Types of Welding Used in Pressure Vessels

Pressure vessels have been around since the dawn of the industrial revolution. They can be used to store a higher volume of products than would be possible at normal atmospheric pressure, and since pressurisation lowers boiling points, pressure vessels can also be used to process products more quickly or for a lower energy cost than would be possible otherwise. They are used in food, pharmaceutical, and chemical processing and in nuclear power both to generate power and to store liquids and gases.

With such a diverse range of applications, the types of pressure vessels in use and the types of welding used in pressure vessels are incredibly diverse. The main commonality of pressure vessels is that they store large amounts of potential energy. A flaw or weak point in a vessel can cause the vessel to fail and release all of its energy at once in an explosion. As a result, it is important that pressure vessel welds be reliable and free of defects. The type of welding used to create a pressure vessel will be determined by the purpose of that pressure vessel.

Considerations for the Welding of Pressure Vessels

The chief consideration for welding pressure vessels is containing the pressurised contents. There is a long history of poorly built or poorly maintained pressure vessels experiencing a loss of containment and a resulting explosion.

Pressure vessel explosions are commonly thought to be a problem of the early industrial period, but they still occur regularly. These incidents underline the importance of quality welding — and of choosing the right welding process for the application. In industries such as biopharmaceutical and semiconductor manufacturing, cleanliness requirements limit the types of welding that can be used to those processes whose final welds can meet sanitary standards as well as pressure-containment requirements.

The Types of Welding Used in Pressure Vessels

Pressure vessels are almost always spherical or cylindrical, as this shape has no corners that could form stress points in the structure. To contain high-pressure products, pressure vessels are almost always built using metal 3 mm (approximately ⅛ inch) or more in thickness.

The edges of the plates used to build a pressure vessel are usually machined to create an angled bevel that allows the root pass of the weld to fully penetrate through the joint. When two component plates are placed side by side they will form a deep V-shaped or U-shaped groove. Once the root pass connects the two plates, subsequent passes fill in this groove with metal, resulting in a smooth, continuous expanse of uniform metal. This is much the same welding process as that used in big bore pipe welding and the narrow groove welding process. Familiarity with those two types of welding can give welders and weld supervisors a good working understanding of how to approach pressure vessel welding.

The following types of welding are used to weld pressure vessel joints:

• Shielded Metal Arc Welding (SMAW) — uses a flux-coated consumable electrode. Highly portable and the equipment is inexpensive and widely available. However, it requires a high degree of skill and the quality of the resulting welds is dependent on the skill of the welder. View ESAB stick welders
• Flux-Cored Arc Welding (FCAW) — uses a consumable wire-fed electrode with a flux core. Similar to SMAW in weld formation but, due to the continuous wire feed, requires less skill to operate. For guidance on flux-cored wire selection and flux handling, see our flux-cored wire guide and flux baking procedures
• Gas Metal Arc Welding (GMAW) — uses a continuous wire-fed electrode and inert shielding gas. One of the easiest welding processes to perform and makes welds that are cleaner than SMAW or FCAW. However, it is known to suffer from unpredictable fusion errors. See our guide on shielding gas management for wire welding
• Gas Tungsten Arc Welding (GTAW) — uses a tungsten electrode and inert shielding gas. Produces very clean welds with excellent fusion and penetration, but it is a slow process and difficult to master. View ESAB TIG welders
• Submerged Arc Welding (SAW) — the weldment is submerged in flux to shield it from oxygen and other atmospheric gases. A clean welding process with a high deposition rate; however, it is not a portable or field-ready process. For flux handling and storage guidance see our guide to managing welding fluxes

Of the types of welding used in pressure vessels, the one chosen for a specific project should suit the needs of the product the pressure vessel will store or produce and the pressure it will be kept at. In the oil industry, pressure vessels are kept at relatively low pressure, and while corrosion can be an issue, sanitary considerations are not. As a result, quicker and more expedient processes like SMAW and FCAW are frequently used.

In higher-specification projects with higher pressures, welding processes that render more consistent results, such as GMAW or SMAW, are more likely to be used. In industries where both pressure and sanitation are a major concern, the root pass — which will be in contact with the product — is very likely to be welded using GTAW, as it provides a far cleaner and smoother weld than any other process. Given the amount of welding that a pressure vessel requires, however, this is likely to be quite time-consuming if done manually. Mechanised welding processes like orbital welding can help increase the speed of GTAW welding used in pressure vessels. For guidance on managing WPS and PQR documentation for pressure vessel welding procedures, see our WeldCloud Notes guide.

Using Orbital Welding to Weld Pressure Vessels

Orbital welding was developed due to the need to create high-quality welds on hydraulic and aviation tubing for aerospace projects. The cylindrical nature of these welds posed a challenge for manual welders, and the changes in pressure as planes changed altitude tended to cause leaks. Orbital GTAW successfully produced high-quality welds that maintained integrity through these intense pressure changes. In the time since, it has been used to create equally high-quality welds in process piping and pressure vessels for the petroleum, biopharmaceutical, semiconductor, and other industries.

Orbital welding is just one of many types of welding used in pressure vessels, but it is by far the best choice for pressure vessels with a high requirement for cleanliness and strength. The biopharmaceutical and semiconductor industries provide two examples of applications that benefit from the consistency and hygiene of orbital welding. The AMI range from ESAB — the world's broadest and most advanced range of automated orbital welding equipment — covers the full spectrum from small-diameter, thin-wall high-purity tube welding through to heavy-application cladding and narrow-gap wire-fed pipe welding. As the requirements for manufacturing products become more stringent and regulatory bodies become less tolerant of accidents, the value of GTAW orbital welding in pressure vessel construction will only grow.

Related Resources

• The benefits of GTAW wire feed pipe welding for high-specification projects
• What is zirconiated tungsten used for
• Understanding the basics of the stainless steel cladding process
• TIG tungsten size: how to choose the right electrode
• What is the difference between cladding and overlay
• Flux-cored and metal-cored wire guide
• Managing welding fluxes: storage, baking and handling
• WPS and PQR management with WeldCloud Notes