Influence of Bottomed Out Spring Hanger Support on Steam Generator Nozzle Weld

Abstract of the technical paper presented at:
SMiRT 25 Conference
Charlotte, North Carolina, USA
August 4-9, 2019

Prepared by:
Khalid Chaudhry
Canadian Nuclear Safety Commission

Abstract

Variable spring hangers are widely used in the nuclear industry as piping supports. The design of variable spring hangers allows the support to be flexible and accommodate thermal expansion and contraction of pipes when they shift from cold to hot position and vice versa.

One such variable spring hanger in the primary heat transport system piping was found to be ‘bottomed out’ in a CANDU nuclear power plant (NPP). In its ‘bottomed out’ state, the spring hanger was not able to accommodate the pipe thermal expansion and, as a result, acted as a rigid restraint. By acting as a rigid pipe restraint, the variable spring support experiences an increased load, and the resistance to pipe thermal expansion also exerts extra loads to the neighbouring pipe supports and equipment. In the affected CANDU NPP, the bottomed out support lies in close proximity to the steam generator nozzle; therefore, the nozzle and its weld will also experience increased loads that were not considered during the design phase.

The full paper will provide the details of structural integrity analysis of heat transport system steam generator nozzle weld to the primary system piping for as “designed” and hanger ‘bottomed out’ conditions. The analysis results will demonstrate the increased loads, experienced by the nozzle and its weld, are acceptable. The effect on fatigue ‘usage factor’ of the nozzle, along with a revision of this ‘usage factor’ for the hanger ‘bottomed out’ condition will be demonstrated.

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