Pressure Vessel Weld Joint Category as per ASME Section VIII Div 1 UW-3

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Pressure Vessel Weld Joint Category as per ASME Section VIII Div 1 UW-3

Pressure Vessel Weld Joint Category is a very important aspect of Pressure Vessels Fabrication. Weld Joint Category classification is done on the basis of joint strength, In this classification, the weaker joint has given more concentration and it is included in Higher Category.

Before learning to Weld Joint Category, We must have basic knowledge of Longitudinal weld joint and Circumferential weld joint, So lets Learn this joint Types and then we will learn Weld Joint Categories.

As we know manufacturing pressure vessels is a very precious job. Because one mistake during manufacturing causes a big problem to the user as well as to the manufacturer. The manufacturing pressure vessel in a single sheet is not possible and it also costlier. So we required cut sheets to joint each other to get the desired pressure vessel. So qualitative welding of these sheets is a very important task.

Here we have two main types of weld joint.

  1. Longitudinal weld Joint
  2. Circumferential Weld Joint

Longitudinal Weld Joint :

Weld Joint is done along the length or along the longitudinal direction is called as Longitudinal Joint. This Joint is also Called as L – Seam Joint or Long Seam Joint.

The name itself describes about longitudinal joint that these weld joints are joining two sheets horizontally, vertically or in any direction. In short, we can say these weld joints are long joints which are joining two sheets. If we are joining two sheets in dish also a L-Seam joint.

Circumferential Weld Joint :

Weld Joint is done along the circumference or along the circumferential direction is called as Circumferential Joint. Circumferential weld joint is also known as C-Seam weld Joint or Circum weld joint.

A circumferential weld may be a sort of weld that’s wont to join two round objects around their circumference. In simple language, we can say C seam joint is used to join two sheets in the round or circumferential direction. E.g. joining of the Shell to Dish End. or Shell to Shell Joint.

A circumferential weld are often administered by many various sorts of welding processes, and a spread of fabric diameters and material types are often joined together by a circumferential weld. The weld area may be prone to corrosion. The inside of the tubing or pipe can collect debris round the weld area, increasing the danger of corrosion. Furthermore, the fabric properties of the tubing or pipe near the circumferential weld area might be altered during the weld process, allowing differing types of corrosion to occur around the weld, both on the within diameter and therefore the outside diameter. It may be necessary to apply a coating to the circumferential weld area after the welding has been performed to protect the weld.

Longitudinal Stress Vs Circumferential Stress:

While designing the pressure vessel consideration of longitudinal & circumferential stress is very important.

Longitudinal Stress:

Longitudinal stresses developed in the vessel’s shell by the wind and earthquake loads must be added to the longitudinal stresses attributable to the internal pressure.

Circumferential Stress:

circumferential stress, or hoop stress, traditional stress within the tangential (azimuth) direction. axial stress, a traditional stress parallel to the axis of cylindrical symmetry. Radial stress, the stress or a force in directions of coplanar with but perpendicular to the symmetry axis.

 By the below equation, we can understand the relationship between these stresses which effects on the weld joint.

σL = 2 x σC


σL = Longitudinal Stress

σC = Circumferential stress

From the above equation, we can learn longitudinal stress is always double circumferential Stress. Hence longitudinal weld joints have more stress or pressure, hence chances of vessel bursting are more on longitudinal weld joints than circumferential weld joints.

longitudinal joint, circumferential joint

As per ASME Section VIII Div. 01 UW-03, there are four types of weld joint categories

  1. Category A Weld Joint
  2. Category B Weld Joint
  3. Category C Weld Joint
  4. Category D Weld Joint

To help you better understand the weld joint category, we’ve taken a snap from ASME sec VIII Div.1 which shows the sort of Joint category as shown below,

weld joint category as per asme section viii div UW-3

Category A Weld Joint :

Longitudinal welded joints within the main shell of the vessel, a dish of the vessel, transition in diameter, or nozzles; any welded joint within a formed or flathead or within the side plates of the flat-sided vessel comes under category A.

In Short :

  1. All longitudinal welds joints in shell and nozzles
  2. All welds joints in heads, Hemispherical head to shell weld joint

All Category A Weld Joint is shown in the above figure and Highlighted by encircled A. So According to above figure Category A Weld Joint is for shell or dish longitudinal joints or L – Seam Joints and All the L-seam joints are coming under category A weld joint.

