ASME B31.1 against B31.3 9

[pegus]

Good day,

I am new using these two standards for power piping, and I wonder if you could help me answering a few doubts of them.

What is the difference between these 2 standards? I made a quick scan to both of them and notice that they include different materials.

I was told that the maximum allowable stress for the B31.1 was has a safety factor of 4 while the B31.3 is 4, I was looking for that in the standard, but there is not an explicit part that say so, Is there a certain section that say this?

Thanks in advance!

 

[XL83NL]

Have you read both the Introduction of the standards and their paragraphs about their scope? This will give you a good idea.

Short, the difference is the application; power piping is for power plants (main application is steam), process piping is for a whole lot of other things, but mainly porcess.
Safety factors differ due to intended service of the plant, and its potential risks involved.

 

[LittleInch]

No there is no explicit section on safety factors. This is all done by use of the S figure in the tables in the code.

I've never really understood why the two codes exist as 31.3 covers all the piping in 31.1 but not vice versa. 31.3 is more common in the process industries with 31.1 limited to the power sector.

When ever anyone "tells you" a code does x,y or z, ask them for the reference section. 9 times out of 10 they won't be able to because it doesn't actually say whet they think it does or applies to some other situation.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[weldstan]

The general safety factor for B31.3 is 3 not 4. B31.3 is essentially a Users' Code. The Owner has much more authority because the processes for which the Code is used are highly different and subject to a host of corrosion issues, flammability issues and toxic issues.

B31.1 was developed by engineers to provide safety in steam generating power systems as an offshoot of ASME I. The Owner operator does not have latitude in its use, especially in locations designated as Boiler External Piping.

 

[TGS4]

The numbers that you quoted and were quoted back to you, are not Safety Factors. They are margins against engineering ultimate stress for general membrane (hand calculated) stress in a cylinder. There is a multitude of failure modes, of which burst (what this design jargon protects against, is but one. The margin against ratcheting is exactly 1.0. The margin against fatigue depends on your definition of fatigue, but is greater than one.

There is, in fact, no such thing as a safety factor.

I agree with the previous commentors that you need to examine the appropriate spite of each Code. Often, the selection of a Code is mandated by legislation.

 

[KevinNZ]

B31.3 does allow higher design stress at lower temperatures but does have higher NTD requirements. B31.3 covers more fluid types and assumes some of these are can be nasty if they leak out. B31.1 has few extra design rules that B31.1 has not picked up on. e.g when to apply corrosion allowance and supports that lift off in the operating condition. You can have higher over pressure in B31.3.

http://www.psig.sg/A1.pdf

 

[curtis2004]

Pegus,

1. Forget about safety factors when are you determining which code to use. They are mostly meaningless.
2. Most important thing in determination of whether to use B31.1 or B31.3 is the only owner of the plant can determines what code to use during the design.
3. If the owner doesn't have people in its organization who thoroughly understands and extensively used provisions of B31.3, most likely plant shall be designed to B31.1.
It is true that B31.3 covers much more than B31.1.

Regards,
Curtis

 

[pegus]

Thanks for the responses,

LittleInch, Regarding the safety factor, I was wondering that because I would like to know what happen if the maximum allowable stress that this standard establishes are exceeded.
I was reading the ASME B31.1 looking for any allowable variation, and I just found in paragraph 102.2.4 "Ratings: Allowance for variation from Normal Operation" that you can exceed the design pressure or temperature only if the circumferential pressure stresses does not exceed the maximum allowable stress 15% if the event duration occurs for no more than 8hr at any time.

If I understand this correctly, this means that the resultant stresses on the piping can exceed the maximum allowable stress at that temperature for 8hrs maximum only if the circumferential pressure stresses does not exceed the maximum allowable stress by 15%, is this correct?

Regards!

 

[KevinNZ]

You need to satisfy 102.2.4 for hoop stress where just the pressure has increased and 102.3.3 for longitudinal stress where other loads have been applied.

What I do understand. is how much over temperature you are allowed. Higher temperature without a pressure increase does not change hoop stress. Temperature will at some point change the allowable stress. With higher temperature are you still limited by 102.3.2 (B)?