
II-D (Material Properties) to perform
mechanical design. However, TE
at MMT below 20°C (68°F) is not contained
in ASME Section II-D because the
Materials Code only provides TE down
to 20°C (68°F), while for this type of design
condition it is necessary to use a TE
value of far below 20°C (68°F).
If the TE at a sub-zero MMT is unavailable,
other references should be explored
to source TE at a sub-zero MMT
to use in the analysis rather than using the
TE at 20°C (68°F). This critical information
is available in ASME B 31.3 (Piping
Code)5
was clarified by ASME Code1
or any other reference book. This
in the following
interpretation number 14-1240.
Standard designation:
BPV Section VIII Div. 1
Edition 2010/Addenda 2011
Paragraph/Figure/Table No.:
UHX-13
Subject description:
Axial differential thermal expansion
between tubes and shell
Date issued: 04/08/2015
Record Number: 14-1240
Question (1): Are the rules in Part
UHX valid if the MMT of the shell or
tubes, or both, are below 20°C (68°F)?
Reply (1): Yes
Question (2): In accordance
with paragraph U-2(g) and the
Introduction to Subpart 2 Physical
Properties Tables in Section II, Part
D, may other sources be used for
values of TE when the material or
temperature is not listed in Tables TE
of Section II, Part D?
Reply (2): Yes.
In one project that contained fixed
tubesheet exchangers in cold service, it
was observed that the design calculation
report mentioned a TE value at 20°C
(68°F), even if the value of MMT was in
negative single or double digits because
the computer program referred to ASME
Section II, Part D database in which the
lowest temperature for TE is 20°C (68°F),
and the same was used for developing the
expansion bellow design. This led to a
bellow design with an incorrect number
of bellow convolutions.
Length of tube expansion for UHX
calculations. ASME and TEMA codes1,2
specify the tube expansion details with or
without grooves; however, light expansion
is not mentioned in either of these
codes. API Standard 6603
mentions
FIG. 2. Tube-to-tubesheet joint details: welded with light expansion (A) and expanded with
two grooves (B).
38 APRIL 2022 | HydrocarbonProcessing.com
light expansion as one where tube wall
thinning is < 2%. In many instances, the
exchanger data sheet specifies strength
welded tubes to be lightly expanded for a
tube-to-tubesheet joint (TTSJ).
The purpose of light expansion is to
establish metal-to-metal contact between

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