Please Note: To Listen to the Webinar You Must Place a ... · PDF file16 Situations That Involve 100.000% of Allowables: 1) Application of safety factors (on top of other safety factors),

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What is 100% of the Allowable Stress for Pipes and Pressure Vessels?

Paulin Research Group

Tony Paulin

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What is 100% of the Allowable for Pipes and Pressure Vessels?

Calculated Stress = Allowable Stress

i.e.

35202.0 = 35202.0

35201 = 35202 … OK

35203 = 35202 … NOT OK

Is 35203 “equal enough” to 35201 to be OK?www.prg-software.com Copyright © 2008 Paulin Research Group. All Rights Reserved. 1-281-920-9775

4www.prg-software.com Copyright © 2008 Paulin Research Group. All Rights Reserved. 1-281-920-9775


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Know where the cliff is.

How “safe” do you want to be.

When designing, it’s someone else’s safety you’re providing for.


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Unnecessary safety requirements can sometimesmake it difficult to do a job. Making welds bigger than what they need to be, or adding pads where they are not needed can create more room for error – especially where fatigue is a concern.




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When We Break the Rules – Just a Little

(or Get Close to the Rule)


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3/4” Drain Line on 24” Header:

Refer to WRC 329 Para 4.4 Run Moments:

“To illustrate a large inaccuracy in all [B31] Code i-factor equations … we give the following example … UFT [28x0.375 and 2x0.25] … it is intuitively apparent that the … i-factor [for this example is off by 10].

“In discussing this kind of example with users of B31.3 and B31.1we have been told, in effect, that the Code requirements are obviously silly; the piping analyst should use his judgment and ignore the vent or drain line in his … analysis.”


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When We Break the Rules Just a Little:

(example with the drain line.)

If we’d left the drain line of of the analysis everything would have been ok and we’d be out having an early lunch.

Since the drain line was inadvertently added, the line showed to be overstressed because of the large SIF, now we have to:

1)Remove the drain line or enter i-factors calculated from ASME Section III NC or ND, or from FEA … <or>

2)Explain that fatigue is the failure mechanism of concern for the large i-factor, and that the line is not cycling (if it’s not), site WRC 329 as a reference, discuss experience...

3)Can the same think happen with small d/D when we’re interested in the attached pipe?


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When We Break the Rules Just a Little:

1) Rely on successful past experience. If there’s no past experience think twice about breaking or stretching the rules, or approaching Code limits.

2) Discuss the problem with as many experts as you can to get as many opinions:

A) Blogs, Google, or Newsgroups

B) Vendors

C) Colleagues

D) Industry magazines, other industries having the same problem (auto)

3) Look for ways to avoid breaking the rules. (rerouting pipe, friction, stiffnesses)

4) Apply added inspection, monitoring, operating procedures, etc. (for many situations the analyst must only have “considered” the problem to satisfy the Code.)

5) Document clearly the decision process and make sure everyone is aware of it.

6) Consider if it means breaking the rules only once, while it’s easy to be safe, or if it means that after you’re long gone, the rules will still be broken.




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Situations That Involve 100.000% of Allowables:

1) Application of safety factors (on top of other safety factors), result in ridiculous decisions. (“You want to be safer, just add another safety factor – design for 1600F!” – play the “safety card” to get your way.)

2) Use good engineering judgment when close to the allowable and take all necessary, reasonable precautions and a few additional ones.

3) A Lawyer, my boss or the client says that judgment is Not Allowed. (Ethics)

4) When we play the “numbers game” to get a calculated stress below the allowable, i.e. adjust the friction coefficient a little, support stiffnesses, FEA averaging methods. (See #2 – don’t let anyone get too close to the cliff.)

5) Judgment is required, (flange bolt loads, SIFs for large D/T, hillsides, laterals, drain lines or similar etc. - the rules aren’t clear or don’t exist.)

6) Sometimes yea or nea (is a system OK or not), is political. The job is late and my boss needs someone else to blame. (Ethics)

7) When probability is involved (fatigue). There are a lot of similar, high stressed welds in a cyclic system, what probability of failure can be tolerated? Is using inspection to affect probability of failure reasonable – yes.



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Issues that arise when stresses are Close to, or Greater Than 100% of the Allowable

1) How realistic is the calculated stress? (Will SIFs or Flexibilities affect the predictions? Does my calculation fit within recommended parameter ranges, i.e. D/T<100, etc. PIPE STRESS program may not help anymore!)

2) How accurate is the allowable? (Who set the design loads? What is the failure mechanism? Is the Code accurate for this failure mechanism, i.e. fatigue, buckling, burst, distortion, ratcheting?)

3) Is it easy to reduce the stress? Can the pipe be rerouted or can the number be tweaked (friction, flexibilities)? If it’s easy to reduce a questionable stress – this is often the simplest, and best thing to do.

4) Is the answer to this question political or does it have legal implications? (Ethics – do you want, or have, to put your job on the line?)

5) How bad are the consequences if I’m wrong?

6) Do we have long safe histories of operation for this type of system?

7) Is the owner/operator/client knowledgeable, easy to work with, and available for discussions?


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1) How realistic is the calculated stress? (Use the PRG Pipe Checklist for guidance, Read WRC Bulletins, Attend Code Meetings.)

