Allowable secondary stresses around supports of Cold Stretched Vessels 1

[MrPDes]

I have a cold stretched vessel that is to be fabricated from 304 Stainless. The pressure envelope is designed to ASME VIII Div 1 Appendix 44 (Previously Code Case Code Case 2596-1). This allows for using an allowable design stress S of 270MPa (rather than the usual 138MPa for non-Cold Stretched vessels).

For the stresses around the vessels supports the fabricator/Designer is also using an allowable stress of 270MPa. This means that the allowable local Primary stresses can be 1.5×270=405MPa and that the allowable Secondary Stresses can be 3×270=810MPa.

The problem I have is that the Ultimate Tensile Strength of the material is 515MPa. The UTS is unaffected by cold stretching. The design analysis results show secondary stresses around the supports of 650MPa. It doesn't make sense to me to allow the secondary stresses to go above UTS.

The support in this case is located on the cold stretched cylindrical shell and applies a radial force into this cylindrical shell.

I would have thought that Appendix 44 is purely for the design of bolloon type pressure components where general membrane stress is much more dominant than any secondary bending stresses.

I think that Mandatory Appendix 1 of ASME II Part D should apply to Local Primary and Secondary stresses in the shell that are caused by loadings on Supports (as per a regular non-cold stretched vessel). That is:
S_yeild = 1.5×270=405MPa. (S yield would usually be 205MPa for a non-cold stretched vessel. This increase is due to work hardening of the area where the support will be attached.)
S_ultimate = 515MPa.
Therefore allowable design stress should be S = Min(405/1.5, 515/3.5) = 147MPa. (it would usually be = Min(205/1.5, 515/3.5) = 138MPa for a non-cold worked vessel.)

This would result in an allowable local primary stresses of 1.5×147=220.5MPa and allowable secondary stresses of 3×147=441MPa.

Appendix 44 does not state how to handle allowable stresses for local loadings on the cold stretched shell. The closest that I can get is Appendix 44-1(g) which specifies that openings shall be designed using regular Section II stresses (138MPa). No mention of the design of supports.

The fabricator argues that the allowable stresses are in accordance with the words written in the code. I am rejecting the design because they have interpreted the code in a way that that causes the design to not comply with the laws of nature. Is there any clause in the code or interpretation that clarifies what allowable design stress to use for local shell stresses caused by loads on supports?

 

[TGS4]

You raise some very valid points, especially having Sps > UTS (and in your case, substantially above UTS). There is some work at the Code Committee level being done about this, but you example is a perfect situation for clarifying the rules.

Please submit an inquiry to the Boiler & Pressure Vessel Standards Committees as soon as possible - in the 2013 Edition of Division 1, the instructions are found on page xxx through xxxi. Once you have done so and have a tracking number from ASME, please let us/me know (here) and I will try to follow that up at the next meeting. This is certainly something that WG-DBA (VIII) should be working on.

I think that your interpretation of 44-2(g) can safely be extended to support loads. But, unanswered is the question about how to design for external nozzle loads.

If it were me, I would not be using an elastic stress analysis method for these vessels - they have been highly plasticly deformed. I woudl start with an elastic-plastic analysis approach, including the stretching operation. The allowable range of the pseudo-elastic secondary stress range to prevent ratcheting may be less than the greater of 3S/2Sy.

 

[MrPDes]

Thanks TGS4,

If I put my self in the ASME committee's shoes and try and think of how I would word a clause in Appendix 44 that addresses this situation, it would be very difficult to not open a can of worms. In ASME VIII Div 1, if there is mention of Primary or Secondary Stresses or Elasto-Plastic analysis then that would be venturing into the complex world of Div 2. On the other hand, if stresses relating to support loading were encompassed within 44-2(g), this may be over conservative for many examples and as a result incorrectly prevent the use of Cold Stretching. I suspect the ASME committee decided to put in the too hard basket and let Fabricators and Purchasers fight it out on a case by case basis.

I have once sent a query to ASME twice with no luck in having a productive response. A year or so ago I e-mailed the ASME committee about UG-40(d)(2) asking if all of the metal on the nozzle pipe extending inside a vessel shell should contribute to the nozzle reinforcement if the pressure inside the nozzle pipe is different to the pressure inside the vessel shell (and the atmospheric pressure out side the vessel was different again). I thought my e-mail was well structured and specific to a particular sentence of text in the code but got a simple reply saying that I should hire a consultant to answer my query. I work for a large company with a dozen Pressure Vessel Engineers with many decades of advanced experience, one of which is a top dog of the European Pressure Vessel Committee. There was always a common sense solution which I used in the end however this solution is not specified in ASME VIII. The wording in UG-40(d)(2) can result in a non-conservative design. I will make a separate thread about it.

