Intergraph - Piping Stress Analysis Using CAESAR-II PDF
Intergraph - Piping Stress Analysis Using CAESAR-II PDF
Intergraph - Piping Stress Analysis Using CAESAR-II PDF
CAESAR II
What do we do?
How do we model the piping system?
How do we document the work?
1
Pitfalls of
Piping Flexibility Analysis
3D Beam Element
2
3D Beam Element
Behavior is
dominated by
bending
Efficient for most
analyses
Sufficient for
system analysis
3D Beam Element
Whats missing?
g
No local effects (shell distortion)
No second order effects
No large rotation
No clash
No accounting for large shear load
Where wall deflection occurs before
bending
As in a short fat cantilever (vs. a long
skinny cantilever)
Centerline support
No shell/wall
13-Feb-08 Introduction to CAESAR II and Pipe Stress Analysis
3
3D Beam Example
Si l cantilever
Simple il bending:
b di
L P
L3
= P
3 E I
(x = F )
K
13-Feb-08 Introduction to CAESAR II and Pipe Stress Analysis
How Do We Represent
Stress?
4
Evaluating Stress at a Point
Stress Element
Longitudinal stress
F/A, PD/4t, M/Z (max. on outside surface)
Hoop stress
PD/2t
Radial stress
0 (on outside surface)
Shear stress
T/2Z, (V=0 on outside surface)
5
From 3D to 2D
Equilibrium
6
Mohrs Circle
7
Mohrs Circle
Representation
Principal Stresses:
S1, S2, S3
8
How Do We Measure
Failure?
9
Other Failure Concerns
Maximum p
principal
p stress S1 ((Rankine).
)
Principal stress alone causes failure of the element.
Wall thickness calculations due to pressure alone.
Maximum shearing stress max (Tresca).
Shear, not direct stress causes failure.
Common stress calculation in piping.
M i
Maximum distortion
di t ti energy wd (von
( Mises).
Mi )
Total distortion of the element causes failure.
Octahedral shearing stress (Gmax) is another measure
of the energy used to distort the element. This is
known as equivalent stress.
13-Feb-08 Introduction to CAESAR II and Pipe Stress Analysis
10
How Do We Measure Failure?
Energy
e gy oof d
distortion
sto t o iss tthe
e most
ost accu
accurate
ate
prediction of failure but maximum shearing
stress is close and conservative.
Piping codes often utilize their own mix (through
the term stress intensity).
CAESAR II can print either Tresca or von Mises
stress in the 132 column stress report.
Our (code) focus is maximum shearing stress.
11
From Lab to Field
How Do We Compare
F il
Failures?
?
Material Characteristics
12
Material Characteristics
Lab Failure
If failure occurs at
yield, the appropriate
stress is calculated
using the yield load
Sy = Py/a
And this is our limit
max Sy/2
13
Field Failure
Us g tthe
Using e maximum
a u sshear
ea cacalculation
cu at o
max is the radius of Mohrs circle.
max = (S1-S3)/2.
So, (S1-S3)/2 Sy/2.
Or (S1-S3) Sy
Piping codes define (S1-S3) as stress intensity.
Stress intensity must be below the material
yield.
14
More Simple?
Hoopp stress (S
( H) is positive
p and below yyield due
to wall thickness requirements (design by rule).
Radial stress is zero, assume this is S3.
Longitudinal stress (SL), assumed positive, must
be checked only if it exceeds hoop stress, then
S1=f(SL,) and (S1-S3)= f(SL,).
S with
So, ith h
hoop stress
t accounted
t d with
ith wallll
thickness, you need only evaluate longitudinal
and shear stresses and compare the results with
the material yield, Sy.
13-Feb-08 Introduction to CAESAR II and Pipe Stress Analysis
15
Or So You Might Think
16
Effects of thermal strain were investigated
and addressed by A.R.C. Markl et. al. in
the late 40s and into the 50s.
Yield is a primary
primary concern for force-
force
based loads which lead to collapse.
But other, non-collapse loads exist.
17
Non-collapse Loads?
18
Are There Strain Limits?
19
Shakedown and Its Limits
20
But Were Not Done
Material Fatigue
21
Piping Material Fatigue
22
Bend Failure
Component Fatigue
23
Stress Intensification
S bw
i=
S el
In-Plane/Out-Plane
24
In-Plane/Out-Plane
25
B31.1 Appendix D
B31.3 Appendix D
26
B31.3 SIF Example
B31.3 Sample
p SIF Calculations
Welding elbow or pipe bend Reinforced fabricated tee with pad or saddle
Input Input
Pipe OD : 10.75 10.75 10.75 10.75 Pipe OD : 10.75 10.75 10.75 10.75
Pipe wall : 0.365 0.365 0.365 0.365 Pipe wall : 0.365 0.365 0.365 0.365
Bend radius : 15 10 30 50 Pad thickness : 0 0.25 0.365 0.5
B31.1 Sample
p SIF Calculations
Welding elbow or pipe bend Reinforced fabricated tee with pad or saddle
Input Input
Pipe OD : 10.75 10.75 10.75 10.75 Pipe OD : 10.75 10.75 10.75 10.75
Pipe wall : 0.365 0.365 0.365 0.365 Pipe wall : 0.365 0.365 0.365 0.365
Bend radius : 15 10 30 50 Branch OD : 4.5 4.5 4.5 4.5
Branch wall : 0.237 0.237 0.237 0.237
Branch OD at tee : 5
Pad thickness : 0 0.25 0.365 0.5
27
To Summarize:
Piping Code
Implementation
28
A Review of the Basic
Concerns
Force-based
Force based loads are limited by yield
But also! Permanent or temporary?
These are primary loads and they produce
sustained and occasional stresses
Strain-based loads are limited by fatigue
These are secondary loads and they
produce expansion stresses
Power Piping
B31.1, ASME III, B31.5, FBDR (, EN-13480?)
Most stringent limitations
Sample Equations
Sustained: Slp + (0.75i)Ma/Z < Sh
E
Expansion:
i iMc/Z
iM /Z < f(1.25Sc
f(1 25S + 1
1.25Sh
25Sh Sustained)
S t i d)
Sustained + Occasional:
Slp + (0.75i)Ma/Z + (0.75i)Mb/Z < kSh
29
Piping code equations:
Process Piping
B31.3, ISO 15649
Wider applications
Sample Equations
Let Sb = {sqrt[(iiMi)2+(ioMo)2]}/Z
p + Fax/A + Sb < Sh
Sustained: Slp
Expansion:
sqrt(Sb2 + 4St2) < f(1.25Sc + 1.25Sh Sustained)
Sustained + Occasional:
Slp + (Fax/A + Sb)sus +(Fax/A+Sb)occ < kSh
Transportation Piping
B31.4, B31.8, TD/12, Z662, DNV
Based of proof testing and yield limits
Addresses compression
Sample Equations
Let Sb = {sqrt[(iiMi)2+(ioMo)2]}/Z
Sustained: Slp + Sb < 0.75Sy
Expansion: sqrt(Sb2 + 4St2) < 0.72Sy
Operating: Sustained + Expansion < Sy
30
Piping code equations:
An Overview of
th D
the Design
i P
Procedure
d
31
Pipe Stress Analysis
and
Design by Analysis
32
Design by Analysis
33
Is It a Good Model?
Focus on stiffness
stiffness, boundary conditions and
loads.
A Sensitivity Study
34
Verifying Results
Design Limits
Pipe Deflection
Equipment loads
35
Use a Sensitivity Study:
To improve the
values
To improve
p your
y
confidence
Which Is Better
a complex model
or a simple model?
36
Summary
37