Dear Friends,

I am analyzing a 36” vapor line with a design pressure of 45 psig. The standard schedule of 36” Pipe thickness is 9.52 mm, but at present the measured pipe thickness is 4.5 mm (60 deg) at 6 o’ clock position and 9.52mm (300 deg) at 3 o’ clock, 12 o’ clock and 9 o’ clock positions. I run the model with 9.52 mm thickness and noticed the stresses are well within the limit, after that I reduced the thickness gradually and checked the system then I found failure at the nearby branch joint, when the thickness is below 8 mm. Therefore I took the average of the thickness at bottom portion of pipe to the top portion of pipe (4.5mm (60 deg) +9.52mm (300 deg) = 8.7mm) and did the analysis. Our Client asks us to check the system how far to the minimum thickness it can withstand. Required thickness as per ASME B31.3 for the given pressure is 1.08 mm without corrosion allowance and mill tolerance.

Questions:

1. Is it possible to model the cross section of pipe with different thickness in CAESAR II? If yes, please advise me how to model it?

2. Average of pipe thickness used is correct or incorrect? Please correct me if i am wrong.

3. Keeping the thickness as 4.5 mm and increasing the remaining portion (9.52-4.5 mm = 300 degree) weight in pipe insulation and fluid density produces different value compared to the average thickness.

4. Keeping the thickness as 4.5 mm and increasing the remaining portion (9.52-4.5 mm = 300 degree) weight in pipe dead weight produces almost the same result compared to average weight thickness. In this case I observed same sustained case values and slightly different expansion case values compared to the average thickness results.

Please help me and give your valuable suggestions.

1) Not for what you describe. CAESAR II deals with pipes that have a uniform cross section. You can change the thickness in the longitudinal direction, on the "next" element, but for a given element the thickness is assumed constant and uniform circumferentially.

2) I would not attempt this. Do you think that thinner section of 4.5mm is going to hold up under your load/pressure combination? What about other conditions?

3+4) You might have adequate "weight supports" such that the 4.5mm thickness is acceptable for this case. You adjusted the weight so that "W" in both systems is about the same, I would expect about the same results.

Thank you Mr. Richard Ay,

Is there any methodology adopted in CAESAR II to check like this kind of scenario with partially corroded pipe.

I recommend you to take a look to API 579-1/ASME FFS-1, Fitness-For-Service (2007).

"The Fitness-For-Service assessment procedures in this Standard cover both the present integrity of the component given a current state of damage and the projected remaining life. Assessment techniques are included to evaluate flaws including: general and localized corrosion, widespread and localized pitting, blisters and hydrogen damage, weld misalignment and shell distortions, crack-like flaws including environmental cracking, laminations, dents and gouges, and remaining life assessment procedures for components operating in the creep range. In addition, evaluation techniques are provided for condition assessment of equipment including resistance to brittle fracture, long-term creep damage, and fire damage.

The Fitness-For-Service assessment procedures in this Standard can be used to evaluate flaws commonly encountered in pressure vessels, piping and tankage. The procedures are not intended to provide a definitive guideline for every possible situation that may be encountered. However, flexibility is provided to the user in the form of an advanced assessment level to handle uncommon situations that may require a more detailed analysis."

Please note that now it is available also FFS-2, 2009, Example Problem Manual.

Best regards.

There is also B31.G. CAESAR II has a module to address B31.G under the "Analysis" menu. While API-579 is much more recent, CAESAR II does not address this.

Hi Kausik,

May i know, for which project u are working for it (i.e.) KOC or KNPC.