Anyway..... I've since been reluctant to submit any other queries to the ASME committee. If I was to put together a short precise query about local stresses in cold stretched vessels and post it here, will you be willing to check it before I submit it to ASME?

 

[TGS4]

If I was to put together a short precise query about local stresses in cold stretched vessels and post it here, will you be willing to check it before I submit it to ASME?

Absolutely. And I will follow it up there.

 

[MrPDes]

See below my proposal. I've tried to make the Background and Question as simple as possible. The examples provide some reasoning behind why the question is being asked. Do you think that this is likely to be considered by the ASME committee? Make suggestions, gramma, spelling corrections if you wish.

Maybe the question should be "Please provide guidance on calculating maximum design stress that is used to evaluate local stresses in the shell that are the result of loads on vessel supports?"


Subject: Section VIII, Division 1 (2013 Edition); Mandatory Appendix 44

Background:
44-5(a) specifies that the wall thickness of the vessel shall be calculated using the maximum design stress values shown in table 44-4-1.
44.2(g) specifies nozzles and opening reinforcement components shall be designed using stress values specified in Section II, Part D.
There is no clause in Mandatory appendix 44 specifying what maximum design stress shall be used for evaluating local stresses in the shell that are the result of loads on vessel supports.

Question 1:
What maximum design stress shall be used to evaluate local stresses in the shell that are the result of loads on vessel supports?

Example 1:
A cylindrical shell of 304 stainless steel has a maximum design stress of 270MPa from table 44-4-1. The supports apply a radial force into this cylindrical shell.
For a design case of the vessel in service an elastic analysis of the stresses in the shell local to the supports is conducted.
The maximum allowable stress for evaluating Primary stresses will be 1.5×270=405MPa.
The maximum stress for evaluating Secondary stresses will be 3×270=810MPa.
The Minimum Ultimate Tensile Strength in ASME II Part D is 515MPa.
It is difficult to understand how the maximum stress for evaluating secondary stresses can be greater than the UTS.

Example 2:
The above example has a minimum calculated design thickness of 4.1mm and a selected plate thickness of 5mm.
During the cold stretching procedure, the cylindrical shell will be pressurised to 1.5 to 1.6 times the design pressure per 44-5(c). (Note that the design pressure is not the same as the MAWP.)
Due to the excess plate thickness, the material will not be stretched to 1.5 to 1.6 times 270MPa (=405MPa). The stress during the cold stretching procedure will actually be =1.5 to 1.6 times 220 (=330MPa).
The material has not been cold worked enough to justify using 270MPa, therefore the maximum design stress will be 220MPa.
Furthermore, the maximum allowable stress for evaluating Primary stresses will be 1.5×220=330MPa. The maximum stress for evaluating Secondary stresses will be 3×270=660MPa. This is still greater than the UTS of 515MPa.

 

[TGS4]

See my mark-up below. I think that Example #2, while relevant, detracts from the primary issue.

Subject: Section VIII, Division 1 (2013 Edition); Mandatory Appendix 44

Purpose: New or additional Code rules

Background:
44-5(a) specifies that the wall thickness of the vessel shall be calculated using the maximum design stress values shown in tTable 44-4-1.
44.2(g) specifies nozzles and opening reinforcement components shall be designed using stress values specified in Section II, Part D.
There is no clause in Mandatory aAppendix 44 specifying what maximum design stress shall be used for evaluating local stresses in the shell that are the result of loads on vessel supports or nozzles. For consistency with UG-22, these loads must be considered.

Question 1:
What maximum design stress shall be used to evaluate local stresses in the shell that are the result of loads on vessel supports or nozzles?

Example 1:
A cylindrical shell of SA-240, Type 304 stainless steel has a maximum design stress of 270MPa (39.3ksi) from tTable 44-4-1. The supports apply a radial force into this cylindrical shell.
For a design case of the vessel in service an elastic analysis of the stresses in the shell local to the supports is conducted. Following the standard approach (consistent with U-2(g), Tthe maximum allowable stress for evaluating Primary Local Membrane stresses will be 1.5×S, or 1.5×270=405MPa. Again, following the standard approach, Tthe maximum stress range for evaluating Secondary Membrane-Plus-Bending stresses ranges will be 3×S, or 3×270=810MPa.
However, tThe Minimum Ultimate Tensile Strength in ASME II Part D is 515MPa.
It is difficult to understand how the maximum stress for evaluating secondary stresses can be substantially greater than the UTS.

