Difference between inches and mm in B31.3 3

[ukmet]

Dear Contributors,

I am confused when I see the Table 331.1.1 of B31.3. Thickness range for carbon steel where PWHT gets compulsory is greater than 20mm but in inches its 3/4in which equals 19mm. What to follow? Or it is one and the same thing? Please guide

 

[BigInch]

Most conservative practice would be to consider it as 19mm.

Do you in fact have a pipe with a >= 20mm wall thickness? What is the exact wt you are considering?

 

[metengr]

Yes, they are considered the same based on conversion - hard versus soft metric conversion.

 

[DekDee]

In this Edition of the Code, SI metric units are given first, with U.S. Customary units in parentheses. Appendices H and X, the table in Appendix K, and Tables C-1, C-3, and C-6 in Appendix C are exceptions. A portion of the allowable design values in Appendix A are given in both SI metric and U.S. Customary units. Except for Appendix A, values in metric units are to be regarded as the standard, unless otherwise agreed between the contracting parties. In Appendix A, the U.S. Customary units are to be regarded as the standard. Instructions are given in those tables for converting tabular data in U.S. Customary units to appropriate SI metric units. Interpretations, Code Cases, and errata to the B31.3 Code on Process Piping are published on the following ASME web page:

http://cstools.asme.org/csconnect/CommitteePages

ASME B31.3-2012 was approved by the American National National Standards Institute on May 9, 2012.

Hope that helps,
Cheers,
DD

 

[jte]

I'm more of a vessel guy than a piping guy... I find Annex 1.C in Section VIII Div. 2 and the accompanying tables useful in understanding the "hard" and "soft" unit conversions. I've also used this Annex to convince our internal standards editors that it is ok to use soft conversions when converting some of our internal standards from US Customary to SI. For what its worth, Table 1.C.1 shows a conversion from 3/4" as 19mm. Maybe this just highlights the subtle distinctions between the vessel codes and piping codes.

 

[ukmet]

Dekdee

Is there any reference which shows or proves that we had to use metric units as standard. And what do we mean by hard and soft conversion?

 

[DekDee]

ukmet,
I thought I had given you a reference - copy and paste straight from 2012 edition ????

 

[BigInch]

AS the code was originally based on 3/4" (19mm) many years ago, how did they all of a sudden justify making it 20mm, but only if you happen to have a metric measuring device in your hand???

 

[DekDee]

Good question Big Inch.
3/4"is actually 19.05 mm so if you were going to round something off you would presume it would be to the nearest whole number.
I wonder if it was done to clarify PWHT requirements (there are various pipe sizes with 19.05 wall thickness) and in the old B31.3 PWHT was required for greater than/equal to 19 mm ???
Cheers,
DD

 

[BigInch]

In cases where I've had an option I've always managed to avoid using 3/4" wall thickness specifically to avoid the need for heat treatment, preferring to design a system to use a slightly lower design pressure, or by selecting the next lower pipe diameter thereby and perhaps even operating at a slightly higher pressure, but still needing less than 3/4" wall. PWHT on pipelines is too slow and expensive, as it would need some 40000 welds for only a 500km length of pipeline. I think the best practice is to avoid designing a system that needs PWHT, unless you can afford it, whereby you would not have to ask these kinds of questions at all.

 

[jte]

Is there any reference which shows or proves that we had to use metric units as standard. And what do we mean by hard and soft conversion?

First, I'll stay out of the 19mm vs 20mm for PWHT issue... Not my expertise. As I mentioned earlier, I'm more of a vessel guy - so for B31 things may be similar but different.

For vessel design, either SI or US Customary can be used. But only one can be the system of record. In other words, once you choose a system of units to use, you must complete the design using that system. For example, if we had the same PWHT cutoffs at 0.75" and 20mm, I could not choose to use USC as the system of record and calculate a wall thickness of 0.77" and then claim to be exempt from PWHT on the basis of being les than 20mm

I've already mentioned that I like the info in VIII Div. 2; an alternative is VIII Div. 1 Appendix GG which goes through philosophy of conversions with a focus on implied precision of significant digits (e.g. 14.7 psi converts to 101 kPa while 15 psi converts to 100 kPa).

As for what's required in B31.3, perhaps the final paragraph in the Foreword would provide some help:

In this 2014 Edition of the Code, SI metric units are given first, with U.S. Customary units in
parentheses. Table K-1 in Appendix K, and Tables C-1 and C-6 in Appendix C, are exceptions,
containing only U.S. Customary units. The allowable design values in Tables A-1 and A-2 in
Appendix A are given in [highlight #FCE94F]U.S. Customary units and are the required values[/highlight]; the SI metric values
in Tables A-1M and A-2M are for information only. Except for Tables A-1M, A-2M, C-1, C-6, and
K-1, [highlight #FCE94F]values in metric units are to be regarded as the standard[/highlight], [highlight #8AE234]unless otherwise agreed[/highlight] between
the contracting parties. Instructions are given in those tables for converting tabular data in U.S.
Customary units to appropriate SI metric units.

As for "hard" vs. "soft", see part 1 at

http://metricationmatters.com/mm-newsletter-2011-03.html