Failure behavior of carbon steel pipe with local wall thinning near orifice

Monotonic four-point bending tests were conducted using pipe specimens having an orifice undergoing local wall thinning. The effects of local wall thinning on the fracture behaviors of pipe were investigated. Local wall thinning was machined on the inside of pipes in order to simulate erosion corrosion metal loss. The configurations of the eroded area were l = 100 mm in axial length, d/t = 0.5 and 0.8 in thickness ratio, and 2θ = 180° in angle. The area undergoing local wall thinning was subjected to either tensile or compressive stress. Failure type could be classified into ovalization, local buckling, and crack initiation, depending on thickness ratio, and stress at the eroded area. Three-dimensional elasto-plastic analyses were also carried out using the finite element method, which is able to accurately simulate fracture behaviors. Failure analysis map was constructed for pipes with or without orifice based on the results of finite element analyses in order to investigate the effect of orifice on the failure behaviors.     

Piping response testing associated with pipe rupture

EPRI has sponsored an experimental program in the pipe whip impact and pipe rupture and depressurization areas. Sixteen pipe whip tests were performed with 3 in Schedule 80 (or 10) carbon steel pipes impacting on rigid target or concrete slab. The major testing parameters include distance, impact location, pipe rupture location, and concrete slab thickness and strength. The piping crushing at impact correlates with impact force and target response behavior. Conservatism was established by comparing measured and calculated impact forces. The pipe rupture and depressurization tests were carried out using 6 in stainless steel and carbon steel pipes under either PWR or BWR fluid conditions. These tests are of axial crack with initial machined-in surface flaw. It was found that pipe rupture would occur only if a long unstable through-wall crack was embedded in a sufficiently long unstable part-through crack (in the pipe wall). All other flaw configuration tested led to pipe leakage only. Reaction forces were measured which show conservatism of simplified method for fully ruptured condition. No good crack propagation information was obtained.     

Crack growth and fracture behaviour of pipes with circumferential defects under internal pressure and superimposed alternating bending load

Pipes made of 20 MnMoNi 5 5 steel and an MnMoNiV special melt, with an external diameter of 800 mm, wall thickness of 47 mm and length of up to 5500 mm, were provided with circumferential defects of defined length and depth. The pipes were loaded by internal pressure and a superimposed alternating bending moment. During the tests, deformation and crack growth were determined in the wall thickness and circumferential direction, and these were compared with calculated values. Pipes with an outer diameter of 226 mm and a wall thickness of 20 mm were used to investigate the leak-before-break behaviour in the dynamic sphere. These pipes also were made of 20 MnMoNi 5 5 steel and an MnMoNiV special melt, and were loaded with internal pressure and an alternating bending moment. The excitation took place at the resonance frequency of the pipes. The pipes also contained circumferential defects of defined length and depth.     

Fracture and deformation behaviors of tee pipe with local wall thinning

Monotonic four-point bending tests were conducted using tee pipe specimens having local wall thinning. The effects of local wall thinning on the fracture behaviors of tee pipes were investigated. Local wall thinning was machined on the inside of pipes in order to simulate metal loss due to erosion corrosion. The configurations of the eroded area were l = 100 mm in eroded axial length, d/t = 0.5 and 0.8 in eroded ratio, and 2θ = 90° in eroded angle. The area undergoing local wall thinning was subjected to either tensile or compressive stress. It was found that the type of fracture could be classified into ovalization, local buckling, and crack initiation, depending on pipe shape, eroded ratio, and stress at the eroded area. Fracture behaviors of the tee pipes were compared with those of straight pipes. Three-dimensional elasto-plastic analyses were also carried out using the finite element method, which was able to accurately simulate fracture behaviors.     

Crack initiation, crack growth and fracture behaviour of large diameter pipes with circumferential defects under internal pressure and superimposed alternating bending load

Pipes made of steel 20 MnMoNi 5 5 and MnMoNiV-special melt having an external diameter of 800 mm, wall thickness of 47 mm, and length of up to 5500 mm were provided with circumferential defects of defined length and depth. They were loaded by internal pressure and a superimposed alternating bending moment. During the tests deformation and crack growth were determined in the wall thickness and circumferential direction. Pipes with an outer diameter of 226 mm and a wall thickness of 20 mm were used to investigate the leak-before-break behaviour in the dynamic sphere. These pipes were also made of steel 20 MnMoNi 5 5 and a MnMoNiV-special melt and were loaded with internal pressure and an alternating bending moment. The excitation took place at the resonance frequency of the pipes. The pipes also contained circumferential defects of defined length and depth.     

