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Presentation on
Applied Local Loads
And of
Induced Stresses
SHANMUGAM VAITHIYANATHAN
P&E / DEME (STATIC)
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Objectives
To Study the need for Local Load Analysis
To Study the various industrially approved methods for Local Load
Analysis.
Methods to reduce stresses due to Local Loads
Application of Local Load Analysis
Petronas Technical Standards (PTS) on the Local Loads
Overview of Engineering Software’s for Local Load Analysis.
Points to consider during Performing and reviewing Nozzle Local
Load Analysis report.
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Define Local Loads
• During operation, Nozzle is subjected to
loads. (apart from internal and External
pressure).
• When a Load is applied over a limited area
with respect to the entire vessel (i.e Pressure
Vessel, Heat Exchanger, Boiler Drums,
Tanks etc) it is called a Local Load
• Examples of Local Loads include
External Local Loads
Piping Loads on Nozzle
Reaction Forces acting on vessel supports
P
Axial (Radial) Forces
Forces acting on Lifting Lugs
VC
Circumferential Forces
Loads acting on Nozzle due to pump vibrations
VL
Longitudinal Forces
MT
Torsional Moments
MC
Circumferential Moments
ML
Longitudinal Moments
Internal Local Loads
Loads on Vessel Internal Support attachments.
Loads on Support rings.
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Local Load Analysis
• These loads cause local stresses,
attenuate rapidly as the distance
from the point of application of load
increases.
• These
Stresses
cannot
be
analyzed by normal (ASME
Calculation) methods. They need
to be analyzed by special method
called Local Load Analysis.
• We should ensure that all major
attachments to the shell are
designed adequately by taking into
consideration the local loading
imposed.
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Available Methods
• There are various methods for carrying out Local Load
Analysis
For Nozzle and similar attachments:
WRC 107
Local Stresses in Spherical and CylindricalShells due to
External Loadings
WRC 297
Local Stresses in Cylindrical Shells, Supplement to WRC-107
• These methods are based on certain assumptions
respectively. Hence each has its own applicability and
limitations.
• All these methods calculate stresses based on
1. Imposed Loads
2. Vessel Geometry
3. Attachment Geometry
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Method WRC 107 Applicability
• Identifies stresses in shell only, not
in nozzle.
• Identifies
stresses
at
the
attachment to shell junction (i.e at
the nozzle edge and pad edge)
• Identifies stresses in shell for
circular as well as rectangular
attachments, solid as well as
hollow.
• Can be used to find stresses in
spherical as well as cylindrical
shells.
• It can be applied to Ellipsoidal
heads.
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Method WRC 107 Applicability
Identifies stresses at eight point (8) in the
attachment Junction
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Method WRC 107 Limitations
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• For d/D <= 0.25 in cylindrical shells.
• Can be used up to d/D <=0.6 for
cylindrical
shells
but
may
be
inaccuracies.
• d/ D Limit for spherical shell is 0.33
• For D/T <= 600
Where :
d = Nozzle OD,
D = Shell OD,
T = Shell Thickness
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Method WRC 297 Applicability
• It’s used to find stresses in shell as
well as nozzles.
• This method is applicable only for
nozzle perpendicular to the axis of
the cylindrical shell.
• Provides better readability for small
values of d/D by plotting the curves
using l = d / Ö DT
• This method allow stress calculation
for thinner wall shells than WRC
107, such as thin walled vessels
and Tanks.
Method
D / T Ratio
WRC 107
D / T <= 600
WRC 297
D / T <= 2500
• Provides data for larger D/T ratios
than in WRC 107
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Method WRC 297 Limitations
• For d/D <= 0.5
• 20 <= D/T <= 2500
• Can be analyzed radial nozzles only on
cylindrical shells.
• It cannot be applied for nozzles on
Spherical shells or Ellipsoidal heads.
• It’s only for round hollow attachment.
• Can’t be used for rectangular
attachments.
Where : d = Nozzle OD, D = Shell OD, T = Shell Thickness
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Brief on the categories of Stresses
STRESS CATEGORIES
PRIMARY
MEMBRANE STRESS
GENERAL (Pm )
SECONDARY (Q)
PEAK
BENDING STRESS (PB)
LOCAL (PL)
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Brief on the categories of Stresses
PRIMARY GENERAL MEMBRANE STRESS (Pm )
This stresses act over a full cross section of the vessel. They are produced my mechanical
Loads and are most hazardous of all types of stress.
