Breaking the Metallic Path: Easier Said Than Done
Author: Tim Hurley
Corrosion prevention in flanged pipeline connections in naval and commercial vessels has been a constant maintenance and safety challenge for more than 200 years. The various technologies commonly used to mitigate this problem are almost as old.
Phenolics were invented in 1900. Phenolic gaskets are used in many flange joint applications including:
- The interior face of ring-type joint flanges.
- Full-faced gaskets cut to the outside diameter of the flange with bolt holes.
- Gaskets for the raised face portion of the flange
- Cathodic water meter isolating gaskets for use with dissimilar metals including steel or cast iron to brass.
Fiber Gaskets were invented in 1899 by an Austrian company. These gaskets were used widely across industrial and naval applications, have been used by the U.S. Navy as recently as the 1980s and are still in service in the current fleet. Fiber gaskets and phenolics are often vulnerable to chemical attack, creep relaxation, deformation and other failures.
In other flange isolation technologies, metal parts may be shielded with a sprayable polymeric resin coating.
The suboptimal nature of materials used for flange isolation results in flanges that are apparently isolated but still corrode. Gasket materials may contribute to failure due to:
- Cracking of the gasket, the sleeve or the washer
- Creeping of the gasket material
- Creeping of the flange bolts, leading to gasket relaxation
- Moisture absorption
But any isolation technology is only as good as the technicians who install and maintain it. Bolts that are not tensioned correctly, gasket material that is cut inadequately, bolts not specified for a flange isolation setting – there are many ways for human error to cause isolation materials to fail and lead to electrochemical corrosion.
For a flanged joint, a modern approach may be the installation of a flange isolation kit designed to electrically separate the bolts from each side of the flange, while the isolating washers provide electrical isolation for each of the nuts attached to the bolts. This kit would be combined with an isolating gasket that, for maritime use, should also be suitable for high-chloride environments to avoid cracking.
But gasket-based flange isolation kits, as outlined above, are necessarily a temporary solution and will always require periodic inspection, maintenance and replacement. A completely modern approach would involve doing away with the flanged joint entirely and replacing it with a monolithic isolation joint (MIJ) where access is not an issue or a pre-fabricated flange kit where access is desired. Both are factory-made components and the MIJ is protected by a polymeric resin coating. Both are designed to be welded in place and the MIJ is durable enough to go 20-30 years without maintenance. While the individual components of the MIJ must be the subject of detailed due diligence and review, an MIJ ought to not only eliminate risks presented by installer/technician error but also reduce requisite preventive and reactive maintenance.