Storage Tank Design

Standards:

National design standard:  GB50341-2003 design standard for vertical cylindrical steel welding oil tank
National construction standard:  GB 50128-2005 Construction and acceptance standard for vertical cylindrical steel welding oil tank

Design specifications

Viewing from all over the world, there are mainly API650, JISB8501, BS2654 and GB50341 as specifications of large oil storage tank. Each formula is concluded according to the hoop stress of membrane theory, combined the loading features of oil storage tank, meanwhile, and added the permissible stress, welded seam efficiency and addition of wall thickness, which can reach the complete computational formula of wall thickness. Each specification determines its own permissible stress and welded seam efficiency based on the properties of materials and the construction technology level of its own country. So even the storage tank has the same volume, the result of wall thickness has great difference.

Specifications of construction

• Construction method: for common vertical storage tanks generally adopt chain flip-chip method. construction preparation→material inspection→cutting→machining→inspection→welding of the center plate at the bottom of tank→welding of the arch edge plate at the bottom of tank→setting up the center post and umbrella stand→installation and welding of top ring→installation and welding of edge and steel angle→assembling of temporary arch bracket→installation of leading chain lifting device→installation and welding of arch roof→installation and welding of the lower circle→Jack-up the top circle shell plate→welding of circular seam→installation and welding of every circular seam→installation and welding of bottom circle, bottom plate and angle seam→installation and welding of edge plate of bottom plate→inspection→installation of accessories→vacuum test→test→settlement observation→hand over.
• Storage tank prefabricating: prepare three are sample plates, one sample plate for checking curvature of arch roof which the length is more than 2 meters, one inner arc sample plate for checking the deformation of the edge of vertical welded seam which the length is no less than 1 meter and one outer arc sample plate for checking the deformation of vertical welded seam which also the length is no less than 1 meter.
• Storage tank assembling: before assembling the bottom plate, the lower surface of it should be done anti-corrosion based on the sequence of installing storage tank. A cross center line should be drawn on the surface of the bottom plate on the basis of acceptance. When the deviation of the center line remarked by civil construction is large, it should be adjust to the minimum.
• Welding and weld seam inspection: according to JB4708-92 to evaluate the welding process of butt joint, T fillet weld and tube plate, and to work out the guide book for welding process on site and the person who does the NDT for weld seam of storage tank. Radiographic inspection is done based on the specification of GB3323-87 Radiogram and quality of butt joint welded by steel melt weld.

Inspection Methods

• The tank body test: bottom board vacuum test adopts vacuum process to test the leak of all bottom board weld seam. And the vacuum degree should be more than 53Kpa, for reinspect the penetrate test will be used.
• Tank water filling test: to observe the foundation settlement according to the up and down test of floating disc. Close the holes of ground part temporarily and open the light hole of the tank roof and then fill the tank with water. The water for test should adopt fresh water and the temperature should be higher than 5 ℃. The pressure should be kept for 48 hours.
• The leak test, strength test and stability test for wall and roof of storage tanks: to close the light hole of tank roof, foam holes and vent holes when the water level is higher than 1 meter, then to fill water continuously, and to observe the pressure through the pressure gage setting on the tank roof. After water filling test is the stability test. Cloth all the opening when the water reaches the maximum operation liquid level. And then drain the water slowly, meanwhile, monitor the negative pressure number strictly, stop drain water when the number of negative pressure reaches 200mm, and to fill water into the tank in order to make the pressure normal.
• Crack detection: To do penetration test or magnetic powder inspection for the fillet weld seam of joint and stiffening plate on the tank.