FAILING CHIMNEY LINER INSPECTION

Industrial Access performed a drone inspection on a decommissioned brick chimney. The structure was found to be in very poor condition, with brickwork severely cracking and crumbling, posing an active falling object hazard. Later, the client contracted Industrial Access again to perform chimney demolition services. The drone inspection allowed Industrial Access to safely and quickly determine the structure’s situation and help the client develop the best strategy for the chimney moving forward.

A power plant in northern India was in need of investigative inspection services for their double-liner reinforced concrete exhaust stack of 902’ height comprised of two flue-gas desulphurization (FGD) carbon steel liners of 23’ diameter. Both flue cans have titanium linings to prevent corrosion while handling wet flue gas. The smokestack operated successfully as a dry stack since 2009, but after local regulations changed to require the facility to use scrubbers, within a year the chimney began experiencing severe deterioration.

The client started noticing leakages in the flue cans and, upon observation, reported liner damage with carryover moisture spillage on support structures and severe corrosion in the flue can shell. Industrial Access (IA) was contracted to perform a comprehensive condition assessment of one of the liners, provide a detailed study of the root cause(s) of observed anomalies and develop a feasible solution strategy for the facility moving forward.

The inspection was carried out in the liner interior as well as liner exterior. IA utilized a pre-built motorized swing stage, existing platforms and rope access techniques to facilitate safe and efficient access to the entirety of the flue can. The annular space of flue can exterior was inspected visually from ground and available platforms.

IA mobilized to perform a detailed condition assessment that includes a visual photographic inspection, a drone inspection, and Ultrasonic Testing (UT) readings. In addition, a section from the flue can inlet duct was cut out and submitted to Applied Technical Services (ATS) for a metallurgical failure analysis to provide further information about the potential cause of rapid liner failure.

The interior inspection of the liner was carried out at approx. EL. 131’ – 623’ from grade, and the condition was concluded to be poor. IA crew noted severe corrosion in the titanium cladding in many locations, at some of which holes were present, revealing the oxidized underlying steel and compromising the structural integrity of the liner. The most compromised areas were at the circumferential splices between adjacent cans and at the liner support locations (vertical and horizontal). Such severe localized corrosion at support points is a major concern as it could lead to catastrophic failure of the structure as a whole. Additionally, IA observed some improper welds of the titanium plates that had been installed over corroded areas. There were gaps present in the welding, which could be allowing flue gases and condensates to penetrate behind the titanium sheets and accelerate corrosion. This could also become a potential safety hazard, since the flowing gas could cause the unwelded edge to flutter and result in titanium sheeting pieces getting ejected from the top of the chimney.

The exterior inspection of the liner was carried out from the annulus at approx. EL. 98’ – 623’ from grade. IA crew observed damaged and missing insulation in multiple locations of the flue can splices and reinforcement. These locations provide increased surface areas for the dissipation of heat, so leaving these surfaces uninsulated could allow acidic condensation to develop and increase the chances of forming corrosion. Widespread holes with leaked flue gases and condensates were noted on the liner exterior, which have further corroded the structure and in some cases dripped onto the support framing, causing oxidation. IA detected overstressed buckling stiffeners, thinning plate, compromised lateral support bumpers, and signs of wildlife entry. Moreover, the welding of the titanium plates on the interior has been done without patching and reinforcing the liner body itself. Therefore, the titanium plating was visible through the holes at many locations, which is a serious concern for the structural integrity of the liner.

UT readings were mostly consistent with the design thickness of the liner. Nonetheless, low readings were still observed in multiple areas, although these were in all probability the thickness of the titanium cladding and not the steel liner. The readings were consistent with the fact that the observed corrosion is highly localized due to chemical attack, and general surface erosion or corrosion of the liner did not appear to have taken place.

Based on the information provided by the client, it appears that the liner experiences several temperature excursions throughout the year. In general, while titanium cladding in wet FGD liners tends to exhibit good resistance at moderate PH levels and low temperatures, hot concentrated reducing acids, however, are known to have very detrimental effects on titanium linings. Since the client’s structure has multiple uninsulated areas acting as a heat sink, the rise in temperature inside the liner slows down, which enables longer exposure to highly concentrated hot reducing acids. This creates an extremely aggressive environment for the liner, particularly in already compromised locations. In addition, repeated temperature cycling due to the frequent start-ups and shut-downs of the system subject the liner to thermal shocks, which increases the chances of differential expansion between the carbon steel and the overlay titanium, and this could further exacerbate the corrosion and deterioration faced by the structure.

The ATS analysis of the liner sample coupon provided additional potential contributing factors to the corrosion damage of the titanium lining. The ATS report revealed that the areas close to corrosion damage have demonstrated loss in thickness of up to 22%. Additionally, the analysis indicated the presence of chlorine in the sample coupon, possibly from the flue gases. This contributed to accelerated corrosion since chlorine is known to be detrimental to titanium which has low resistance to chloride species. Lastly, the sample coupon exhibited cracks along the intermetallic phase between the steel and the titanium overlay. The cracks in the intermetallic phase resulted in delamination and flaking-off of the titanium cladding and exposure of the base metal. This resulted in a repeating cycle of further corrosion and delamination, thus causing extensive damage to the liner.

The inspection revealed multiple interconnected likely causes of the smokestack’s liner degradation, one of the primary being dissipation of heat through uninsulated or inadequately insulated areas, which increases acidity inside the liner and drives corrosion. IA also made some non-scope observations, such as the severely damaged concrete at the top of the chimney column, to make sure the facility is aware of potential safety hazards onsite. Lastly, IA’s experienced engineers developed a maintenance recommendation and risk mitigation strategy for the client. The plan involves reinforcement repairs for the corroded plating, splices, support rings and stiffeners, patching of holes, replacement of insulation on liner exterior and most steel elements, replacement of failed titanium patches, installation of a protective coating or lining, and more. Regardless of the situation, IA can provide thorough condition evaluations that will identify any immediate issues as well as long-term areas of concern, enabling every facility to always remain one step ahead.