Introduction
Glass-lined steel (GLS) offers benefits such as excellent corrosion resistance to a broad range of chemicals, product purity, a non-stick surface, and lower purchasing costs compared to nickel alloy or reactive metals. Unlike thermoplastic-lined options, the glass lining will not permeate and can handle full vacuum. However, glass is susceptible to mechanical damage, has limited allowable bolt load and gasket types for its flange connections, and any lining damage can only be covered, not truly repaired. Due to the breadth of its chemical resistance, GLS can shine in reactor applications where the chemistry can vary greatly during its residence time or as a multi-product vessel. GLS can provide reliable service for decades when the glass is not attacked and flange edge corrosion is mitigated, barring mechanical damage.
The glass application process entails the manual spraying of multiple layers of glass frit with a firing occurring after each layer or “coat”. A typical vessel has one to two ground coats and three to six cover coats; the former’s purpose is to provide an interface layer between the steel substrate and the cover coats, with the latter providing the chemical resistance. Only a few equipment manufacturers have glass-lining capabilities suitable for the chemical processing industry.
The current industry standard glass lining repair techniques or methods do not actually repair the glass lining but instead cover the damaged location with PTFE or tantalum, which have mostly similar chemical resistances to glass. (For more information and images on the types of repairs, their installation techniques, and their unique damage mechanisms, refer to MTI’s publication “Repair and Damage Assessment for Glass-Lined Equipment”). The available glass lining repair techniques depend on the location, size of damage, and operational risk. All repairs take skill, are seen as temporary, and require proper monitoring.
Overview of MTI Project 266
The glass lining repair technique reviewed in this project is novel in that it would reestablish a continuous glass barrier at the damaged lining location, versus the current method of covering up the damage. This project's objective was to quantitatively evaluate the robustness of this new repair technique via a review of the repair process and comparison testing between original and repair glass. The glass lining repair technique mirrors original glass fabrication in that a ground coat and cover coat are applied separately with localized firing occurring after each coat is applied. At the time of sample preparation, there were opportunities to refine the application method.
The project was executed in two phases. In phase 1, local repairs were completed and assessed using standard NDE techniques for GLS. The repair areas were deemed
acceptable from a 6 kV spark test, visual assessment, coating thickness gauge measurements, statiflux testing, and compression testing in a flange assembly with a maximum target stress of 6000 psi (41.4 MPa).
For phase 2, repairs were performed on GLS blind flanges with the repair surface oriented parallel to the ground. Visual examination showed that the repair glass successfully fused with the original glass; the repair location was clearly visible due to having a slightly different shade of blue. Samples passed 6 kV spark testing prior to being subjected to environmental tests.
Individual sets of repaired and unrepaired samples underwent multiple aggressive tests to assess the repair method’s ability to handle chemical and mechanical damage mechanisms:
• Exposure to 20 wt% hydrochloric acid, 212 °F (100 °C), both liquid and vapor space
• Exposure to 30 wt% sodium hydroxide, 122 °F (50 °C), both liquid and vapor space
• Thermal shock testing
• Abrasion testing
The chemical exposure tests were deliberately selected to be aggressive to glass so that attack would occur and allow a performance comparison. While the repair glass did not perform as well as the original glass in chemical exposure, there were no breaches in the lining. Depending on the driving force for using glass, a glass repair with lower chemical resistance than the original lining may be acceptable for the end user (e.g., glass was selected to provide a non-stick surface). In the abrasion testing and thermal shock testing, the repair glass performance was equivalent to the OEM glass and published limits from the OEM product literature.
Results from Project 266 testing indicate that this glass lining repair technique has potential for use as an alternative to the traditional repair techniques. When selecting repair methods, each technique offers advantages and disadvantages and will need to be applied on a case-by-case basis. It is also worth acknowledging that any field application of this glass lining repair technique will likely have additional considerations and complications when compared to the controlled experiment process; this is further discussed in the project conclusions.
For more details on the testing setup, results, and conclusions, MTI members can visit the MTI Technical Resource Library and access the MTI Project 266 report.