Hydrocarbon Processing - November 2022 - 60

Heat Transfer
* The primary membrane, primary
+ secondary + bending stresses
and fatigue stresses for Occasional
loads are tabulated in TABLE 12.
The calculated stresses at the inlet
and outlet shells, and the inlet
and outlet nozzles are within the
allowable limits.
Since the induced stresses in the shell
and inlet and outlet nozzles are wthin allowable
limits for all the three load combinations
(i.e., Sustained, Thermal and
Occasional), a further increase in thickness
from 40S to 80S is not required.
Load Case 1, where the Thermal load
is considered at design temperature, results
in stresses exceeding the allowable
limits with thickness higher than the
standard thickness (40S) of pipe shell.
An increase in shell thickness of more
than 80S is infeasible since that results
in the inner diameter of the shell becoming
smaller than the outside diameter of
the heater bundle, thereby making the
insertion of the heater bundle inside the
shell impossible.
Load Case 2, where the Thermal load
is considered at operating temperature,
is a more practical scenario compared to
Load Case 1 since the heater nozzles and
shell will be under operating conditions
most of the time-this is particularly
true in combination with the wind load,
which by definition is considered an Occasional
load. This load combination also
enabled the adoption of a standard pipe
wall thickness by keeping the induced
stresses within the allowable limits.
Takeaways and recommendations.
With cost-competitive and scheduledriven
projects, detailed engineering contractors
(DECs) are expected to specify
all technical requirements correctly and
completely in the requisition document
to avoid changes at a later stage that may
affect the project cost and schedule. To
avoid post-order changes, DECs should
TABLE 10. SCH 40S thick shell: Calculated stress and limits of equivalent stress for Load
Case 2 (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, %
8
9
SPL (MPa)
200
200
63
63
17
17
Remarks
Under limit
Under limit
Under limit
Under limit
TABLE 11. SCH 40S thick shell: Calculated stress and limits of equivalent stress for Load
Case 2 (Thermal loads)
Nozzle ends Components
Shell (6 in.)
Inlet
Outlet
Conical nozzle
(6 in./3 in.)
Shell (6 in.)
Nozzle (6 ft)
PL (MPa)
151
153
36
36
Primary local membrane stress
Ratio, %
75
76
SPL (MPa)
200
200
63
63
57
57
Remarks
Under limit
Under limit
Under limit
Under limit
not over-specify the external loads (e.g.,
specifying standard piping loads for vessels
and exchangers for electric heaters).
Instead, DECs should evaluate various
external loading combinations and specify
the most feasible load combination.
As explained above, a simulation
study with appropriate load combinations,
considering operating temperatures
instead of design temperatures, can
help avoid increased hardware costs, and
(in this case) increase the thickness of
the electric heater shell during the project's
detailed design stage, preventing
both cost and schedule overruns.
To avoid late changes in the supplier
design with a conservative load combination,
it is recommended to consider
thermal loads at operating conditions-
the more likely scenario-rather than at
design conditions, which tend to be conservative
and may result in a late change
in form and an increase in the thickness
of the shell and/or nozzles.
NOTE
a
Paulin Research Group, Codeware Nozzle
PRO v15.0 Build 16
NOMENCLATURE
PL = Primary local membrane stress
Pb = Primary bending stress
Q = Secondary membrane + bending stress
F = Peak fatigue stress
S = Allowable stress at design/operating temperature
SPL
= Allowable primary local membrane stress
SPS = Allowable primary + secondary membrane
and bending stress
Sa = Allowable peak fatigue stress
Savg = Average allowable stress between design/
operating temperature and ambient temperature
LITERATURE CITED
1
Pramanik, R., " Challenges in design and engineering
of electric heaters, " Chemical Engineering World,
September 2017.
2
3
ASME Boiler and Pressure Vessel Code Section II,
" Materials, " 2019.
ASME Boiler and Pressure Vessel Code Section
VIII Division 2, " Rules for construction of pressure
vessels, " 2019.
Nozzle
ends Components
Inlet
Shell (6 in.)
Conical nozzle
(6 in./3 in.)
Outlet Shell (6 in.)
Nozzle (6 ft)
TABLE 12. SCH 40S thick shell: Calculated stress and limits of equivalent stress for Load Case 2 (Occasional loads)
Primary local membrane stress
SPL
PL
(MPa)
184
185
54
54
(MPa)
200
200
63
63
Ratio,
%
92
92
86
86
60 NOVEMBER 2022 | HydrocarbonProcessing.com
PL
Remarks
Under limit
Under limit
Under limit
Under limit
+ Pb
Primary + secondary + bending
+ Q
SPS
(MPa)
277
279
63
63
(MPa)
407
407
270
270
Ratio,
%
68
69
23
23
Remarks
Under limit
Under limit
Under limit
Under limit
PL
+ Pb
+ Q
+ F (MPa)
145
147
28
28
Peak fatigue stress
Sa
(MPa)
971
971
971
971
15
15
3
3
Ratio,
% Remarks
Under limit
Under limit
Under limit
Under limit
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