Many structures built in the 1970s through the mid-1980s were equipped with free-standing, acid-resistant brick linings. Although some of these independent brick-lined chimneys operate dry and encounter minimal problems, many work downstream of old-generation wet FGD systems still using bypassed flue gas for reheat.
At its inception, the acid-resistant brick liner was touted as the answer to one of this type of chimney's biggest maintenance problems - liners made from carbon steel that were quickly damaged by the harsh acidic environment found downstream of the wet scrubbers.
Unfortunately, though, the new brick liners were not a panacea. After two or three decades of service, the majority of these liners have developed a lean or deflected position as a result of operating wet. Studies indicate that numerous complex factors can influence this deflection, but there is very rarely a common contributor.
Liner lean
Only two common factors were identified in all leaning liner cases: the use of bypass gas for reheat and the application of red shale brick in the liner construction. Other varied factors that can influence liner lean include the quantity and composition of moisture deposited on the liner impingement area, composition of the coal used, flue gas reheat methods, and fluctuating flue gas temperatures. While some chimneys can operate for years without any problems, the effects are quick for others, resulting in deflection in just one year.
One variable reason for liner leaning is the impact of differing temperatures of wet flue gas exiting the main scrubber duct and then hitting the liner wall at 180° F from the breech opening, a fairly large area commonly referred to as the impingement zone. Over time, moisture expansion - irreversible growth - of brick and/or mortar occurs. Expansion of the liner wall in the impingement area causes lateral deflection at upper elevations.
The significance of the liner being in a deflected position is that during periods of high wind loading, movement of the column could result in contact with the stationary brick liner. The possible resulting contact between the column or platform and the liner - usually at the top of the structure -could cause partial failure of the liner. Dislodged bricks may fall inside the annulus, knocking out or damaging emissions monitoring equipment, which would result in an unscheduled shutdown.
Solving the problem
Solutions for correcting the leaning liner condition are limited and can be expensive to implement. When the liner has moved dangerously close to the column interior or internal platforms, a counterweight system incorporating heavy concrete weights connected to the liner with cables and pulleys can pull the liner to an acceptable position within the column. However, the counterweight system should be used only as a last resort because the additional weight imposed on an isolated portion of the liner can result in structural damage to its base.
If deflection of the liner has been identified but not yet dangerous, studies have indicated that installing a target wall - an area in the liner that helps prevent temperature fluctuations or transport of moisture in the flue gas into the liner wall - halts additional lean of the liner. Placing a target wall at the base of the liner helps prevent temperature fluctuations or transport of moisture into the wall, which helps alleviate additional lean of the liner. Regular inspections are essential to monitoring any liner deflection problems. Without these regular inspections, deflection may increase rapidly, requiring drastic action once detected.
In addition to deflection, brick liners are commonly damaged by corrosion of the liner-reinforcing system. Brick liners incorporate a system of circumferential steel bands to limit the expansion of vertical cracks that inherently develop, particularly in thicker areas of the liner wall that are susceptible to a greater thermal gradient.
Corrosion damage
Corrosion damage to the carbon steel liner bands occurs when moisture passes through the liner wall from the interior.
Condensation formed on internal liner surfaces is forced through the liner wall in vapor form by a function of thermal conductivity. Depending greatly on ambient conditions, crystalline accumulations develop as the vapors condense on the cooler liner exterior. This accumulation grows, feeding on any present moisture, and the results are rather acidic.
Damage is slow but constant - the lower the temperature, the greater the accumulation - resulting in corrosion of liner bands in many cases. At locations where liner bands are exposed to liquid flowing through defects in the liner wall, corrosion damage to the liner bands progresses rapidly. In essence, any penetration - cracks, test ports, or areas around a flexible breaching seal -in the liner may provide an area for condensation or carryover water to flow through the liner wall.
Oftentimes, corrosion damage results in failure of the affected liner bands. Without the entire liner band reinforcing system being 100% effective, existing vertical cracks in the liner expand in width and length. In extreme cases, serious structural damage to the liner can occur.
Protecting the liner
Most brick-lined chimneys operating in a wet environment incorporate an annulus pressurization system. Although the system does not prevent moisture from migrating through the wall, it stops flowing water and reduces rapid damage. Keeping the annulus pressurization fans in good working condition is important, as is careful monitoring of the system to maintain the liner bands and reinforcements to 100% capacity.
In addition, certain materials are available to seal the internal liner surface to prevent the passage of moisture through the wall. However, these materials tend to be time-consuming and expensive to install, so some operators have elected to completely replace the existing carbon steel liner bands with corrosion-resistant bands.
An area sometimes overlooked during inspections is the base of the liner and surrounding floor. During initial construction, a lead pan is installed inside the base of the liner to contain condensation and carryover water inside the liner. Because it is fragile, lead is easily damaged to the point of failure. Water accumulated inside the liner can drain through fractures in the lead pan into the annulus, where it can migrate into cracks or construction joints in the concrete column, resulting in structural damage to the floor and column.
Problem and solution
Overall, a lack of maintenance or incorrect repair procedures can result in near catastrophic conditions for brick liners. Such was the case for a power generation company in the Southwest.
The brick liners in a pair of 400-ft tall chimneys were designed with two bypass breechings and one scrubber breeching. Hot, dry flue gas hit one side of the internal liner wall, and cooler wet flue gas hit the opposite wall, imposing tremendous stress on the liner walls and causing numerous vertical cracks.
Corrosion, caused by liquid flowing through cracks in the wall, destroyed the liner-reinforcing bands. Existing vertical cracks continued to expand. Movement of the liner wall displaced lintel beams over the bypass breech openings. The liner wall above the bypass breech opening shifted downward.
In one of the liners, a 30-ft-high, 13-ft-wide pie-shaped area below the scrubber duct, held in place by extremely strained liner reinforcing bands, was leaning outward and in danger of falling out of the liner wall. Brick in the area of the breech openings had disintegrated. The two 4-in. inside rows of brick in the 16-in.-thick liner wall were removed from a core sample as rubble.
The structurally deficient liners were stabilized by placing a reinforced concrete sheath around the bottom 155 ft of the liners to an elevation above the breech openings. Dead loads on the deficient liner wall were transferred to the sheath through hundreds of steel dowels.
At the same time, areas of disintegrated brick were removed from the internal liner wall surface, and the resulting cavities were filled with structural gunnite to the original thickness of the liner wall. In the vertical area of the sheath, a sacrificial insulating lining system protected internal liner surfaces.
The cost of replacing the brick liners would have been several million dollars and would have taken several months. However, with proper inspections and maintenance, the extensive repairs required to stabilize the liners likely would not have been necessary at all.
About the Author
Kenny Kendall is a special project coordinator for Pullman Power, a specialty contractor that offers inspection, design, repair, rehabilitation, and demolition of chimneys, silos, observation towers, steel stacks, building foundations, and cooling towers for industrial facilities, commercial properties, public infrastructure, and municipal buildings.