Those wondering how to weld pressure vessels may already know that the answer to this question varies depending on the type of pressure vessel in question. In the U.S., the relevant standard from the American Society of Mechanical Engineers (ASME) defines a pressure vessel as any container with a maximum operating pressure of 15 pounds per square inch or more. This includes power generation pressures that average around 2,000 PSI or higher as well as home water heaters that are usually rated up to 150 PSI.
Clearly, the term “pressure vessel” encompasses a wide range of containers of various types and, as a result, the ways pressure vessels are welded are equally diverse. In this article, we’ll focus on the welding of very high-strength, high-PSI custom pressure vessels. Welding these types of pressure vessels requires highly specialized welding processes and carefully developed welding parameters in order to meet the rigorous specifications for these containers.
Welding Requirements for High-Spec Pressure Vessels
Their ability to contain very high pressures isn’t the only consideration for high-performance specification pressure vessels. Pressure vessels used in the biopharma, food, and electronics industries must contain high pressures while also meeting extraordinary standards for product purity and cleanliness. Similarly, in the nuclear industry, most pressure vessels are required to contain substances pressurized to the middling 2,000 to 3,000 PSI range. These vessels must retain this pressure while being subjected to heat, the corrosive action of steam, potentially highly corrosive gases, and the effects of neutron irradiation, which reduces the ductility of many metals. This loss of flexibility must be engineered into the design of nuclear pressure vessels before they are ever welded.
The decision on how to weld pressures vessels with these very high specifications will depend on the vessel’s engineering design and will ultimately be determined by the following:
- Materials: High-specification pressure vessels have unique material requirements. Steel or stainless steel is common, but for high heat or pressure vessels that contain corrosive materials, exotic alloys like Inconel® or Monel® are used. Heat exchangers may use multiple materials, with aluminum or another metal with high thermal conductivity inside a steel or stainless steel housing.
- Fabrication: Due to the way mechanical forces are exerted, most pressure vessels are cylindrical or circular in shape. The materials for smaller pressure vessels like gas tanks are mass-manufactured, but for larger, specialized pressure vessels, the metal must be cut and rolled from a flat plate, or specially extruded for the work.
- Machining: The materials for a pressure vessel must be cut and machined to allow for intake and output valves, gauges, heat exchangers, and the other devices that make it possible for a pressure vessel to function.
- Preparation: Once the components of a pressure vessel have been fabricated and machined, they need to be thoroughly prepared by cleaning and carefully measuring their dimensions for compliance with the engineering specifications for the pressure vessel.
The factors above will partly determine the specifications that a pressure vessel weld must meet. For example, the materials used will determine the weld filler material needed, and the shape of the final pressure vessel will determine the amount of filler required. The weld type that is needed on the basis of those two factors will in turn determine how the pressure vessel’s components are machined. The way pressure vessels are welded is a complex interplay of these factors, with some requirements ruling out certain fabrication methods, and some welding types, in turn, ruling out particular methods of joint machining.
How to Weld Pressure Vessels
In the U.S., the relevant standard for welding pressure vessels is the ASME Boiler and Pressure Vessel Code (BPVC), which has a 364-page subsection concerning welding. This exacting detail is necessary because, in pressure vessels, the forces are uniformly applied across the interior surface of the vessel. Any point of weakness will be a focal point of stress that could result in an escaping jet of liquid or gas. Improperly welded or damaged pressure vessels have also been to explode with lethal consequences even today.
The goal of any welding is to produce a smooth, continuous piece of metal in which the properties of the weld differ as little as possible from the properties of the workpiece. However, creating a truly uniform weld that matches the workpiece exactly is a theoretical goal; there will always be some variations in the weld and in its strength compared with the workpiece. The goal is to keep these variations to an absolute minimum.
The best way to reduce variability that can weaken welds and pressure vessels overall is through automated welding processes. Machine weld heads are steadier than even the most accomplished welder’s hands and aren’t subject to fatigue, which can cause variations in welding speed, pattern, or arc voltage. These variations, which are inevitable in manual welding, may result in flaws and stress points in pressure vessels. One of the biggest advantages of automated welding is that it offers excellent repeatability, which means that a high level of consistency can be maintained not only during a single weld, but also during subsequent welds of the same type. Automation also eliminates many of the training and qualification barriers that hamper high-specification welding in general.
Why Choose Automated Orbital Welding for Pressure Vessels
In the past, fixed weld heads were used, combined with hydraulic equipment that moved the pressure vessel itself, to weld pressure vessels using automated welding equipment. This method is still used for very large pressure vessels. However, the disadvantages of moving very heavy pieces of steel connected only by tack welds should be obvious. Movement can alter fit-up and even result in significant rework if the tack welds fail.
Automated orbital welding allows the pressure vessel being assembled to remain firmly in place as the orbital welder orbits the workpiece. Orbital weld heads range from heavy-duty devices capable of welding the thickest pressure vessels to small-scale fusion welders that are suitable for welding leak-free joints in the heat exchangers that go inside some pressure vessels. Using a weld head capable of moving around the workpiece makes the welding setup process easier and helps prevent time-consuming rework.
While the best welding proce`ss for a particular pressure vessel will depend on the type of vessel being welded and its specifications, orbital gas tungsten arc welding (GTAW) provides the most reliable, hygienic welds for demanding, high-specification applications. There are a myriad of options for welding pressure vessels, but automated GTAW orbital welding represents one of the best.
Arc Machines, Inc. provides high-quality automated orbital GTAW welders and access to staff who know how to develop welding parameters for pressure vessels of all kinds. For inquiries regarding products, contact sales@arcmachines.com. For service inquiries, contact service@arcmachines.com. Arc Machines welcomes the opportunity to discuss your specific needs. Contact us to arrange a meeting.
