The storage tanks They are large metal structures used mainly by the oil, chemical, paper and food industries, which can have various configurations, depending on a series of parameters, such as dimensions, orientation, construction and type of body or external 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 with a global reach: the American Petroleum Institute and the American Welding Society (respectively API and AWS).
The AWS regulatory intervention is due to the fact that the construction of storage tanks involves various welding processes, which by virtue of API regulation 650 displace the old riveting technique, currently in disuse. This is because, due to the size of the tanks, there are no metal plates with the dimensions required to form a single piece, so several large plates must be used that are welded together until the complete tank is assembled. Therefore, it is clear that automation and experience in tank welding In the field it is becoming more and more popular, in view of its many advantages over traditional manual welding.
Pay attention to this, before knowing the types and equipment of automated tank welding currently offered by the market, let us examine some preliminary considerations.
Structure and assembly of storage tanks
Regardless of design and intended use, all tanks have three main components: the bottom or floor, the Body and the ceiling, as we see in the figure below.
The material used for the construction of these metal tanks consists of large steel plates and their variants, such as galvanized steel, stainless steel and nickel steel. Each will have a different application, as for example stainless steel is resistant to corrosion, while nickel steel confers sufficient ductility and structural integrity at extreme sub-zero temperatures.
The body of the tank is made up of a series of rings welded together (known as “ferrules”), and depending on the application it can consist of one or two walls. Thus, the most common and used bodies are those with a single wall, while those with double walls are used in applications where storage at low temperature or high pressure is required. The case of cryogenic tanks that store liquefied natural gas at -196 ºC is the most representative of double-walled tanks, where the internal wall is made of nickel steel and the external wall is made of steel and prestressed concrete.
Tanks can be built above or below ground level, their orientation can be vertical or horizontal, and the roof can take on a variety of shapes, for example, it can be conical, dome-shaped or floating, all depending on the actual needs and applications.
The three components of a tank require the welding of the steel plates, either between the bottom and the body, between the body and the roof, as well as between the plates that make up the bottom, the body or the roof.
The vast majority of horizontal tanks are built in two ways:
a) Traditional type: from bottom to top, consisting of the following stages:
- The bottom of the tank is mounted on a prefabricated base reinforced with steel plates that are welded together.
- The ferrules are assembled by vertical welding, from steel plates up to 3 m wide.
- The next ferrule is welded on top of the previous one by horizontal welding and the roof is the last thing to be installed.
b) Jack-up type: from top to bottom, consisting of the following stages
- Identical to the traditional type, that is, the bottom of the tank is mounted on a prefabricated base reinforced with steel plates that are welded together.
- Along the perimeter of the bottom a system of Hydraulic jacks.
- The roof is assembled separately and is raised by means of a crane. At the same time, the ferrule that will be attached to the roof is welded by vertical welding and mounted on the hydraulic jacks. The roof is welded to the assembled ferrule and the assembly is raised with the hydraulic jacks.
- The following shells are assembled, they are raised with the hydraulic jacks, they are welded to the previous ones by horizontal welding, and so on until the lower shell is welded to the tank floor.
We can see the complete jack-up construction in the following video.
Welding processes and equipment used for tank welding
The various stages of welding involved in the assembly of storage tanks employ various techniques and equipment depending on the tank component involved. However, automation has imposed the use of tractors or welding carts that move on rails, and the size of these tractor systems will depend on the component of the tank to be welded.
For the ceiling and the floor, for example, mini-tractors automated welding machine, while an automated welding system is used for the body ferrules. horizontal welding carriages Y vertical welding carriages. None of these sophisticated systems require expert operators, so in addition to providing the highest quality and repeatability, they reduce construction time and therefore increase productivity.
Let’s take a brief look at how each system works.
