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Research Papers: Materials and Fabrication

Leak Before Break: Studies in Support of New R6 Guidance on Leak Rate Evaluation

[+] Author and Article Information
J. P. Taggart

 Serco Assurance, Birchwood Park, Warrington, Cheshire WA3 6GA, United Kingdom

P. J. Budden

 British Energy Generation Ltd., Barnett Way, Barnwood, Gloucestershire GL4 3RS, United Kingdom

J. Pressure Vessel Technol 130(1), 011402 (Jan 17, 2008) (6 pages) doi:10.1115/1.2826422 History: Received February 09, 2006; Revised December 19, 2006; Published January 17, 2008

The concept of leak-before-break (LBB) is often used in safety cases for pressure systems, particularly, in the nuclear industry. An important factor in making a LBB case is in the prediction of the leak rate of fluid through a crack. This paper presents a summary of a program of work, which had the aim of improving guidance on leak rate evaluation for the LBB procedures in the R6 defect assessment methodology. Methods of calculating leak rates have been reviewed, and this has led to a crack morphology model being proposed, which represents single-phase isothermal compressible flow through a crack. In the crack morphology model, the flow is assumed by default to be fully rough turbulent, and the effective roughness to vary between a local roughness value for narrow cracks and a global value (i.e., the overall crack contours) for wide cracks. The effect of pressure drops due to changes in the flow direction at crack turns has also been included. Calculations using the model show that the friction factor relation due to Spence (1991, “Leakage Flow Through Small Cracks—Report of Second Stage of Experimental Work  ,” unpublished) gives better agreement with measured flow rates than that due to Button (1978, “Gas Flow Through Cracks  ,” ASME J. Fluids Eng., 100, pp. 453–458), which tends to overestimate the flow rate for the examples studied. The inclusion of an inertial pressure term arising from changes in overall flow direction appears to be justified.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figure 3

Tests on a fatigue crack in a wild plate: measured and calculated flow rates as a function of pressure difference

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Figure 2

(a) Schematic of flow around two crack turns: crack width much less than roughness amplitude (Wc<Rglamp). (b) Schematic of flow around several crack turns: crack width much greater than roughness amplitude (Wc>Rglamp).

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Figure 1

1∕λ versus log10(W∕Ra) for various friction factor relations

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