Primary stresses are generally due to internal (or) external pressure. The basic characteristic
of a primary stress is that it is not self-limiting.
Example are :
Circumferential and longitudinal stress due to pressure.
Membrane stress in the nozzle wall within the area of reinforcement due to
pressure or external loads.
Compressive and tensile axial stresses due to wind.
PRIMARY GENERAL BENDING STRESS (Pm )
This Stresses are capable of causing collapse of the vessel.
Examples are
Bending stress in the center of a flat head or crown of a dished head.
Bending stress in the ligaments of closely spaced openings
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Brief on the categories of Stresses
LOCAL PRIMARY MEMBRANE STRESS (PL)
An Example of a local primary membrane stress is the membrane stress in a shell produced by
external load, and moment at a nozzle connection.
SECONDARY STRESS (Q)
The basic difference between a secondary stress and a primary stress is that secondary is
largely self-equilibrating (or) Self limiting, which allow them to exceed yield since small plastic
deformation will redistribute the stresses to other parts of the structure.
Example are :
• General thermal stress and bending stress at a gross structural discontinuity.
• Radial Loads on nozzles produce secondary stresses in the shell at the junction of the
nozzle.
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Allowable Stresses
Snap Shot From
ASME Section VIII
DIV 2, Table 5.6,
Page No 553
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Allowable Stresses
Snap Shot From
ASME Section VIII
DIV 2, Figure 5.15,
Page No 557
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Allowable Stresses
Definition Allowable Stress Limit SPS
As Per ASME Section VIII DIV 2, Para 5.5.61 (Page No : 540)
PVELITE & CODECAL Computes the allowable stress S PS for Induced primary +
secondary stress as 3 x average of stresses allowed at minimum (38 oC) and at
maximum (design temperature) INSTEAD of 3 x average of the hot and cold
temperature during the operating cycle as stated by ASME Code.
Reason from PVELITE Support Forum : Secondary stresses are derived principally
from the attached pipe work as the piping thermally expands. The thermal expansion
naturally occurs between two temperatures. Thermal Stresses are defined as (among
other consideration) as secondary stresses
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Allowable Stresses
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As a Nut Shell, Below Table gives the Type of Stresses induced during Local
Load Analysis and their allowable Limits
Classification
Type of Stress
Allowable
Limit
Pm
General Primary Membrane Stress
S
PL
Local Primary Membrane Stress
1.5 * S
PB
Primary Bending Stress
1.5 * S
PL + PB
Primary Membrane + Bending Stress
1.5 * S
PL + PB + Q
Primary & Secondary Membrane + Bending
Stress
SPS
Where,
S
Allowable Stress at Maximum Design Temperature
SPS
Average Allowable Stress at Maximum and Minimum Design Temperature
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Concerns of WRC 107 & WRC 297
The Following list summarized areas where
WRC 107 & WRC 297 are considered weak,
• d / D > 0.5
• t / T < 1.0
• Hillsides or Laterals attachments
• Larger D / T
• Temperatures are approaching the
creep regime.
• Cyclic conditions
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Method to Reduce Stresses
1. Stress in Shell (at the edge
of attachment)
Whenever the stresses in
shell exceed the allowable
limit
§ Provide an Reinforcement
(or) Compensation pad if
not provided
§ Increase the pad Thickness
§ Increase the
size if feasible
attachment
§ Increase the shell thickness
locally
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Method to Reduce Stresses
2. Stress in Shell (at
the edge of Pad)
§ Increase the pad
width,
this
will
reduce the stresses.
§ Increase the shell
thickness locally.
3. Stress in Nozzle
§ Increase the nozzle
thickness
§ Use Self reinforced
type nozzles
SELF REINFORCED
NOZZLE
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Method to Reduce Stresses
EXAMPLE : INCREASEING THE SHELL THICKNESS TOTALLY FOR REDUCING
STRESSES
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Method to Reduce Stresses
• RF Pads remove stresses at the Nozzle-Shell
Juncture.
• Reinforcement decreases the flexibility of the
nozzle-shell attachment.
