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Research Papers: Codes and Standards

Ranking of Creep Damage in Main Steam Piping System Girth Welds Considering Multiaxial Stress Ranges

[+] Author and Article Information
Marvin J. Cohn

Fellow ASME
Intertek AIM,
3510 Bassett Street,
Santa Clara, CA 95054
e-mail: marvin.cohn@intertek.com

Fatma G. Faham

Intertek AIM,
3510 Bassett Street,
Santa Clara, CA 95054
e-mail: fatma.faham@intertek.com

Dipak Patel

Intertek AIM,
3510 Bassett Street,
Santa Clara, CA 95054
e-mail: dipak.patel@intertek.com

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received July 2, 2015; final manuscript received March 16, 2016; published online April 28, 2016. Assoc. Editor: Allen C. Smith.

J. Pressure Vessel Technol 138(4), 041202 (Apr 28, 2016) (10 pages) Paper No: PVT-15-1147; doi: 10.1115/1.4033077 History: Received July 02, 2015; Revised March 16, 2016

A high-energy piping (HEP) asset integrity management program is important for the safety of plant personnel and reliability of the fossil plant generating unit. HEP weldment failures have resulted in serious injuries, fatalities, extensive damage of components, and significant lost generation. The main steam (MS) piping system is one of the most critical HEP systems. Creep damage assessment in MS piping systems should include the evaluation of multiaxial stresses associated with field conditions and significant anomalies, such as malfunctioning supports and significant displacement interferences. This paper presents empirical data illustrating that the most critical girth welds of MS piping systems have creep failures which can be successfully ranked by a multiaxial stress parameter, such as maximum principal stress. Inelastic (redistributed) stresses at the piping outside diameter (OD) surface were evaluated for the base metal of three MS piping systems at the piping analysis model nodes. The range of piping system stresses at the piping nodes for each piping system was determined for the redistributed creep stress condition. The range of piping stresses was subsequently included on a Larson–Miller parameter (LMP) plot for the grade P22 material, revealing the few critical (lead-the-fleet) girth welds selected for nondestructive examination (NDE). In the three MS piping systems, the stress ranges varied from 55% to 80%, with only a few locations at stresses beyond the 65 percentile of the range. By including evaluations of significant field anomalies and the redistributed multiaxial stresses on the outside surface, it was shown that there is a good correlation of the ranked redistributed multiaxial stresses to the observed creep damage. This process also revealed that a large number of MS piping girth welds have insufficient applied stresses to develop substantial creep damage within the expected unit lifetime (assuming no major fabrication defects). This study also provided a comparison of the results of a conventional American Society of Mechanical Engineers (ASME) B31.1 Code as-designed sustained stress analysis versus the redistributed maximum principal stresses in the as-found (current) condition for a complete set of MS piping system nodes. The evaluations of redistributed maximum principal stresses in the as-found condition were useful in selecting high priority ranked girth weldment creep damage locations. The evaluations of B31.1 Code as-designed sustained load stresses were not useful in selecting high priority creep damage locations.

Copyright © 2016 by ASME
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References

Figures

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Fig. 1

Circumferential stress due to pressure—elastic and inelastic

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Fig. 2

Axial stress due to pressure—elastic and inelastic

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Fig. 3

SHOH connection creep crack

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Fig. 4

Auxiliary piping connection creep crack

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Fig. 5

MS-1 piping system isometric

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Fig. 6

MS-1 range of as-designed piping SL stresses

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Fig. 7

MS-1 range of as-found piping stresses (redistributed maximum principal stresses)

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Fig. 8

Grade 22 base metal creep rupture data—log stress versus LMP curves

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Fig. 9

Log stress versus LMP curves and range of stresses for the MS-1 as-designed piping analysis

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Fig. 10

MS-1 range of as-found piping stresses (redistributed maximum principal stresses for weldments)

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Fig. 11

Log stress versus LMP curves and range of stresses for the MS-1 as-found piping weldments

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Fig. 12

MS-2A piping system isometric

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Fig. 13

MS-2A MS girth weld crack at the west SHOH reducer connection

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Fig. 14

MS-2A range of as-found piping stresses (redistributed maximum principal stresses)

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Fig. 15

Cross section of MS-2B boat sample

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Fig. 16

MS-2B microcracks at 25 mm depth (MAG: 125×, 5% Nital Etch)

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Fig. 17

MS-2B range of as-found piping stresses (redistributed maximum principal stresses)

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