Butt welded steel tanks have been a main stay in the field of constructing above ground storage tanks for decades. Around 1936, welded steel tanks overtook riveted tanks as the preferred method in new above ground storage tank construction. In step with this change, API standards were published to guide construction of such tanks for adequate safety and reasonable economy. Now in its twelfth edition, API 650 is the current standard to which welded steel tanks are built. This standard covers the minimum requirements for materials, design, fabrication, erection, welding, and inspection for constructing a new above ground storage tank.
Following this trend in constructing new welded storage tanks, between 1960 and 1990 many of the old riveted tanks were being cut down and reconstructed as welded tanks. The economics of this process became less attractive after the catastrophic tank failure of a reconstructed tank in 1986. In January of 1991, the API 653 Standard was published and addressed the criteria by which welded or riveted tanks should be dismantled and reconstructed. This Standard also addresses the need for tank inspections, repairs, or alterations on existing tanks.
In building welded steel above ground storage tanks, different welding processes can be used. Shield Metal Arc Welding (SMAW) or better known as stick (arc) welding was the leading welding process and still very common today for construction of above ground storage tanks. The SMAW process is versatile and very well suited to the environment of field erected work.
Submerged Arc Welding (SAW) became popular in the tank industry in the early 1980s. This process was used on lap welded bottoms and roofs and butt welded shell joints. The first use of this process took many years of education to develop a method of welding that would be clean of porosity under the surface of the weld. Many tank builders abandoned this method of welding on bottoms due to the inability to control moisture from the bottom side of the lap plates. However, the use of preheat in the winter time has made this a very successful way of the welding the girth (round) seams connecting one shell course to the next. This method of welding, known as Three O’Clock welding (3-O’Clock), delivers good clean and smooth penetration and provides a more uniform weld. The submerged arc weld blends and merges well with the two plates being joined and also provides X-ray quality welds.
API 650 section 9 requires all welding on above ground storage tanks to be done in accordance with the manufactures Weld Procedure Specifications (WPS) and the supporting Procedure Qualification Record (PQR). Welding operators are tested and certified in accordance with the tank builders welding procedures. Welds on newly constructed above ground storage tanks are subjected to various types of inspection.
Radiographic (x-ray) testing is performed to prove that the welding operators are in compliance with the welding procedure. The radiographic method is used on the shell butt weld joints, annular plate butt joints, and flush type connections with butt joints. This method is not used on the bottom and roof lap joints, top angle joints, man-way necks, shell to bottom joints, structural joints, and appurtenances. Welds examined by radiography are judged as acceptable or unacceptable by the standards of ASME section VIII. Section 8 of API 650 provides criteria for determining the number of shots to be taken for each welding operator based upon the thickness, number of shell courses, number of T joints, and the linear feet of vertical and horizontal welds completed. The storage tank owner/operator has the right to select the locations of the x-rays based on the Section 8.1.2 criteria.
All tank shell welds should also be cleaned and visually checked from both sides of the joint making sure weld joints are free of defects such as craters, pinholes, undercut, and under fill. Excessive weld reinforcement or overlay should be ground smooth along with the removal of all arc strikes, weld spatter, fit up, and scaffold strap burrs to include replacing any parent metal that might have been lost in the process of removing the bracket straps, nuts, and fit up equipment.
Vacuum box testing is the most common way of testing the tank floor weld seams for leaks. Care should be taken in ensuring all floor welds are visually checked to be complete and free of any slag, weld burrs, and other defects such as pinholes, undercut, and under fill before the vacuum testing is performed.
The shell to floor/bottom joint is visually inspected in the same manner. When one side of the welded joint is complete, the most common weld test is to apply a high flash point penetrating fluid such as diesel fuel to the opposite side. The welded side is then checked for any visible signs of wicking. If any repairs are required, the penetrating fluid test is performed again before the other side is welded.
Appurtenances, including inlet and outlet nozzles, should also have a good visual check making sure welds are free of defects. Air pressure is then applied through a quarter-inch pipe threaded tell-tale hole in the re-pad. Soap suds are then brushed or sprayed onto all sides of the re-pad and both sides of the penetration where the weld attaches the pipe nozzle to the shell. The welds are visually checked for small air bubbles penetrating the soap solution.
Once the welds on a new above ground storage tank pass all of these weld inspection processes, the tank is ready for service. If repairs are needed on a tank during its lifetime, similar weld inspection methods are used for the particular type of weld repair performed.
If you have any storage tank weld questions, contact us at 800-774-3230 or through our website.