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Sticky Wickets: Correcting Trashrack Failure at a Pumped-Storage Plant

After the December 2005 failure of the 450-MW Taum Sauk Pumped-Storage plant, Ameren Missouri personnel discovered portions of the draft tube trashracks from the lower reservoir while cleaning out the tailrace. An investigation indicated the failures, brought on by corrosion-accelerated fatigue, occurred at the area experiencing the highest velocities during generation. A new trashrack design was developed, and the new trashracks were placed into service when the plant began operating in 2010. The trashracks have operated reliably, with no indications of cracking or degradation.

The lower portions of the trashracks at the 450-MW Taum Sauk pumped-storage plant failed during service. The insets show a bar from the lower remaining portion of the trashrack (on right) and several individual bars from the upper part of the remaining trashrack (on left).
The lower portions of the trashracks at the 450-MW Taum Sauk pumped-storage plant failed during service. The insets show a bar from the lower remaining portion of the trashrack (on right) and several individual bars from the upper part of the remaining trashrack (on left).

Background

The Taum Sauk plant has two 225-MW units, each with a draft tube with dual exits. Trashracks are located at the exit of the draft tubes into the tailrace of the lower reservoir, and each slot contains a single steel trashrack 16 feet high by 13 feet 3 inches wide.

During cleanout of the tailrace in 2006, bar sections from the steel draft tube trashracks were found. Inspection revealed that a similar section of the lower half of the draft tube trashracks was missing in the left trashrack of each unit.

Maintenance records documented a similar failure occurring several years earlier, when detached trashrack sections were discovered during an outage inspection. At that time, the trashracks were replaced with racks of the original design. The time of failure for these replacement trashracks could not be determined, but it most likely occurred after the two pump-turbines were replaced with higher-capacity runners in 1998 and 1999.

Draft tube trashracks are subjected to significant hydraulic forces. The manufacturer of the new Taum Sauk runners, American Hydro Corporation (now Weir American Hydro), confirmed that its computational fluid dynamics modeling showed the failure location for the trashracks occurred in the region of highest velocities during generation.

Failure forensics

A preliminary examination of the failure sections, performed by Hydro Performance Processes Inc. (HPPi), suggested the failure may have occurred in three phases. Initially, fatigue cracks propagated from both sides of the lower ends of the vertical bars, likely due to excitation from flow-induced vibrations. The cracks met in the middle of the bars, and the lower ends of the bars were sufficiently weakened to allow detachment at the bottom. Presumably, hydraulic forces tore away the upper portion of the detached section and swept the piece downstream.

Subsequent evaluations by Ameren Missouri metallurgists confirmed the failure as corrosion-accelerated fatigue.

Structural Technology Corporation (STC) performed modal analysis testing on an undamaged trashrack at Taum Sauk. Results were used to identify likely natural frequencies of the trashrack that could be excited by flow-induced vibrations. This testing used the impact-response technique,1,2 with frequency response functions measured by tri-axial accelerometers at 119 measurement points with 114 impact points. Modal frequencies and shapes for the trashrack were computed from the impact-response transfer functions using STC software.

Excitation from vortex shedding can cause either forced vibrations at the vortex shedding frequency or hydroelastic vibrations. Trashracks typically experience hydroelastic vibrations, during which one or more natural frequencies of the trashrack interact with the vortex shedding to further organize the excitation, strengthen the excitation, and widen the velocity range over which the resonant vibrations occur.3,4,5,6

The new trashrack, being unloaded at the plant before installation, features varied bar dimensions and spacing, increased bar thickness and other attributes designed to deal with the unique site conditions.
The new trashrack, being unloaded at the plant before installation, features varied bar dimensions and spacing, increased bar thickness and other attributes designed to deal with the unique site conditions.

American Society of Civil Engineers' recommended nomenclature for vibration modes of trashracks includes "heave" for vertical vibration, "lateral" for vibration transverse to the flow, and "plunge" for vibration in the flow direction.3,4,7

From the modal field testing, 29 vibration modes - ranging in frequency from 19 Hertz (Hz) to 289 Hz - were identified. To evaluate the potential for flow-induced vibrations, HPPi compared the measured natural frequencies and mode shapes for the trashracks to vortex shedding frequencies estimated using the dimensionless Strouhal Number.

