Carbon and stainless steel aboveground storage tanks are constructed in accordance with guidelines set forth by API-650 Welded Tanks for Oil Storage and other standards based on the intended service [1-5]. In the context of aboveground storage tanks (ASTs), a door sheet refers to a temporary opening created in the tank's shell to provide access for inspection, maintenance, repairs, or cleaning. These door sheets are typically cut into the side of the tank to allow personnel or equipment to enter the tank without dismantling it entirely. This article discusses some of the main features to consider during storage tank door sheet inspection and repair.
Aboveground Storage Tank Door Sheets
In the context of aboveground storage tanks (ASTs), a door sheet refers to a temporary opening created in the tank's shell to provide access for inspection, maintenance, repairs, or cleaning. These door sheets are typically cut into the side of the tank to allow personnel or equipment to enter the tank without dismantling it entirely. Common uses for tank door sheets include:
Inspection and Maintenance Access: Door sheets enable access to the interior of the tank, where it might otherwise be difficult to enter. This is especially important in larger tanks used to store fuel, chemicals, or other liquids, where regular inspections are necessary to check for corrosion, leaks, or structural issues.
Cleaning: For tanks that need to be cleaned internally, such as after storing certain chemicals or during routine maintenance, the door sheet provides a larger, more accessible opening to get equipment in and out.
Repairs: If a section of the tank’s internal structure or lining needs to be repaired, the door sheet can allow workers to safely reach the damaged areas and carry out the necessary fixes.
Replacement of Components: Sometimes, interior components like pipes, valves, or structural supports may need to be replaced. The door sheet provides a way to remove and install these parts without compromising the tank's overall integrity.
After the work is completed, the door sheet is typically reinstalled or replaced, and the tank is resealed to ensure it remains watertight and structurally sound.
Aboveground Storage Tank Door Sheet Specifications
Insertion of tank door sheet is a major alteration and must be carried out in accordance with the relevant standard. API-653 is the only standard of references cited in this article that explicitly defined and address door sheets. API-653 defines a door sheet as “A plate (or plates) cut from an existing tank shell to create a temporary access opening. After planned work is completed, the door sheet(s) shall be reinstalled or replaced”. The standard provides the details for door sheets in riveted seam tanks, lap-weld seam tanks, and butt weld shell seam tanks. Since the door sheet is basically removal and replacement of shell plate, the standard rules for storage tank shell removal and replacement must be followed. The thickness of the door sheet must be equal to or greater than greatest nominal thickness of any plate in the same course adjoining the replacement plate except where the adjoining plate is a thickened insert.
The minimum dimension rules set forth in API-650 must also be followed. The include the minimum size of round, circular, and rectangular repair plates and their weld boundary spacing from the tank bottom seam, annular ring welded joint, vertical seams, horizontal seams, and adjacent shell alterations. All weld joint design shall be in accordance with API-650 or comparable construction code with plates being joined by complete penetration and fusion welds.
The section of the shell plate removed for a door sheet in a butt-welded tank can either be reinstalled in its original position or replaced with new material. In both cases, the installation of the door sheet must use joints that ensure full penetration and complete fusion.
If the vertical cut of a door sheet crosses an existing seam in a butt-welded tank without an offset and the removed section is reinstalled, additional weld inspection is required where the new vertical weld seam meets the existing horizontal seam. Along with the standard examination procedures, the back gouged surface of the root pass and the final pass (on both sides) of the new welds must be inspected using magnetic particle or liquid penetrant testing methods. Additionally, the existing horizontal weld seam at the intersection must also be inspected using the same methods for a distance of 6 inches on both sides of the intersection.
If the vertical cutline of a door sheet crosses an existing seam in a butt-welded tank with an offset, the minimum offset must meet the dimension specified by the code. Before welding the new vertical seams, the existing horizontal seam must be cut at least 12 inches beyond the new vertical joints. The horizontal seam should be welded last.
Aboveground Storage Tank Door Sheet Inspection
The aboveground storage tank door sheet shown in Figure 1 requires further alteration to be compliant with the codes referenced in this article. Ultrasonic thickness testing did confirm the plate satisfies the minimum thickness requirements. However, the exact steel alloy is unknown and addition positive material identification (PMI) may be required to ensure the correct carbon steel alloy was used.
The bottom seam of the door sheet was not cut flush with the tank bottom or annular plate and must be at least 6” from this seam. The spacing between the tank bottom seam and door varied between 4-6” and is not compliant. While the door sheet does not need to be cut flush with the thank bottom the top and bottom corners must be rounded in accordance with the relevant construction code.
The tank door sheet is cut across a vertical seam which is allowable and the minimum spacing V between the vertical weld and door sheet vertical welds are acceptable.
It appears the door plate was cut as a single sheet that extended 2-3 inches above the first course horizontal butt weld. The door must confirm with the API-653 Details for Door Sheet in Butt Weld Shell Seam Tank—No Vertical Seam Offset rules. In this case, the repair must be at least 6” above the weld seam to satisfy the requirement to be equal to greater than the minimum corner radius of 6” or 3” height.
The door plate vertical weld in the second course is less than 6” away from the course vertical weld. The door replacement would have to extend at least 6” pass the vertical weld while complying with all other horizontal and base weld spacing rules.
Conclusion
The cost to replace and/or repair non-compliant tank door sheets often exceeds the initial cost as the repair area, welding requirement, engineering and design time increase. Consulting and understanding the construction and repair code prior to starting an aboveground storage tank shell alteration will reduce this risk of non-compliance and incurred cost. This article is concise introduction that addresses tank door sheets. This basic rules that apply to door sheet apply to most tank shall alterations. All repairs must be complete penetration full fusion welds. Welds must be non-destructively testing with magnetic particle testing, liquid penetrant testing and in some cases ultrasonic testing or phased array ultrasonic testing.
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