Proposed Revision/Addition: I believe that SG-D(VIII), in conjunction with WG-DBA needs to develop the appropriate stress levels for such a problem, consistent with U-2(g) and the rules in ASME Section VIII, Division 2, Part 5.

I would not include this second example - it slightly detracts from the original issue
Example 2:
The above example has a minimum calculated design thickness of 4.1mm and a selected plate thickness of 5mm.
During the cold stretching procedure, the cylindrical shell will be pressurised to 1.5 to 1.6 times the design pressure per 44-5(c). (Note that the design pressure is not the same as the MAWP.)
Due to the excess plate thickness, the material will not be stretched to 1.5 to 1.6 times 270MPa (=405MPa). The stress during the cold stretching procedure will actually be =1.5 to 1.6 times 220 (=330MPa).
The material has not been cold worked enough to justify using 270MPa, therefore the maximum design stress will be 220MPa.
Furthermore, the maximum allowable stress for evaluating Primary stresses will be 1.5×220=330MPa. The maximum stress for evaluating Secondary stresses will be 3×270=660MPa. This is still greater than the UTS of 515MPa.

 

[MrPDes]

It took a while, but the Record Number is 14-646.

It will go through the "request for a code change" procedure (not "request for interpretation" procedure).

It should be discussed at a meeting in three weeks. I didn't submit example 2.

 

[TGS4]

Thank you. I will follow-up with WG-DBA and SG-D(VIII). We will continue this discussion off-line.

For the rest of the community, we will provide updates. This is an example of the correct procedure to follow when there is a lack of clarity in the ASME Codes. Hopefully we can get a reasonably fast (ie less than two years) resolution.

 

[MrPDes]

Another point I have recently leart is that during the cold stretching procedure the material is cold worked in the hoop direction ONLY and therefore has the enhanced properties specified in Appendix 44 in the hoop direction ONLY.

In the Longitudinal direction, the tensile stress only reaches the 0.2% proof stress during the cold stretching procedure and therfore receives no cold working and therefore does not have the enhanced material properties.

The shell stress from supports loads is measured using Von-Vises Equivilant Stress which is scaler value that does not differentiate between hoop and axial principal stress. There is no method in ASME VIII Div 2 to accomodate material with different properties in hoop and axial directions, therefore I believe that conservatism requires that the conventional properties be used.

In other news, the enquiry is in the system. I am not certain of progress but I understand it is being looked at.

 

[TGS4]

The item is being looked at and is the subject of some robust discussion.

Just a correction to your post. You said:

There is no method in ASME VIII Div 2 to accomodate material with different properties in hoop and axial directions...

What is correct, however, is

There is no elastic method in ASME VIII Div 2 to accomodate material with different properties in hoop and axial directions

As I have said before, it is my opinion that the elastic-plastic methods in Part 5 of VIII-2 are more than suitable for handling this situation, provided that the cold-stretching operation is included in the simulation. Besides, the material does not become plastic in the hoop direction only. Our current understanding (and the mathematics a mechanics of materials that we use to simulate plasticity) is that once the invariant exceeds yield, then the entire material becomes plastic isotropically. It does not behave anisotropically. So, there are NOT different properties in the hoop and axial directions.

Furthermore, the concern with secondary stresses is the failure mode of ratcheting. The methodology in 5.5.7 should be sufficient for this problem.

The biggest issue that that I have with an elastic analysis of this type of problem is that it really is inappropriate - elastic-plastic is the only appropriate means to solve this problem - because it involves substantial plasticity to begin with.

 

[MrPDes]

Thanks TGS4,

I have very little experience with elastic-plastic analysis but my understanding is that Table 5.5 of Part 5 of VIII-2 says that 2.4 times the pressure and any loads during operation shall be applied to the FEM model. The final stress of the analysis shall then be no greater than UTS.

Because Div 1 has a design margin of 3.5, the FEM model would need to have the pressure and loads multiplied by 3.5× instead of 2.4×.

