Hydrocarbon Processing - January 2022 - 80

Heat Transfer
P. K. SINGLA, Worley, Al-Khobar, Saudi Arabia
Bolting material bolts-up the exchanger design
Shell-and-tube heat exchangers use
multiple girth flange joints based on their
Tubular Exchangers Manufacturers Association
(TEMA) type to assemble shell
and tube sides. This permits their disassembly,
inspection and cleaning.1
Peripheral
gaskets and fasteners play a key role
in finalizing the geometry of any flange.
Corrosive and environmental cracking
services may dictate the requirements for
directly exposed bolting materials to specific
grades, per international standards,
such as NACE MR0175/MR0103.2
This article details a case where the
use of special service bolting material
grade led to an excessively large external
flange joint, resulting in heat exchanger
overweight. It also describes the proposed
solution by adjusting the extent of
insulation on the exchanger, directly exposing
the bolting material to atmosphere
rather than the insulated sour service,
which allowed the use of alternative higher
strength bolting material grade.
Case description. This exchanger serves
as a kettle-type chiller and propane-based
TABLE 1. Table A.4 from NACE MR0175:
Acceptable bolting materials in
wet H2
S service
Bolts
ASTM A193 grade B7M
ASTM A320 grade L7M
Nuts
ASTM A194
grades 2HM, 7M
liquid refrigerant on the shell side, which
extracts heat from tube-side, wet hydrogen
sulfide (H2
S) (3,500 ppmv) hydrocarbon
gas. Because of the upstream process,
this complies with NACE MR0175.
The main process and design parameters
from the exchanger datasheet include:
* TEMA type/size:
° TEMA type: AKU
° Size: 2.15 m/2.92 m-6.1 m
* Heat duty: 8.3 MW
* Design code: ASME Sec VIII,
Div. 2, Cl 1
* Operating temperature:
° Shell side: 26°C/28°C
° Tube side: 46°C/32°C
* Design conditions:
° Shell side: 30 barg/150°C/(-) 21°C
° Tube side: 90 barg/150°C/(-) 21°C
* Shell and flange materials:
° Shell side: SA-516 Gr. 70N,
SA-350 LF2 Cl 1
° Tube side: SA-516 Gr. 70N
(HIC), SA-350 LF2 Cl 1
* Bolting material: SA-320-L7M/
SA-194-2HM
* Insulation:
° Shell side: Cold conservation
° Tube side: Cold conservation.
Problem statement. The heat exchanger
size and quantity are finalized in a frontend
engineering design (FEED) package as
an input document from the owner. Upon
verifying the mechanical design-before
issuing the datasheet in the material requisition
in the detailed design phase-it was
found that flange outside diameter (OD)
with SA-320-L7M bolting material went
beyond 4.5 m, with a weight of 63 metric
t (tonnes), while the complete exchanger
weight was approximately 350 metric t.
These dimensions and weights were
unacceptable from plot space (the exchanger
is installed in a module structure),
transportation, maintenance,
and even
fabrication and cost perspectives.
The prime reason for this is the NACE
bolting material grade SA-320 L7M used.
ASME Section II limits its size to a maximum
of 64 mm, and it has a lower allowable
stress compared to L7 and L43
grades. Due to its lower strength and size,
and to meet the root bolting area requirement,
the quantity of studs had to be increased,
leading to a higher bolt circle
diameter (BCD) and larger flange OD,
compounding the problem.
The FEED package states that the exchanger
will be completely insulated to
TABLE 2. Comparison of flange
dimensions
Parameter
Flange ID [B], mm
Flange thickness
[t], mm
Bolts quantity
Bolt diameter, mm
Bolt circle diameter
[C], mm
FIG. 1. Proposed extent of insulation.
80 JANUARY 2022 | HydrocarbonProcessing.com
FIG. 2. Utilizing alternate bolting materials.
Flange weight,
metric t
L7M
2,150
640
104
64
L7
2,150
550
84
64
L43
2,150
370
44
90
4,420 3,570 2,535
Flange OD [A], mm 4,550 3,690 2,703
63
31
7
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