A metal framework’s design plan (technological document) and Work Execution plan together form the storage tank construction. There must be enough space on the construction site to have sections for general layout, operating materials and machinery, utility systems and safety equipment.
The first step of constructing a storage tank requires the arrangement for basement and foundation to be completed.
- Work Execution Plan:
A work execution plan is created by an engineering design organization and helps in the welding and assembly work for the storage tank construction. It requires the use of special facilities and equipment’s. It includes:
• Procedural descriptions of the tank’s elements space stability
• Measures load bearing capacity of the construction elements during the process of erecting the storage plan
• Safety regulations
• Labor protection norms
• Environmental protection norms
• Order of tank testing procedures
• Scope of control measures for erection
Under the Work Execution Plan, the technological standard for welding that are included are:
• Welding materials
• Order of procedures
• Requirements for heating the junctions
• Welding methods
• Preparation of edges for welding
• List of required equipment for the tank construction - Welding
Electric arc welding methods are used during tank erection. They can be automatic sub merged arc welding, manual arc welding, mechanized arc welding with self- clinching flux cored wire etc. The climatic conditions play a vital role in this process. 1.3 – 2.0 is the stipulated limit for the shape coefficient of the built-up seam. - Assembly of tank bottom:
There are 2 main elements of the bottom namely, segmental ring with the welded first ring of the shell and the central part that is welded and assembled individually. Only after the complete installment of these 2 parts the welding joint is welded. This helps in connecting the other 2 elements.
The bands are welded with the overlap seam from the middles of the band to its edges, using a back step procedure. Lower overhead joints are welded post this.
Post this the coiled bottoms are assembled which consist of 2 main parts. This is placed on the first half of the bottom that acts as the outside cover of the coil.
When the coil is fully unrolled a brace is welded in the center of the top-half bottom to fix the rope for a hauling winch. A cool prime coat is used to avoid corrosion which is generally a thin layer of primer. Two layers of bitumen with filling material is also added to the bottom. - Storage tank shell:
The tank shell is installed by a plate-by-plate method in which the first ring is welded and installed completely followed by the 2nd ring on top of the first moving in an upward direction.
It is important to fix the shell plates with each other as well as the bottom section. The work execution plan will stipulate the extreme deviations of the 1st ring. Temporary stiffing rings are used to withstand wind stress.
The tank shell can also be installed in when it is in coiled form by lifting the coiled shell panel to a vertical position and uncoiling it. The end areas are shaped and the butt-joint is assembled finally. - Tank roof and pontoon:
Fixed roof- these can be installed from top, when the roof is conical or spheric or from inside the tank when the roof has separate elements for framework and decking A central supporting stand can also be used to assist in the instillation process.
It is important to keep in mind the assembly load on the roof separately as well as on the construction elements. To prevent deformation, temporary bracing equipment, ties and supporting devices can be used.
A pontoon or a floating roof is installed at the bottom of the tank and is leak proof. - Manway plates and fitting pipe:
Manway plates are placed not less than 100 mm away from the butt-joints of the shell. It is important to meet the distances between welding joints. A radiographic method is used to control the covered parts of the joint. - Quality control:
This includes the use of use of welding methods, scope of control of the welding joints, carrying out technical and design construction supervision. The use of effective welding procedures and materials based on the Work Execution Plan. Areas of joint crossing are given extra attention in this step. The results of the quality control tests are fixed in certificates and act as supplements to the document to the tank. - Final test
The last test is a hydraulic pressure test done to check the tightness of connections and durability of the entire construction. For tanks with pontoons this test is done before installing the rim seals and these tanks are subject to relatively under pressure from the inside.
