Among the various projects taking place at Gavins Point Power Plant, generator unit number three was recently dewatered to allow project crews to inspect the draft tube for damage caused by cavitation and make the necessary repairs.
Cavitation describes the state-change from liquid-to-gas that occurs when water flows quickly through a confined space. As the water moves, the vapor pressure drops and the water changes directly to a gas. (Similar to water boiling without a temperature change). Cavitation is a common occurrence in hydroelectric turbines, and generally appears around guide vanes, wicket gates, the turbine blades, and in the draft tube.
Cavitation is always occurring as water pushes through the unit. When implosions, which occur during the liquid-to-gas state-change, take place near turbine structures, flow-surfaces can be damaged and eroded. Over time, the resulting pits or cavities multiply and require repairs. This type of cavitation can also occur on boat propellers.
If left unrepaired, the erosion damage can reduce the efficiency of power generation, potentially lead to further damage and ultimately cause major damage to the rest of the turbine.
According to Steve Neumann, Senior Mechanic at Gavins Point Dam, “We try to perform cavitation repairs every year on at least one unit. Each year, we un-water the units, one at a time and perform an internal inspection of the draft tube and scroll case, which includes taking blade clearance readings, inspecting the wicket gates, taking wicket gate clearance readings, and evaluating the overall condition of the scroll case.”
The repair process does not always involve the detailed work that is taking place this year. “If we only have a unit down (dewatered and not generating power) for three or four weeks time constraints mean we don’t typically install the scaffolding and stage the welding equipment,” said Neumann.
Cavitation repairs had been delayed in recent years because of time constraints caused by the flooding in 2011, repairs following the flooding, water requirements and other impacts.
“Safety requirements have changed since the last time the scaffolding was placed at Gavins Point,” said Mike Welch, Power Plant Superintendent at Gavins Point. “The safety requirements for inspecting the brackets that support the scaffolding have become more specific in how they must be inspected, just within the past year.”
The cavitation repair process involves closing the intake gates; dewatering the draft tube; placing the scaffolding deck and scaffolding, which provides workers access to the areas requiring inspection and repair; performing the inspection; marking every pit on exposed surfaces in the draft tube that are larger than a salad plate; removing and cleaning damaged surfaces; and welding and grinding stainless steel to repair the damaged areas. Because of the changes to the safety requirements, the entire process is being documented and closely monitored.
The original draft tube scaffolding was wooden and hung from brackets mounted to the draft tube walls. In 1973, Gavins Point Dam began using steel scaffolding that was designed to provide a safer, more stable work surface.
“The documentation that is taking place with this installation will be used as a training tool for future project teams and will help others better understand the cavitation repair process,” said Welch.
“We are watching the scaffolding placement from different perspectives by using still photos and video cameras to record the work taking place inside the draft tube as well as having District experts evaluate the scaffolding structure and the team’s safety practices,” Welch added.
The hydropower mechanic career field is a unique one with its own learning curve and among the project team members is Dale Stibral, a rehired annuitant, who retired after 35 years as the Senior Mechanic at Gavins Point.
“Bringing Dale back has been an asset on a number of different levels because each plant has its own special operating or maintenance needs or history. There are some things a person may only see once or twice in a career and he has that history and background,” said Neumann. “Dale knows where things are and when we ask for something, he knows what we need and where to find it. Together he and I can provide mentoring and training for the rest of the crew to prepare them for additional responsibilities in the future.”
Structural engineers and District safety personnel traveled to Gavins Point from the Omaha District to watch the project crew place the scaffolding and determine if there were any recommendations for improving safety.
“The scaffolding is well made and quite sturdy,” said Sean Denning a structural engineer from Omaha. “This is the first maintenance platform we're evaluating with the new safety criteria, so we'll use what we learn from Gavins Point to assess scaffolding at the other main stem dams.”
Annette Fowler, a safety specialist from the Omaha District was also on hand. She was also looking for ways to improve worker safety for the operation.
New requirements for Turbine Maintenance Platform safety are currently in draft status and scheduled for publication this spring. The Omaha District Operations Division, which oversees project operations at the six main stem dams along the Missouri River, is proactively evaluating current processes and equipment against the forthcoming requirements.
“The project provided an excellent opportunity for me to observe the installation process and to compare it to the new Turbine Maintenance Platform safety requirements,” said Fowler. “Installing the platforms and scaffolding can expose personnel to hazardous falls. Among my observation goals was to focus on fall hazards and providing methods to improve fall protection systems.”
Cavitation repairs in the draft tube for unit three will continue through mid-March.
Additional maintenance and rehabilitation projects are also underway at the Gavins Point project including replacing three power transformers and rehabilitating the spillway’s tainter gates. Work will move to unit two for annual generator maintenance, to install new un-watering and drain valves, to install a new motor control center, and replace switchgear equipment.