ASME PRESSURE VESSELS
The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America. For more information about our products, heavy plate & custom fabrication services or fabrication capabilities contact us today!
SHELL REPAIRS TO ABOVEGROUND ATMOSPHERIC TANKS
API-653, Tank Repair and Alteration, intends that repairs and alterations made on an Aboveground Atmospheric Storage Tank (AST) result in general equivalence to API-650 requirements. Section 9 of API-653, contains specific requirements that are intended to meet this objective. After the AST has been in service for some time, thinning due to corrosion may require that sections of the shell be repaired in order for the tank to have adequate strength for its next period of operation. Such repairs are generally done by installing insert plates into the shell using full-thickness butt welds.
Before Addendum 1 issued in December 1996, API-653 did not permit the use of lap-welded shell patch plates under any circumstances. The primary concerns with using lap-welded patch plates relate to the quality and strength of the attachment fillet weld, the possible increased risk of initiating a brittle fracture at the fillet weld (especially in steels that do not meet current fracture toughness requirements), and the ease with which the patch plate is installed (which may lead some individuals to not take it as seriously as they should). Addendum 1 relaxed this general prohibition on the use of lap-welded patch plates provided that the owner specifies their use and they meet requirements stated in API-653. Despite this relaxation, the specified requirements for lap-welded patch plates will still limit their widespread use. As noted below, the maximum shell and patch plate thickness limit is a significant constraint.
Lapped patch repairs may be considered permanent repairs subject to an established inspection and maintenance program. The requirements specified in API-653 may be used to evaluate preexisting lapped patch repairs, except that the various plate thickness limits need not be applied.
The following highlights several of the requirements for using lap-welded patch plates. This is not an all-inclusive list; so, refer to API-653 for additional details.
- All repair material must meet the requirements of the applicable construction standard and API-653.
- Lap patches cannot be used on any shell course whose original construction thickness is greater than 1/2 in. (12.5 mm), and cannot be used to replace door sheets or shell plates. The 1/2 in. (12.5 mm) thickness limit relates to potential brittle fracture concerns. There has never been a documented case of a brittle fracture in plates less than 1/2 in. (12.5 mm) thick. Therefore, limiting patch plate thickness to 1/2 in. (12.5 mm) minimizes the brittle fracture risk. This maximum thickness limit effectively limits the use of lap patches to small diameter tanks, or the upper courses of large diameter tanks, where the shell plate thickness is not over 1/2 in. (12.5 mm).
- The lap patch must be at least 3/16 in. (4.8 mm) thick, but no thicker than the smaller of 1/2 in. (12.5 mm) or the thickness of the plate to which it is being welded. Therefore, if the design calculations conclude that a lap patch plate must be over 1/2 in. (12.5 mm) thick, a butt-welded insert patch must be used instead.
- The shape of the lap patch may be circular, oblong, square, or rectangular. All corners must be rounded to a 2 in. (50 mm) minimum radius (except at the bottom-to-shell joint). This reduces local stress concentrations and facilitates welding.
- Minimum spacing limits between the lap patch welds and shell seams are specified. The size and placement of the lap patch might require adjustment to respect these limits.
- A lap patch may extend to and intersect with the external bottom-to-shell joint at a 90° angle. Acceptable attachment details are specified in API-653.
- Shell openings and their reinforcement shall not be positioned within a lapped patch shell repair. The main concern here is the additional loads that could be imposed on the lap patch fillet welds.
- The area on the shell where the lap patch welds are to be made must first be ultrasonically examined for plate defects and remaining thickness. The plate size must be changed (probably increased) as needed to weld to sound shell plate material.
- Lap patches shall not be lapped on lap-welded shell seams, riveted shell seams, other lap patch repairs, distorted areas, or unrepaired cracks or defects.
- Minimum and maximum lap patch size limits are specified (see API-653 for details). The patch plate must be formed to the shell curvature in all cases.
The following Table highlights additional requirements that must be met when lapped patch repairs are used for particular situations.
| Repair Situation | Additional Requirements |
| Closure of holes caused by removal of: | • Continuous fillet welds in outer perimeter of plate and inner perimeter of hole. |
| • Existing shell openings | • 2 in. (50 mm) minimum hole diameter. |
| • Thinned areas of shell | • 2 in. (50 mm) minimum corner radius of shell hole. |
| • Complete removal of original nozzle neck and any associated reinforcement plate. | |
| • Complete removal of original nozzle neck and any associated reinforcement plate. | |
| • Repair plate thickness ≤ nominal thickness of shell plate adjacent to repair. | |
| • Full-thickness fillet welds. | |
| • 4 in. (100 mm) minimum repair plate dimension. | |
| Repair Situation | Additional Requirements |
| Reinforce thinned areas of shell without removal | • Repair plate thickness calculated using E ≤ 0.35. Here again, using this weld joint efficiency may result in the lap patch being thicker than the shell plate. If this results in a thickness greater than 1/2 in., a lap patch cannot be used. |
| • Full-thickness fillet weld. | |
| • Repair plate thickness cannot exceed: | |
| o shell plate thickness by more than one-third, but by no more than 1/8 in. | |
| o 1/2 in. (12.5 mm) | |
| • Strength of remaining shell under repair plate cannot be considered effective in carrying calculated service or hydrotest loads. | |
| Repair small leaks or minimize potential for leaks at isolated or widely scattered pits | • Existing shell thickness meets minimum thickness requirements. |
| • Repair plate thickness calculated using E ≤ 0.35 (same comments as above regarding resulting patch plate thickness). | |
| • Full-thickness fillet weld. | |
| • Repair plate thickness cannot exceed: | |
| o Shell plate thickness by more than one-third. | |
| o 1/2 in. (12.5 mm) | |
| • Minimum repair plate thickness ≥ 3/16 in | |
| • Cannot be used if: | |
| o Crevice corrosion or concentration cell corrosion is a concern if fillet weld is exposed to stored liquid. | |
| o Presence of stored liquid between shell and repair plate will prevent tank gas freeing for hot work. | |
| • Thickness of shell plate under repair plate must be evaluated for acceptability during future inspections. |
As is evident from the above summary, many of the lap-welded patch plate requirements can easily be met. However, others [e.g., the maximum 1/2 in. (12.5 mm) thickness limit] effectively precludes their use for tank shell repairs.
Source: API 653
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The sizes of these projects are up to 200’ in length, 350 tons, 16’ diameter and 4” thick.