Dear seniors,
We have a line in our plant which got rupture during the start-up after a shutdown. The line data is as below:
1. Diameter 2"
2. Material API 5L Gr. B
3. Sch. 40
4. Insulation thickness 70mm (128kg/m3). Steam traced line.
5. The line is running on rack with spans at a few places as 7.5 meters.
6. Design temperature 135 Celsius and design pressure 10 bar.

I modeled the line in CAESAR II and it showed failure in Sustained case. I was wondering why 2" line is running on the rack. Checked the company standard and came to know that the standards allow minimum 2" line on the rack. I increased the thickness of line in CAESAR II to sch. 80 and the stresses came down to around 50% of allowable. I wrote a report concluding that the line should have either more thickness or it should be having intermediate support in this condition. I got the blast from operation that there is a similar line and running on the same condition in another similar unit and it is not failing why it would fail. I told them that as per analysis, the stress are more than the allowable values so if you run it at these condition it would fail. They were not satisfied at my answer and still insist that it will not fail.
My question to you all is: How this "non-failure" of similar line can be explained? Please note that the stresses were around 107% of allowable values.

If the line did fail, whom would your Ops guys blame ... the stress engineer or themselves ??

Your stresses are roughly 107% of (2/3) yield stress. Simply becuase Caesar II says a pipe will fail a stress check does not mean it will or should fall down, but you are clearly in the danger zone. You are eating up the safety built margin into design codes such as ASME B31.3. You may well end up with a real failure if other effedts work against you, such as poor wall thickness control, unexpected loads etc.


Just out of interest, in my opinion your pipe has an excessive span. I would not exceed 6m with insulated 2" pipe, particularly since it is carrying a tracer line. I imagine that the mid span deflection must be quite noticable too.

@MoverZ,
Thanks for reply!

Yes you are right, operation should consider this that there already is a failure and as you said, this line IS already in a danger zone.

For your point about excessive span and mid span deflection, I shall check it.

Aarif,

Just want to add two points:
1- You may consider some earthquake, wind, corrosion and ... that has not happened to the existing line yet, but who can gurantee that certainly they will not happen for the life time of the line?
2-On the other hand most of the time the design pressure and temperatures are higher than the actual operating data.