Category B Weld Joint :

Circumferential welded joints or C seam joints within the main shell of vessel communicating chambers, nozzles of the vessels, or transitions in diameter including weld joints between the transition and a cylinder at either the large or a small end; circumferential welded joints or c seam joints connecting formed heads other than hemispherical to main shells to transitions in diameter to nozzles or to communicating chambers.

In Short :

  1. All circumferential welds in shell and nozzles.
  2. Head to shell joint other than Hemisphere Head.

All Category B Weld Joint is shown in the above figure and Highlighted by encircled B. So According to the above figure All the circumferential joints or C-Seam joints will be in Category B weld joint.

Category C Weld Joint :

Welded joints in connecting flanges of vessel, Van stone laps, tube sheets, or flatheads to the main shell of vessel, to formed heads, to transition in diameter, to nozzles or to communicating chambers any welded joints connecting one side plate to another side plate of a flat-sided vessel comes under Category C.

In Short:

All Flange Weld Joints.

All Category C Weld Joint is shown in the above figure and Highlighted by encircled C. So According to above figure All Flange weld Joints coming under Category C weld joints.

Category D Weld Joint :

The welded joints of nozzles to main shell or dish comes under Category D. Manhole, inlet nozzle outlet nozzles, agitator entry nozzles joints to shell or dish are the example of the D category nozzle.

In Short :

All Nozzle Attachment Weld Joints.

All Category D Weld Joint is shown in the above figure and Highlighted by encircled D. So According to above figure All Nozzle attachment weld Joints coming under Category D weld joints.

This All about Category A, Category B, Category C, Category D weld joints in pressure vessels.

Interlinking Between Weld Joint Category, Join Efficiency and Radiography :

The American Society Of Mechanical Engineers (ASME) formed some rules regarding the manufacturing of pressure vessels.

Let us start out by briefly explaining those terms:

Radiography test is a non-destructive testing method of inspecting some hidden cracks in the material by using radio magnetic radiation to penetrate through the materials. The purpose is to make sure the structural integrity of the weld joint. As per ASME, there are four sorts of Radiography test (RT), i.e. RT1, RT2, RT3, and RT4.

Joint efficiency is that the number won’t define welded joint strength which basically depends on the RT type. This Joint efficiency plays an important role in determining the thickness of the pressure vessel components in Mechanical calculation. For Radiography Test 1 (RT1) and Radiography Test 2 (RT2), joint efficiency is 1.0, for RT3 efficiency is 0.85 and RT4 efficiency is 0.70.

The Weld Joint Category is how each weld on a vessel is assessed to a Joint category, supported the criticality. As per ASME, there are four sorts of weld joint categories, Category A, Category B, Category C, and Category D.

Now we will see how these three are interlinked as per ASME Sec VIII Div.1

As per ASME Sec VIII Div.1 the RT Type 1 or 2 or 3 or 4 are going to be selected supported the sort of weld joint category A or B or C or D., In turn, the sort of RT 1 or 2 or 3 or 4 will decide the Joint efficiency value 1.0 or 0.85 or 0.7. So this is often how the Radiography test, Joint efficiency, and Weld Joint category are interlinked as per ASME Sec VIII Div.1.

To give an example, we’ve detailed the RT type with the Joint efficiency supported weld category.

Category A and category D butt welds shall be fully radiography (RT1) hence the Joint efficiency are 1.0.

Category B and category C butt welds shall be spot radiography (RT3) hence the joint efficiency are 0.85.

This is because longitudinal joints or L seam joints are more critical than circumferential joints.

In general, heads connecting with shell will fall on Category B (RT-3 Spot Radiography), however, just in case of hemispherical head, the top with shell joint will fall on Category A (RT-2 full radiography), because this joint is more critical or it’ll be under double stress.

This all about Longitudinal Joint, Circumferential Joint, Longitudinal Stress, Circumferential Stress, Weld Joint Category, Weld Joint efficiency, and Radiography, we hope you will learn all concepts in detail.

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Imran Pinjara

Author of Book " Master in Fabrication Layout Development " Published worldwide. Founder and CEO of Let'sFab Educational Services. More than 8 Years of Professional Experience in Field of Pressure Vessel, Heat Ex changer, Storage Tanks, Piping and other Process Equipment Fabrication Industry. He had worked in many Fabrication Industry from small workshop to MNC Company. He had Completed PGDM in Process Piping Design and Engineering as per ASME B31.3 and Bachelor of Mechanical Engineering.