By Ray Chao
Tank bottoms in atmospheric storage tanks (ASTs) are susceptible to both external and internal corrosion, and an earlier article discussed tank bottom inspection and evaluation. Repair of corroded tank bottoms in old ASTs is thus a common maintenance undertaking by tank owners. API Standard 653, Tank Inspection, Repair, Alteration and Reconstruction, provides guidance on the repair of tank bottoms in ASTs. The rules in API 653, however, contain certain restrictions on weld repairs and, in some cases, require that the shell penetrations be raised. This article will examine these requirements and discuss safe repair of tank bottoms without undue costs or prolonged tank downtime.
API 653 restricts welding in the “critical zone” when making repairs to a portion of the tank bottom. The “critical zone” is defined as: within the annular plate ring, within 12 inches of the shell, or within 12 inches of the inside edge of the annular ring. No welding, welded-on patch plates, or weld overlays are permitted within the “critical zone” except for welding of widely scattered pits, cracks in the bottom plates, the shell-to-bottom weld, or where the bottom or annular plate is being replaced. If more extensive repairs are required, the bottom plate under the tank shell would have to be cut out and a new plate installed.
The “critical zone” is supposed to be based on stress considerations. It is well known that the maximum stress in a tank bottom exists at the toe of the inside shell-to-bottom fillet weld at the annular plate or sketch plate. However, this is a very localized bending stress and it decays rapidly with distance from the shell-to-bottom junction. Indeed, the annular plate width requirement of API 650 was established by determining the required distance for the edge moment to damp out completely using the beam on an elastic foundation approach. Therefore, the "critical zone" should only be the immediate area of the shell-to-bottom junction. Yet, API 653 does not prohibit repair of the shell-to-bottom weld.
Weld repairs in the bottom “critical zone” should be treated no differently than those permitted during construction of new tanks. The weld restrictions in the “critical zone” are unwarranted, except that if welded-on patch plates are used, they should be located several inches away from the shell-to-bottom fillet weld in order to minimize the compound effect of two stress risers located near each other. All weld repairs within 12 inches of the shell-to-bottom junction should be examined by the magnetic particle method.
The following provides examples of what should be permissible. More than 25 years ago, the author led an effort to relocate two new 600,000 Bbl (260 ft. diameter by 64 ft. high) floating roof tanks due to foundation failures that occurred when the tanks were undergoing hydrostatic tests. The moves were successfully accomplished by using the water flotation method which subjected the tank bottoms to much higher stresses than those under normal operating conditions. In order to protect the lap weld between the annular plate and the bottom plate from potential failures, 6-inch wide by 1/4-inch thick patch plates covering the lap weld over its entire circumference were installed. The tanks were subsequently placed into service without any ill effects. It may also be noted that weld overlay to restore base metal thickness for strength considerations is permitted by the ASME B&PV Code Section VIII for pressure vessels.
When a tank bottom is corroded beyond repair, a new bottom must often be installed over the existing corroded bottom. This is done by slotting the new bottom through the tank shell several inches above the existing bottom. The rules in API 653 require the spacing between the existing welds around nozzles and the new bottom-to-shell weld to comply with the requirements of API 650. Trimming of the existing nozzle reinforcing plate is permitted to satisfy the weld spacing requirements, provided that the modified detail complies with the requirements of API 650. In many instances, these requirements would necessitate raising the existing shell nozzles.
With an API “Regular Type” nozzle, if the weld spacing requirements cannot be met by trimming the existing reinforcing plate, the lower half of the existing reinforcing plate can be removed and replaced with a new plate to form a “tombstone” shaped reinforcing plate. In essence, this converts the “Regular Type” nozzle to a “Low Type” nozzle. However, with an API “Low Type” nozzle, the distance from the tank bottom to the center of the nozzle would be reduced to less than the minimum required by API 650 and the nozzle would have to be raised. Raising existing shell nozzles involves cutting and welding of shell plates which may be of unknown toughness, thereby increasing the risk of a brittle fracture. One must therefore examine the technical justification for raising nozzles.
The reason for raising nozzles is to comply with the nozzle details given in API 650 which provide reinforcement of shell openings generally in accordance with the well-established area replacement rules of the ASME Code. However, nozzles in an AST that are located near the tank bottom are subject to much lower stresses under the hydrostatic head pressure than those at higher evaluations. In fact, because the bottom of the shell is restrained from radial displacement by the bottom plates, the circumferential membrane stress in the tank shell at the bottom of the tank is essentially zero. Therefore, the requirements for nozzle reinforcement should be based on stress analysis, considering the cross-sectional area available in the shell plate for reinforcement, as allowed by the ASME Code, as well as by API 650. In most cases, there should be no need to raise the nozzles.
In summary, the API 653 restrictions on weld repairs and requirements for raising shell penetrations when making tank bottom repairs should be reexamined. Relaxation of these rules should be possible to facilitate tank bottom repairs without affecting tank safety.