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#75263 - 12/16/20 11:43 PM ASME VIII UG-28 clarification
Jouko Offline
Member

Registered: 01/11/04
Posts: 383
I normally work with pipes but this is more of a vessel related and I do not have PV Elite to check what it does. I hope that a vessel designers would answer.

Not one of the standards are clear. I have a question how to understand ASME Factor B graph/table together with UG-28.

If we assume the following:
Graph and table: CS-2
A value: 0.0034 (or any value that intersects 300 F line)
Temperature: 299 F

Round dimensional values used
Do: 98.5"
t: 1"
L 40"

In the graph there is a note for the 300 F line Upto 300 F. In UG-28 Step 4 it says "Move vertically to an intersection with the material/temperature line for the design temperature." Table has no word UpTo. Design temperature is below 300 F. There is no line 200 F or similar below 290 F but there is that UpTo so it is valid for all temperatures below and including 300 F.

Step 4 last sentence: For values of A falling to the left of the material/temperature line, see step 7. This is 100% clear if the A value would be less than 0.176 10^-4 which is the lowest A value.

If I use Step 7 formula the maximum allowable external pressure is around 900 psi. Using the 300F graph max pressure is about 210 psi. That is a serious difference. Material values will not change between 299 to 300 F that much.

Please consider above and let me know if Step 7 formula should be used or 300 F line where A crosses the line. I tried to look at code cases but I could not find a solution. Literature I found is silent.

Thanks
_________________________
Regards,

Jouko
jouko@jat.co.za

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#75283 - 12/25/20 01:41 PM Re: ASME VIII UG-28 clarification [Re: Jouko]
mariog Offline
Member

Registered: 09/29/07
Posts: 798
Loc: Romania
Dear Jouko,

I tried to understand why you involved Step 7, which is used only in case the value of A is falling to the left of the applicable temperature line in Figures of ASME II D. Do you suspect that might be a curve (applicable to 290 F for example) to the left of the curve specified for "up to 300 F"?- or I totally failed to understand the reason?

In my opinion the reason for specifying the curve applicable "up to 300F" is the fact the modulus of elasticity value is considered as constant up to that value (and the value is 29*10^6 psi for curve CS-2, for example). So in my interpretation, indeed that curve is "up to 300F" and must be considered as applicable for cases when the vertical line "A' intersects that curve and T<=300F.
For the actual cases when A is to the left of the curves. B should be taken as B=A*E/2 (comparing the equations in step 6 and step 7).
By the way, for A=0.176E-04 and E=29E06 psi, the value indicated in table CS-2 is 0.250E3 (which is an approximation of AE/2) but CS-2 figure indicates a value of 2500 (at least in 2019 edn).

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#75285 - 12/26/20 05:00 AM Re: ASME VIII UG-28 clarification [Re: Jouko]
mariog Offline
Member

Registered: 09/29/07
Posts: 798
Loc: Romania
Some additional information

1. There are alternative rules in Case 2286 (it seems that Rev6 is the last one, so it's 2286-6), which are intended to deal with the excessive conservatism in the original ASME methodology. In PV-Elite the Case 2286 is supported but I haven't accurate information which Case revision because I have access to PV-Elite few revisions old.

2. An old book, Brownell and Young- Process Equipment Design Handbook has a chapter that explains the roots of the method. For me, it seems that today the safety factor between theoretical pressure and allowable pressure is a factor of 3, not 4 as in the original work, but this is just a detail.
One can understand that A is in fact f/E where f is stress, A is strain, E is the modulus of elasticity.
The CS-2 curve is just an attempt to calculate B=f/2 when you know A=f/E. Maybe for us, today, the graph CS-2 is rather confusing but in fact is just the stress-strain graph (scaled down with a factor of 1/2) considering also the influence of temperature on E. Anyway, the shape looks like steel stress-strain non-linear idealized curves, no doubt. What today seems to be strange is the fact graphs are not drawn for small strain, that's why (for cases when A is to the left of the curves) they called (in background) the rule of B as taken directly B=A*E/2=f/2 assuming that you know E in this limited linear domain. I mean it appears that has been decided is useless to improve the graph as drawn starting in zero- stress and zero-strain with additional 5 strait lines (case in which no A will be to the "left" of the curves)

I think the above will clarify your doubts. For your case, the troubles are not due to temperature but to curves themself. For "A" value of 0.0034 you are in the non-linear domain stress-strain and you cannot compare the value with those given by "step 7" which is intended only for linear domain not shown in figure CS-2 (also theoretically in agreement with the linear domain shown in CS-2).

