Table of Contents
Introduction
Although first welding technology ever developed has been reported as back as 4000 BC in Egypt but actual joining of metals especially steel gained momentum during world war I & II. Until as recently as 19th century, various types of welding machines were developed to meet different welding process resulting into a weld joint that can be trusted for its strength of parent metals (Dunn & McClelland, 2015).
Welding processes for a variety of joining of similar & dissimilar metals of all shapes and sizes through minimum to maximum thickness are of utmost importance for achieving industrial development (Utech, 2013). Dissimilar-metal welding refers to the joining of two different alloy systems. All fusion welds are dissimilar-metal welds (DMW’s) because the metals being joined have a cast structure. In dissimilar metal welding, the properties of three metals must be considered: the two metals being joined, and the filler metal used to join them. For example, if one of the metals being joined is welded using preheat when welding to itself, preheat should be used in making a DMW (Malone, 2011).
Introduction to Tank Welding
The storage tanks are large metal structures used primarily for oil, chemicals, paper and food industries, which may have various configurations, depending on several parameters, such as dimensions, orientation, and type of construction body or outer wall. The constant search for efficiency in the manufacturing process and the guarantee of the quality of the product they contain has imposed the application of strict standards that today govern the construction of storage tanks, dictated by two entities of worldwide scope: the American Petroleum Institute (API) and the American Welding Society (AWS) (Groover, 2007).
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The regulatory intervention of the AWS is since the construction of the storage tanks involves various welding processes, which under the API standard 650 displace the old riveting technique, which is currently in disuse. This is because, due to the size of the tanks, there are no metallic plates with the required dimensions to form a single piece, for which several large plates must be used that are welded together to assemble the entire tank. Therefore, the automation and experience in tank welding in the field is becoming increasingly important, in view of its many advantages over traditional manual welding (Kou, 2003). Before knowing the types and equipment of automated welding of tanks currently offered by the market, let’s examine some previous considerations.
Tank Joint Welding
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 (Kou, 2003). 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 650 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 (Kou, 2003).
Shielded Metal Arc Welding: SMAW, Stick, Arc
In building above ground welded steel storage tanks, different welding processes can be used. Shielded 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
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 (Kou, 2003). The first use of this process took many years of education to develop a method of welding that would be clean of porosity in the weld joint. 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.
With SAW technique, circumferential joints connecting one shell course to the next is also done with ease and perfection. 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 provides X-ray quality welds.
Flux-Cored Arc Welding: FCAW
Flux core welding or tubular electrode welding has evolved from the MIG welding process to improve arc action, metal transfer, weld metal properties, and weld appearance. It is an arc welding process in which the heat for welding is provided by an arc between a continuously fed tubular electrode wire and the workpiece. Shielding is obtained by a flux contained within the tubular electrode wire or by the flux and an externally supplied shielding gas (Zinn & Semiatin, 1988).
Flux-cored electrode welding can be done in two ways: carbon dioxide gas can be used with the flux to provide additional shielding, or the flux core alone can provide all the shielding gas and slagging materials. The carbon dioxide gas shield produces a deeply penetrating arc and usually provides better weld than is possible without an external gas shield. Although flux-cored arc welding may be applied semi automatically, by machine, or automatically. The process is usually applied semi automatically.
