Hydrocarbon Processing - June 2022 - 87

Heat Transfer
is SA-387-11-2 plate and the tubesheet is
integrally welded to the shell. The corresponding
tubesheet forging normally
adopted is from SA-336-11-2, and the
tubesheet
is provided with a hub for
welding with the shell from SA-387-11-2.
The tubesheet hub thickness was inadvertently
maintained the same as the
shell thickness without realizing that the
allowable stresses of LAS Class 2 forging
are lower than that of LAS Class 2 plate.
For such constructions, the tubesheet
forging should be from SA-336-11-3
(Class 3), which will match the allowable
stresses of the LAS Class 2 plate; the hub
can be from the same thickness as the
shell. FIG. 7A shows the tubesheet hub-toshell
attachment detail when both thicknesses
are equal (the hub material allowable
stress is equal to the shell material
allowable stress), while FIG. 7B shows the
attachment detail when the hub (material
with a lower allowable stress) is welded
to the shell (material with higher allowable
stress compared to the hub).
This will be difficult to correct (or
weld repair) once the tubesheet hub is
Limitations about pressure vessel
bolting diameters. Pressure bolting
materials for ASME pressure vessel applications
are limited by the diameters
of bolt material specifications listed in
the " Allowable Stress Table 3 " of Section
II-Part D. Whenever bolt specifications
from ASME Section II-Part A or Part B
are used for pressure vessel application,
only those diameters of the material
specifications that are listed in Table 3 of
Section II-Part D5
should be used.
The authors encountered a heat exchanger
in which the body flange used a
2.75-in. diameter SA-320-L7 as pressure
bolting. ASME Code Section II-A may list
mechanical properties and chemical composition
of a bolt specification for a diameter
that may be outside the size range
listed in ASME Section II-D Table 3; this
does not mean that such a bolt diameter
and specification are permitted for use in
machined out and it is determined that
the Class 2 forged hub thickness is maintained
the same as the cylindrical shell
thickness from a Class 2 plate.
pressure vessel applications.
Even if ASME Code Section II-A permits
a 2.75-in. diameter SA-320-L7 bolt
for manufacture, allowable stresses for
SA-320-L7 bolts are only provided up
to 2.5-in. diameter maximum in the Allowable
Stress Table 3 of Section II-D4
.
Therefore, 2.75-in. diameter SA-320-L7
bolts are not permitted for pressure vessel
applications.
For any bolt material, the size/thickness
column should be used as a reference
for diameter limitations of bolts. Refer to
the extreme right column in TABLE 3, an extract
from Table 3 of ASME Section II-D4
.
SA-456-660 bolts are required to
be impact tested at minimum design
metal temperature (MDMT) of
≤ 20°C. SA-456-660 A or B bolts are
used as pressure bolting on high-alloy
body flange joints at high temperatures
[> 650°F (> 343°C)], particularly under
high design pressure (> 50 barg). The
advantage of Alloy 660 is that there is no
diameter restriction in Section II-D, and
the allowable stresses are steady up to
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Hydrocarbon Processing | JUNE 2022 87
Hydrocarbon Processing - June 2022

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