Proof of the fracture resistance of the pipes in the THTR 300 plant using high temperature burst tests with X 20 CrMoV 12 1 components

An adequate safety margin was demonstrated by fracture mechanics investigation between postulated flaws in components and the critical crack length. These calculations were experimentally verified by pipe burst tests under service-like conditions of the THTR-300 for the provided X 20 CrMoV 12 1 in the begin-of-life condition.The tests show that flaws with a length of the pipe diameter and a depth of 90% of the wall thickness

• - oriented axially in the base metal, and

• - circumferentially in the weld

surely satisfy the leak-before-fracture criteria. The strength behaviour of the pipes can be described conservatively within the relevant technical region by common fracture mechanics failure criteria except the plastic instability criteria.     

Experimental studies of 4-inch pipe whip test under BWR loca conditions

Pipe whip tests or jet discharge tests have been performed at the Japan Atomic Energy Research Institute, which simulate the instantaneous circumferential guillotine break of primary coolant piping in nuclear power plants. The present paper describes the results of the pipe whip tests using test pipes of 4 inch diameter, under the BWR LOCA conditions, which were performed from 1979 to 1981. The tests were carried out at an initial pressure of about 6.8 MPa and an initial temperature of about 285°C.The test pipe was 114.3 mm (4 in) in diameter, 8.6 mm in thickness and 4500 mm in length. The four pipe whip restraints used in the tests were the U-bar type of 8 mm in diameter and fabricated from Type 304 stainless steel. The experimental parameters were the clearance (30, 50 and 100 mm) and the overhang length (250, 400, 550 and 1000 mm).The main purpose of these tests is to investigate the effects of the clearance and the overhang length on the pipe whip behavior. It has been clarified from the test results that a smaller clearance and a shorter overhang length causes the deformation of the pipe and restraints to be minimized, and the test pipe collapses near the setting point of the restraints with the overhang length of 1000 mm.     

Fracture strength and behavior of carbon steel pipes with local wall thinning subjected to cyclic bending load

The power plant piping is designed to withstand seismic events using the design fatigue curve. However, the fatigue strength of a pipe with local wall thinning caused by erosion/corrosion is not clear. To evaluate the fatigue strength of pipes with local wall thinning, low cycle fatigue tests were conducted on 100A carbon steel pipes with local wall thinning. In load controlled tests on these pipes, ratcheting deformation was observed, and the fatigue strength became lower than that of cracked pipes. In displacement controlled tests, the fatigue strength of eroded pipes with 100 mm in eroded axial length, 0.5 in normalized eroded depth and 90° in eroded angle was almost equal to that given by the design fatigue curve in ASME B&PV Code Sec. III. To evaluate the local strain range in the maximum wall thinning area, the finite element analysis was conducted on the eroded pipes in the displacement controlled tests. It is concluded that the Mises strain range in the maximum wall thinning area and the low cycle fatigue curve can be used to conservatively estimate the low cycle fatigue life of an eroded pipe and the validity of estimated results can be confirmed experimentally.     

Crack growth measurements on components under cyclic loading: comparison of different methods

The cyclic crack growth behaviour was measured by means of d.c. potential drop, a.c. potential drop, ultrasonic and crack-opening displacement (COD) methods. The methods were applied to component tests on straight pipes with an outer diameter of approximately 800 mm and a wall thickness of approximately 50 mm. The pipes were subjected to constant internal pressure (about 15 MPa) and either an alternating (or pulsating) or a quasi-static bending moment using d.c. potential drop, a.c. potential drop, ultrasonic and flaw-opening (COD) methods. The efficiency of the particular methods has been proved by comparison with fractographical analysis of the fracture surfaces.     