Over a wide range of conditions applicable to vortex shedding from trashracks, the Strouhal Number ranges from about 0.2 to 0.22 for rectangular members and from about 0.19 to 0.21 for circular members.3,4,5,6,8

Expected vortex shedding frequencies for all the primary elements of the trashrack - square bars, round rods and rectangular bars - were computed based on assumed Strouhal Numbers and approach velocities determined from flows at the most efficient load (MEL) and maximum load. Figure 1 on page 114 shows correlation between the expected range of vortex shedding frequencies and observed lateral natural frequencies for the original trashrack bars. The thickness of the horizontal shading for the lateral mode frequencies represents the small anticipated decrease in frequency associated with the added-mass effect when the trashrack is vibrating in water.5,6,9 Similar diagrams were developed and evaluated for heave modes and plunge modes at MEL and maximum flows.

Figure 1 shows the lateral modes, which are consistent with the fatigue cracking in the lower portion of the bars. The first lateral mode, at about 50 Hz, lines up with the vortex shedding frequency produced by the average approach velocity during normal generating. However, if this mode was responsible for the trashrack failure, it would not have been localized to the velocity "hot spot."

The second lateral mode (at 106.5 Hz) is a more likely candidate for involvement in the trashrack failure. Figure 2 shows the mode shape for the second lateral mode, based on the modal field tests, and a simplified schematic that superimposes the second lateral mode shape onto the missing trashrack section.

Re-design of trashracks

HPPi provided a preliminary trashrack design based on previously successful designs and design criteria developed from the forensic analysis and ASCE task force and American Society of Mechanical Engineers recommendations.3,7,10 Features of the design criteria are:

- Varied bar dimensions and spacing to minimize effects of compounding stimuli from vortex shedding on adjacent bars;

- Increased bar thickness to increase stiffness;

- Increased section dimensions and weld geometry for design flows and trash loading; and

- Minimized cross section area to limit effects on unit performance.

HDR Engineering performed static and dynamic finite element analyses (FEA) on the preliminary design, and HPPi and HDR collaborated in iteratively improving the design. Design changes resulting from the FEA and a constructability review were incorporated into the design drawings and fabrication specification. The dynamic FEA results were used to determine modal frequencies and mode shapes, and the correlation diagrams developed were evaluated for lateral vibration, heave modes and plunge modes at MEL and maximum flows.

Installation and experience

The Gateway Company fabricated the new trashracks. Work was monitored and inspected throughout the process to ensure quality, and the fabrication proceeded smoothly. Personnel from Taum Sauk installed the new trashracks in two days (i.e., one day for each unit).

The trashracks were returned to service in spring 2010. Cursory inspections have been performed two to four times a year by divers during unit outages. No failures or indications of problems have been found. In March 2012, divers from Three Rivers Diving performed an extensive underwater inspection of the trashracks. All section members were inspected and visually confirmed by camera, and no degradation, indications of cracking, or other problems were found.

- By Patrick March, president and principal consultant, Hydro Performance Processes Inc.; Matthew Ratliff, mechanical project engineer, Ameren Missouri; and Phillip Kirchner, senior civil/structural engineer, HDR Engineering

Acknowledgments

The authors gratefully acknowledge Chris Powell from Structural Technology Corporation for conducting modal tests, Steve Phillips of Three Rivers Diving for performing detailed underwater trashrack inspection, and The Gateway Company for fabricating the new trashracks.

Notes

1Ewins, D.J., Modal Testing: Theory and Practice, John Wiley & Sons Inc., New York, 1984.

2Schohl, G.A., and P.A. March, "Modal Testing of Trashrack at Hiwassee Dam," Proceedings of ASCE Hydraulics Division Specialty Conference, American Society of Civil Engineers, Reston, Va., 1982.

3Crandall, S., S. Vigander, and P.A. March, "Destructive Vibration of Trashracks due to Fluid-Structure Interaction," ASME Journal of Engineering for Industry, November 1975.

4March, P.A., and S. Vigander, "Some TVA Experiences with Flow-Induced Vibrations," in Practical Experiences with Flow-Induced Vibrations: Proceedings of IAHR/IUTAM Symposium, Springer-Verlag, New York, 1980, pages 228-236.

5Blevins, R.D., Applied Fluid Dynamics Handbook, Van Nostrand Reinhold Company Inc., New York, 1984.

6Blevins, R.D., Flow-Induced Vibration, Second Edition, Van Nostrand Reinhold Company Inc., New York, 1990.

7Hydraulic Design of Reversible Flow Trashracks, American Society of Civil Engineers, New York, 1993.

8Hoerner, S.F., Fluid-Dynamic Drag, Hoerner Fluid Dynamics, Vancouver, Wash., 1965.

9March, P.A., and S. Vigander, "Model/Prototype Comparisons of Trashrack Vibrations," Proceedings of ASCE Hydraulics Division Specialty Conference, American Society of Civil Engineers, Reston, Va., 1982.

10The Guide to Hydropower Mechanical Design, PennWell Corporation, Tulsa, Okla., 1996.


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