ASME B31G-2012 pdf – Manual for Determining the Remaining Strength of Corroded Pipelines
ASME B31G-2012 pdf – Manual for Determining the Remaining Strength of Corroded Pipelines.
(Ii) Flow stress is a concept relevant to fracture mechanics and is used in the Level 1, Level 2, and Level 3 evaluations. It is not a property specified in a material grade or finished product standard. Research indicates that it may be defined variously as given below.
(1) Sfl0 for plain carbon steel operating at temperatures below 250°F (120°C) may be defined by S1101 =
1.1 X SMYS. Sfl0r shall not exceed SMTS.
(2) SfI0 for plain carbon and low-alloy steel having SMYS not in excess of 70 ksi (483 MPa) and operating at temperatures below 250°F (120°C) may be defined by
= SMYS + 10 ksi (69 MPa). S110, shall not exceed SMTS.
(3) for plain carbon and low-alloy steel having SMYS not fri excess of 80 ksi (551 MPa) may be defined by S0 = (SYT + SUT)/2, where SYT and S- are specified at the operating temperature in accordance with the ASME Boiler and Pressure Vessel Code, Section 11, Part D; applicable pipe product specification; or room temperature strength multiplied by the temperature derating factor specified by the applicable construction code. Linear interpolation of strength values is allowed between listed temperatures.
(c) This document does not prescribe which definition for flow stress should be used where more than one definition applies. Where more than one definition applies, the various definitions produce acceptable though not necessarily identical results when used with any given evaluation method. It is noted that S110 was defined as 1.1 x SMYS in previous editions of B31G. This definition remains an inherent element of the Level 0 assessment and is recommended with the Level 1 assessment performed in accordance with para. 2.2(a).
(d) Only the specified nominal wall thickness shall be used for the uncorroded wall thickness when conducting a Level 0 evaluation. If kflowfl with confidence, the actual uncorroded wall thickness may be used with a Level 1, Level 2, or Level 3 evaluation, with a suitable adjustment of the hoop stress due to internal pressure.
(e) Pipe body material may he considered to have adequate ductile fracture initiation properties for purposes of this Standard if the material operates at a temperature no colder than 100°F (55°C) below the temperature at which 85% shear appearance is observed in a Charpy V-notched impact test.
Metal-loss corrosion anomalies indicated by inline inspection may be evaluated by a Level I or Level 2 evaluation method. The user is cautioned against overstating the precision of evaluations applied with flaw dimensions indicated by inline inspection without adequate calibration or verification of actual flaw sizes by investigations carried out in the field.
1.12 Flaw Interaction
The methods described herein are suitable for evaluating isolated areas of metal loss. Corrosion may occur such that multiple areas of metal loss are closely spaced longitudinally or transversely. If spaced sufficiently closely, the metal loss areas may interact so as to result in failure at a lower pressure than would be expected based on an analysis of the separate flaws. The following guideline is suggested with reference to Fig. 1.12-1, based on limited testing and analysis:
(a) Flaws are considered interacting if they are spaced longitudinally or circumferentially from each other within a distance of 3 times the wall thickness (3t). Interacting flaws should be evaluated as a single flaw combined from all interacting flaws.
(b) Flaws are considered noninteracting if spaced outside of the above dimensions. Noninteracting flaws should be evaluated as separate flaws.
Care should be exercised when grouping or clustering anomalies indicated by inline inspection for purposes of evaluating interaction during the prioritization process. Consideration should be given to minimum thresholds of metal loss for reliable detection and sizing, minimum thresholds for reporting, and the expected mode of coating failure (e.g., localized failure versus disbondment over large areas). Methods employed for clustering of inline inspection anomalies should he validated by field verification of actual flaw dimensions and spacing.
1.13 Flaw Orientation
Corrosion caused by disbondment of continuous wrapped coatings may exhibit a helical pattern. If the helical pattern lies at an angle less than 45 deg to the pipe axis, the overall length of the corroded area indicated as L1 in Fig. L13-1 shall be considered in the evaluation. If the helical pattern lies at an angle of 45 deg or greater to the pipe axis, it is sufficient to consider the most severe longitudinal section through the corroded area having a length L2 in Fig. 1.13-1.