Investigation of discrepancies in waterhammer structural integrity analysis simulation and actual vibration testing

An abstract of the technical paper presented to:
17th International Conference on Nuclear Engineering
(ICONE 17 – paper 75100)
Brussels, Belgium
July 12–16, 2009

Prepared by:
Khalid Chaudhry and Andrei Blahoianu
Canadian Nuclear Safety Commission (CNSC)
Ottawa, Ontario, Canada

Abstract

While verifying the primary heat transport (PHT) piping design for increased waterhammer loads due to sudden relief valve opening, it was discovered that linear piping analysis program, which was relied upon extensively in the past, predicted overly conservative results.

By overestimating the piping stresses, the stress results did not satisfy the ASME code, section III, subsection NB-3652 equation 9 limits for level B service loading. During the course of investigation to meet ASME code limits, the licensee carried out a series of controlled actual waterhammer tests on thoroughly instrumented PHT piping and recorded the measured piping displacements.

Waterhammer pressure-time histories created from these actual tests were then used as input into the standard linear piping analyses to compare analysis simulation results with the actual measured displacement data. It was observed that the analysis simulation results overestimated the piping displacement results by a large margin, i.e., by a factor of five.

A further insight into the analysis results indicated the presence of a single, the so-called “killer” mode of vibration which accounted for nearly all of the PHT piping displacement response to test waterhammer loading. On a hypothetical basis, a restraint was applied in the direction of vibration of the pipe and the linear analysis was repeated.

It was discovered that the simulated analytical piping response using a modified restraint had a much better match with the displacement results obtained during the actual test. From this hypothetical restraint application, it was inferred that friction between the supports and the pipe is the key ingredient which dampens the pipe oscillations and hence a lower response during the test than the linear analysis which does not consider the friction between the pipe and its guide support.

This paper further investigates the contribution of structural damping, friction effects between the pipe and its supports (use of contact elements), fluid structure interactions and issues related to application of friction to carry out modified nonstandard analyses to better predict the piping response to waterhammer transient loading.

Acronyms

ASME – American Society of Mechanical Engineers
PHT – Primary Heat Transport

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