Table of Contents
- Door Sheets in Above ground Storage Tanks.
- Specifications for Tank Door Sheets
- Checking and Testing of Door Sheets.
- Conclusion
Above ground storage tanks are usually made of carbon steel or stainless steel to comply with the provisions of API-650 Welded Tanks to Oil Storage and other relevant standards, according to conditions of service [1-5]. In these tanks, a door sheet is a provisional opening made in the shell to allow access to the shell to facilitate inspection, cleaning, maintenance or repair. These are available in the form of openings made in the tank wall to allow the movement of personnel or equipment without the need to dismantle the structure. Here, the paper has described the important considerations regarding inspection and repair of tank door sheets, according to the generally adhered to industry practices.
Figure 1: Steel above ground storage tank inspection door sheet.
Door Sheets in Above ground Storage Tanks
In an above-ground storage tank (AST), a door sheet is merely a temporary cut-out in the shell to enable internal access to work on the inside of the tank, which may be inspection, maintenance or cleaning or repair. These openings are usually found along the tank wall, allowing access without the need to disassemble the main structural components.
The following are some of the common uses of door sheets :
1. The first is the Inspection and Maintenance Access :
They also offer a convenient way to reach inside the tank which is otherwise hard to reach. This is particularly relevant to large tanks containing fuels, chemicals, or other fluids, where inspection should be carried out periodically to detect corrosion, leakages, or structural inadequacies.
2. Internal Cleaning :
The door sheet provides enough room to maneuver personnel and equipment in the tank during cleaning operations, which can be either routine or post-storing of certain items.
3. Repair Activities :
In cases where the internal parts or linings are in need of repair, the availability of door sheet makes it easy to reach damaged parts and enhance effectiveness in carrying out maintenance work.
4. Component Replacement :
Internal components like piping, valves or supports may require change with time. The door sheet can be easily removed and installed without affecting the overall tank structure.
After the necessary work has been done, the opening is re-installed or replaced, and the tank is well sealed up to ensure that the tank regains its structural integrity and is leak-tight.
Figure 2: Above ground storage tank door sheet with vertical and horizontal braces.
Specifications for Tank Door Sheets
The production of a door sheet can be regarded as one of the major changes and it has to be in accordance with standards. The presence of API-653 very specifically covers door sheets, which are defined as plates taken out of an existing tank shell to create a temporary access door, to be reinstalled or replaced at the end of work.
The standard offers advice on various types of tank construction, such as rivets, lap-welds, and butt-welds. As installing a door sheet requires shell plates to be removed and replaced, all the general requirements of shell plate replacement have to be adhered to. The thickness of the replacement plate must not be less than the maximum nominal thickness of plates in the same course, except where there is bare insert plates of greater thickness.
Minimal dimensional and geometrical requirements are also specified in API-650. These consist of acceptable repair plate shapes (circular or rectangular) and minimum distance between critical features (bottom seams, annular ring welds, vertical seams, horizontal seams). The design of all weld joints should meet API-650 or other relevant construction codes in order to be totally penetrated and fused.
In the case of butt-welded tanks, the section of the shell removed can either be re-installed or a new plate can be installed. In either scenario, welds should be able to attain complete penetration and fusion to ensure continuity in the structure.
Figure 3: Example API-653 compliant above ground storage tank alteration for a door sheet.
When a vertical cut of the door sheet cuts through an existing seam in a butt-welded tank with no offset and the original plate is used again, further inspection is needed. In addition to normal examination procedures, the back-gouged root surface and final weld passes (on each side) should be analyzed with magnetic particle testing or liquid penetrant testing. Besides, the intersecting horizontal seam should also be examined with the same techniques at least 6 inches on both sides.
In the event that the vertical cutline crosses an existing seam with an offset, the offset should comply with the minimum requirements given in the code. In this situation, one should cut the horizontal seam at least 12 inches above the vertical point of welding then proceed to weld the new vertical joints. The vertical welds are then completed and the horizontal seam is then welded.
Liquid penetrant testing is one of the key methods used to inspect door sheet
weld seams under API 653 here is a full breakdown of how the process works
→Liquid Penetrant Testing — Method, Process and Applications
Checking and Testing of Door Sheets.
The door sheet structure depicted in Figure 1 needs to be further adjusted to comply with the requirements of the code. Ultrasonic thickness testing ensures that plate thickness is minimally acceptable, but not material composition. Positive material identification (PMI) can thus be required to ensure that the material is of the required grade of carbon steel.
The lower edge of the door sheet is not extended to the bottom of the tank and the annular plate and falls short of the minimum spacing 6 inches. The measured distances were 4-6 inches, which meant non-compliance. Whereas, it is not compulsory, it is recommended to round off the top and bottom corners of the door sheet with regards to the relevant code provisions.
The intersection of the vertical seam with the door sheet seems to be acceptable, and the distance between the vertical weld and the door sheet is within acceptable limits.
The deleted plate seems to have been cut in one piece about 23 inches above the first course horizontal butt weld. But API-653 butt-welded tank requirements which do not have vertical seam offset state that repair must extend at least 6 inches above the weld seam to meet minimum corner radius and height requirements.
Further, the vertical weld of the door plate in the second course is not more than 6 inches above the adjacent seam as it is not in accordance with the requirements of the code. The replacement door sheet should also be at least 6 inches in length beyond the vertical seam, and meet all spacing requirements with horizontal seams, and the tank base.
Conclusion
The costs involved in repairing or replacing non-compliant door sheets within the storage tanks may incur extra costs than what was originally expected because of the extra repair, welding and extra engineering work. Effective interpretation and implementation of the appropriate construction and repair codes prior to making the modifications can greatly decrease the chances of non-compliance and the accompanying costs.
This article summarizes some of the important considerations in tank door sheets, and can be applicable to other shell modifications. The repairs must be made so that there is full penetration and fusion of the weld. Moreover, welds should be subjected to the proper non-destructive test procedures, including magnetic particle testing, liquid penetrant testing and, where necessary, ultrasonic or phased array ultrasonic testing, in line with the established industry standards.
Door sheets are just one part of above ground storage tank compliance
tank floor inspections carry their own set of code requirements worth knowing
→Tank Floor Inspections — Requirements, Methods and Compliance
References
- API-650 API Std 650 Welded Tanks for Oil Storage, Thirteenth Edition, Includes Errata 1 (2021)
- API-653 Tank Inspection, Repair, Alteration, and Reconstruction (2021)
- API-620 API Std 620 Design and Construction of Large, Welded, Low-Pressure Storage Tanks, Twelfth Edition, Includes Addendum 1 (2014) Addendum 2 (2018), and Addendum 3 (2021)
- AWWA D-100 AWWA D100-21Welded Steel Tanks for Water Storage
- NFPA-22 NFPA 22 Standard for Water Tanks for Private Fire Protection
Author:
1. Thomas R. Hay, Ph.D., P.E., President (TechKnowServ)
