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Joint Efficiency
•Definition:
•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.
•How do we indicate “Joint Efficiency” as per ASME SEC 8? - “E”
•Why Joint Efficiency is so important?
•Example: A chain is as strong as its weakest link.
BREAKS
WEAKEST LINK
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Joint Efficiency Matters
•Note the Similarity Below:
WELD
• “The Weld can also be the weakest link”
•In simple term, joint efficiency is how much faith do we have in a weak link(Basically weld in our case). More
like confidence factors that we use during RBI’s.
•Joint Efficiency = Confidence on the weakest link
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Joint Efficiency “E”
•HOW CONFIDENT ARE WE IN THE QUALITY OF THE WELD ?
•E = STRENGTH OF THE WELD / STRENGTH OF PARENT METAL
•IF WELD IS AS GOOD AS PARENT METAL E = 1.00 / IF WELD IS WEAKER THAN PARENT METAL E = < 1.00
•HOW DO WE DETERMINE THE QUALITY OF THE WELD? BY USING RADIOGRAPHY.
•Joint Efficiency is based on radiography - Weld type / Category.
•Weld Type ?
•Weld Category?
•
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Weld Category / Type
•Category = Location Type = Geometry
Pressure Vessel Weld Joint Category is a very important aspect of Pressure Vessels
Fabrication. Weld Joint Category classification is done based on joint strength, In this
Welded From Both Sides.
classification, the weaker joint has given more concentration, and it is included in Higher
Category. Welded from one side with Removable
backing.
Backing that remains in place.
Welded from one side only.
Double fillet welded.
Single Fillet With Plug Welds.
Single Fillet Weld.
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Joint Efficiency “E” Let us see the simplified version of the table
•JOINT EFFICIENCY ‘E’ IS DETERMINED BY TWO THINGS: Table UW-12 as per ASME SEC 8
•1 - WELD TYPE Type Joint Category a)Full b)Spot c)None
1 A,B,C & D 1.00 0.85 0.70
•2 - DEGREE OF RADIOGRAPHY 2 A,B,C & D 0.90 0.80 0.65
3 A,B & C N/A N/A 0.60
LET US LOOK AT TABLE UW-12 from ASME SEC VIII:
LET US LOOK AT AN EXAMPLE:
Let us have a TYPE 2 joint, subjected to spot radiography.
What will be the JOINT EFFICIENCY?
E = 0.80
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Where you will be using this “E”?
•Joint Efficiency is one of the key factor in determining the thickness of the vessel, under the design values of
pressure and stress value(Maximum allowable stress)
•
•“E” replaced with 1, t = 0.172 in.
•Difference = 0.202 – 0.172 = 0.03 in = 0.762mm.
•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.
•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%.
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Examples:
Lot more considerations are applicable as per ASME Sec – VIII, the above snips are
examples used for discussions.
For Example:
1- All lethal services(UW-11) should be fully RT’ed and should be Type 1 welds.
2- How to take SPOT radiography and location for Spot radiography.
3- Naming’s based on radiography, RT-1, RT-2, RT-3 & RT-4- Usually this can be found on
the name plate.
4- Refer UW-12 for complete details.
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Considerations:
LETHAL SERVICE
Full Radiography
Categories B and C butt welds in nozzles and
communicating chambers that neither
exceed NPS 10 (DNS 250) nor 1-1/8 in.
(29mm) wall thickness do not require any
radiographic examination;
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