Joint Efficiencies and Radiography in Pressure vessels 

Joint efficiency is a factor required in all head and shell calculations that accounts for how closely a finished weld joint approximates the quality of the seamless parent material. Without further inspection it is assumed the welded joint is weaker than the material around it due to potential defects such as porosity, slag inclusions, and others. Shell thickness and therefore weld quantity is increased to account for this reduction in strength. Code welders following a qualified weld procedure are tested to weld a finished joint that maintains 100% of the parent material strength, but without further testing the allowed strength of a production joint is reduced to 70%. For some design conditions, such as lethal service, the Code requires the designer to specify full radiography. However, when not required, the designer can specify optional radiographic examination to increase joint efficiency and reduce the required thickness of shells and heads. The designer weighs the material and welding costs against inspection costs to determine which course is best suited for the application.  The figures below show the ASME VIII-1 joint efficiency values based on Type 1 joints (butt joints fully welded from both sides or equivalent) and degree of radiographic examination. The information is generated using the radiography logic diagrams and samples from Part 7 of PTB-4-2013 ASME Section VIII – Division 1 Example Problem Manual – the PTB-4 ‘E7.1’ through ‘E7.4’ example numbers are indicated where applicable. Radiography test is a nondestructive testing method of inspecting some hidden cracks in the material by using radio magnetic radiation to penetrate through the materials. The purpose is to ensure the structural integrity of the weld joint. As per ASME, there are four types of Radiography test (RT), i.e. RT1, RT2, RT3 and RT4. Joint efficiency is the number used to define welded joint strength which basically depends on the RT type. This Joint efficiency plays a vital role in determining the thickness of the pressure vessel components in Mechanical calculation. For RT1 and RT2 joint efficiency is 1.0, for RT3 efficiency is 0.85 and for RT4 efficiency is 0.70. The Weld Joint Category is how each weld on a Pressure Vessel Engineering is classified to a Joint category, based on the criticality. As per ASME there are four types of weld joint categories, Category A, Category B, Category C, and Category D. Now we can 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 will be selected based on the type of weld joint category A or B or C or D. In turn the type 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 how the Radiography test, Joint efficiency and Weld Joint category are interlinked as per ASME Sec VIII Div.1. To give an example, we have detailed the RT type with Joint efficiency based on weld category. Category A and D butt welds shall be fully radiography (RT1) hence the Joint efficiency is 1.0. Category B and C butt welds shall be spot radiography (RT3) hence the joint efficiency is 0.85. This is because longitudinal joints are more critical than circumferential joints. In general, heads connecting with shell will fall on Category B (RT-3 Spot Radiography), however in case of hemispherical head, the head with shell joint will fall on Category A (RT-2 full radiography), because this joint is more critical or it will be under double stress.