The construction of large, low-pressure storage tanks for liquids like oil, chemicals, or refrigerated gases demands rigorous engineering standards. Welding forms the critical bond that ensures these structures maintain their integrity under pressure and contain their contents safely.
Adherence to specific welding protocols is essential for tank reliability and longevity. These protocols are defined by the American Petroleum Institute standard known as API 620.
Qualification of Welders and Procedures:
API 620 mandates strict qualification for both the welding process and the individuals performing the work. Companies must develop and qualify detailed Welding Procedure Specifications (WPS) for each joint type, material thickness, and welding method used. These WPS documents outline essential variables like welding current, travel speed, and electrode type.
Qualified welders then demonstrate their skill by performing test welds under the WPS. They must pass visual inspections and destructive testing, such as bend or tensile tests, to prove they can produce sound, code-compliant welds consistently.
Materials and Welding Methods:
The standard applies to tanks constructed from carbon steel, stainless steel, and other alloys. The choice of welding consumables electrodes, wires, and fluxes must match or exceed the base metal’s properties, particularly for strength and corrosion resistance. API 620 commonly references methods like Shielded Metal Arc Welding (SMAW), Submerged Arc Welding (SAW), and Gas Metal Arc Welding (GMAW).
The selected method must provide the controlled heat input and deposition rates necessary for the specific materials and joint geometries, minimizing distortion and preserving material strength.
Inspection and Non-Destructive Testing (NDT):
Quality assurance in welding under API 620 relies on comprehensive inspection. Visual examination occurs throughout the welding process. The standard then requires specific Non-Destructive Testing based on the tank’s design criteria and the joint’s importance. Radiographic Testing (RT) or Ultrasonic Testing (UT) often examines full-penetration welds in critical shell and bottom seams.
Magnetic Particle Testing (MT) or Dye Penetrant Testing (PT) checks for surface defects on attachment welds and other components. The extent and location of NDT follow a detailed acceptance criteria, ensuring flaws exceeding code limits get repaired.
Repair of Defective Welds:
The standard provides clear directives for addressing welds that fail inspection. Procedures require removing the defective material completely, typically by grinding or gouging, before re-welding. The repair process must follow a qualified WPS and may need additional preheat or stress relief. Inspectors re-examine all repair welds using the original NDT method, and sometimes a more stringent one, to confirm the repair meets all acceptance standards.