Verification test results of a cutting technique for the ITER blanket cooling pipes

For replacement of the first wall (FW) of the international thermonuclear experimental reactor (ITER), cutting and welding tools for the cooling pipes must be able to access a pipe from the surface side of the FW and cut/weld the pipe from the inside the cooling pipe (inner diameter: 42.72 mm, thickness: 2.77 mm). The cutting tool for the pipe end is required to cut a flat plate circularly from the surface side of the FW (cutting diameter: approximately 44 mm, plate thickness: 5 mm). To determine the specifications for both the tools and the blanket hydraulic connections, the ITER Organization (IO) and the Japan Domestic Agency (JADA) conducted research and development activities regarding the FW replacement. This paper describes the current status of the development of cutting tools for the cooling pipe connection.     

Fatigue behavior of pipes containing multiple flaws in inner surface

In this work was studied the growth behavior of multiple cracks in the inner surface of pipes. The fatigue tests were performed using two kinds of test pipes, i.e., the straight pipes and bend pipes of AISI Type 304L stainless steel, having 320 mm in outer diameter and 35 mm in thickness approximately.The crack growth curves obtained by the fatigue tests were compared with the analytical curves of two kinds of crack growth prediction methods. One method is based on the ASME Boiler and Pressure Vessel Code, Sec. XI. Another method is based on the procedure in which the crack growth formula is applied to both the surface and thickness directions. The analytical crack growth curves predicted by the ASME Code are conservative for the test results of the straight and bend pipes. However the results of bend pipe test suggest that the procedure of the ASME Code may give an unconservative fatigue life under the certain condition.On the other hand, the test results of the straight pipes can be evaluated reasonably and those of the bend pipes can be evaluated conservatively by the latter method.     

Comparison of experimental and finite element analytical results for the strength and the deformation of pipes with local wall thinning subjected to bending moment

Fracture behaviors of pipes with local wall thinning are very important for the integrity of power plant piping system. In this study, monotonic bending tests without internal pressure are conducted on 48.6 mm diameter Schedule 80 (thickness, 5.1 mm) STS370 full-scale carbon steel pipe specimens. Fracture strengths of locally wall-thinned pipes were calculated by elasto-plastic analysis using finite element method. The elasto-plastic analysis was performed by FE code ANSYS. We simulated various types of local wall thinning that can be occurred at pipe surface due to coolant flow. Locally wall thinned shapes were machined to be different in size along the circumferential or axial direction of straight pipes. We investigated fracture strengths and failure modes of locally wall thinned pipes by four point bending test. And, the allowable limit of pipes with local wall thinning was investigated. In addition, we compared the simulated results by finite element analysis with experimental data. The failure mode, fracture strength and fracture behavior obtained from FE analyses showed well agreement with experimental results. From the test results, we identified three types of failure modes into ovalization, local buckling and crack initiation. These failure modes could be classified according to thinned depth, thinned length and thinned angle of a pipe. For locally wall-thinned specimens, maximum moments (M_max) were estimated by using the net-section stress criterion. Pipes with local wall thinning can be estimated using σ_u instead of σ_f because of 1.19σ_f  σ_u. Also, the axial strain affects failure modes occurred on local wall thinning. the allowable limit of local wall thinning for carbon steel pipe used can be given as follows; in the case of M_max ≥ M_y, if 10 ≤ l < 25 mm, d/t can be allowed to about 55%, and if 25 ≤ l < 100 mm, d/t can be allowed to about 50%. Also, if 100 ≤ l ≤ 120 mm, d/t can be allowed to about 29%.     

