Pressure vessels are one of the critical mechanical equipment that store and transfer fluids under high pressure. According to ASME Section VIII, if the maximum allowable working pressure of a closed container is greater than 15psi, the vessel is termed as the pressure vessel.
Pressure vessel comprises of a wide range of containers of different types and has vast oil and gas industry applications, fertilizer industry, process industry, FMCG, etc. They are designed as reactors, separators, blowdown drums, columns, and heat exchangers containing hot compressed fluids.
Importance of Pressure Vessels Welding
Pressure vessels are manufactured in different shapes and sizes, depending upon the applications. Because of the critical nature of pressure vessel applications, welding of pressure vessels is done by paying the utmost attention to the minor details. It must be of very high quality.
If pressure vessel welding is not adequately performed, then there can be potential health and safety hazards of leaking vessels, including suffocations, fires, poisoning, and explosion hazards.
Pressure Vessel Welding Processes
Pressure vessels are manufactured by bending and rolling of a steel plate, and then butt-welded at its ends. Welding is a critical step in the fabrication of pressure vessels. The material of pressure vessels is mainly carbon steel or stainless steel.
Following types of welding processes are employed in pressure vessel fabrication, which are as follows:
Plasma welding is a complex welding process in pressure vessel welding techniques. In this pressure vessel welding technique, a plasma jet is formed due to an extremely high temperature. This plasma jet provides a medium to transfer an arc to the workpiece.
Plasma welding requires highly-skilled operators to monitor the welding parameters. The plasma welding process brings high-quality standards to pressure vessel fabrication, and it helps make precision and strong welds on both thick and thin metal.
It is a versatile welding process that is suitable for all welding positions. It uses a non-consumable tungsten electrode. It produces the highest quality weld and has become a replacement for gas metal arc welding in pressure vessel fabrication. TIG welding is mostly used where small welds are needed on stainless steel.
It is a high energy density variant of GTAW. It is a full penetration keyhole welding that is performed on stainless steel and titanium and nickel during pressure vessel fabrication. Keyhole TIG welding is a revolutionary technology that guarantees a low level of weld seam crowning, no slag or spatter, and low energy requirement.
K-TIG welding technology offers enormous productivity, cost, and quality advantages while ensuring x-ray quality welds with superb root and cap aesthetics.
Factors Governing Pressure Vessel Welding Specification
There are different factors that you should take into account before selecting a particular welding process and welding parameters for a pressure vessel. These factors include:
- Operating Conditions, i.e. maximum allowable working pressure and design temperature
- The thickness of the metal plate to be welded
- The material of the plate to be welded.
- Type of service
Pressure Vessel Welding Inspection
Since pressure vessels are highly pressurized and can be dangerous if not properly welded, the inspection of welded pressure vessels is necessary to ensure the quality of welded joints. Following pressure vessel welding inspection technologies are applied on pressure vessels:
This pressure vessel welding inspection technique can detect welding defects beneath the surface. It is an extremely expensive technique that is used for critical weld joints as in nuclear power plants.
This pressure vessel welding technique can detect surface as well as subsurface welding defects. In this technique, a high-frequency sound wave is detected through the base metal and the welded joint. When a wave hits a discontinuity in a weld joint, part of the wave is reflected back. In this way, a welding defect can be detected.
Both welding inspection technologies have pros and cons that need to be considered based on applicable standards and customer requirements.