What is Welding Joint Efficiency?
Joint Efficiency is a term used in ASME codes for the design of welded joints in pressure vessels. Joint Efficiency refers to the strength of a welded joint with respect to the strength of the base metal.
A Joint Efficiency of 1.00 (100%) indicates that the weld has the same strength of the base metal, and it is assumed as if it is seamless.
Concurrently, an efficiency of 0.50 (50%) indicates that the joint has half the strength of the base metal.
Joint efficiency is an important factor to be considered in the design stage. When designing the minimum thickness of the shell or the design pressure, the strength of the base metal should be multiplied by the Joint Efficiency value. According to ASME, the strength of the welding electrode is only considered during the selection of the electrode. Its strength should be more than or equal to the strength of the base metal. If this condition is guaranteed, then the design should consider the weakest link in the chain, which is the base metal.
With that being said, ASME proposes statistical values for this parameter in Table UW-12 based on the type of joint, and the extent of examination. Joint efficiency depends upon the type of weld joint and the extent of the NDT. A summary of welding joint types, and NDT scope defining the various joint efficiencies according to ASME BPVC Section VIII Div. 1 is shown in the below table.
Type of Joint
The selection of weld type, such as butt or lap joint, depends on the Joint Location, and Service Restrictions. The location of joints in equipment is grouped into categories. These categories are described in UW-3 and further shown in Figure UW-3. For example, longitudinal joints are under category A, circumferential is under B, the weld in a welded neck flange is under C, and welds connecting nozzles to shells are under D.
Service Restrictions include Potential Hazards, Temperature, Pressure, and Direct Firing. Based on the service and joint category, the joint type selection is restricted in UW-2. This restriction should provide the best option against the anticipated types of stresses. ASME Table UW-12 includes the following joint types:
- Butt Joint – Double Welding
- Butt Joint – Single Welding without backing strip
- Butt Joint – Single Welding with backing strip
- Lap Joint – Double Fillet
- Lap Joint – Single Fillet with Plug Welds
- Lap Joint – Single Fillet without Plug Welds
- Corner Joint
- Angle Joint
Extent of Radiography
In some cases, it is not practical to perform radiography to the whole length of welded joints. Therefore, ASME proposes a selection criterion in UW-11 to whether perform a Full Radiography, Spot Radiography, or No Radiography whatsoever. Full Radiography implies that the full length of the welded joints is to be inspected by Radiography. Spot Radiography is to examines sample spots of the weld to represent the whole weld. The extent of examination is selected in UW-11 based on:
- Type of joint
- Lethality of fluid
- Thickness of joint
- Design pressure
- Type of Welding, for thick plates (Electrogas/Electroslag)
If spot radiography is permitted, then according to UW-52, at least one spot should be examined for each 50ft increment of the weld length. The length of each spot should be at least 6”. Moreover, at least one spot per increment per welder should be examined to represent the weld quality of each welder. Regardless of the number, the spots are recorded as a representation of the weld quality of the 50ft weld increment. If the spot is accepted, then the represented weld increment is also accepted. If not, then two additional spots need to be examined. If both additional spots are accepted, then the whole increment is accepted, but the rejected spot still has to be repaired. On the other hand, if any of the additional spots were rejected, then the whole increment is consequently rejected. The fabricator has two options, either the whole rejected weld increment needs to be removed and welded again or to be fully examined and repaired at the defective locations only.
With the previous being said, Ultrasonic Testing can be used instead of radiography in case the material thickness is equal or greater than 1/4”. The same rules apply.
After deciding the required type of joint and extent of examination, joint efficiency can be obtained from Table UW-12, as mentioned. It can be seen in the Table that some types of joints are not Applicable for radiography, such as fillet and angle joints. This is due to the geometry of their arrangement. The varying thickness of metal will cause difficulty in interpreting the RT Imagery. In addition, these types of joints are usually designed for components that are under low loading. Therefore, these are deemed noncritical and do not justify the number of resources being put to perform the examination on them.
Weld joint Category
The term “Category” as used herein defines the location of a joint in a vessel, but not the type of joint.
(1) Category A. Longitudinal welded joints within the main shell, communicating chambers, transitions in diameter, or nozzles; any welded joint within a sphere, within a formed or flat head, or within the side plates of a flat-sided vessel; circumferential welded joints connecting hemispherical heads to main shells, to transitions in diameters, to nozzles, or to communicating chambers.
(2) Category B. Circumferential welded joints within the main shell, communicating chambers, nozzles, or transitions in diameter including joints between the transition and a cylinder at either the large or small end; circumferential welded joints connecting formed heads other than hemispherical to main shells, to transitions in diameter, to nozzles, or to
(3) Category C. Welded joints connecting flanges, Van Stone laps, tubesheets, or flat heads to the main shell, to formed heads, to transitions in diameter, to nozzles, or to communicating chambers any welded joint connecting one side plate to another side plate of a flat-sided vessel.
(4) Category D. Welded joints connecting communicating chambers or nozzles to main shells, to spheres, to transitions in diameter, to heads, or to flat-sided vessels, and those joints connecting nozzles to communicating chambers (for nozzles at the small end of a transition in diameter, see Category B).