What is Joint Efficiency?
Joint efficiency of welds can be defined as the reliability that can be obtained from the joints after welding. This coefficient can adopt values smaller than 1, and to some extent it can be said that the joint efficiency is a way of reducing the allowable stress of the material. Therefore, the joint efficiency depends on the level of Non-Destructive Examination (NDE) and the Category and Type of the welds joints of two pieces of equipment.
The joint efficiency is defined to account for different weld configurations, ability to transfer the loads to be withstood by the pressure vessel in different degrees and uncertainties in the quality of the joints. If a weld joint is inspected in its full length, defects (if any) will be detected as long as the NDE is properly carried out. Now, when only partial inspection is applied results could be defect free and this means that probably the rest of the vessel will be too, but there is not 100% certainty. In this case, due to the uncertainty by using only a partial inspection (not all welds have been inspected), the joint efficiency will be less than one and will decrease the allowable stress of the material.
The greater the level of NDE and the better joint preparation of the pieces to be welded, the more certainty about the condition of welds. Therefore, it is not necessary to penalize the thickness due to “uncertainties”. One of the first aspects to be taken into consideration when designing a pressure vessel is to define the joint efficiency for different parts of the equipment. In some cases the values of joint efficiencies will be already defined and in some others they should be selected according to the equipment type, operating conditions and the designer “know-how”.
The Full or Spot Dilemma
When not specified or when it is not a code requirement to apply an NDE throughout the entire length of the weld joint, it is up to the designer to apply a total (Full) or partial (Spot) NDE approach. Logically, this decision will result in different joint efficiencies depending on the alternative chosen, and thus will result in thinner or thicker plates. Designing with Spot approach will result in greater thicknesses compared to the Full approach; however, the cost of a Spot NDE is less than the cost of a Full NDE. The questions is, does the thickness reduction by applying Full NDE offset the increase in NDE cost?
There is no rule or sound engineering practice to determine whether to use either alternative, only recommendations that calls for the consideration of the designer. The decision should be solely based on the cost and materials availability.
If you want to know more:
ASME VIII | Design of Pressure Vessels
TEMA | Design of Shell & Tube Heat Exchangers
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