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#75288 - 12/27/20 10:27 AM Re: ASME VIII UG-28 clarification [Re: mariog]
Jouko Offline
Member

Registered: 01/11/04
Posts: 383
Thanks for the comments.

As I read the bases of the Factor B graphs I take the B value from the vertical A to Upto 300 F line in all cases where there is intersection. If there is no intersection then it is Step 7. There is commercial software where Step 7 is used for all A values if the temperature is below 300 F. So I had my opinion in conflict with the software. Literature is totally silent on this issue.

I will contact the software supplier and will ask them to look into this issue.

I cannot use the Code case but I had a look. Step 2: "Move vertically to an intersection with the material temperature line for the design temperature" 300 F is the lowest curve. What to do if the temperature is 250 F? My argument is that the word "Upto" on 300 F line (CS-2) is "Design temperature" line for all temperatures <=300F. I understand that your opinion is the same.
_________________________
Regards,

Jouko
jouko@jat.co.za

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#75289 - 12/27/20 01:12 PM Re: ASME VIII UG-28 clarification [Re: Jouko]
mariog Offline
Member

Registered: 09/29/07
Posts: 798
Loc: Romania
Yes, I agree the curve "up to 300F" is conservative taken, neglecting the variations of properties (E and limit of stress-strain elastic domain) for temperature less than 300F, so you can call the curve "design values for temperature up to 300F".

Probably, in the same degree of approximation, for cases when "A" is to the left of graph and temperature is less than 300F, the modulus of elasticity may be E=29*10^6 psi as is written on graph (despite there are accurate "E" values in Table TM-1 of ASME II D), and one would consider that using more accurate values for "E" is not really an improvement. But, as you said, literature is silent also on this issue.

By the way, for such cases ("A" is to the left of graph) is strange (but not wrong) that the Table CS-2 and Figure CS-2 starts in different points. Table says A=0.176*10^(-4) and B=250 psi and graph starts with 0.000176 and 2500 psi.
Both pairs are "correct" (in the limit of work approximation), just graph is more truncated than the table.

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#75290 - 12/27/20 01:38 PM Re: ASME VIII UG-28 clarification [Re: Jouko]
mariog Offline
Member

Registered: 09/29/07
Posts: 798
Loc: Romania
Same graph in ASME BPVC 1956 was more generous, more explicit and combined. Probably ASME changed the presentation to avoid discussion is working with plastic strain and made it more digestible...

About the software you've mentioned, I guess that their interpretation was indeed that "less than 300 F" must be a curve to the left of 300F curve, hence they thought "to the left" brings-up automatically the rule of "step 7". Understanding the reason of that rule (basically is dedicated for the linear part of curves, going to 0,0 point- not shown in graph- and "to the left" is just a visual indication) may clarify the matter. In addition "to the left" in the today graph is different from "to the left" in 1956 edition, because the older edn had more "linear" part in the graph.


Attachments
B graph BPVC 1956.pdf (261 downloads)


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#75293 - 12/28/20 10:44 PM Re: ASME VIII UG-28 clarification [Re: mariog]
Jouko Offline
Member

Registered: 01/11/04
Posts: 383
I captured quite a few tables into electronic format. Then I had to check all of those values. I used the data to create graphs for checking purposes. In some cases these graphs differ quite a bit. Basically tables do not capture the points in correct spots to get the same graph shape. There are also other differences as you point out below.

I had a look into the metric edition. There are some rounding issues like between the allowable stresses in imperial and metric editions.

I did contact the software supplier. They are still of the opinion that at temperatures below 300 F irrespective of A value Step 7 formula is to be used. I will try to get an opinion from ASME engineers.



Originally Posted By: mariog


By the way, for such cases ("A" is to the left of graph) is strange (but not wrong) that the Table CS-2 and Figure CS-2 starts in different points. Table says A=0.176*10^(-4) and B=250 psi and graph starts with 0.000176 and 2500 psi.
Both pairs are "correct" (in the limit of work approximation), just graph is more truncated than the table.
_________________________
Regards,

Jouko
jouko@jat.co.za

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#75294 - 12/29/20 12:00 AM Re: ASME VIII UG-28 clarification [Re: Jouko]
mariog Offline
Member

Registered: 09/29/07
Posts: 798
Loc: Romania
And what you've written in the first post is not a warning bell for them?
Quote:
If I use Step 7 formula the maximum allowable external pressure is around 900 psi. Using the 300F graph max pressure is about 210 psi. That is a serious difference. Material values will not change between 299 to 300 F that much.


They are just counting the stress- strain as elastic even the value of strain is in plastic domain, which is highly non-conservative. That's exactly what the graph tries to avoid and the reason for which such graph exists!

My best regards.

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