Classification of Pressure Vessel Welded Joints

In our daily sales work, customers often inquire about ultrasonic flaw detectors and probes. They usually tell us which type of welded joint they need to inspect. Today, let’s take a closer look at the classification of welded joints.

In order to specify different requirements for factors such as joint misalignment, heat treatment, nondestructive testing, and weld size, GB150 classifies welded joints between pressure-bearing components of a vessel into four categories—A, B, C, and D — according to their position, the structural type of the connected components, and the stress level, as shown in Figure 1.

1. Category A welded joints include the longitudinal welds of cylindrical sections (including nozzles) and conical shells (excluding multilayer wrapped vessel plates), the circumferential welds connecting spherical heads to cylindrical shells, all welded seams within formed heads (both dished and flat), as well as butt joints between inserted nozzles or flanges and the shell.
2. Category B welded joints include the circumferential welds of cylindrical shells, the joints connecting the small end of a conical head to a nozzle, the joints connecting long-neck flanges to shells or nozzles, the butt joints between flat covers or tube sheets and cylindrical shells, as well as the circumferential butt joints between nozzles. However, those already specified as Category A joints are excluded.
3. Category C welded joints include the non-butt joints connecting torispherical heads, flat covers, or tube sheets to cylindrical shells; the joints connecting flanges to shells or nozzles; the lap joints between inner heads and cylindrical shells; as well as the longitudinal welds of multilayer wrapped vessel plates. However, those already specified as Category A or B joints are excluded.
4. Category D welded joints include the connections of nozzles (including manhole cylinders), flanges, reinforcing rings, and similar components to the shell, except for those already specified as Category A, B, or C joints.
5. Category E welded joints refer to the connections between non-pressure components and pressure components.

Note: The classification of welded joints is based solely on their location within the vessel rather than on the structural form of the joint. Therefore, when designing welded joint configurations, the importance of the vessel, design conditions, and welding conditions should be taken into consideration. As a result, welded joints of the same category may adopt different structural forms under different vessel conditions.