3.5×270 MPa for 304 Stainless = 945 MPa. This is well above the UTS of the material. I have come across comments and clues form various sources that say the UTS of the material does increase by up to 30% following the Cold Stretch procedure but I can't see it being remotely close to 945 MPa. An elastic-plastic analysis of a VIII-1 Appendix 44 vessel fails with a simple hand calculation.

Elastic-Plastic analysis may be the most appropriate means to solve the problem but if my understanding of elastic-plastic analysis is correct then both elastic and elastic-plastic analysis seem to both need some rule adjustments to accomodate a cold stretched vessel.

 

[TGS4]

You are focusing on the "Protection Against Plastic Collapse" portion of Part 5. However, the subject that we are discussing with respect to secondary stresses is "Protection Against Ratcheting". That is why I referred you to paragraph 5.5.7.

3.5×270 MPa for 304 Stainless = 945 MPa. This is well above the UTS of the material. I have come across comments and clues form various sources that say the UTS of the material does increase by up to 30% following the Cold Stretch procedure but I can't see it being remotely close to 945 MPa. An elastic-plastic analysis of a VIII-1 Appendix 44 vessel fails with a simple hand calculation.

I'm not sure what these values are, but I am quite certain that you are misapplying the elastic-plastic analysis methodology. Indeed, for Protection Against Plastic Collapse for a Division 1 vessel, the internal pressure load needs to be factored by 3.5 rather than 2.4. However, for these stainless steel materials, most of the allowable stress is governed by the 1.5 margin against yield - hence cold-stretching is a way of improving the yield strength of these materials, and perhaps making the margin against ultimate govern.

Another error in your "hand calculation" is that these calculations use true stress and not engineering stress - which makes a significant difference in materials with low yield-to-ultimate ratios such as stainless steels.

 

[MrPDes]

TGS4,
I’m try to interpret your post. Are you saying that assessment of primary stresses and “Plastic Collapse” is taken care of in during the VIII-1 Appendix 44 design and therefore conducting a general elastic plastic analysis per VIII-2 Section 5.2.4 to assess plastic collapse would possibly be a waste of time?

And that VIII-2 Section 5.5.7 should be purely used to assess “excessive plastic deformation” of the shell around the area local to the loaded supports because all bending around the supports is secondary?

I would completely agree with you, however...... I think that the Primary bending stresses around the supports need to be considered in a “Plastic Collapse” assessment and Appendix 44 for doesn’t do that.

WRC 429 specifies that “bending at a discontinuity cannot be assumed to be entirely secondary without appropriate evaluation”. It also says “At discontinuities the secondary bending stress is normally significantly greater than the primary stress, and considering all bending stress as Primary is considered too conservative.”

Many vessel components are an obvious gross structural discontinuity, such as a nozzle or cylinder-cone junction. My application is hardy a gross structural discontinuity. It consists of a flawless shell of 2.5m diameter and 5 mm thick with a 3” diameter thin pipe welded onto it. (The pipe is the support with radial load applied to it). Many in the vessel design community seem to abide by a “design by rule” within the “Design by Analysis” Part of VIII-2 that specifies that primary bending only occurs in flat heads and the crown of a dished head. I’ve never been able to find this rule or any rule relating to categorising bending stresses around supports.

However PD 5500 does consider bending stresses around doubler pads and supports to be primary (with an option of increasing allowable bending to 2.0× if allowable membrane is correspondingly reduced to 1.2×).

Are you aware of any resource that clearly justifies stresses around supports as secondary?

A question: If say a round bar made of rubber (not welded to the shell) applied a radial load onto the shell would there be any secondary bending stresses?

Even without the support and its loading, I can’t get a cold stretched cylinder under design internal pressure only to be compliant with a general elastic-plastic analysis per VIII-2 Part 5.2.4. From Appendix 44, for a 304 Stainless steel cylinder, S = 270 MPa. This means that the maximum principal hoop stress = 270 MPa and corresponding principal axial stress = 135 MPa. The General Membrane Von-Mises equivilant stress is calulated to be = 234 MPa. If the 5% reduction in thickness is taken in to account the true stress will slightly increase to approximately 245 MPa. 3.5×245 MPa = 857 MPa (per table 5.5 of VIII-2). This is 66% higher than the conventional UTS of 515 MPa for 304 Stainless. There is no published values for the UTS of cold stretched 304 stainless.

