Experimental Evaluation of the Bending Load Effect on the Failure Pressure of Wall-Thinned Elbows

Abstract

During normal operating conditions, piping systems in nuclear power plants are subject to internal pressure and bending loads induced by deadweight, thermal expansion, and internal pressure. Thus, understanding the effect of bending load on the failure of wall-thinned elbows is important to understand failure behavior and to evaluate the failure pressure reliably. This study includes a series of burst tests using full-scale 4-in. schedule 80 elbow specimens with local wall-thinning under combined internal pressure and in-plane bending load. The results are compared with those tested under simple internal pressure only. In the tests, various circumferential thinning angles (θ/π=0.125, 0.25, 0.5, 1.0) and thinning locations (intrados, extrados, and full-circumference) were considered. Each specimen was initially subjected to a displacement controlled in-plane bending load, closing mode for extrados wall-thinned elbows, and opening mode for intrados wall-thinned elbows, and then internal pressure was applied up to the point of final failure. The results showed that the effect of in-plane bending on the failure pressure and failure mode was minor under all wall-thinning conditions. In addition, the dependence of failure pressure on the circumferential thinning angle and thinning locations was identical to that observed under simple internal pressure.

Low Cycle Fatigue Behavior of Elbows with Local Wall Thinning

“…The maximum values of the internal pressure (4 MPa) were limited due to the use of the hand pressure pump, as at high pressures it was impossible to avoid large pressure fluctuations. Similar problems were reported in [6], where both manual and high-pressure hydraulic pumps were used to pressurize elbows for low and high pressure levels, respectively. Therefore, higher as well as burst pressures were not investigated in the current experimental program.…”

“…Local wall-thinning due to flow-accelerated corrosion/erosion has been recently realized as one of the main degradation mechanisms. It was stated that the effect of the wall thinning on strength and integrity represents a significant issue, specifically for nuclear power plants, since wall-thinning has already led to the failures of carbon steel piping systems in several nuclear power plants in the past [6]. A serious concern is also associated with the operation, behavior and integrity of piping undergoing progressive wall thinning under extreme loading conditions [7], such as the 2011 massive earthquake in Japan.…”

“…There were several recent studies both experimental and numerical addressing the problem of structural integrity and life of carbon steel elbows under severe loading conditions [6][7][8][9][10][11][12][13][14][15][16]. Fatigue life was mainly considered around 50-300 cycles, as this range is representative of a typical earthquake loading.…”

“…The difficulties in prediction methodologies are largely attributed to numerous factors, which influence the strength and integrity of elbows with wall thinning defects [7]. Some of these factors are currently disregarded in design and evaluation procedures or incorporated through the use of large safety factors [6][7][8]. In addition, the current standards, which are oriented for the design of nuclear piping components [17,18], are mainly focused on carbon steel materials providing design fatigue curves for steel elbows in high and low-cycle fatigue regimes.…”

Effect of Ovality in Inlet Pigtail Pipe Bends Under Combined Internal Pressure and In-Plane Bending for Ni-Fe-Cr B407 Material

“…Besides shape irregularities, pipe diameter, wall thickness and bend radius also effect a change in stress in bend region [4]. However, the majority of the existing solutions have been developed either empirically based on test data or analytically based on a simple equilibrium stress field and yield criterion considering without the effect of ovality in pipe bends [5][6][7][8][9][10], thus the resulting solutions tend to under predict the actual limit loads, which lead to failure in bend region. Effect of ovality in pipe bend should be quantified and included in limit loads solution to avoid failure.…”