Case Studies
Westinghouse Turbine Flange Repair
PROBLEM
During Westinghouse Turbine overhauls, all manway doors and flange are removed for access. When reassembling the units, these flanges and manway doors must be sealed to prevent hydrogen gas leakage during operation. Their existing sealer, a gasket varnish, Westinghouse Part Number S/N 32101, is no longer being manufactured and a replacement product has not proven satisfactory.
Waterbox Lining
PROBLEM
In 1993, Israel Electric Corporation was concerned about the failing rubber lining and subsequent pitting and corrosion throughout the circulating water systems especially in their salt water cooled power plants. Specifically, the debris filters and waterboxes all showed varying degrees of rubber lining failure. They decided to repair the rubber lining where possible and reline where necessary.
Phosphate Railcar Lining
PROBLEM
Carbon Steel Railcars are used to transport wet, coarse Phosphate Rock from the mine separation plant to the Chemical Plant for processing into Phosphoric Acid. The existing railcars were lined with a 40 mil Urethane coating which was failing after a few years of service. The failure was from a combination of abrasion and the acids leaching from the course, wet Phosphate Rock.
Hastaloy Stack Repair
PROBLEM
A South-Eastern Utility installed limestone SO2 scrubber systems on their 280 MW units between 1977 and 1981. The outlet ducts were wallpapered with Hastaloy, to prevent corrosion, from the scrubber outlets to the stack inlet. They found that the stack wall directly opposite the outlet duct was being subject to excessive corrosion/abrasion from impingement of the moist flue gas. The Plant decided to install a partial Hastaloy stack liner at that point. However, they were concerned that the edges of the new Hastaloy would be subject to the same accelerated corrosion rate as they were seeing at the edge where the existing Hastaloy lined outlet duct ended.
FGD Reaction Tank Lining
PROBLEM
A South-Central Utility installed limestone SO2 scrubber systems on their 720 MW units in 1985 and 1987. The Reaction Tank walls and ceilings were originally lined with a thick FRP mat product. This lining started to fail within the first year of operation. The failure mode was delamination between coats and disbondment between the lining and the substrate (carbon steel). The linings on all nine tanks on each unit were failing and the tanks corroding.
FGD Outlet Duct Lining
Problem
A Mid-Central Utility installed limestone SO2 scrubber systems on their three 700+ MW units in 1978, 1980 and 1984. As early as 1982, the ductwork from the scrubber outlet to the stack inlet was showing severe corrosion and metal loss. A particular problem with finding a suitable lining product was that the duct temperatures reached 400°F during startup and shutdown.
Demineralizer Floor Repair
PROBLEM
A Mid-West Utility had installed an “acid resistant” tile floor in their demineralizer room to resist attack from concentrated sulfuric acid and caustic spills. In 1983, much of the tiled floor exposed to chemical spills was failing due to the attack on the grout and subsequent undercutting which resulted in release of the tile and attack on the concrete subfloor. Water leaking through the floor also added to the problem.
Coal Mill Classifier Repair
PROBLEM
The Coal Mill Classifier Cone directs the pulverized coal out of the mill and into the furnace. Hot air, from the air side of the air preheater, is pulled through the coal mill by the exhaust fan (refer CH-801) picking up the fine coal dust and feeding it to the furnace. The classifier cone is generally lined with thick Alumina wear plates to protect the cone from the abrasive coal particles. With time, these plates wear down and must be replaced. Replacement is done both in the field and at the manufacturer. The replacement wear plates are metal backed with a hole in the center to allow welding to the metal classifier cone. The problem is to protect the weld holes after installation, and to a lesser extent, the edges where each plate meets the next one.
Coal Exhauster Wear Plate Repair
PROBLEM
A southeastern utility was experiencing excessive material loss on the wear plates around the outlets of the coal crusher exhausted fans. Since these plates were made of special hardened alloys, welding repair was expensive and time consuming. Attempts at rebuilding with a troweled on epoxy mastic over thick metal support screens, only gave a short time protection. Also, the work involved in welding the support screens and then troweling the material to a depth of up to 1 inch was still time consuming and expensive.
Nuclear Plant Waterbox Rebuild
PROBLEM
By 1983, at a New England utility, the cast iron waterboxes, were subjected to many years of salt water attack. The corrosion was almost to the failure point, and the waterboxes were leaking at a few points. Replacement of the old waterboxes, ieven if possible, would take from 1.5 to 2 years to have them fabricated. Welding was not considered as the heat stress might cause cracking and complete failure of the waterboxes and shutdown of the plant.
Cast Iron Waterbox Rebuild
PROBLEM
In 1985, the Cast Iron Waterboxes at the San Juan Station of Puerto Rico Electric Authority, after being subjected to many years of salt water attack, were corroded to the failure point and were leaking in numerous areas. Replacement of the old waterboxes, if even possible, would take about 2-3 years, but the power generation was needed immediately.
Bottom Ash Elbow Repair
PROBLEM
A Western Utility experienced leaking in their bottom ash transfer line. This allowed flue gas to be released into the air, and there was concern of an immediate pipe failure requiring an emergency shutdowm. Since this was the weekend, there was no chance of getting an immediate replacement. This is a common problem as transfer lines carrying pulverized coal, fly ash, or bottom ash are subject to serious abrasion effects especially in the elbows and transition points. The enhanced abrasion occurs as the particulate stream is forced to change direction, impacting much of the particulate on the turning radius. Of the three materials, bottom ash is the most severe. Bottom ash, in addition to being extremely hard and crystalline has a wide range of particulate sizes. For example, “Black Beauty”, a commonly used Grit-Blast Media, is made from screened bottom ash.
