Hydrocarbon Processing - May 2022 - 55

Process Optimization
FIG. 1 shows the acting loads on the nozzles. The shear
stress caused by the transverse force and the torsional moment
at the nozzle-shell intersection are neglected, since it is
assumed by this simple approximation that their contribution
will normally not exceed 0.15 f.
In case of doubt, the maximum total shear stress in the shell
at the outer diameter of the nozzle should be calculated according
to clauses 16.4.5.7 and 16.5.5.7, respectively, of EN
13445-3:2014 / A8:20197
5500 clause G.2.8.2.3 (f) and G.2.8.3.3 (f)2
, or alternatively according to PD
. If it appears that
the limit of 0.15 f is exceeded, a more rigorous analysis should
be considered. The applicable formulas for calculating the
permissible individual nozzle loads are included in TABLE 1.
Nomenclature is listed in TABLE 2.
Note: In the case of relatively thin-walled nozzle necks-
i.e., the ratio of shell thickness and nozzle neck thickness
exceeding 1-it is recommended to divide the permissible
individual loads at the nozzle-shell intersection by that ratio
(TABLE 3). This is due to the possibility that the stress in the
nozzle neck may be a determining factor.
Assessment protocol. The following step-by-step approach
explains the roles of the vessel design engineer and the piping
stress analyst.
Vessel design engineer:
* Step 1: The vessel design conforms to the applicable code
or standard considering only internal design pressure.
* Step 2: The allowable individual loads for each process
nozzle are calculated, including the flange.
* Step 3: The vendor/vessel manufacturer furnishes
the individual allowable forces and moments for each
process nozzle-vessel intersection, as well as for the
nozzle flange, in a tabular form on the appropriate
drawing of the relevant equipment item. Normally this
should be done twice: in the preliminary bid phase,
and in the final mechanical design phase.
* Step 4: The information compiled in Step 3 must be
transferred to the piping stress analyst.
Piping stress analyst: The starting point for the piping stress
analyst is the compliance of the connected piping with the applicable
design code:
* Step 5: Determine exerted nozzle loads (piping
reactions) using an accepted software program.
* Step 6: Prepare a summary of piping reactions for the
relevant process nozzles.
* Step 7: Provide load interaction checks at each process
nozzle-vessel intersection and flange connection. In
practice, this often means a joint effort of the vessel
design engineer and the piping stress analyst. Both
TABLE 4. Load interaction rules
At nozzle-shell intersection
At flange-facing nozzle
Nozzle on spherical part of head
Factual
+
Fallowable
Mactual
Mallowable
Factual
Fallowable
+
≤ 1
Factual
Fallowable
Mactual
Mallowable
Hydrocarbon Processing | MAY 2022 55
≤ 1
Nozzle on cylindrical shell
Ml actual
+
Ml allowable
+
Mc actual
Mc allowable
≤ 1
TABLE 3. Stress intensity left over for external nozzle loads
Stress intensity left over
for external nozzle loads:
σ = 3− 2
⎛
⎝
Pd
MAWP
⎞
⎠
f
f = Design stress (MPa) according to the applicable design code
MAWP = Maximum allowable working pressure at the nozzle
intersection (MPa)
Explanation: Satisfies the elastic
shakedown criteria of 3 f
disciplines must be convinced that simultaneous action
of internal pressure and external loads are acceptable.
Linear load interaction rules that apply are shown in
TABLE 4. The result of the load interaction rules must be recorded
in the piping stress report.
Note: In cases where the piping reactions are not permissible,
re-routing of the piping system or rearrangement of the
pipe supports should be considered. Often, discounting the
Symbol
F
Ml
Mc
M
σ
Do
T
Tpad
Dn
Dpad
Pd
Pr
A
B
C
t
G
W
Kf
dbh
dbh*
TABLE 2. Nomenclature
Description
Axial force
Longitudinal moment
Circumferential moment
Meridional (bending) moment
Available stress intensity for external loads
Outside diameter shell/sphere
Wall thickness shell/sphere
Thickness of reinforcing plate
Outside diameter nozzle neck
Outside diameter reinforcing plate
Internal design pressure
Unit
N
Nmm
Nmm
Nmm
MPa
mm
mm
mm
mm
mm
MPa
Rated pressure (ASME B16.5 or ASME B16.47) MPa
Outside diameter of flange
Inside diameter of flange
Bolt circle diameter
Flange thickness
Effective sealing diameter
Flange width: 0.5 (A-B)
" Koves " factor
Bolt hole diameter
Max [ dbh (1 - B/1000) ; 0.5 dbh]
mm
mm
mm
mm
mm
mm
-
mm
mm
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