Outline of nuclear piping research conducted in Italy

Following an early phase of limited activity at the University of Pisa on small stainless steel pipes containing axial cracks, in 1981 ENEA, the Italian Committee for Research and Development of Nuclear Energy and Alternative Energies, has started a massive research campaign on fracture of carbon and stainless steel piping containing through and part-through cracks loaded either under pressure or in bending. The purpose of the program was to develop a better understanding on pipes fracture behaviour in order to set new design criteria more realistic, yet conservative, than the guillotine break and prepare acceptance criteria for in-service flaws particularly under the growing pressure of IGSCC that has merciless affected worldwide practically any BWR piping system.The analysis of more than 100 tests carried out at CISE research centre, in Milano, on 4 inch, 6 inch, 8 inch and 10 inch pipes has indicated that unstable fracture requires at least 150° through wall crack under ASME maximum design stresses. The leak area, before instability takes place, is always less than 10% of the net cross section area of the pipe. This has led ENEA to consider a 10% break area as a reasonable value to calculate jet forces. Further, it was found that the net section collapse load criterion by far underestimates the actual collapse load and that 360° part-through cracks tend to switch from ductile to brittle failure mode of a pipe loaded in bending.Further work is planned for the next 3 years including high temperature tests, stainless steel weldments and HAZ tests, high compliance tests and eventually burst tests. Besides the ENEA's research program, Ansaldo AMN, the Italian Nuclear Architect Engineer, is developing theoretical studies and codes to treat the problem of pipe fracture.     

Simplified analysis of shrinkage in pipe to pipe butt welds

Generally some shrinkage is typical of butt welding of pipes. Shrinkage due to butt welding could be more pronounced and significant in thin wall stainless steel pipes because the thermal expansion coefficient is roughly one and half times that of carbon steel. An axisymmetric finite element evaluation of hoop shrinkage associated with circumferential butt welds in thin wall stainless steel pipes was performed. Actual shrinkage data for a larger (24 in. diameter, 0.375 in. wall thickness) pipe and a smaller (4 in. diameter, 0.237 in. wall thickness) pipe were utilized. The results indicate that very localized residual stresses in excess of yield strength produced during cooldown of metal in the weld and heat affected zones cause redistribution of the stresses. A simplified elastic–plastic analysis approach was developed with adjustments for section modulus and Poisson’s ratio, and the strains due to radial shrinkage were calculated for inside and outside surfaces of the pipe at the weld center line. From the strain point of view, the strain values in the circumferential direction were about 1.4% for the larger size pipe and 3.4% for the smaller size pipe. The strain values in the axial direction were 2.5% for the larger pipe and 5.9% for the smaller pipe. It is concluded that these levels of strains are not detrimental in nature. However, for the smaller pipe they are on the high side and it is recommended not to use the pipe for elevated temperature service. Residual stresses were also calculated for inside and outside surfaces of the pipe at weld center line using a simplified elastic–plastic approach and a bilinear stress–strain curve and compared with published data indicating a general agreement.     

Experimental studies of PWR primary piping under loca conditions

The experimental program performed on AQUITAINE-II facility is directed to study the mechanical behavior of primary PWR pipes and the forces exerted on the neighbouring structures as a consequence of a breach opening. It is jointly financed by the Commissariat à l'Energie Atomique, Framatome, Electricité de France and Westinghouse. Some forty tests have been carried out with different pipe configurations (straight tube, elbow, S-or U-shaped tube) and different break types (single or double guillotine). The following aspects are investigated: (1) the dynamic behavior of the pipe and in particular the formation of a plastic hinge at the restraint; (2) impact function of a pipe on an energy-absorbing bumper; (3) lateral stability of both ends of a pipe, after a double-guillotine break.     

Acceptance Size of Erosion Thinning in Carbon Steel Pipes Subjected to Internal Pressure and Tensile Load

Structural integrity evaluation of local wall thinning caused by erosion is important for maintaining the integrity of piping systems in power generating plants. The pipe of interest is a STS 42 carbon steel pipe loaded by an axial load and subjected to an internal pressure. Acceptable thinning length and width were determined from the allowable size of circumfer- encial and axial cracks in pipes, and the wall thickness is determined from the local membrane stress rule. Thus the acceptable extent and depth of wall thinning were proposed. Double-ended fracture can then be prevented if the local wall thinning is kept within this acceptable size.     

Prediction of leak areas and experimental verification on carbon and stainless steel pipes

This paper presents a study on the pipe crack opening area which is a task in the frame of the six-years Pipe Test Program that ENEA (Comitato Nazionale per la Ricerca e lo Sviluppo dell' nergia ucleare e delle nergie lternative) started aiming to address some of the uncertainties related to Leak-Before-Break (LBB) concept.The study focuses on the opening generated in a pipe containing a through wall circumferential crack loaded in pure bending, trying to assess the relative leak area of paramount importance in any evaluation of dynamic effects, i.e. thrust forces and jet impingement, and leak detection systems capability as well.The purpose of this report is to check some available analytical tools against the experimental results obtained on stainless and carbon steel pipes.In particular two different methods are considered, that is:

1. (a) Tada/Paris formula,

2. (b) General Electric Engineering Approach.

