Fire Tube collapse due to hot spot in a Free Water Knockdown

[JeSan]

Fire Tube collapse due to hot spot in a Free Water Knockdown Vessel.
I have a Fire Tube (U Shape) that collapsed due to a problem with a “hot Spot” located 2 meters from the burner (in the hot area) of the Fire Tube. The collapse caused that the wall thickness got reduced to 0,160 inches. (Very similar to a dent caused by a mechanical damage). Tube Nominal thickness was 0,625 inches. The tube is subject to an external pressure of 70 psi., which is the actual vessel internal working pressure. The maximum allowable working pressure (MAWP) is 110 psi @250 oF. I need to establish which is the maximum working pressure under the actual condition (reduced wall thickness). I have used the following approach, using clause UG-28 of ASME BPVC VIII, Div.1 with the following results:

I.D. (ft) 2,40 ID
Do (in.) 30,00 OD
# Stiffeners 5 Flange, Elbow, supports
L. (in.) 76,8 Pipe length between stiffeners
t min. (in) 0,156 Minimum wall thickness measured
L/Do 2,6
Do/t min 192,3
A 0,0002 Factor A. Fig. G - Subpart 3 of Section II, Part D
B 2500 Factor B. de Fig. C-2 - Subpart 3 of Section II, Part D
MAWP= 17,33 psi (*)
19,21 psi (*1)

Do/t > 10 20

E= 27,3E+06 Elasticity modulus of steel SA516 Gr.70

Formulae: (*) Pa = 4B/3(Do/t)
(*1) Pa = 2xAxE/3(Do/t)For values of A falling to the left of the applicable material /temperature line

Question: Is this approach correct?. The actual maximum working pressure appears to be very low. Can someone help with the approach using Code Case 2286-1. Is there any other approach available? API.?
Many thanks in advanced for your assistance.

 

[metengr]

Jesan;
Not sure that I picked up on the extent of damage in your post. Can this damage be described as a locally thin area (LTA) versus assuming a uniformly thinned tube? LTA analysis is best described in ASME/API FFS-1, 2007 Edition

 

[bernoullies123]

Very similar to a dent caused by a mechanical damage

or maybe API 579-1/ASME FFS-1 2007 Fitness-For-Service Part 12 Assessment of dents, gouges, and dent-gouge combinations.

 

[JeSan]

MetEng.

Yes, it is a LTA. Thanks for your advise of FFT. I had some difficulties getting the codes in my country due to ex-change restrictions. The LTA is about 3 inches long by 0,5 inches wide.

 

[metengr]

JeSan;
You would be wise to use API FFS-1 for this evaluation.