differences between charts and graphs

[McJe]

Did anybody noticed the differences between the charts and the graphs of several material groups? (we use the metric ones)
I even think there are some errors in the charts.

We use the charts in our excel calculation sheets, and we came across some deviations.

Does somebody else uses the charts?
And what do you do with the incorrect values?

Is it allowed to correct these values in the charts?
Or do we have to calculate with the (in my opinion) faulty values?


Jeroen.

 

[saplanti]

You do not give your references for the charts or the graphs you mentioned. Therefore no-one would be able to answer your question unless one do read your mind.

To cut short if you like you can refer your spreadsheet by attaching it, and referencing the codes you are using.

Otherwise, my guess, you will not get the answer you are looking for.

Hope it helps.

 

[McJe]

My fault...

I'm looking through the external pressure charts in ASME.
The charts I have found 'errors' are:
- CS-6
- NFN-12
- NFN-14
- NFN-15
- NFN-16
- NFN-18

The graphs I created in Excel are crossing each other, some worse then others, but it's certainly not very good looking. The values I entered are checked and double checked. You can verify my graphs easily by entering the tables in excel.

I think it would not be that difficult for ASME to create a nice set of matching graphs and tables.

Next to these errors mentionned, there are also some typo's in the tables:
- NFN-9: metric - 705°C - "A=3.0 e-3" should be "3.0 e-2" (this is correct in the customary table)
- NFN-18: metric and customary: B-value corresponding with the lowest A-value and lowest temperature: B should be 20MPa / 2.84psi. I assume this is a typo in the customary table, copied to the metric table. (see graph to verify)
- NFN-18: metric - 260°C - lowest A value misses a '0' (zero).


The biggest issue would be that it isn't allowed to alter these tables when using them. OK, I *might* be correct here, but I can imagine allowing corrections leads to custom adaptations to correct calculations....

 

[saplanti]

There are some difference between your charts and ASME charts. ASME is using logaritmic scale on the horizontal and different(?) scale on the vertical. Your charts (vertical values and scale not given) are using only linear scale . The ASME charts are smooted by some formulae (I am just guessing this). You can see this by comparing intermediate values of table with the chart values.

I always used the charts not the tables. However, I would expect they are more or less identical.

Hope this helps.

 

[saplanti]

I might be wrong on the scale issue of your chart horizontal, because I can not see the intermediate values.

 

[McJe]

The scale is trivial in this case.
But, they are both logatythmic in my examples, just like in the ASME book, to show similar graphs.
It is true that we need to interpolate logatythmic between points, and that the results will be less rugged.

But, all this does not solve the fact that a point of a higher temperature line jumps over a lower temperature line.
(example: NFN12 - 95° line, NFN16 - 425° line)
Or, the other error on NFN18: the 20° line should be colinear to the 95° line according to the graphs.
And for difference n°3 : the charts should result in a sort of 'smooth' graph. Those jumpy charts will lead to faulty interpolations.
And finally: I think it is necessary to have a known point at the chart where the line buckles. This is missing in many cases. This will lead to faulty interpolations between the 2 surrounding points.

 

[TGS4]

If you think that this is truly an error, then follow the instruction in Division 1, and submit an inquiry to the Code Committees - mistakes happen and it's through the careful review process that these things get fixed (wonder why there's so many revisions every year - this is why).

The ASME Code Committees are volunteers who usually have full-time jobs, so don't get all bent out of shape is there is an error - report it and move on.

 

[jte]

TGS4 makes a good point. As one of my managers used to tell me, “Don’t come to me with a problem, come to me with a solution.” The curves which you seem to be referring to are in ASME B&PV Section II Part D. The contact info is on page 934 of the 2008 ASME Section II-D pdf, page 887 of the hard copy in 4-600.

Read Appendix 4 for preferred format of the inquiry, but rest assured that your concerns will be addressed even if you don’t follow the format exactly.

As TGS4 pointed out, the Code folks are volunteers, so consider joining in the volunteer effort and coming up with a correction which can be easily implemented.

jt

 

[McJe]

Thanks for the answer,
I'll send this to the right person.

But how about the second part of this issue?
Is it allowed to use (until corrected by ASME) my own corrected tables? Of do we need to keep working with faulty values?

 

[doct9960]

Hi McJe,

This is where "engineering judgment" comes to play. If you believe the tables in ASME are incorrect and your corrected tables are MORE conservative than ASME, I would use the conservative one.

Cheers!

 

[jte]

I think doct9960's response is the way to go. No one will fault you for being more conservative than the Code. But if you go on your own and do something which is less conservative and something happens (even unrelated to the issue at hand), then you will be in the position of having to defend the "not Code compliant" design. And the folks listening will not be engineers who have a hope of understanding the technical stuff which you are telling them.

jt