Local Thin Area in a Pressure Vessel Shell Under Insulation
An inservice inspector is reviewing a vertical pressure vessel, 60-inch inside diameter, SA-516 Gr. 70 shell, design 150 psig at 400 degrees F, joint efficiency 1.0, corrosion allowance 0.125 inch. Original shell thickness was 0.500 inch. Insulation was removed from a 12-inch by 18-inch area where staining was found. Ultrasonic thickness readings in the area range from 0.46 to 0.33 inch; the lowest reading is at the center of a 2-inch-diameter pit. Nearby readings outside the pit are 0.44 to 0.46 inch. The vessel has no prior repairs. The owner wants to return it to service at 150 psig. The inspector must determine whether the shell is acceptable, what additional evaluation is needed, and what records should be made.
1. Calculate the minimum required shell thickness including corrosion allowance and compare with readings. Use UG-27 and state assumptions.
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Using UG-27(c)(1) for circumferential stress: t = PR/(SE - 0.6P) + CA. P = 150 psi, R = 30 in, S = 20,000 psi assumed allowable for SA-516-70 at 400 degrees F, E = 1.0. t = (150)(30)/[(20,000)(1.0) - 0.6(150)] + 0.125 = 4,500/19,910 + 0.125 = 0.226 + 0.125 = 0.351 in. The lowest reading is 0.33 in, which is below the 0.351-in required thickness. Therefore the shell is not acceptable at 150 psig based on the general shell formula alone. The 0.44 to 0.46 readings outside the pit are above required, so the pit is the local governing feature. I would not accept the vessel for return to service without a local thin-area evaluation or repair. The calculation is only for the shell; nozzle, head, and support effects also need review.
2. Does the 0.33-inch pit govern? What evaluation or repair options exist?
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The 0.33-in pit governs because it is the thinnest point and it is 2 in in diameter. A local thin area can sometimes be accepted by fitness-for-service, such as API 579 or an equivalent evaluation, if the remaining strength is adequate for pressure plus other loads, but that is an engineering evaluation, not an inspector's field call. Options are: blend the pit out if the resulting thickness remains at or above 0.351 in and the profile is smooth; weld repair by an R-stamp holder if the material and procedure are suitable; rerate the vessel to a lower MAWP if the owner can accept reduced service; or replace the shell section. I would require UT mapping of the pit and surrounding area, and I would not accept a simple average thickness.
3. What documentation and follow-up inspection would you require?
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Records should include the UT readings with grid locations, the calculated required thickness and assumptions, the fitness-for-service evaluation if used, the repair R-1 if repaired, WPS/PQR and NDE results, and the hydrotest record. Follow-up should include re-inspection of the area at a shorter interval, insulation and coating repair to stop corrosion, and a review of the corrosion mechanism. If the vessel is returned to service under a fitness-for-service acceptance, the inspector should document the acceptance basis and any operating limits. If repaired, the inspector verifies the R stamp and that the repair restored the required thickness.