Chimney maintenance can be one of the most challenging of plant MRO activities. Their height, form, and functions make them quite unique from other structures. Whether it be a complex chimney with a reinforced concrete shell surrounding one or more flues - or a simple unlined steel stack - there are many factors that contribute to deterioration.

Unfortunately, the pace of chimney deterioration is exponential, so in the first years of operation there may be no evidence of changing conditions. Later, deterioration accelerates rapidly, catching some facility managers off guard and requiring major repairs instead of minor maintenance.

The causes of deterioration in reinforced concrete chimneys are divided into three classifications. One results from the flue gas itself: acid-forming components that are extremely corrosive. These also contribute to the breakdown of the calcium silicate hydrates that give strength to concrete. Events like downwash over the top of the chimney, deficient breaching seals, leaking sample ports, or a breach in the liner itself can be very detrimental to the concrete. If they bleed into the annular space between the liner and the chimney shell, the gases will condense, forming an acidic liquid. This will degrade the concrete, and if it reaches the reinforcing steel, it can lead to corrosion-induced cracks, delamination and spalling.

A second form of deterioration occurs when the liner itself begins to deteriorate and the chimney is exposed directly to the hot gases. When the insulation becomes damaged or saturated, or if the original acid-resistant coating material begins to fail, holes can develop in the liner, thus allowing the process gas to come into direct contact with the chimney shell.

A third form of deterioration relates to carbonation, which is typically the result of natural environmental factors, and thus considered to be the "natural and expected" mode of deterioration of a concrete structure.

Deterioration: Probably Worse than you Think

In the natural environment, most chimneys will experience a lowered pH through carbonation at the rate of approximately one millimeter per year - hardly a significant deterioration rate. But consider that hundreds of chimneys in the United States that are approaching their 25th or 30th anniversary - by which time a full inch and a half of concrete will have been carbonated. An inch and a half is the average embedment depth of the outer curtain of reinforcing steel in most chimneys. As a result, the reinforcing steel will often be exposed to rust and corrosion induced by the moisture inherent to the concrete. Next, the rusting metal undergoes a volumetric change - sometimes growing to as much as 12 to 14 times the size of the original steel. Spalling, marked by the concrete falling away, is the end result.

In some instances, facilities such as power plants may be required to employ wet scrubbers to remove certain sulfurs from flue gases. The residue from scrubber operations combines with water to form sulfuric acid, which in turn creates an environment that is extremely corrosive to most metallic components.

The detrimental effects of wind, rain and fluctuating temperatures on reinforced concrete chimneys should also be taken into account. When wind velocity is greater than zero (the winds are not calm), an airflow pattern quickly develops around the stack. The illustration shows the effects of positive wind velocity: because of aerodynamic volume displacement, an increase in wind velocity occurs on both sides of the stackpipes. This creates a zone of negative pressure on the leeward side, in which wind turbulence occurs. Flue gas is drawn into this turbulent negative-pressure zone. Since the surface temperature of the chimney shaft is much lower than the temperature of the emerging flue gas, it condenses on the leeward side, in highly noticeable yellowish, brownish and black deposits.

Chimney construction techniques themselves contribute to deterioration risk. The jump-form construction technique commonly used for building reinforced concrete stacks up to 500 feet high calls for the chimney to be stage-built in 7- to 10-foot sections. As a result, there are cold joints between sections where the concrete is not well bonded. This creates a natural path for air and water to intrude, attacking the reinforcing steel where it bisects the construction joints. The sand and cement used to cover these joints during initial construction lasts for only a few years before it begins to weather away, thus creating a natural pathway for corrosion ... and structural deficiency.

In metal stacks often designed for chimneys under 200 feet high, the steel is welded together and exterior stiffeners are added. While these weld joints might appear tight, that's often not the case. Small cracks in the welds caused by the thermal stresses of construction and wind loads on the steel shell itself can form. And once cracks begin to occur, corrosion soon follows ... usually developing in places that are not visible, especially in crevices where corrosive materials can accumulate and become more concentrated.

The Importance of Chimney Coatings

A variety of coatings have evolved over the years for chimney protection. Some are quite specialized and are able to withstand a wide range of thermal conditions.

There are three reasons to coat a concrete chimney: to protect it from environmental elements; to protect it from process gases; and to comply with Federal Aviation Administration requirements for incorporating aviation warning devices. In order to select the most appropriate coatings for a particular chimney, it is best to consult a chimney expert.

In some instances, an acrylic-based aviation warning paint will be sufficient, providing proper visibility for seven to ten years. But there are many situations where aviation warning paint will not provide proper resistive qualities, and it is not full-bodied enough to seal construction joints, thus leading to deterioration and subsequent costly repairs. In such cases, a more chemically resistive epoxy coating system can be applied to the higher zones of the chimney, while a less resistive coating will generally suffice for the lower zone.

Unfortunately, if you're only concerned with the exterior of a chimney when selecting coatings, mistakes can be made. Some chimney liners are located very close to the shell, thereby transmitting more heat during operation. In such cases, provision should be made for the concrete to "breathe" - to allow moisture vapor to pass through the concrete. If the improper selection of an exterior coating prevents breathing from occurring, it can create a bigger problem inside the chimney than on the outside, trapping moisture that will corrode the reinforcing steel more quickly. To prevent this, a breathable epoxy coating with a urethane topcoat is often a good solution.

For metal stacks, alternate coatings have been developed that are engineered for higher heat resistance both for interior and exterior applications. These are made from vinylester and phenolic compounds. Another coating solution is to "metalize" the interior of the stack by applying a pressurized molten zinc or aluminum alloy onto the steel. This will provide enhanced cathodic protection at high-temperature operations.

Taking the Best Approach

Generally speaking, by the time you notice you may have a problem with your chimney, you already have more than just a "maintenance" problem. To minimize the risk of costly repair or rebuild, shift your focus from "short-term maintenance costs" to "long-term lifetime cost." Track down the original chimney design drawings and study them. Look especially at the interface between the liner, side clearances, and other elements. Also, be wary of "patch" repairs conducted by internal maintenance, which can easily upset the delicate balance between the liner and the shell (and often do more harm than good). Don't settle for testing simply one portion of the structure for corrosion; in addition to testing the liner, a hammer inspection of the chimney shell is always advisable.

Chimney maintenance contracts are becoming more common in the industry. These allow for predictive planning and budgeting. In addition, such programs establish a maintenance history on the chimneys, which can be a very valuable reference in subsequent years.

Whatever approach you decide to take, don't simply rely on the level of inspection and maintenance activities that may have been carried out in the past to be the guide for the future!