Uticaj ispitivanja probnim pritiskom na nastanak i rast prslina u zavarenim spojevima opreme pod pritiskom

Abstract

The applied stress level which will produce plastic deformations in WM is lower than the BM yield strength, not to mention effect of over-loading. One should also keep in mind that welded joints are places with non-uniform stress distribution because of stress concentration even if residual stresses are released. Therefore, simultaneously introduced different influencing factors will produce difficulties for stress calculation of welded joints, so that the design of pressure vessels becomes more complicated than usual. Sometimes, it is inevitable to test the produced pressure vessel, using different techniques, needed for stress calculations and assessment of weldments behaviour under different service conditions. Reversible HPP (RHPP) “Bajina Basta” is in operation since 1982, [2]. Taking into account that the failure of the most critical part, the pipeline (Fig. 1), would cause water overflow of the surge hub in the surrounding area, and the collapse of the entire plant, it is clear that special attention is still need to prove its structural integrity [3]. One of the most important effects is the proof testing, i.e. over-pressure, which is the main concern in this paper. Namely, there is a history of welded joint failures due to proof testing, e.g. spherical pressure vessels for liquefied natural gases (LNG) [4], caused by unnecessary plastic deformation acting as crack initiation and propagation mechanism. Some details are given below, as taken from monograph [3].. It is shown, using different approaches, that the effect of over-pressure is detrimental from the point of view of structural integrity of pipeline in RHPP. Two approaches, one more sophisticated (micromechanical modelling), and the other, simpler one (FAD), lead to the same conclusion, that proof over-pressure is unnecessary challenge to the safety of the critical component in a very important power plant.