Based on concerns about fumes, smoke, and dust caused by welding inside its Raccoon Mountain underground powerhouse, the Tennessee Valley Authority (TVA) installed a portable filtration system to provide a safe working environment.
The welding work was required as part of TVA’s efforts to replace turbine runners at the 1,600-MW pumped-storage project on the Tennessee River in Tennessee.
Situation at Raccoon Mountain
Ventilation of welding fumes in a hydro plant can be difficult. With an underground plant, ventilation can be especially difficult because of the long runs of hose required to reach outside the plant.
In 2004, personnel at Raccoon Mountain undertook the first of four runner replacements. During the first outage, for Unit 1, personnel determined that special rigging would be required to remove the runner. This runner is different from those in the other three units, in that it is oversized. In fact, because of its size, the runner would have to be tilted. Developing the rigging and removing the runner would add several days to the outage.
The portable filtration system installed at the 1,600-MW Raccoon Mountain pumped-storage project removes fumes and smoke generated during welding and plasma cutting.
One option to reduce the time required to remove this runner was to use plasma cutting. This would allow the runner to be removed in pieces, using the existing lifting device in the plant.
This plan was implemented in September 2005, during the outage to replace the Unit 1 runner. However, plasma cutting in the turbine spiral case and draft tube resulted in a significant amount of fumes and smoke in the plant. It was clear that the normal method, ventilating into the plant, was unacceptable.
Investigating a solution
To improve ventilation for the plasma cutting, TVA assigned Terry Rogers, hydro tech IV, to research solutions. He recommended installing a portable filtration system, the AT-9 from Amtech.
The system consists of a portable cabinet that contains a system blower, 18 filter cartridges, a 22-inch fan, and an exhaust silencer.
The system blower draws air through ducts into the top of the collector cabinet, then distributes that air uniformly across the cabinet interior. The air then moves downward into the filter section, taking with it most of the dust and fumes. The filters collect the dust and fumes as the air passes through them. Finally, the clean air passes through the system fan and is exhausted back into the plant. The clean air also can be ducted to another location if desired.
Once the filters become loaded, the reverse jet pulse cleaning system is activated, either manually or automatically. Upon activation, the system forces compressed air from the clean air side through the filters. The particle-laden air then is expelled into the downward air flow inside the cabinet. Gravity assists in conveying the contaminants to the dust drawer, where they can be removed and disposed of (usually after 60 days of operation).
Installing and operating the system
This portable filtration system was connected to the opening of the spiral case for Unit 1 via a series of hoses and a dog-house attached to the spiral case door. The wooden dog-house acts as a pressure boundary for personnel to enter and exit the spiral case to perform work, while still maintaining a negative pressure to ensure fumes exit the area. All welding and plasma-cutting fumes were drawn through the filtration system, then exhausted into the plant. When the head cover was removed, a working platform designed for stator inspections was lowered into place. This provided adequate restriction to ensure a negative pressure in the spiral case.
With the portable filtration system operating, WeldMart personnel were able to plasma cut the runner in place. This allowed it to be removed using the normal lifting device, saving a couple days of outage time. This savings alone paid for the portable ventilation system, which cost about $20,000.
Although only Unit 1 has the oversized runner, work needed to be performed on the other units for which the portable filtration system was invaluable. For example, the new runners for Units 2 and 3 were longer than the old runners. To make room for them, it was necessary to cut out the draft tube liner and weld in a new liner. Plant personnel also used the filtration system for the associated outage work, such as stay vane cavitation repair and welding the nose cone. The final runner replacement was completed in June 2007.
TVA foresees using the filtration system during future cavitation repair.
When the standards for removal of hexavalent chromium were implemented in 2007, TVA purchased a $6,000 high-efficiency particulate air (HEPA) post-filter for the portable filtration system. This removes hexavalent chromium, which is not filtered by the main system.
– By Mitchell M. Samuelian, hydropower O&M program manager, Tennessee Valley Authority, RAC 1A, 1101 Market Street, Chattanooga, TN 37402; (1) 423-413-5400; E-mail: email@example.com.