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September 20, 2017

12 inch welded-on patch plate overlapping a bottom seam – in accordance with API 653.

12 inch welded-on patch plate overlapping a bottom seam – in accordance with API 653.

Does Size Matter?

When it comes to repairing tanks, it does! Looking at this question from a quality control perspective this can be a very critical choice. Let’s talk specifically about bottom plate repair. This year is not different than any other year in the tank world and we see many of our clients (and potential clients) in need of completing repairs on their storage tanks after completing the API 653 out of service inspections. One typical repair that we see often are deep corrosion pits or even through holes in the bottom plates. I think we have all seen tanks that are thirty or forty years old that may have gone through many out of service inspections and repairs done in past years. And I have most definitely seen some interesting ways that repairs have been made.

Let’s look at one of the proper code requirements for a welded on patch plate. First, we need to reference the appropriate code to complete these repairs. API 653 “Tank Inspection, Repair, Alteration, and Reconstruction” is where we need to be looking in. Specifically, in section 9.10.1.1 General Repair Requirements The use of welded-on patch plates for repairing a portion of uniformly supported tank bottoms is permitted within the limitations given in this section and 9.10.1.2. See Figure 9.13 for acceptable details for welded-on patch plates. a) The minimum dimension for a welded-on patch plate that overlaps a bottom seam or existing patch is 12 in. The welded-on patch plate may be circular. oblong, or polygonal with rounded comers. b) A welded-on patch plate smaller than 12 in. in diameter is permitted if: it is equal to or exceeds 6 in. in diameter; it does not overlap a bottom seam; it is not placed fully or partially over an existing patch; and it extends beyond the corroded bottom area, if any, by at least 2 in.

So, we can see here that “Yes” size does matter! Joking aside, this is a very brief look at the requirements and there are many other restrictions and rules that apply just to properly install and place patch plates on the bottom plates. It can often be tempting to just throw some material down and cover up the corrosion or stop a leak but by doing this we could potentially be causing more harm to the tank by not strictly adhering to the codes and standards put in place to effectively do so.

 

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