Basic Equation for Hoop Failure Stress

Understanding Basic Equation for Hoop Failure Stress

• Used to calculate the failure stress level of a pressurized pipe containing a longitudinally oriented crack or defect. The length is taken as L, along the pipe axis, even if the corrosion pits are not lined up along the pipe axis.
• Developed on the basis of rather sharp flaws, whereas corrosion usually produces rather blunt defects. These two factors tend to add conservatism to the analysis.

Figure 1 shows an example of how corrosion might occur and the parameters of metal loss used in the RSTRENG analysis.

• L is the axial length of corrosion
• d is the maximum depth of corrosion
• wt is the pipe wall thickness
• A is the corrosion profile area projected into the axial plane

Figure 2 shows how the corrosion area, A, is defined as a parabola for the B31G Criterion.

Values of the Equation Factors were established by Committee Consensus

• “M” was defined as (1 + .8L2/Dt).5
• “S” the flow stress was assumed to be 1.1 x SMYS
• To simplify the analysis of the area of metal loss, “A” was defined to be the area of a parabola that presumed the deepest pit was exactly in the middle of the projected length of corrosion. The metal loss then becomes A = (2/3)Ld
• A0 is the corrosion length times full wall thickness (Lt)

Figure 2 below shows how the area of projected metal loss A, is defined as a parabola for the initial B31G Criterion.

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