• Width of the RF Pad, should not exceed
1.65√RT, Beyond this range pad has been
shown to be ineffective.
A
Shows Max. Membrane stresses
accuring at the nozzle # Shell
Juncture.
B
Reinforcement thickness increases,
the maximum stress shift towards pad
edge
C
Max. Stresses shift to edge of pas as
T/t and M approaches critical value.
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Method to Reduce Stresses
• Pads can be even dangerous on thin-walled shells, Pad could very
easily transfer the maximum loading to the pad edge, resulting in
crack propagation or even rupture.
• Caution should be taken in working with thin walled shells, where
the flexibility of the shell is often sufficient to decrease induced
stresses from external loadings.
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WRC 297 & WRC 107 Applications
WRC 297 can only be applied for Local Load analysis of Nozzles.
WRC 107 has a wide range of applications as listed below
Analysis of Lug Supports
Analysis of Lifting Lugs
Analysis of brackets
Analysis of Leg Supports
Analysis of Clips
Analysis of Tailing Lugs
For all these types of attachments, it is only required to convert the
given attachment size into an equivalent rectangle which has
The Same Moment of Inertia
The Same ratio of length to
width as the original attachment
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Applications
WRC Analysis of Attachments
LADDER AND PLATFORM CLIP ATTACHMENT
PIPE SUPPORT CLIP ATTACHMENT
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Applications
As since no hard and fast rules have yet been determined, it would
seem reasonable to apply the factors as outlined. Very large or
critical loads should, however, be examined in depth
Reference : From Book Pressure vessel design Manual (3rd Edition) By
Author Dennis Moss (Page No : 268)
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PTS on Nozzle Local Load Analysis
PTS 31.22.20.31 (Pressure vessels) Para 4.12.15, 4.12.16 & Annex VIII,
Covers about this analysis
Detailed design & analysis of nozzle connection in accordance with WRC
107, 297 or other analytical method as per Annex VIII of the PTS
Equipment Nozzles with agitator
or large valve
Nozzles with PRV, also consider
the force due to an open relief
valve
Nozzles attached to piping
subject to movement / loads
Manways or Inspection opening
(hand-holes), Thermo well
Nozzles, Pressure indicator
nozzles not required analysis
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PTS on Nozzle Local Load Analysis
1.
Above table is not applicable to pressure vessel constructed of non-ferrous
materials, austenitic steel less than 6 mm thick.
2.
The value on nozzle loadings used for design purposes shall be clearly stated
on the vessel drawings for future reference
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Alternative Methods (FEA)
Various Geometries (with or without Pads) Covered by FEA Methods
Head Structural
Support
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Alternative Methods (FEA)
Reviewing Nozzle Local Load FEA Analysis Stress Calculation Report
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Alternative Methods (FEA)
Sample Finite Element Local
Load Stress Analysis For the
Nozzle on Dish Head
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Overview of Engineering Software’s for
Local Load Analysis
There are many engineering software's to perform the Equipment
attachment Local Load Analysis effectively, Each software has it own
limitations. Most common software's used by our Local vendors are
Compress, PVELITE & CODECAL.
Below Listed Few Software's
WRC 107
WRC 297
FINITE
ELEMENT
ANALYSIS
COMPRESS
(CODEWARE)
Ö
Ö
Ö
PVELITE
Ö
Ö
Ö
CODECAL
Ö
Ö
x
FINGLOW
Ö
Ö
x
NOZZLE PRO
x
x
Ö
SOFTWARES
Note : For correctness of this table please refer the Latest software release updates
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Points to Consider during performing and reviewing Nozzle
Local Load Calculation
1. Direction of Loads to be Checked
Nozzle Load Directions As Per
Petronas Standard
Nozzle Load Directions As Per
PVELITE Software
Nozzle Load Directions As Per
Compress Software
• Our PTS Nozzle direction should match with the software Load direction used for
Analysis.
• In PVELITE / COMPRESS Software, Radial Load are towards nozzle inward
direction and in PTS its outward pull from the Nozzle.
• During Performing / Reviewing the above software calculation, Kindly ensure that
the radial load should be negative to match with PTS Loads direction
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Points to Consider during performing and reviewing Nozzle
Local Load Calculation
2. In the WRC Calculation, Design Internal Pressure shall be MAWP (Maximum Allowable
Working Pressure) of the vessel, If the Equipment is rated for MAWP.
