investigation of the interaction between steam …703.1 investigation of the interaction between...

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Investigation of the Interaction between Steam Generator Tubes and

Foreign Objects Resulting in Tube Wear

Omar Alquaddoomi, Ivan Catton University of California, Los Angeles

Department of Mechanical & Aerospace Engineering Los Angeles, CA 90045, USA

[email protected], [email protected]

Heather Feldman Electric Power Research Institute 1300 West W.T. Harris Boulevard

Charlotte, NC 28262 [email protected]

ABSTRACT

Foreign objects in the secondary side flow of steam generators have caused significant damage to steam generator tubes. In each case, the foreign object interaction is induced by the secondary side water flow, which accelerates the object into contact with the tubes and forces object motion relative to the tube surfaces. The resulting tube wear may eventually lead to failure of the tube structure, leakage of the primary fluid, or a possible forced outage. The purpose of this project is to document foreign object motion in prototypical steam generator conditions. Computational fluid dynamics (CFD) and experiments are implemented towards this goal. Experiments have been done to observe and describe the motion of various foreign objects as situated under typical flow conditions in the outer tube bank region of the steam generator, just above the tubesheet. Such experimental results are necessary to both determine parameters that are used to calculate wear rates on the tube structures as well as to provide a physical validation for the CFD analysis.

1 INTRODUCTION

Steam generator tube wear caused by foreign objects is a continuing challenge for the efficient operation and maintenance of nuclear power plants. A great deal of effort is focused on identifying foreign objects within the secondary side of the steam generator, determining if the foreign object is potentially damaging, and if so, predicting the extent of the damage and the rate of wear on the steam generator tube structures. Several existing methods are available to identify if a foreign object is in a steam generator. The more difficult problem is to reliably predict continuing rate of wear, thereby providing an estimate of how long the steam generator can continue to operate before the wear becomes unacceptable. The decision to continue operation or to remove the foreign object thus depends on wear prediction: developing an accurate and robust model of the tube wear rate for a specific foreign object within given steam generator conditions.

Models [1] to predict tube wear have been developed based on rough approximations of local flow magnitudes, turbulence intensity, and object geometry. An immediate improvement

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Proceedings of the International Conference Nuclear Energy for New Europe, Portorož, Slovenia, Sept. 8-11, 2008

to the existing wear rate estimation models is thus to provide empirical values for these parameters obtained from experimental and numerical simulation of the steam generator flow conditions and interaction of the foreign object. This research work is directed towards such a wear rate model improvement, based on the information obtained from two sources: computational simulation of flow in the steam generator and observations of foreign object motion within an experimental test section.

The steam generator of a PWR plant encompasses a wide variety of thermal and flow

conditions under normal operation. Flow near the tubesheet is typically single-phase flow, whereas at higher elevations the flow may be predominantly two-phase. A large majority of the foreign objects have been located at the bottom of the steam generator near the tubesheet. Therefore, attention is focused on the axial locations just above the tubesheet. The region of interest for the study is further limited after observing that many tube wear events occur near the tube bundle periphery. Flow from the downcomer results in a maximum cross-flow velocity magnitude at the tube periphery. The region of interest is centered about the tube periphery in the radial direction, and just above the tubesheet in the axial direction (Figure 1).

Full Scale Steam Generator

Region of Interest

Tubesheet

Tube

Inner Shroud

Downcomer

Flow DistributionPlate

Figure 1: The Region of Interest within the Steam Generator

2 COMPUTATIONAL STUDY

The design of the experimental test section was evaluated using CFD and changes in the design were made a priori to fabrication. This evaluation was done by comparing the numerical results from the test section model to the results of a more realistic steam generator model. SC/Tetra [2] is a comprehensive CFD program developed by Cradle Software and is used exclusively within this work. It uses a finite element model based fluid dynamics solver to compute flows in two and three-dimensional domains.

The basis for the experimental test section design is a steam generator with 0.6875 inch diameter tubing and a pitch of 0.98 inch in a rectangular pattern. The downcomer window is 12 inches high. The distance from the shell to the shroud is 2.82 inches and the distance from the shroud to the first row of tubes is 5 inches. It is not feasible to simulate the flow in the entire domain of the steam generator; thus, a sub-section of the steam generator is chosen as a preliminary flow domain and gradually increased in size to allow for more realism in the simulation. The boundary conditions used for the steam generator flow simulations are derived from the ATHOS/SGAP software [3].

