Article clip: === Will Oroville Spillway Gates Fail in Heavy Flows? Design Flaws & Fixes Risk Gate Binding?
Has DWR missed for over 40 years the accumulation of mud, debris, and silt that ended up jamming a Radial Gate? For many years Inspectors believed that extensive leaking from Gates were normal. What changed this belief in the failure of a binding Gate during a test? Are the Gates closer to Failure with these new findings?
LOS ANGELES, CA (California Network) - In 2007, during an official Federal Energy Regulatory Commission (FERC) Inspection, Radial Gate 4 jammed after only lifting 6 feet of its 33 foot travel [1]. From the perspective that all components of the Spillway Radial Gates are considered a High-Reliability System, what ensued in the subsequent Engineering Failure Analysis Report findings could only be read as an engineering nightmare. Quoting the Report: "During Federal Energy Regulatory Commission required operational testing Spillway Gate No. 4 would only open to approximately 7 feet at which time the motor would trip offline from overload." "Initial inspection found heavy galling marks on the right side wall plates as well as protruding bolts on the seal assembly directly adjacent to the wall plates. A bronze plate was also found between the wall and seal. This plate was later determined to be from a retrofit done in 1974." "The seal assemblies were removed and disassembled. A large amount of mud and debris was found behind the seal. The seal inflation piping was completely filled with mud and debris also. One bronze guide shoe was damaged beyond repair." "Two main items were attributed to the gate binding: 1. Lack of maintenance caused the system to degrade and become clogged with mud and debris. 2. Due to irregularities in the seal assemblies - it appears they were not properly adjusted for the proper clearance over the entire length of the seal."
This event uncovered an astonishing amount of damage, broken parts, and design issues so serious that it led to the complete refurbishment to all eight of the Spillway Radial Gates. These issues are considered FERC Category 1 Failure modes that are of the highest rated level. Failures of Radial Gates have been known to lead to potential catastrophic outcomes including dam failures [10][11]. DWR's own report to FERC noted that a major cause was from "Lack of maintenance" - which caused the system to degrade and become clogged with mud and debris. How could a High-Reliability System be neglected to the degree to where the accumulation of mud and debris became a contributing failure mode? How many years of this type of maintenance neglect of this mud and debris have been going on? Forty years?
DWR Engineers also discovered that silt had entered into the piping that pressurizes the rubber seals, rendering the inflation/deflation system inoperable. The hydraulically operated piping & rubber seal systems did not have any silt filtering nor any method of flushing the tubing for maintenance cleaning. This design flaw wasn't discovered for forty years until gate 4 failed. Apparently, there was no maintenance inspection program nor were there a DSOD inspection program of the hydraulic tubing systems that could have alerted engineers to this internal pipe & seal clogging issue.
The most severe findings were loose bolts and heavy scraping wear on the stainless steel side plates and the bronze guide shoe bars (see Fig 5). The stainless steel plates are imbedded into the pier concrete column walls to form a smooth surface for the inflatable rubber seals and the bronze guide shoes to slide upon. The scraping and galling marks were so significant that the stainless steel plate surfaces had to be repaired. How could DSOD or Maintenance miss these dramatic scraping marks for so many years? It finally took a broken bronze shoe plate becoming jammed into the sidewall seam for the combined failure of Gate 4 to awaken DWR to these serious and numerous issues. DSOD did a "special spillway" inspection report soon after the Gate 4 failure and found that most of the gates had visible sign of "galling" (rubbing wear) on the side seal stainless steel plates [2]. DSOD and Maintenance further discovered that the side seal assemblies were highly irregular, bent, unaligned, and were conducive to be "imprecise to where seal leakage can be expected" [2].
Evidence suggests that DSOD inspectors had become accustomed to excessive gate seal leakage for many years [4][5][6][7][8][9]. Yet this "extensive leakage" was considered normal. In a High-Reliability System, this should have triggered alarms as the seals were designed to be water tight with minimal seepage. But, this serious multi-gate failure mode was considered "normal" by the consistent "satisfactory" approvals in the decades of Inspection reports. This is very difficult to fathom in a high quality engineering background.
= end clip More at link -Article link:
Will Oroville Spillway Gates Fail in Heavy Flows? Design Flaws & Fixes Risk Gate Binding?
Fig 1. DSOD Inspection photograph of "alignment bolts" that secure the Side Seal Assembly and are intended to maintain a precise alignment gap during the operation of the heavy approx 20 to 30 ton Radial Gates. This image reveals a better alignment, or minimal offset, of the welded radial gate mounting flanges/plates. Other Gates have very large offsets of these mounting points with respect to the bolt holes in the Side Seal Assembly - thus creating the "fix" extensions and spacers employed by DWR. The base design is a poor design as torsional or operational side forces may "shift" the eyelet at the "hold bolt" - thus opening a gap for mud and debris to wedge into while in flow conditions.
Fig 2. DSOD 2015 Feb 3 Inspection photograph reveals a Gate Failure Binding Risk in the angled attachment "fixes" to secure the Gate Side Seal assembly strips to a Radial Gate. Any strong forces from Gate Flow operation may risk shifting of the alignment bolting design of the side seal assembly, resulting in excessive frictional forces from side seals and bronze guide shoes on the sidewalls.
Fig 3. Radial Gate Design Flaw. A series of bolts are used to align the side seal assembly to the approx 20 to 30 ton Radial Gate. Poorly aligned gate mounting flanges and side seal assembly bolt holes have led to bolt bending, eyelet bolts with spacers, and extensions of bar or metal. Any vibration or torsional forces by the massive gate could shift these mounting points to where there could be a risk of binding of the gate via excessive frictional forces on the sidewalls.
Fig 4. DWR Gate 4 2008 Failure analysis report identifying past used bent bolts due to alignment problems. New bolts wouldn't fit until eyelets used with spacers. DWR notes that the reassembled side assemblies pressed hard against the sidewalls while noting this as a source of a potential gate binding issue in the past and present. DWR worked the array of adjustment bolts to get both sides aligned to 0.040 inch gap from the sidewalls.
Fig 5. Severe scraping damage and heavy wear (drag marks) to a bronze guide shoe on the failed Gate 4 (binding failure). Bolts found to be scraping the sidewalls and were cut away to remove the guide shoe. Mud and debris found caked within the inflation seal assemblies rendering the seals inoperable.