3. In PVELITE Calculation, Ensure that the pressure thrust force is included depending on the
requirement.
Pressure Thrust Force
Example Cases to Consider Pressure Thrust Load
• Nozzle with Blind Flange will Experience this Load
• Nozzle with Straight run pipe to an elbow, with no intermediate restraints.
To know further example cases on this Load, have a look on the below PVELITE forum Link in
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Internet http://65.57.255.42/ubbthreads/ubbthreads.php?ubb=showflat&Number=331
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Points to Consider during performing and reviewing Nozzle
Local Load Calculation
Some more Example Case : Possibility of Pressure thrust Load
Pressure Thrust Load on the Filter Vessel Nozzle
due to the Elbow
Pressure Thrust Load on the Filter Vessel Nozzle
due to blind flange & Valve.
Above Pictures from our KAKG-A offshore platform, from the Utility Area
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Points to Consider during performing and reviewing Nozzle
Local Load Calculation
4. For the Offset Nozzles,
Hill side Nozzles,
Closely spaced Nozzles
based on the
Complexity, check for
the option of FEA
analysis by
understanding
Limitation in WRC 107
& 297
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Points to Consider during performing and reviewing Nozzle
Local Load Calculation
Sample Vessel Where the Nozzles are Closely Located, May need of
FEA Analysis
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Points to Consider during performing and reviewing Nozzle
Local Load Calculation
5. Consider other Miscellaneous Loads on the Nozzles based on the situation
Other than the standard Nozzles Loads given in PTS 31.22.20.31, Table VIII
Below Shown Picture is the Reaction Furnace (Located in Bottom) & Steam
Drum (Located in Top) , We can see that Steam Drum load is transferred on
the process nozzle of Reaction Furnace, In this case Reaction Furnace Nozzle
Loads can be higher than Standard nozzles Loads given in PTS.
Another Example is from our KAKG-A platform, Where heavy valve induces
Longitudinal Moment on the vessel
KAKG-A PLATFORM
Other Example Case : Reboiler (Heat Exchanger) attached to the Splitter Columns42
Points to Consider during performing and reviewing Nozzle
Local Load Calculation
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6. Special Consideration shell be given when there is Bigger Size Nozzles (or) More number
of Nozzles on The Dish Head.
While Performing Local Analysis on Bigger Nozzles in the Dish head, If there is a need
to provide Reinforcement pad (or) Insert Plate, there might be possibility of Welds on the
Dishend Knuckle region. To avoid the weld in Knuckle Region listed below some
solutions,
•
Increase the Dish Head Thickness.
•
Check with Reinforced Nozzle.
This case is more frequent in our offshore vessels, So special consideration shall be
given in the Initial stage itself.
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Points to Consider during performing and reviewing Nozzle
Local Load Calculation
7. While performing / reviewing the Local Loads
analysis calculation for
Lifting Lugs,
Trunnions & Tailings, Kindly ensure below
listed points.
•
Analysis shall be carried out at at ambient
temperature (i.e Design Temperature = 30o
C)
•
Allowable Stress limit shall be corresponding
to the ambient temperature.
•
There will no Internal design pressure.
•
Respect to the direction of lifting the
equipment Loads and Moments shall be
applied
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Points to Consider during performing and reviewing Nozzle
Local Load Calculation
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Points to Consider during performing and reviewing Nozzle
Local Load Calculation
8. Warnings in the Nozzle Local Load Analysis Calculation report, When the results
exceeds the Graph curve value in WRC 107 or WRC 297.
PVELITE WRC 107
CALCULATION
SNAP SHOT
Solution : When WRC 107 Graph curve value exceed, we can request the vendor to
perform the calculation in WRC 297. If the WRC 297 Graph curve value also
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exceeds the only alternative is FEA.
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Facts to understand about Local Load Analysis
Local Loads on Nozzles are not transferred to the
vessel itself. For example supports and cone
junctions will not see those loads.
Its necessary to add these loads separately to the
vessel.
Nozzle Loads are usually small compared to the
vessel weight
WRC 107 & 297 does not
recognize any stiffeners as it is
not included in the WRC Bulletin
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THANK YOU
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