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A computational domain of 30 degrees was created and used for the simulation of the flow in the steam generator design. This computational model is the basis for comparison with the normal square tube bundle configuration of the experimental test section design. The 30-degree section is determined to be a sufficiently large domain to be used as the basis for comparison with the much smaller (5 by 10 array) test section flow domain. The area of interest here is located at the midpoint of the model, i.e. at the region around the horizontal line bisecting the domain: the “0-degree” radius.

Figure 2: Steam Generator periphery, 30-degree section. Magnitude of velocity contours for the 0-degree (normal square configuration) simulation (top pair) and the velocity magnitude

for the 45-degree (rotated square configuration) simulation (bottom pair). Left images are viewing the tube array from the top (left), at a distance of about 3 in. above the tubesheet; right

images are in the plane bisecting the domain, perpendicular to the top view.

The flow in this region is observed to be predominantly radial, with maximum velocity

located between the tubes. This is the representative flow pattern for this region of interest and therefore the flow pattern against which the experimental test section design is compared. An additional CFD simulation is obtained for the region of the steam generator where the flow at the periphery is incident on the tube array at a relative angle of 45 degrees. The computational volume of this simulation is identical in size to the simulation above (centred about the positive x-axis), the difference being that this region is centred about a 45-degree radius of the steam generator. Figure 2 shows the velocity magnitude for the 0-degree (or normal square configuration) simulation (left) and the velocity magnitude for the 45-degree (or rotated square configuration) simulation (right). The simulated flow at the 0-degree incidence angle differs substantially from the flow simulated at the 45-degree incidence angle.

2.1 Experimental Design Study using CFD

Figure 3 shows the CFD results for the experimental test section design. The size of the test section model is determined by three parameters: the height of the downcomer, the location of the flow distribution plate, and the number of tubes included. A five row by ten column array of tubes in a normal square pattern was chosen for the initial experimental test section design. The most apparent difference between this design and the steam generator geometry is the limitation in the circumferential extents and approximation of the cylindrical plan as rectangular. CFD simulations of the test section model are done for both the normal square configuration and

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the rotated square configuration. Figure 3 shows a velocity magnitude contour plot for the normal square case and for the rotated square case.

Figure 3: Flow simulation showing magnitude of velocity about 5 inches above the tubesheet in the test section for a normal square tube array configuration (left) and the flow for a

45-degree rotated square array (right) under the same conditions. Units are [m/s].

The rotated array simulation was computed using the same flow conditions and

downcomer geometry as the normal square array. Major differences are observed, including the flow distribution at the tube bundle periphery. These results indicate that the flow fields vary significantly with circumferential location, and that independent experiments are required to determine the influence of each flow on foreign object interaction.

Evaluation of each of these two test section simulations with their respective steam generator flow simulations provides a means of validating the test section design. The flow pattern is similar: the maximum flow speeds occur between the tubes (in the pitch regions). Also of note is that the magnitude of the flow velocities in each case agrees well, given the same boundary conditions (the exception being the location of the walls). Similarly, the flow pattern and magnitudes observed in the rotated square test section model agree well with the corresponding region of interest in the simulation of Figure 2.

3 EXPERIMENTAL STUDY

3.1 Construction and Installation of the Test Section

The interior volume of the actual test section exactly matches that of the computational test section model; the actual test section is a 1-to-1 scale with the computational model. Figure 4 shows a photo of the assembled test section and a schematic of the design. The base of the test section models the tubesheet of the steam generator. The base plate is fastened to the test section using removable bolts and is sealed with a rectangular rubber gasket; the base plate may be removed for modification to the tube bundle, adjustment of the foreign object, or to switch one tube bundle configuration with a different pattern. The inlet diffuser is designed to allow the flow to smoothly develop from a round profile in the pipe to a rectangular profile at the downcomer entrance, minimizing flow separation and enhancing the uniformity of the inlet flow. Two flow outlets are provided, one facing in the axial direction, one in the radial direction. Each exit is provided with a flow contraction body to bring the flow back to the pipe profile smoothly. Downstream of each exit contractor are 3-inch globe valves that provide control over the fraction of the flow exiting in each direction; this provides control over the exit pressures in each direction and can be adjusted to match the conditions used in the computational simulations or to approximate different flow conditions as required for various steam generator designs. The flow to the test section is supplied and controlled by the water flow loop facility. This system is

