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.

Related Links

Pipeline HUB — User Resources External Corrosion Direct Assessment Procedure – RSTRENG Additional Information

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