Chicago Pipe Stress Analysis Different Tools Different Results

1 © 2014 ANSYS, Inc. June 3, 2014 ANSYS Confidential

Pipe Stress Analysis – Different Tools, Different Results

John C. Oliva, PhD

Engineering Analyst

Hemlock Semiconductor Corporation

Hemlock, Michigan

Presented at the 2014 ANSYS Regional Conference, Chicago, May 23, 2014


A Little Background on HSC:

Delivering the highest-quality polysilicon for semiconductor and solar wafer manufacturing.

www.hscpoly.com


Specializing in structural and fluids based mechanical simulations.

The Computational Engineering Group:


Presentation Overview • Pipe stress analysis programs are in wide use to

evaluate the structural integrity of piping systems.

• If a system is modeled side-by-side using one of those tools and with a general purpose finite element tool like ANSYS, different results are obtained.

• This presentation will seek to explain why those differences occur.

• Illustrative examples will be used to demonstrate the differences.


Pipe Stress Analysis Programs

• Example Programs:

− CAEPIPE by SST Systems

− Caesar II by Integraph Corporation

− SIMFLEX by Peng Engineering

− AutoPIPE by Bentley Systems

− Many others

• Most pipe analysis programs are simplified finite element solvers that represent piping components as beam elements.

• Such application specific softwares are developed for the sole purpose of evaluating pipe configurations per specific pipe design codes.


All pipe in this example is 4”, schedule 40

Corner 90° bend is a flangeless long radius elbow

Pipe is fabricated from generic carbon steel

Example 1:


Calculating Stresses at the Fixed End:


At the worst case point on the pipe surface, these stresses add together:

Calculating Stresses at the Fixed End:


Calculating stresses immediately before the elbow:


Stresses Within the Elbow:

• Calculating the stress within the pipe elbow is not straight forward as it acts like a stress riser.

• All that can be said with certainty is that the stresses will be higher in the elbow than on either of its ends.


Image shows calculated stresses along inside surface of the piping, sectioned down its midplane.

ANSYS Model Results:


ANSYS Model Results: Max stress at pipe end equals 37,800 psi This is in close agreement to that calculated “by hand”, which was 37,646 psi…less than 1% discrepancy.

Max stress in elbow is found to be 138,940 psi. This would equate to a stress multiplier of 3.7, which is reasonable.


Note the indicated node numbering.

Pipe Stress Analysis Model:


NODE

Bending

Stress

lb./sq.in.

Torsion

Stress

lb./sq.in.

SIF

In Plane

SIF

Out

Plane

Code Stress

lb./sq.in.

10 0 0 1 1 0

18 35464.4 0 1 1 35464.4

18 69238.8 0 1.952 1.627 69238.8

19 71815.6 0 1.952 1.627 72038.4

19 71815.6 0 1.952 1.627 72038.4

20 72882.9 0 1.952 1.627 73198

20 37331 0 1 1 37646

30 37331 0 1 1 37646

Stress at fixed end of 37,646 psi is exactly that which was calculated analytically, which was noted to be less than 1% off from the ANSYS estimate.

Pipe Stress Analysis Calculated Results:


NODE

Bending

Stress

lb./sq.in.

Torsion

Stress

lb./sq.in.

SIF

In Plane

SIF

Out

Plane

Code Stress

lb./sq.in.

10 0 0 1 1 0

18 35464.4 0 1 1 35464.4

18 69238.8 0 1.952 1.627 69238.8

19 71815.6 0 1.952 1.627 72038.4

19 71815.6 0 1.952 1.627 72038.4

20 72882.9 0 1.952 1.627 73198

20 37331 0 1 1 37646

30 37331 0 1 1 37646

Stress within the elbow is calculated to be 73,198 psi. Recall, ANSYS calculated the stress at this same point to be 138,940 psi.

Pipe Stress Analysis Calculated Results:

18 19

20


The Differences Lie in the SIF:

• In solid mechanics, a stress concentration factor (often denoted as k) is used to estimate the maximum theoretical stress in a given geometry. • Stress Intensification Factors (SIF’s) are established to predict a stress value that accounts for both static and fatigue loading in accordance with compliance to a particular piping code. (In the example, ASME B31.3 was used.) • Analogously, ANSYS calculated the maximum theoretical stress in the elbow, the pipe stress analysis program calculated the code stress for that loading.


