B31.8 vs B31.3: Toughness Requirements 1

[Rubje]

In the following thread: thread378-116351 I saw this post by SJones:

"I don't think that there will be a problem as the design factor in 31.3 is lower than B31.8. However, B31.3 may not address Charpy toughness testing in the same manner as 31.8 (in the sense that 31.8 demands the designer to assess the need for it and a lot of people overlook running ductile fracture in rich gas lines for example). Personally, I don't subscribe to the B31.3 approach of allowing A106 Gr B to go to -29 deg C without Charpy testing (in line with a lot of upstream oil companies). In addition, if you have any fittings or pipeline parts that have girth welds, 31.8 demands that they be made in accordance with API 1104 which may not hold with the standard used to manufacture the fittings."

I thought this was interesting and I have a couple of questions:

1. What is the concern with allowing A106 Gr B to go to -29 C without a Charpy Testing? Why was the committee comfortable with this?

2. Lastly, I have a question about this part of the post "B31.3 may not address Charpy toughness testing in the same manner as 31.8 (in the sense that 31.8 demands the designer to assess the need for it and a lot of people overlook running ductile fracture in rich gas lines for example)"

My understanding is B31.8 has more stringent toughness testing requirements compared to B31.3 while B31.3 doesn't always require toughness testing (based upon temperature, fluid, etc). Am I misunderstanding this?

Thank you all for the help.

 

[SJones]

18 years on and you’re forcing me to look at B31.8 Charpy requirements again!

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[weldstan]

The B31.3 allowable stress values are generally based on the ultimate tensile strength with safety factor while B31.8 is based on a percentage of the yield strength (up to 80% of the SMYS) and location of the line. In the thousands of cases that I have performed analyses of B31.3 pipe systems, I have rarely seen stresses in excess of 60% of the MSYS during hydrotest vs >105% of the SMYS during hydrotest of gas pipelines during its pressure test. Also the impact test requirements for B1.3 are based on thickness and minimum design temperature and design pressure. Some of the materials, like A-106, are not permitted for use at a MDMT of -20 F for all thicknesses.

As you stated the Designer/Engineer is responsible for determining the need for impact testing on B31.8 gas pipelines. The basis for impact testing is primarily based on damage mechanisms, total potential damage and fracture mechanics at the involved stress levels near yield strength. Many pipelines in distribution systems and low pressure transmission systems operate below 50% of the SMYS and are not subject subject to impact testing by the designer. Finally B31.8 also permits WPS per ASME IX except in specific instances.

There are simply different philosophies used by both Codes to determine the need for impact testing and the safety and integrity of the piping systems.

 

[Rubje]

@SJones
hehe. Your posts are very informative and I enjoy reading them

@weldstan

Thank you for the response. that makes sense but this quote from the reference post still confuses me:

"Personally, I don't subscribe to the B31.3 approach of allowing A106 Gr B to go to -29 deg C without Charpy testing "

Do you also share the same concern as SJones in that referenced post?

 

[SJones]

https://www.mcaa.org/ncpwb/wp-conte...n_Steel_Piping_During_Hydrostatic_Testing.pdf

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[Rubje]

@SJones

This is an insightful read. thank you!