Typical tank bottom repairs range from patch plates to complete bottom replacement. Bottom failure on a water tank usually becomes evident prior to cleaning or after the bottom is abrasive blasted. Often, patch plates can be lap welded over existing holes. In more extreme cases, the entire floor can be scanned using a Magnetic Flux Leakage Test, a standard part of the API 653 Annual Tank Inspection, which gives an exact record and map of the floor. Per API standards, bottom repair or replacement is required when there is less than a .125 inch thickness in any portion of the tank bottom. The test identifies if uniform corrosion necessitates replacement of the entire bottom, or if there are pitted areas or pinholes that require patching.
The most common area for roof corrosion at the center vent, column cap, and rafter connections where there is the greatest air flow. Unfortunately, this is also a difficult area to access. The most typical repair is to replace the center vent and a diameter of roof plate. In some cases the rafter end will get cut shorter, to remove the rusted rafter ends, and modify the center column cap plate to accommodate the shorter rafters. This prevents removing the entire rafter.
Shell plate repairs can range from a simple patch plate for an isolated pinhole, or a complete strengthening to comply with current seismic requirements. Seismic repairs can involve a completely new tub-ring, a lap and plug welded sheet, or bolt-on seismic strengthening bands.
Common piping repairs include overflow modifications, reducing height in the tank for seismic reasons, creating a proper air gap for Department of Health requirements, and replacing an under-tank penetration with a shell penetration that utilizes new flexible nozzles.
Standard additions to existing tanks include increasing the size of manways or adding additional manways to create at least two access points at ground level.