Plastic Collapse:
If during a series of shake down cycles, primary stresses remain greater than the materials yield stress then the material will collapse. To prevent plastic collapse Pm, Pl and Pl+Pb shall be less than Sy following Shake down.

For a conventionally designed vessel, in ASME II-D Mandatory Appendix 1, there is an added requirement S ≤ UTS/3.5 for areas of cylinders and spheres remote from gross structural discontinuities. VIII-1 Appendix 44 seems to overrule II-D Appendix 1 and in the process remove the S ≤ UTS/3.5 requirement and anything relating to the UTS from the design of cold stretched pressure vessels. There of course may be something relating to UTS going on behind the rules that the ASME committee have decided not to publish.

Appendix 44 seems to use values of S for various Stainless Steels obtained from Cold Stretching experience. For 304 Stainless Steel, S and is set to 270 MPa (up from 138 MPa for a conventional vessel). During the cold stretch procedure the principal hoop stress is 1.5×S = 405 MPa.

I interpret the cold stretching as a procedure where the cylinder is shaken down. In the process almost the entire vessel will be work hardened with a resulting increase in yield stress. For the cylinder, the yield stress increases from the conventional 205 MPa to 405 MPa.

This is in accordance with VIII-1 because the design by rule requirement that the design principal hoop stress in the cylinder be no greater than Sy/1.5 = S = 270 MPa is being met.

In terms of an eleastic analysis this is also in accordance with VIII-2 Part 5 because the design by analysis requirement that the Von-Mises value for General Primary Membrane Stress Pm in the cylinder is no greater than 1.0×S = 270 MPa is being met. For this application hoop principal stress = 270 MPa and Axial = 135 MPa, therefore the Engineering value of Pm = 234 MPa.

In terms of an elastic-plastic analysis, the 3.5×Pm shall be no greater than UTS. The material thins by approximately 5% therefore the true Von-Mises value of Pm is approximately 234+5% = 245 MPa. 3.5×245 = 857 MPa. This is 66% higher than the conventional value of the UTS of 515 MPa.

A post on this forum that I have read says that ASME committee has written a report with findings that UTS increases by up to 30%. EN 13458-2:2002 also presents a figure that indicates an increased UTS after cold stretching. Nothing out there suggests the UTS increases by a whooping 66%.

Is there something flawed in by calculations above or is Appendix 44 just a "special" set of rules?

 

[MrPDes]

Clarification:
For elastic-plastic analysis, I have written 3.5×245 = 857 MPa. I should actually say the applied internal pressure in the FEA is multiplied by 3.5 per Table 5.5 of VIII-2 and then as a result the general membrane stress in the Analysis becomes 3.5× greater at 857 MPa.

I'm possibly not handing the True stress correctly. But when the membrane stress from the FEA is greater than the UTS, I'm not sure if the true stress can be handled. It would just rupture.

 

[TGS4]

A couple of considerations:

1) I do not believe that any sort of elastic analysis is appropriate for cold-stretched vessels. Therefore, any reference to elastically-calculated stresses or elastic stress analysis classification/categorization are inappropriate in this discussion. In that vein, the approaches described in WRC 429 would not be appropriate.

2) Your question with regards to P+Q stresses is considering the results of an elastic analysis, which I have said is not appropriate. That said, when one does an elastic analysis, P+Q stresses are used in the consideration of Protection Against Ratcheting.

3) The assessment of Protection Againsy Plastic Collapse is very different from Protectuon Against Ratcheting. For starters, the former considers design loads and design load combinations and factors on those, whereas the latter is concerned with operating load ranges.

I do take your point about the design margins. Using your example of a vessel fabricated from SA-240 Type 304, the revised allowable stress is 270 MPa. At room temperature, the UTS of the same material is 515 MPa, resulting in a design margin of 1.9. This is vastly different than the otherwise design margin in VIII-1 of 3.5 on ultimate. This would indeed be problematic for performing any type of analysis.

In discussing this item with the volunteer Project Manager in SGD-VIII, I understand that a resolution to close this item without providing any guidance other than to say that your issue is not addressed in Appendix 44 and you should see U-2(g) was passed at the meeting. While I agree with the literal approach to this resolution, it still does not provide any guidance on how to consider U-2(g). I will take this up with the Project Manager.

 

[TGS4]

The only other thing I would mention are the additional rules that ABSA in Alberta, Canada impose on Appendix 44 construction -

http://absa.ca/IBIndex/IB13-016.pdf