Calculaion are compared with experimental results from pure bending pipe tests, run at room temperature.     

Experimental investigation of the failure behavior of notched wall-thinned pipes

The present study performed full-scale pipe tests using 100A Schedule 80 pipe specimens with simulated notched and circular wall thinning to investigate the failure behavior of notched wall-thinned pipes. The tests were conducted under both monotonic and cyclic bending moments at a constant internal pressure of 10 MPa at room temperature. The failure pattern, load carrying capacity, deformation ability, and fatigue strength of the notched wall-thinned pipes were evaluated by comparing results to those of circular wall-thinned pipes. The investigation showed that the effect of the type of thinning on the failure behavior was more sensitive under cyclic loading conditions than under monotonic loading conditions. The load carrying capacity of pipes with notched wall thinning was approximately the same or slightly less than that of pipes with circular wall thinning when the thinning area was subjected to tensile stress. However, when the thinning area was subjected to compressive stress, the load carrying capacity of pipes with notched wall thinning was greater than that of pipes containing circular wall thinning. The deformation ability and fatigue strength increased proportionally with the axial length of the thinning defect, and thus these properties were significantly reduced in notched wall-thinned pipes.     

Evolution of the structure of a gas–liquid two-phase flow in a large vertical pipe

The evolution of the structure of a gas–liquid flow in a large vertical pipe of 195 mm inner diameter was investigated at the TOPFLOW test facility in Rossendorf. Wire-mesh sensors were used to measure sequences of two-dimensional distributions of local instantaneous gas fraction within the complete pipe cross-section. The sensors own a resolution of 3 mm at a frequency of 2500 Hz. Superficial velocities were varied in a range covering flow regimes from bubbly to churn-turbulent flow. The distance between the gas injection and the sensor position was changed using a so-called variable gas injection system. It consists of six gas injection units, each equipped with three rings of injection orifices in the pipe wall (orifice diameter: 1 and 4 mm), which are fed from ring chambers. The gas flow towards these distributor chambers is individually controlled by valves. Measured bubble-size resolved radial gas fraction profiles reveal differences in the lateral migration of bubbles of different size starting from the injection at the wall. The evolution of bubble-size distributions allows to study bubble coalescence and break-up. The influence of the physical properties of the fluid was studied by comparing cold air–water experiments with steam–water tests at 65 bar.     

Evolution of interfacial area concentration in a vertical air–water flow measured by wire–mesh sensors

An application of wire–mesh sensors to obtain the interfacial area concentration in vertical pipes is presented as an alternative to the widely used multiple-tip electrical or optical fibre probes. The measuring data of a mesh sensor consists of a three-dimensional matrix of local instantaneous gas fractions measured at each crossing point of the wires and recorded as a time sequence. Bubbles are clearly distinguishable in this data matrix, since they represent regions of interconnected elements containing the gaseous phase. The method to deduce the interfacial area concentration from this data is based on a full reconstruction of the gas–liquid interface, where the interfacial area of each bubble is recovered as the sum of the surface area of all surface elements belonging to the given bubble. The new method can be applied to large bubbles with an arbitrary shape. To study the change of the interfacial area concentration along the pipe the distance between sensor and gas injection was varied. The axial development of the interfacial area density measured in the test pipe of 195.3 mm inner diameter was compared to the measurements carried out by Sun et al. [Sun, X., Smith, T., Kim, S., Ishii, M., Uhle, J., 2002. Interfacial area of bubbly flow in a relatively large diameter pipe. Exp. Thermal Fluid Sci. 27, 97–109] in a pipe of 101.6 mm diameter, which is the largest pipe for which interfacial area densities are presented in literature. An acceptable agreement was found, whereas deviations are consistent with the differences in the boundary conditions of both experiments.