If you are using a tombstone patch to repair an area in the critical zone you have to follow API-653, 9.10.1.2. Point f states the bottom plate under the perimeter of a welded-on patch plate shall meet the thickness requirements in 4.4. If you have an isolated hole through a plate in the critical zone and only half an inch of plate from the corner weld that meets this thickness requirement, is it acceptable to use a tombstone patch. The other option is to remove and replace this plate but it would spark a hydro or fitness for service. Please advise.
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Tombstone Patch
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We have decided to replace the corroded sheet based on the following:
Per API-653, Section 9.10.1.2 Repairs Within the Critical Zone “The use of welded-on patch plates is permitted for repairing a portion of tank bottoms within the critical zone (see3.10 for definition) provided 9.10.1.1 requirements and the following additional requirements are met”. Section 9.10.1.1 b states that the patches must extend “beyond the corroded bottom area, if any, by at least 2 in”. In this case, this could not be achieved.
Per API-653, Section 9.10.1.2 Repairs Within the Critical Zone “The use of welded-on patch plates is permitted for repairing a portion of tank bottoms within the critical zone (see3.10 for definition) provided 9.10.1.1 requirements and the following additional requirements are met”. Section 9.10.1.1 b states that the patches must extend “beyond the corroded bottom area, if any, by at least 2 in”. In this case, this could not be achieved.
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