Pipe Whip Tests Research Articles

Modeling of Pipe Whip Phenomenon Induced by Fast Transients Based on Fluid–Structure Interaction Method Using a Coupled 1D/3D Modeling Approach

The sudden increase in the operating pressure of nuclear power plants (NPPs) is due to the water hammer phenomenon, which tends to produce a whipping effect that causes serious damage to the pipes and their surroundings. The mechanical response of these pipelines under the influence of such fast fluid transients can be estimated using the fluid–structure interaction (FSI) method. The computational time and expense are predominantly dependent on the number of finite elements developed in the model. Hence, an effective modeling technique with limited and efficient nodes and elements is desired to obtain the closest possible results. A coupled 1D/3D finite element modeling approach using the FSI method is proposed to determine the influence of fast transients on the mechanical pipe whipping behavior of gas pipelines in NPPs. The geometric coupled modeling approach utilizes the presence of both the 3D solid elements and the 1D beam elements sharing a local conjunction. The computational model is modelled for a pipe-to-wall impact test scenario taken from the previously conducted French Commissariat a l’Energie Atomique (CEA) pipe whip experiments. The results of displacement, stresses, and impact velocity at the 3D section featuring the elbow are compared for the change in the 3D solid length varied at the juncture of the elbow. The computed results from the Ansys FSI coupling method using the Fluent and Transient Structural modules provides fair validation with the previously conducted experimental results and correlates with the CEA pipe whip tests on pipe-to-wall impact models. Thus, the 1D/3D coupled modeling approach, which minimizes the area of the solid region by constricting it to the impact area with appropriate contact modeling at the junctures, can be considered in the future for decreasing the computational time and the creation of finite elements.

Pipe Whip: In-Plane Whipping of Bent Cantilever Pipes

The dynamic elastic-plastic behavior of a bent cantilever pipe subjected to an in-plane force pulse at its tip is described. A theoretical model based on a large deflection formulation of dynamic beam theory is described. This takes into account the plastic hardening-softening behavior which is characteristic of a pipe when it is subjected to large changes in curvature. This beam model was first formulated and applied by Reid et al. (1995b, 1996), who demonstrated that it was able to describe quite accurately the characteristics of freely whipping, straight cantilever pipes. The present paper extends this model to enable it to be applied to bent cantilever pipes and covers cases in which the bend angle is opened or closed. Previous attempts (Wang, 1991) to analyze these problems using a rigid, perfectly plastic model were not completely successful due to the inability to identify admissible modes of deformation for all circumstances. A systematic program of pipe whip tests has been carried out in UMIST on right-angled bent pipes. To illustrate the validity and use of the theoretical model, two typical cases are considered and the results compared with the experimental data extracted from high-speed films of the tests. Excellent agreement between the two is demonstrated.

Large-Deflection Analysis of Whipping Pipes. I: Rigid, Perfectly-Plastic Model

A theoretical model is constructed for a large-deflection analysis of a rigid, perfectly-plastic cantilever with a tip mass loaded by a transverse follower force pulse. The purpose of building and analyzing such a model is to provide an analytical basis for studying unconstrained motions of a whipping pipe. Such motion can arise from the failure of high-pressure piping systems and is consequently a safety-related phenomenon. A guillotine break in the pipe leads to the expulsion of a jet of high-pressure fluid, and the reaction force pulse exerted on the pipe causes it to accelerate and deform. Generally, the motion of the pipe consists of an initial transient phase and a modal, root rotation phase. The equations for the initial traveling hinge phase and root rotation phase are derived and solved numerically. Pipe kinematics and deformations are deduced and comparisons are made with observations from pipe-whip tests. Finally, the model is used to give indications of the variation in the zone of influence (i.e., the region where the whipping pipe could strike another pipe or other components) for different ratios of tip mass to pipe mass.

Experimental and analytical studies of pipe whip tests under PWR LOCA conditions

Abstract A series of pipe rupture tests has been performed at the Japan Atomic Energy Research Institute (JAERI) to demonstrate the safety of primary coolant circuits in the event of pipe rupture in nuclear power plants. Pipe whip tests and jet discharge tests have been conducted under boiling water reactor (BWR) and pressurized water reactor (PWR) loss-of-coolant accident (LOCA) conditions. The present paper describes the experimental and analytical results of the pipe whip tests performed under PWR LOCA conditions using 4, 6 and 8-inch test pipes. The tests were carried out at an initial pressure and temperature of 15.7 MPa and 325°C, respectively. Moreover, a dynamic analysis of pipe whip tests was carried out using the general purpose finite element program ADINA.

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.

Moment-rotation relationship considering flattening of pipe due to pipe whip loading

It is important to take flattening of pipe into consideration in order to obtain pipe deformation due to pipe whip loading. An experimental relationship between the flattening of pipe and the pipe surface strain was used to derive the moment-rotation relationship of whipping pipe. The derived moment-rotation relationship was used to calculate the pipe strain in the pipe whip tests using a simplified energy balance method. A comparatively good agreement was obtained between the analytical and experimental results.

Analytical studies of four-inch pipe whip tests under BWR LOCA conditions

The purpose of pipe rupture studies in JAERI is to perform model tests on pipe whip, restraint behavior, jet impingement and jet thrust force, and to establish a computational method for analyzing these phenomena. This report presents the analytical results of 4-inch pipe whip tests under BWR LOCA conditions. Dynamic response analyses were performed using the general-purpose finite element program ADINA. The test pipe was modelled by straight beam elements and the four restraints were modelled by a single truss element. The analytical results were compared with the experimental results. Impact time and maximum total restraint force showed good agreement with experimental results. On the other hand, pipe strain and pipe deflection could not be predicted so well. The reason for this is that the sliding of the restraint during the pipe whip movement cannot be considered in the analyses.

