A-106 Gr. B Allowable Stress in ASME B31.4

[cingold]

Hi, I couldn't get an answer on the Intergraph forums so I'm hoping someone on here can help me with this.

Recently, I ran an analysis of a pump station piping system in CAESAR II. The code I used was ASME B31.4 and the material was A-106 Gr. B. The temperature was 100F and the pressure was 1440 psig.

What I'm curious about is that the program showed the allowable Sustained stress was 18,900 psi (which is 54% of the yield stress).

I'm still learning ASME B31.4, but from what I read, the standard design factor is 0.72. The code lists the allowable stress for A-106 Gr. B as 25,200 psi. I've also read that the design factor should not exceed 0.72, so the 0.54 is acceptable.

My question: why is the default design factor in CAESAR II 0.54 for this material? I'm assuming that the design factor is the reason the allowable stress is so low.

Please help me. Maybe there's something I'm missing in the code.

 

[BigInch]

There are different allowable stresses for different load conditions, ie. internal and external pressure, allowable expansion stress, additive longitudinal stress and effective stress at crossings and location of the pipe, ie. offshore, onshore, navigable waters, if the pipe is restrained, or unrestrained.

B31.4 Design factors are 0.60, 0.72, 0.80, 0.90 and 0.54. They are all used for various stresses and conditions.

Yield stress Sy is 35000, Ex/Internal Pressure Allowable stress (HOOP STRESS) is 0.72 Sy
BUT for additive longitudinal stress the allowable is exactly as calculated by CAESAR = 0.54 Sy

All of that is explained quite nicely in TABLE 403.3.1-1

READ the Code cover to cover, understand what you can, then ...
ask the questions.


Independent events are seldomly independent.

 

[cingold]

BigInch,

Thanks for the reply.

I'm reading the 2012 version of the code (and maybe CAESAR is based on an older version), but the allowable stresses are as follows for Additive Longitudinal:

Restrained Pipe: 0.9 Sy
Unrestrained Pipe: 0.75 Sy
Riser Pipe: 0.8 Sy
Slurry Pipe: n/a

Note 1 says to include bending stress into these (I believe this is taking into account internally with the program).

I'm doing an analysis on Unrestrained Pipe but none of these listed values have a design factor of 0.54. If I multiply the yield stress by the above design factor and by the Internal Pressure design factor, I get the allowable listed in CAESAR...(35,000)(0.75)(0.72)=18,900.

But this calculation just seems like double dipping on the allowables. I know you say to read the whole code (which I've done), but I'm not seeing anything in there that tells me to use 0.54 as a design factor for Sustained stresses. Then again, I've misread things before as I'm sure we all have. If you know the lines in the code that tells me to use 0.54 as a design factor please let me know so I don't overdesign or underdesign something.

 

[BigInch]

You'll see 0.54 in the 2009-2010 editions. CAESAR (at least that version) has not been updated to the 2012 issue of API 2012.

Independent events are seldomly independent.

 

[jkal321]

cingold (Mechanical)/

Could you please let me know which piping code and where is the formula below located in the code:

St=0.9Sy for retrained pipe

Thanks

 

[cingold]

I don't have a copy of ASME B31.4 in front of me, but when I posted this I was using the most recent version of it. If I remember right, that was from Table 403.3.1-1 as mentioned in BigInch's post.

 

[jkal321]

Could view the attachment. The highlighted section. I couldn't find even there?

Thanks

 

[cingold]

jkal321,

I'm not sure what you're talking about. Were you trying to include an attachment?

 

[jkal321]

Here you are. Sorry

 

[cingold]

jkal321,

That's not the same code that we were talking about. We were talking about ASME B31.4 for pipe. That looks like it's referencing ASME Section VIII for pressure vessels.

 

[jkal321]

That is why I am confused. I am basically looking where is the formula located because it shows in these two code refernces and I can' seem to find it.

 

[cingold]

Again maybe I'm confused as to what you're asking, but all of these equations are based on the equations we learned in college. These were developed from experimentation and then modified to use safety factors.

You'll see slightly varied forms of these equations in all the different codes based on particular applications.

 

[jkal321]

cingold,

Sorry to go back and forth. What I am trying to find is where in B31.4 does it specifically state:

Restrained Pipe: 0.9 Sy

I could see the design factor 0.9 Sy, but I can't find where it defines it for restrained pipes vs unstrained pipes as you stated above.

Thanks,

 

[cingold]

Table 403.3.1-1

 

[jkal321]

Thank you so much.

I had an old photocopy of B31.4, so I searched for an updated copy and I found the information I was looking for and concur to your answer above.