2) How bad are the consequences if I’m wrong? (Use the PRG Pipe Checklist for Guidance and Risk Analysis, Use common sense, talk to colleagues, the owner, etc., and check www.chemsafety.gov.)

We’ve Already Talked About Some of These Issues:

We’ve Talked a Little About “Allowables”, But I’d Like to Specifically Address Several Code Allowables to

help us get a “feel” for 100% of an “allowable.”


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Pressure Stress in a Cylindrical Shell

Allowable = Suts / SF

(How close to the cliff? Is 10% overstress going to fail a cylinder under pressure? – NO. Is 300% overstress going to fail a cylinder under pressure – most likely)

Pressure Failure Occurs When PD/2t = Suts … roughly

( 3.5/2.4 = 1.458 )


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Fatigue (Cyclic) Stress in a Piping Component

Is 200% of the allowable for cycles > 3125 going to cause a failure? –it’s going to result in a 50/50 chance of failure.


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Sustained Longitudinal Stress in a Piping System

Allowable = Sh ~ 2/3Sy

(How close to the cliff?) –

“Background for the [Piping] Code Simplified Method for [Sustained] Loads in Piping Systems”

S.E. Moore, and E.C. Rodabaugh.

Longitudinal stresses are principally caused by bending moments and axial pressure thrust.

For bending moments the safety factor for piping components is thought to be around 1.5 when bends are the major contributor to primary external load failure. (See ref. above p.154)


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Stresses in Flanges & Bolts


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ASME Section VIII Division 1, Nonmandatory App. S



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What Does this Mean for the Flange & Gasket:

1) Larger diameter flanges in 150# classes may be overloaded by boltup loads if someone really tries.

2) Soft gaskets can be crushed when flanges areovertightened

3) Target bolt stresses for large bore or ASME flanges should be selected carefully and torques should be properly applied.

4) This is especially important in systems that have cycles, or where the joints must be taken apart and retightened.

5) 40,000 to 50,000 psi bolt preload for A193 B7 studs is reasonable for B16.5 flanges.


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Section III NB/NC/ND Limits

Membrane stress in bolts < 2 x Bolt Allowable

Membrane + Bending stress in bolts < 3 x Bolt Allowable

Pretension < Membrane Allowable

additionally, and separately…

Pressure Stress + Bending Moment < Membrane Allowable

(Pretension is not added to pressure stress + bending moment because the bolt loads do not significantly change when pressure or external loads are applied. Many basis tests performed with asbestos gaskets.) Allowed bending moments may be a little low. ?2 times?

(due to attached pipe, not bending in the bolt)


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Example Flange Calculation (4” – 150#)

Pretension: 11,100 lb.

Pressure: 1053 lb.

Moment: 2525 lb.

“IF” Interaction:

11100 – 1053 – 2525 = 7522 lb.


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Ref. “Evaluation of the Bolting and Flanges of ANSI B16.5 Flanged Joints…” E.C. Rodabaugh, S.E. Moore.


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Everyone Doesn’t Always Agree

When we talk about 100% of the Allowable – We have to get the right load cases and stresses.

* B31.3 S203.6.2 Anticipated Sustained Conditions. (“All anticipated sustained conditions utilizing all possible support scenarios should be considered and either evaluated or “Approved By Inspection.”)

* www pipestress com/pages/PEpapers.html:

“Hazardous situations created by improper piping analyses.”

Discussion ongoing since 1985.

Does the load at the support go into the thermal case, the sustained case or both?


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Conclusions

1) Increasing the design margin does not always mean increasing the safety.

2) The job is a lot harder when you’re close to the allowable.

3) For flanged joints, prestress and operating loads are handled as separate, independant load cases.

4) Bolt prestress < 2Sa(bolt) is OK for the bolt

5) Allowed flange bending stress based on “leakage moment” from Roberts, (NB/NC/ND) may be too conservative.

6) Pipe stress program stresses and allowables are safe, not accurate, (if used properly.) {True or False?}

7) Be careful with run pipe SIFs when d/D < 0.4www.prg-software.com Copyright © 2008 Paulin Research Group. All Rights Reserved. 1-281-920-9775

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Is 35203 “equal enough” to 35202?

How close to the edge of the cliff is 100% of the allowable?

Do I have a client that understands that a calculation is a rough approximation of reality?

Are there other things that make the system safe?

Does the stress coincide with experience I have with this system?

What are the design loads based on?

What are the consequences of failure?


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Tuesday April 8 - External Loads on Nozzles (Comparisons of WRC107, B31, EN13445, VIII Div2 and Mean-Life-to-Failure)(Part 1) [MOVED FROM MARCH 27]

Tuesday April 15 - External Loads on Nozzles (Comparisons of WRC107, B31, EN13445, VIII Div2 and Mean-Life-to-Failure)(Part 2)

Tuesday April 22 – Buckling of Piping Components (Part 1a). Buckling of Pressure Vessel Components (Part 1b).

Tuesday April 29 – FRP Pipe Failures and Lessons to Be Learned.” (Part 1) (Guest Lecturer – Dr. Hans Bos)

Thursday May 1 – “Pipe & Pressure Vessel Ethics – Conditions of Disagreement” (Part 1)

April PRG Webinar Schedule

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Please Note: To Listen to the Webinar You Must Place a ... · PDF file16 Situations That Involve 100.000% of Allowables: 1) Application of safety factors (on top of other safety factors),