INTRODUCTION
The welds used in the fabrication of pressure vessel undergo a rigorous quality control and assurance process, including x-ray inspections, ultrasonic testing and a variety of other forms of non-destructive testing (NDT). If a pressure vessel weld fails to meet these quality control and assurance processes and non-destructive testing, the reflection of the defective weld can be both difficult and expensive to rectify.As such, the use of the best possible pressure vessel welding process is critical-it improves the likelihood of a durable weld, free from defects such as porosity and inclusions.
COMMON TECHNIQUES FOE WELDING PRESSURE VESSEL
Welding is a common process in all kinds of metal fabrication, wherein the pieces are joined by means of heat or pressure or both. Several types of welding processes are employed in pressure vessel fabrication, which are as follow:
GTAW
Plasma welding
K-TIG
GAS TUNGSTEN ARC WELDING (GTAW)
Gas tungsten arc welding (GTAW) is sometimes referred to as tungsten inert gas (TIG) welding. In this process, coalescence of metals is produced by heating with an arc between the base metal and a no consumable tungsten electrode. Weld pool temperatures can approach 2500 °C (4530 °F). The tungsten electrode serves only to maintain the arc. Shielding is obtained from a gas or gas mixture, usually helium, argon, or a combination of the two. Pressure and a filler metal may or may not be used depending on the joint configuration. TIG welding is mostly used where precise, small welds are needed on stainless steel and non-ferrous metals such as aluminium, magnesium, and copper alloys.The practical upper limit for single pass GTAW welding is generally considered to be 2mm. Beyond this thickness, a V-groove root pass is normally applied, followed by filler passes.
PLASMA WELDING
The plasma welding process was introduced to the welding industry as a method of bringing better control to the arc welding process in lower current ranges. Plasma arc welding is a widely utilized welding process in pressure vessel fabrication wherein plasma, which is heated to an extremely high temperature and ionized, is used to transfer an electric arc to a work piece. The plasma welding process brings the highest quality standards to pressure vessel fabrication and it helps make strong and precision welds on both thick and thin metal. The practical upper limit for single pass PAW welding is generally considered to be 8mm to 10mm, although it is more commonly used for root passes of 4mm to 6mm, followed by filler passes using TIG/GTAW.
K-TIG (KEYHOLE TIG)
K-TIG (keyhole TIG) is a variant of GTAW that works across a whole range of applications, typically operating at 100 times the speed of TIG welding(in materials up to 16mm in thickness), and typically operates at twice the speed of plasma welding. It easily handles longitudinal and circumferential welds on pipe, plate, spooling, vessel, tank and other materials. K-TIG only requires a square butt joint but can weld into all standard GTAW preparations—there’s no root gap required. K-TIG works across a whole range of applications, and is particularly well suited to pressure vessel production where defect free welds are a critical requirement. The system welds effortlessly in 1G and 2G welding positions, and is perfect for titanium (at 3mm to 16mm), and stainless steel (at 3mm to 13mm).In tube forming and pressure vessel applications where there is typically tight fit-up at the root and a gap at the face, K-TIG will accommodate a gap of up to 20% of joint thickness gap at the face.
K-TIG performs single pass, full penetration welds in a wide range of materials, including: titanium up to 5/8 inch (16mm) thickness, for stainless steel up to 1/2 inch (13mm) thickness, for duplex & Super Duplex up to 1/2 inch (13mm) thickness, for Nickel alloys & super alloys up to 1/2 inch (13mm) thickness and for high quality carbon steel up to 1/4 inch (6mm) thickness.
Universal Engineering Services has more than a decades experience in the pressure vessel design for clients in UAE, Oman, Qatar, Kuwait, Bahrain, Saudi Arabia, and many more countries all over the world. UES work to many ASME standards to design and validate pressure vessels, boiler, fittings and piping systems. We are also experts in Stress analysis of piping, structural design, ASME Joint review, Design of Storage tanks, code calculations, FEA/FEM, and spotless service on design management.
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