Blowdown thrust force under pipe rupture accident: I. Experimental evaluations of blowdown thrust force and decompression characteristics

Blowdown thrust forces and decompression characteristics were evaluated concerning the jet discharge or pipe whip tests with a 4-inch or 6-inch diameter pipe under PWR LOCA or BWR LOCA conditions related to pipe rupture accidents in nuclear power plants. This paper presents experimental evaluations of time-dependent and maximum blowdown thrust forces, and evaluations of decompression characteristics under instantaneous pipe rupture conditions. The following items are discussed: the peak value of the blowdown thrust force, the jet thrust coefficient for the maximum blowdown thrust force, the pressure recovery after break, and the relationship between the pressure undershoot of the sudden decompression and the decompression rate.

Nonlinear dynamic analysis of high energy line pipe whip

This paper describes a nonlinear dynamic analysis of TVA high energy line pipe whip tests using the ABAQUS-EPGEN code. The analysis considers the effects of large deformation and strain rate on resisting moment and energy absorption capability. The numerical results of impact forces, impact velocities, pipe strains, and reaction forces at pipe supports are compared to the TVA test data. The calculated pipe whip impact time and forces are also compared with those predicted using current industry practice. The calculated pipe support reaction forces are found to be in good agreement with the TVA test data except for some peak values at the very beginning of the pipe break. These peaks are believed to be due to stress wave propagation which cannot be addressed by the ABAQUS code. Both elbow crushing and strain rate have been approximately simulated. The effects are found to be important for pipe whip impact evaluation.

Experimental and analytical studies of four-inch pipe whip tests under PWR LOCA conditions

This paper presents experimental and analytical results of pipe whip tests performed under PWR LOCA conditions using a test pipe of 4-inch diameter and U-shaped restraints. In the tests, the effects of the overhang length on the pipe whip behavior of the pipe-restraints system were studied by measuring the strains and deformations of the test pipe and restraints, and the restraints forces. The equation for predicting the maximum strain at the outer surface of the pipe was derived using a static equilibrium condition. The calculated maximum strains at the outer surface of the pipe agree fairly well with experimental data. The dynamic response analysis of the pipe-restraints system was conducted by the finite element program ADINA. The applicability of the ADINA program to the pipe whip analysis is made clear through this analysis.

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.

ＢＷＲ・ＬＯＣＡ条件下の４ｉｎ．口径パイプホイップ試験 オーバーハング長さの効果

Pipe whip tests or jet discharge tests have been performed at the Japan Atomic Energy Research Institute, which simulate the instantaneous guillotine break of primary coolant piping in nuclear power plants. This paper describes the results of the 4-inch pipe whip tests (RUN 5407, 5501, 5504, 5603), under the BWR LOCA conditions, which were performed from 1979 to 1981. The test pressure was 6.8MPa and test temperature 285°C. In these tests, clearance was kept constant at the value of 100mm and overhang lengths were 250, 400, 550 and 1, 000mm, respectively.The main purpose of these tests is to investigate the effect of overhang length on pipe whip behavior. From the tests results, the pipe movement is effectively limited by the restraints if the overhang length is 250mm or 400mm. The deformation of the test pipe and restraints becomes large with increasing overhang length. It is clarified that the test pipe collapses at the setting point of the restraints with the overhang length more than 900mm.

配管・レストレント系の動的応答解析 ＰＷＲ‐ＬＯＣＡ条件４ｉｎ．口径管パイプホイップ試験の解析

This paper presents the results of the dynamic response analysis of the pipe-restraints system by the general purpose finite element program ADINA. The analysis was carried out for the pipe whip tests performed under the PWR·LOCA conditions using 4-in. test pipe. In the analysis, the test pipe was modeled by an assemblage of the beam elements with the isotropic elastic-plastic material properties and the restraints were represented by the truss elements with the nonlinear elastic material properties including gap effect.The following results are obtained through the analysis. (1) Pipe can be modeled with the beam elements, when the overhang length is short and, therefore, the flattening of a cross-section of pipe is small. (2) The steady state restraint force can be predicted by modeling the restraints with the truss elements.

ＰＷＲ・ＬＯＣＡ条件下でのブローダウン反力の過渡状態解析

The analytical results of blowdown characteristics and its thrust force were compared with the experiment, which were performed as pipe whip tests under the PWR·LOCA conditions on the hypothetical accident of guillotine break of pipes. The blowdown thrust force was obtained by the integral momentum equation about single-phase flow, homogeneous and segarated two-phase flow, assuming critical pressure at the exit if critical flow condition was satisfied. The following results are obtained:(1) The node-junction method is useful for the analysis of water hammer phenomena and of the blowdown thrust force.(2) The Henry-Fauske model for subcooled critical flow is effective for the analysis of the maximum thrust force under the PWR·LOCA conditions. The jet thrust parameter of analysis and experiment is 1.08.(3) The thrust parameter of saturated blowdown has the same one with the value under pressurized condition when the stagnant pressure is chosen as the saturated one.(4) The dominant terms of the blowdown thrust force in the momentum equation are the pressure and momentum terms, while the acceleration term is contributive just after break.