“One of the less well known aspects of piping flexibility analysis per the ASME B31 Codes is that in piping stress analysis, the calculated stress range due to bending loads is about one-half of the peak stress range. This is because the stress concentration factor for typical as-welded pipe butt welds is two. Since the stresses are compared to a butt-welded pipe fatigue curve, one-half of the actual peak stresses is calculated. Thus, the theoretical stress, for example, in an elbow due to bending loads is two times what is calculated in a piping flexibility analysis following Code procedures. This is not significant when performing standard design calculations, because the Code procedures are self-consistent. However, it can be very significant when trying to do a more detailed analysis, for example, in a fitness-for-service assessment.” 1

A critical point that is subtly incorporated into the ASME piping code is the following:

1 “Process Piping - The Complete Guide to ASME B31.3”, Third Edition, by Charles Becht IV, Copyright 2009, ASME Press, Page 81


Back to the Results at the 90° Elbow:

ANSYS stress = 138,940 psi

Pipe stress analysis = 73,198 psi

Per the note on the previous slide, maximum theoretical stress is two times that determined by code procedures, thus:

2 x Pipe Stress Analysis should equal ANSYS

2 x 73,198 psi = 146,396 psi (compare to ANSYS value of 138,940 psi)

The two stress estimates are thus ~5% off from one another.


One can compare stresses from a pipe design program directly to code allowable values.

OR

In most cases, one can calculate the maximum stresses with a general purpose finite element tool (like ANSYS), divide those results by two, and compare those to code allowable values.

In Other Words:


A final note as to how the pipe analyzer calculated the SIF in the example: Per Appendix D of the ASME piping code:

SIF reported by pipe designer was 1.952


Example 2:

When using SIF’s blindly can get you into trouble. Consider the Appendix D entry for a “branch weld on fitting”.


Example 2:

A footnote indicates that this SIF relationship may become non-conservative if the branch line diameter (d) to main line diameter (D) connected by the weld-o-let exceeds a ratio of 0.5. Thus, the SIF relationship is not applicable if the d/D ratio is within the range:

0.5 < d/D < 1.0


Two models run, each in a pipe design program and in ANSYS:

d/D requirement is satisfied d/D requirement is violated


Model Results:

2 inch / 10 inch Joint 8 inch / 10 inch Joint

d/D ratio 0.2 0.8

Pipe Analysis Program 64,605 psi 1,850 psi

ANSYS 85,393 psi 5,042 psi

Pipe Analysis to ANSYS ratio 1.32 2.73

If ANSYS results are assumed to be accurate, then when the d/D ratio requirement is met, the piping analysis program overestimates the stress…a conservative approach.

(Recall that the maximum theoretical stress divided by 2 should equal the code stress, so here the true code stress might be approximately 42,697 psi.)


Model Results:

When the d/D rule is violated, then the pipe stress analysis program reports a stress lower than what may actually be present. True code stress ≈ 5,042 psi / 2 = 2,521 psi. The pipe stress analysis tool reported a value that may be 25% too low.

2 inch / 10 inch Joint 8 inch / 10 inch Joint

d/D ratio 0.2 0.8

Pipe Analysis Program 64,605 psi 1,850 psi

ANSYS 85,393 psi 5,042 psi

Pipe Analysis to ANSYS ratio 1.32 2.73


Concluding Recommendations:

• Those interpreting the output of any simulations need to understand the distinction between pipe code stresses and theoretical maximum stresses.

• The SIF relationships provided in Appendix D of the ASME code must be strictly applied only to those geometries for which they were developed. Ignoring any of the limits of their applicability can result in unsafe and misleading system evaluations.

• Appendix D SIF’s should not be extrapolated to geometries that only resemble the specified configurations in the code. In cases where a component is not explicitly accounted for in the code, SIF’s must be developed for the specific case using either experimental methods, or finite element modeling.


Contact:

e-mail: [email protected]

On Twitter: @Oliva_JC

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Chicago Pipe Stress Analysis Different Tools Different Results

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