Description of Event:
On Friday, 4/24/20 while swapping units at Beta Hydropower Plant (Beta), (shutting down U1 for brake work) the operator heard a noise under the head cover area- possibly in the runner. It appeared as if something may be stuck in that area, making sounds on each revolution. The decision was made to dewater the Beta plant and make an entry to determine the cause of the noise.
What was found was a piece of metal (36” x 3” x 3/4”) on the runner along with a large (12’ x 4’ x 3/8”) plate in the scroll case next to the distributor section.
After researching the drawings and then verified by further inspection the large plate had come loose from the corner of the surge tank opening to the penstock gate for U2 and travelled all the way around to U1 water chamber scroll case.
The metal found on U1 runner was from one of the support bars on the detached deflector plate. One other support bar is missing and not located, believing to have passed thru one of the runners. The plate and support bar were from the deflector plate in front of U2 Penstock gate, but both ended up over at U1 water chamber. In addition, the new tunnel flow transducer cables were cut off apparently when the plate came down from U2 gate.
The maintenance crew proceeded with doing a quick inspection of U1 runner and wicket gates, finding no obvious damage. They then obtained needed tube and clamp scaffolding to access the deflector plate that was still apparently intact on U1 gate for a detailed inspection. Findings were as follows;
• Gate 1 deflector shield was intact but slightly loose with the top mounting nuts/ studs eroded and starting to loosen from the concrete.
• Gate supports that were installed in 2016 looked good, no apparent movement of the gate sliding at all on the supports.
• Both gates D.S. hinge area bearing assemblies looked in good condition, the studs/ nuts were slightly eroded but the 8” bearings did not appear to have any sign of failing.
• The various access square openings on the deflector shield were no longer needed with not using the gates.
• This included a large approx. 2’ square opening on the DS sides. Along with the 2’ openings there are also approx. 2- 3” gaps all around the gate sides for even water pressure distribution on the gates- no upward pressure.
Inspection Findings with Photographs – Attachment 1
Despite a challenging and hazardous work area, in-house crews performed the work safely utilizing the required safety procedures and hazardous work assessment tools. The turbines were placed on-line on May 14, 2020, resulting in outage duration of 20 days. Due to the prompt action of the plant operator to investigate an abnormal condition a catastrophic failure of the turbine or distributor section may have been prevented.
Actions Taken:
Discussions on repair options:
1. NOT SELECTED: Complete removal of the Penstock gate assemblies (Contractor was brought on site to give us an estimate for the future).
2. NOT SELECTED: Removal and replacement of the deflector shields- The removal and replacement would be a significant workload, with rigging new panels due to their weight. Even with fabricating each plate into 5 sections, each section would be over 100 lbs. and then need to be welded/ bolted together once in place. This would occur with the small sections being brought into the chamber thru the small hatches or using crane/ rigging to transport the large plates up onto the surge tank and lowering them in place. In addition, it’s believed these shields might aid penstock hydraulics and debris build-up on the gates.
3. SELECTED OPTION: Removal of the plate still in front of gate 1 and demo gate from U2 that had ended up near U1 scroll case. This is what was ultimately determined to be the simplest and safest alternative. The plates will be cut up into smaller sections that are easily handled and removed from the chamber. In addition, the gate supports that were installed 4 years ago will be welded directly to the gate instead of simply supporting it to keep hydraulic aberrations in the penstock from “working” the gates.
Removal vs. replacement was also based on the high probability that this plate had been off of U2 gate for up to 2 months with no known issues. U2 had several broken wicket gate links on 2/6/20 and U1 had a broken gate link on 2/17/20 so the deflector plate support brackets probably went thru U2 initially and possibly U1 or the metal plate that was found on U1 runner was there for a while.
Both units will be looked over in detail, checking for any possible damage from the loose metal possibly entering the distributor section.
Removal and Repairs – Attachment 2
Safety Summary:
• Dewatering of the tunnel was done per procedure and the required wait period was adhered to.
• Alpha Hydropower Plant Head Water (HW) was lowered to minimum to lower the water level in the floor of the tunnel and penstocks to make it safer and easier to work in (Note: Penstock drains are open to Beta Tailwater (TW) / Alpha Hydropower Plant HW).
• Confined space access procedures were followed.
• Energy Control (Lock-Out Tag-Out) procedures were followed.
Method & Findings:
Upon inspection it was found that the deflector plate on the penstock gates in Unit 2 had come free of its supports and portions were found around the runner and scroll case of Unit 1. The root cause of the failure of the deflector plate appeared to be the concrete anchors shearing and pulling free of the concrete where the plate was mounted overhead. The mechanism of failure is believed to be corrosion of the anchors combined with localized concrete spalling in the anchorage area. These deflector plates have been in service since 1943 and in a submerged environment nearly 100% of the time.
Supporting Materials:
Corrective Actions:
Sequence & Completion Dates for Actions Listed:
Refer to Attachment 2.
Lessons Learned:
A work order will be placed into work management system to perform a thorough inspection of the repairs as well as the integrity of the penstock gates during the next dewatering, which is anticipated to occur again around 2025 during the next Part 12 inspection or before.
The inspection scope will focus on:
• Integrity of anchors on penstock support brackets
• Integrity of pin and bearing connections on the penstock gates
• Integrity of welds to support brackets
If management decides at the time to perform removal of the penstock gates, that could also be accomplished.
Additionally work crews across the hydropower fleet will be briefed on this incident to increase awareness of potential risks and share lessons learned.
Recommendations:
1. During each unit planned outage, inspect all components and supporting structures installed in the waterway for proper integrity.