The mini-tractors used to separately assemble the floor and roof of the tank weld together the corresponding plates of each component and also carry out the interior horizontal fillet welding of the lower shell to the floor. They move in a circular motion along rails or mechanical guides, and the various models are suitable for arc welding, including SMAW (gas shielded metal arc), FCAW (flux-cored arc) and TIG ( tungsten arc protected with gas) or also, to weld the first ferrule to the floor, SAW type (by submerged arc).
In the following figure we see some examples of mini-tractors.
These equipments have a digital control system that can program and store welding parameters, such as number of passes, welding process, diameters, current, voltage or speed, in millimeters per minute. These parameters ensure characteristics that affect stability, displacement, speed, electrode distribution and energy supplied to the process.
Carts for vertical welding
The vertical welding It is used to join, one by one, the steel plates that form each ferrule of the tank body. Generally, the processes used are the electrogas welding (EGW) or by electroslag and FCAW welding.
EGW is a high deposition process that uses retainer shoes to confine a molten metal for vertical progression welding. With this particular configuration, the welding arc travels on the axis of the vertical weld and deposits the filler metal as if the weld were made flat. EGW is considered a highly automated process, requiring closed-loop current sensing, or other means, to control travel speed and automatically maintain electrode extension without operator intervention.
Carriages using EGW welding are operating platforms used for single pass vertical welding. In addition to carts, some manufacturers also offer structures based on a column and a mount (see figure below). Without the requirement for gas shielding, this column structure is a simple and reliable choice for high deposition vertical welding on walls thicker than 15mm.
For its part, the vertical welding of high-strength steel tanks or non-ferrous metals (for example, stainless steel), is carried out with cars that adopt the FCAW process for multiple welding passes.
The equipment used for vertical welding has four main components: the rail, which is adjusted vertically to the plate to carry out the welding from the bottom up, the welding torch, the programming unit and the remote control unit, which has a potentiometer for the respective settings of the welding parameters.
Trolleys for horizontal welding
The horizontal welding or of circumference It is, as the word indicates, the one that is carried out with automatic equipment to join the shells of the tanks to each other during construction in the field. They are equipped with one or two welding heads, displacement control, flux conveyor and flux recovery system for horizontal welding that is carried out by the submerged arc (SAW) process.
The trolleys cover both sides (internal and external) of the plates, moving on adjustable wheels and at a controlled speed.
The basic components that make up a horizontal welding carriage are: the frame, welding heads, controls, flux conveyor, flux recovery system, and welding power source.
Among the advantages of these automated equipment we can mention the following:
- They use the mechanized SAW process to weld horizontal seams of storage tanks 20 times faster than traditional manual welding processes.
- The power-driven frame and operator platform allow you to comfortably perform a variety of welding operations on a variety of tank shells.
- They provide high quality continuous welds around the entire circumference of the tank.
Tank Weld Inspection
The provisions of API Standard 650 also provide for rigorous inspection of welds made on storage tanks made of steel.
In accordance with API 650, radiographic testing is applied to body butt joints, body plate butt joints, and flush connections with butt joints. In contrast, the radiographic method is not used for floor-to-ceiling lap joints, top angle joints, bottom ferrule-to-floor joints, structural joints, and accessories (eg, inlet and outlet nozzles).
For its part, the most common way to verify that there are no leaks from the tank floor weld seams is by means of the vacuum chamber test. These welds are previously visually inspected for slag residue, weld flash, and other defects such as pinholes, undercuts, and lack of fill.
Visual inspection of these characteristics also applies to all welds on both sides of the tank shell, as well as to fittings and joints between shell and floor, where these latter joints are also subjected to liquid penetrant inspection.
Once the welds on a new storage tank pass all of these inspections, the tank is considered ready for service. If repairs are required during their useful life, similar inspection methods should be used for the particular type of repair performed.
Although the tank welding Storage metals have been around for decades, the use of automated equipment offered by current technology has introduced a constantly growing trend. The goal essentially points to time savings, less waste of material, greater productivity and the possibility of providing structures that meet the most diverse requirements in accordance with current regulations. Therefore, the designs of new equipment for automated welding with more and better features.