NB Repair vs Alteration

[GemmellG]

My company is involved with a project to change out four superheater platens that failed a short time after boiler (150 MW Coal Fired) start up. The four superheater platens have multiple materials i.e. SA-213-T22 SA-213-T11 SA-213-T91 & SA-213-TP347H. Due to delivery issues for SA-213-T91 it was decided to substitute SA-213-T91 with SA-213-TP347H. The work is as-per NBIC Part 3, My original assessment was to class replacement as a repair as the material substitution meets NBIC Part 3 (Paragraph 3.3.3 s). The Lloyds AI has classed the work as an Alteration. I am not sure why as the material substitution is an upgrade to the existing. Please provide any comments or am I missing something. As always thanks.

 

[david339933]

3.3.3(s) states the allowable stress of the new material must be equal to or greater than the original. A quick search shows TP347H to be less than T-91. Therefore should be under 3.4.4(g), Examples of Alterations.

 

[FormerAIS]

As david stated, as long as the material being switched is of the same or greater tensile strength, there is no issue. To add to that, the original thickness must be used in order to maintain the 'Repair' classification.

This is all based on the strength of the material already being calculated and verified by an AI during original construction. Once you change the thickness or use a material that has a lower tensile strength for the given temperature, more engineering must be performed to ensure ASME is being complied with.

Quality is not an act, it is a habit - Aristotle

 

[GemmellG]

Guys thanks for the quick response. As-per information below SA-213-TP347H has a higher maximum allowable stress value than SA-213-T91. NBIC has no reference to tensile strength nor does ASME Sec 1 when calculating tube min wall. So can you let me know if I am missing something. Again thanks for the great responses.

ASME Sec II Part D (Customary)2017 Edition Table 1A
Line# 36 SA-213-TP347H @ 1100F is 14.1 Ksi.
Line# 11 SA-213-T91 @ 1100F is 10.6 Ksi.

I have completed the calcs as-per PG-27.2.1
SA-213-TP347H M.W = 0.177".
SA-213-T91 M.W = 0.231".

 

[david339933]

Apologies... I had quickly checked to 800F...didn't go up to 1100F. If your min. thickness is the same or greater than the original, ask the AI why it cannot be classified as a repair.

 

[oknow]

Have you contacted the Insurance Company who insures the Boiler? How about the Jurisdiction?

 

[GemmellG]

Yes It’s Lloyd’s out of China AI who classed work as an Alteration

 

[weldstan]

As long as the tube thickness is the same it's a repair as long as the design allowable stress is equal to or grater than. It you ecalculate the thickness and use a thinner wall, it's an Alteration.