Research Papers: Materials and Fabrication

A Thermal Stress Mitigation Technique for Local Postweld Heat Treatment of Welds in Pressure Vessels

[+] Author and Article Information
Chunge Nie

School of Traffic and Transportation Engineering,
Dalian Jiaotong University,
Dalian, China

Pingsha Dong

Department of Naval Architecture and
Marine Engineering,
University of Michigan,
Ann Arbor, MI 48109
e-mail: dongp@umich.edu

1Work performed as an Exchange Ph.D student at University of New Orleans, New Orleans, LA.

2Corresponding author.

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received September 2, 2014; final manuscript received October 31, 2014; published online February 27, 2015. Assoc. Editor: Haofeng Chen.

J. Pressure Vessel Technol 137(5), 051404 (Oct 01, 2015) (9 pages) Paper No: PVT-14-1142; doi: 10.1115/1.4029097 History: Received September 02, 2014; Revised October 31, 2014; Online February 27, 2015

This paper introduces a novel method for effectively mitigating high thermal stresses caused during local postweld heat treatment (PWHT) of welds in pressure vessels on which traditional heating method such as bull's eye heating arrangement has been proven difficult in meeting Code requirements for avoiding “harmful” temperature gradients. The method involves the use of a secondary heat band (SHB) that strategically positioned at some distance away from primary PWHT heat band (HB) in terms of vessel characteristic length parameter Rt, where R is vessel radius and t wall thickness. The basic principles associated with the SHB based technique are first demonstrated on a simple straight pipe girth weld configuration. Then, applications for treating nozzle welds in more complex spherical vessel, cylindrical vessel, and at end of cylindrical vessel are presented. Finally, a set of recommended guidelines are provided for defining both the SHB size and location for performing local PWHT on welds in three major nozzle/vessel weld configurations.

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Fig. 1

Conventional local bull's-eye PWHT heating configuration allowed by codes and standards (e.g., Refs. [1,8]): (a) spherical nozzle weld; (b) cylindrical nozzle weld

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Fig. 2

A pipe girth weld for concept demonstration (axisymmetric model with linear elements): (a) girth welded pipe; (b) pipe geometry and local PWHT with three adjacent circumferential HBs

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Fig. 3

FE results for circumferential bend heating of a girth weld in a straight pipe: (a) axial thermal stress distribution with only HB1 being activated; (b) HB size effects on thermal stress distributions; (c) HB size effects on temperature distribution

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Fig. 4

Comparison of stress distributions on outer surface corresponding to case 1 and case 5

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Fig. 5

FEA model (local view) of a nozzle weld in spherical vessel and local PWHT heating arrangement: (a) nozzle and vessel dimensions; (b) definitions of PHB and GCB

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Fig. 6

Deformation mode and thermal stress (hoop) at nozzle weld area during local PWHT: (a) deformation mode; (b) hoop thermal stress

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Fig. 7

Thermal stress distributions due to circumferential band heating on a spherical vessel: (a) HB definition and temperature distribution; (b) thermal stress distribution on outer surface

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Fig. 8

SHB definitions for investigating SHB position effects on compressive stress generation at nozzle weld

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Fig. 10

Compassion of stress distributions (hoop stress) for three cases with PHB and SHB at different distance away from weld

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Fig. 9

Comparison of thermal stress distributions resulted from single band heating at different position from nozzle weld toe

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Fig. 11

Nozzle weld in cylindrical vessel: (a) geometry of a nozzle weld in cylindrical vessel; (b) FE model and PHB definition at nozzle weld

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Fig. 12

Comparison of thermal stress distributions along nozzle weld outer surface

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Fig. 13

Schematic diagram of local PWHT on a nozzle weld in a cylindrical vessel with two SHBs

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Fig. 14

FEA model and axial heating band definitions

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Fig. 15

Stress distributions on outer surface as a function of distance away from HB edge on cylindrical vessel

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Fig. 16

Temperature distribution and definitions of PHB and C-SHB

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Fig. 20

Heating band definitions: (a) doubling the baseline PHB size; (b) applying a rectangular SHB (R-SHB)

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Fig. 19

Comparison of thermal stresses along weld toe using conventional spot heating and SHB based local PWHT procedures

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Fig. 18

A nozzle weld near cylindrical vessel head: (a) vessel and nozzle dimensions; (b) primary heating band

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Fig. 17

Definitions of R-SHB



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