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composed of the water reservoir, the pump, the supply line, two return lines, and a flow bypass line. The water reservoir acts as an accumulator for the flow and supplies the necessary inlet pressure to the pump.

Figure 4: Tube array installed and configured for measurements (left). The experimental test section schematic (right).

3.2 Fabrication and Installation of the Foreign Objects

Design information for each of the three foreign objects was obtained from both the Steam Generator Degradation Database and actual foreign objects recovered from steam generators, both of which were supplied by EPRI. Table 1 lists the dimensions and weight of each of the foreign objects used in the experiments. In order to highlight the foreign object for viewing by the digital cameras, the surface of each object was painted using a white enamel base. Each of the foreign objects is placed in the same initial location inside the test section at the start of each experimental run, between the first and second columns of tubes.

Table 1: Specifications for the foreign object models used in the experiments

Foreign Object Dimensions [inches] Mass [grams]

Wire 4.0 long x 0.04 diameter 1.41

Braidwood Weld Splatter 2.0 x 1.29 x 0.068 thick 19.7

Flexitallic Gasket Piece 3.5 x 0.178 x 0.02 thick 0.77

3.3 Data Acquisition and Analysis

A non-invasive optical technique called Diode Array Velocimetry (DAV) is used to measure the average instantaneous velocity of the water within the test section. A single component of velocity may be measured at a single location in the test section. To date, measurements of the radial component of velocity at the periphery of the tube bundle have been made. These measurements provide an estimate of the flow speed that is incident on the foreign objects during testing.

Two high-speed (120 frames per second) CCD digital video cameras are configured to view the foreign object and record it’s motion during the experiments. One camera views the

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object in the direction parallel to the tube axis, from below the test section, while the other camera views the object in the span-wise direction (looking through the gap between the tube columns) from the side of the test section. A custom data processing and analysis technique is used to determine the location of the foreign objects in each of the images. All the image preprocessing operations are done in the Adobe Photoshop program [4]. The preprocessing operations can be predefined and executed automatically for a set of raw input images. The result is a set of processed images that can be analyzed. The purpose of the image analysis is to output the pixel coordinates of the geometric center of the foreign object within each input image. To do so, the program must locate the pixels corresponding to the object and calculate the centroid of the pixel distribution. A custom program is used to do this, implementing the Java programming language and a freely available image-processing package called ImageJ [5]. The output provides the position of the foreign object, relative to the fixed tubes and base plate locations, as a function of time. Information such as the frequency of motion, vertical and horizontal displacements, mode of contact with the tubes (sliding or impact), and other results can be determined from the output.

4 RESULTS AND DISCUSSION

Experiments conducted for each of the foreign objects measure the flow speed in the vicinity of the foreign object and the location of the foreign object, in three directions, as a function of time. Flow speed measurements were made using the DAV probe at a single location upstream of the first row of tubes for each of the flow rate settings used in the experiments. The probe is located 3 inches above the base plate vertically, at the midpoint of the distance between the center tube and the left center tube in the span-wise direction, and approximately 2 inches upstream of the centerline of the first row of tubes in the stream-wise direction. Flow speed measurements were averaged using an ensemble of 25 samples per second over a 20 second interval. Local velocity and motion data were recorded for each of the three foreign objects at six flow rates. The axial and radial displacements of each foreign object are determined from the two components of object displacement that each camera view provides.

