EXPANSION CASES IN BURIED LINES

Dear Experts

I am doing a stress analysis for a buried line (PIG LINE) in code ASME 31.8. My doubt is that as I am using two temperatures maximum and minimum (T1 & T2). When I give recommend in load case Caesar II default shows

• Case 1 (HYD) W+HP
• Case 2 (OPE) W+T1+P1
• Case 3 (OPE) W+T2+P1
• Case 4 (SUS) W+P1
• Case 5 (EXP) L5=L2-L4
• Case 6 (EXP) L6=L3-L4
• Case 7 (EXP) L7=L2-L3

My doubt is that, when case 7 is applicable for buried lines??

any comments?

If LC 7 produces a greater stress range than LC 5 or LC 6, then, yes, it is applicable.

Dear Dave,

We known that LC 7 produces a greater stress range than LC 5 or LC 6,

We are checking LC 5 and LC 6 then why we need to check LC 7 case, its really required for all conditions & piping system. Then why we need to do analysis for LC 5 and LC 6 case, if LC 7 is worst case means directly we can check only LC 7 case.

Kindly explain me when this case will happen, which method is followed to solve this problem algebraic or scalar, in my understanding all expansion case is followed by algebraic methods only.

I have done detailed analysis for 5 pipe line system for same temp. & pres.,

CASE-1

The expansion stress for design condition (LC 5 is 86%) and expansion stress for operating condition (LC 6 is 44%) here one more case is added newly LC 7 for maximum stress range case in this, L2-L3 output for maximum expansion stress range condition (LC 7 is 121.62) in which way this value is coming.

CASE-2

The other 4 lines are expansion stress for design condition (LC 5 is 70%) and expansion stress operating condition (LC 6 is 21%) here one more case is added newly LC 7 for maximum stress range case in this, L2-L3 output for maximum expansion stress range condition (LC 7 is 91%) in which way this this value is coming.

My doubt First case-1 based on the calculation algebraic method SQRT of (86*86+86*86) the CAESAR II out put coming 121.62% it is ok.

The second case-2 is based on the calculation algebraic method SQRT of (70+21) the CAESAR II out put coming 91%

Case-2 is algebraic method????

Why this much difference is coming.

Kindly advice me why this case required when it will happen practically and it is applicable for all piping system.

By

Rajiv

Dear Dave,

• Case 5 is the thermal expansion case caused by temperature T1 (i.e.) from ambient to maximum temperature.

• Case 6 is the thermal expansion case caused by temperature T2 (i.e.) from ambient to minimum temperature.

• Case 7 is the thermal expansion case caused by from T1 to T2 (i.e.) from minimum temperature to maximum temperature.

In a fully restrained pipe say 5 km. Where case 7 will applicable. When this case will happen as pipe is fully covered by soil.

any comments?

It sounds like you are questioning your values for T1 & T2 at this point in your piping system.

Dear dave,

Your reply is not valuable, I don’t understand what you mean please advise me.

regards

rajiv

For a piping system, T1 and T2 are "loads".
If I understand well, your comment is about T1 and T2 in a pipe fully covered by soil- something as "are the loads realistic?"
Would you expect such things we know to be true?

Dear experts,

Read the story fully then reply me one by one correctly ok,

Mr.Dave your reply is not satisfied for me, kindly give me some detailed and acceptable information about this problem.

Your example is unclear to me.

If you are looking at fatigue, then the complete strain range should be used. You may not have a great number of cycles to deal with but fatigue ranges (not states) are used.
If you are not looking at fatigue then the hot state and cold state evaluation may be sufficient.
Check your piping code.

Dear Dave thanks for your reply, I am not performing any fatigue analysis in my analysis. I am only performing stress analysis for a gas pipeline (ASME B 31.8). As per 2012 Edition, ASME 31.8 design Code under Expansion and Flexibility chapter says “The total range in temperature from minimum design temperature to the maximum design temperature shall be considered in all expansion stress calculations, whether piping is cold-sprung or not” is for unrestrained piping only.


In my case major portion around 5 km is underground portion only. Only small amount of portion is above ground. (70 meters) So please suggest me L7 case is applicable in my system are not.

You quoted B31.8, 832.3 Flexibility Requirements, paragraph (f)

"Your case" is clearly described by B31.8

B31.8 832 EXPANSION AND FLEXIBILITY, 832.1 Application
"Part 832 is applicable to piping meeting the definition of unrestrained piping in para. 833.1(c)."

833 DESIGN FOR LONGITUDINAL STRESS, 833.1 Restraint
[...]
(c) Piping that is freed to displace axially or flex at bends is "unrestrained".
Unrestrained piping may include the following:
(1) aboveground piping that is configured to accommodate thermal expansion or anchor movements through flexibility
(2) bends and adjacent piping buried in soft or unconsolidated soil
(3) an unbackfilled section of otherwise buried pipeline that is sufficiently flexible to displace laterally or which contains a bend
(4) pipe subject to an end cap pressure force

It is evident that for "restrained" parts of pipelines you don't need any fatigue calculation. For the rest of pipeline, see the Code!