Hydrocarbon Processing - November 2022 - 57

Heat Transfer
Load cases. The following two load cases
with various combinations of external
loads were considered. While Load Case
1 considered thermal loads at design temperatures,
Load Case 2 considered thermal
loads at operating temperatures (i.e.,
inlet temperature for the inlet nozzle and
outlet temperature for the outlet nozzle).
Load Case 1: At design temperature
conditions:
* Sustained loads (Sustained): Dead
weight (loads from component
weights, fluid and internal pressure)
* Operating loads (Thermal):
Sustained loads + thermal load
(design)
* Occasional loads (Occasional):
Sustained loads + thermal load
(design) + wind load.
Load Case 2: At operating temperature
conditions:
* Sustained loads (Sustained): Dead
weight (loads from component
weights, fluid and internal pressure)
* Operating loads (Thermal):
Sustained loads + thermal load
(operating)
* Occasional loads (Occasional):
Sustained loads + thermal load
(operating) + wind load.
Since dead weight (design)-which by
definition will include the design pressure
of the fluid-cannot be considered as a
sustained load, dead load (design) is not
considered here. As the electric heater is
located on an open structure at an elevation,
the wind load is considered as part
of the occassional load. These load combinations
are presented in TABLE 3.
Allowable stresses. The following allowable
stresses were considered as per
Part 5 of ASME Sec VIII Div 2 in the
stress analysis:
* S = Allowable at design/operating
temperature obtained from
ASME Section II Part D2
[3]
* SPL = 1.5S3
* SPS = 3Savg3
[4]
[4]
* Sa = Allowable obtained from
a fatigue curve for the specified
number of operating cycles.3
[4]
STRESS ANALYSIS
Stress analysis was performed at the
shell-to-nozzle junctions at the inlet and
outlet (referred to as shell here) and the inlet
and outlet nozzles, using a proprietary
finite element analysis (FEA) programa
Nozzle ends Components
Shell (6 in.)
Inlet
Outlet
Conical nozzle
(6 in./3 in.)
Shell (6 in.)
Nozzle (6 ft)
PL (MPa)
157
158
39
38
FIG. 11. Load Case 1 (Occasional):
Fatigue stresses at the inlet and outlet
for SCH 80S.
FIG. 13. Load Case 2 (Thermal):
Primary membrane stresses at the
inlet and outlet for SCH 40S.
TABLE 4. SCH 40S thick shell: Calculated stress and limits of equivalent stress
for Load Case 1 (Sustained loads)
Nozzle ends Components
Shell (6 in.)
Inlet
Outlet
Conical nozzle
(6 in./3 in.)
Shell (6 in.)
Nozzle (6 ft)
PL (MPa)
17
18
11
11
Primary local membrane stress
Ratio, %
SPL (MPa)
Inlet
44
Outlet
44
Shell (6 in.)
Conical nozzle
(6 in./ 3 in.)
Shell (6 in.)
Nozzle (6 ft)
TABLE 5. SCH 40S thick shell: Calculated stress and limits of equivalent stress
for Load Case 1 (Thermal loads)
Primary local membrane stress
Ratio, %
357
359
SPL (MPa)
44
44
44
44
87
86
Remarks
Overstressed
Overstressed
Under limit
Under limit
Hydrocarbon Processing | NOVEMBER 2022 57
PL (MPa)
17
18
11
11
FIG. 10. Load Case 1 (Occasional):
Primary + secondary + bend stresses
at the inlet and outlet for SCH 80S.
FIG. 12. Load Case 2 (Sustained):
Primary membrane stresses at the
inlet and outlet for SCH 40S.
and considering two different pipe thicknesses
of the shell and nozzle, namely 40S
and 80S for Load Case 1 and the thickness
of 40S for Load Case 2. The results are
presented in TABLES 4-12 and FIGS. 2-14.
Results and discussions. Load Case
1 for the SCH 40S thick shell: An FEA
using the softwarea
was carried out, considering
an allowable stress at a design
temperature of 690°C for both the shell
and inlet and outlet nozzles.
* The primary membrane stresses
for Sustained loads are tabulated
in TABLE 4. The calculated stresses
are within the allowable limit.
* The primary membrane stresses
for Thermal loads are tabulated
in TABLE 5.
° The primary membrane
stresses at the inlet shell and
inlet nozzle are ~357% and
~359% of allowable limits,
respectively.
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