Figures 6 shows the axial displacement and frequency response data for the wire foreign object over the range of flow speeds. Displacement and frequency data were measured for each foreign object in each of the three directions. The right column of Figure 6 is the frequency spectra generated by taking the discrete Fourier transform of the corresponding displacement function (to the left). The frequency spectra show the relative magnitude of each component of frequency in the time-dependent displacement function. Peaks at a specific frequency indicate a dominant motion of the object at that frequency and in that particular direction. Non-zero values across a wide band of frequencies indicate more random motion. Furthermore, the area under the spectrum curve gives an indication of the “energy” of the object motion: more area indicates faster and larger motions. The range of frequencies is limited to a maximum measurable value of 60 Hz. Several characteristics were observed concerning the behaviour of each foreign object during the tests.

The wire object exhibits the largest amplitudes of displacements. This displacement is almost entirely manifested in the axial direction, with maximum displacements exceeding 4 diameters of the wire. For the case with the flow velocity of 0.64 m/s, there are very slight displacements. As the flow speed increases, the wire rapidly oscillates, reaching its maximum energy at a flow speed of 1.72 m/s. At this speed a clear peak in the frequency spectrum is observed at a value of 9 Hz. At 1.90 m/s, the motion changes form, and the spectrum indicates that the frequencies of motion breakdown into a very gradual displacement across the entire interval, and higher frequency oscillations.

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0.64 m/s

0.95 m/s

1.50 m/s

1.72 m/s

1.90 m/s

Figure 6: Axial displacement as a function of time (left) and corresponding frequency spectra (right) for the wire foreign object motion.

The energy of the motion at this speed is also relatively high. The wire did not experience

significant motion in the radial direction; instead, the wire is pressed up against the tubes most of the time. This is the case for even the higher velocity flow conditions. The great majority of the energy of the motion is observed to be axial, with contact between the wire and the tubes in the sliding mode. This sliding interaction occurs predominantly at frequencies in the range between 5 and 12 Hz, with an average of 9 Hz.

There is virtually no motion observed in the weld splatter (sheet metal); for the most part the object remains pinned up against the second tube row for the entire duration of the

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experiment. At the maximum flow speed, the object does move in the span-wise direction; this motion is a rapid translation to a new position where the object again comes to rest. Such motion is important to modeling the object interaction, but a larger time interval of observation is required to characterize this phenomena. This behaviour is not necessarily characteristic of the interaction elsewhere in the tube bundle, possibly even one or two rows further into the test section. Additional tests are required to investigate the interaction of this object.

The flexitallic gasket object exhibits a lack of motion similar to the weld splatter; it appears to be pinned up against the upstream tube row. Again, replacement of the flexitallic gasket at a new location in the tube bundle or varying the initial orientation of the object may result in significantly different interactions.

5 CONCLUSION

In order to make accurate predictions of the nature and extent of tube wear, it is necessary to provide any wear rate model with accurate “interaction” input parameters as they depend themselves on the system under consideration. This work provides an empirical basis for these parameters so that such an improved wear rate model can be developed. Computational studies first provided an understanding of the flow field and a validation for the design of the experimental test section. Experimental techniques have been developed and used to measure the amplitude and frequency of motion for several different prototypical foreign object types, as well as the local velocity in the vicinity of the object during motion. Observations of the different foreign objects also allow some qualitative conclusions to be made concerning the interaction. For these tests, the wire object exhibits much higher interaction energy than the gasket or splatter material. This may indicate that the wire type object will cause a greater amount of wear and, when identified in such a configuration, is a high priority for removal. Further work is planned in order to more exactly determine the influence of the various foreign objects on tube wear, including objects of different types and new configurations of tube bundles and steam generator designs.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support of EPRI for providing the guidance and funding for this work.

REFERENCES

[1] Wear Predictions of Foreign Objects in Steam Generators. EPRI, Palo Alto, CA: 2005. 1011798

[2] SC/Tetra Thermofluid Analysis System & Unstructured Mesh Generator; Software Cradle Co., Ltd., Osaka 532-0011 Japan. (software program)

[3] Steam Generator Analysis Package (SGAP) and ATHOS v3.0, EPRI, Palo Alto, CA: 2005. 1003728 (software program)

[4] Adobe Photoshop 7.0, Adobe Systems Incorporates, 1990-2202. (software program)

[5] Rasband, W.S., ImageJ, U. S. National Institutes of Health, Bethesda, Maryland, USA, http://rsb.info.nih.gov/ij/, 1997-2007. (software program)

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