I’ll admit that I’ve always thought my questioning nature was a strength. Philosophically, I hope this leads me to keep an open mind regarding things I don’t know. If I don’t understand something, I learn as much about it as I can to form an accurate opinion. So when I hear tales of mysterious NDT machines that can find FRP laminate defects and determine the remaining service life of FRP equipment using only an external scan, I will admit to you that my interest gets piqued. The industry I work in, which I would broadly classify as the FRP piping and vessel industry, desperately needs improved inspection tools. I applaud everyone who is working to develop these new technologies, however end users should be cautioned to completely understand how these developing inspection technologies relate to their fiberglass equipment.
How do fiberglass items fail? The truth is there is no one answer, there is not even a good common answer. Maybe the original designer of the equipment didn’t fully understand what the corrosion requirements for the vessel or pipeline would be. Maybe the process changed over several years in service at a facility. Maybe a process upset allowed unexpected chemicals to back up into the tank or pipeline. Maybe the low bidder you bought your FRP equipment from gave you exactly what you should have expected from them. In my experience most FRP vessels and pipelines do not fail because of issues within the primary structural laminates, and when those structural failures do rarely happen it is very early in an equipment’s service life. FRP equipment occasionally fails later due to issues with internal and external secondary overlay bonds. FRP equipment occasionally fails later due to cracked flanges from over torqueing the bolts when mating to a raised face connecting flange. Sometimes the equipment fails from a softening of the resin within the internal laminates due to chemical attack, and occasionally FRP equipment fails from a poorly designed or fabricated flat bottom and transition knuckle region.
The shell section of an FRP vessel or the structural layers of an FRP pipe are primarily simple cylinders, and the strength and engineering mechanics of these laminates has been fairly well understood for many decades now. With relatively little engineering knowledge, it is possible to determine the necessary thickness requirements for these structural components. These laminates are some of the best understood laminates within a vessel or piping system. So believing that the estimated remaining structural capacity of a storage vessel’s shell laminate is the best indicator of that vessel’s remaining overall service life, without also considering the condition of the internal laminates can lead you to make a poor maintenance decision. In my experience, FRP equipment fails at a much higher rate from internal corrosion damages or from poor initial construction than from a degradation of the structural properties of the laminates. One of the most critical areas for an FRP storage vessel is the lower knuckle transition region where a storage vessel’s side shell connects to the flat bottom. If this region isn’t sufficiently stiffened and properly constructed, the vessel’s bottom will flex during normal operation, and through repeated flexing in this critical region cracks will develop and allow future leaks. How can you tell if there is any stress cracking occurring in the very critical lower knuckle region of your vessel without a periodic internal visual inspection? How can you tell whether the internal laminates are softening from a chemical attack with any of these externally applied NDT tools? How can you tell if the internal laminates are debonding to the point of concern with any of these externally applied NDT tools? How can you have confidence that performing NDT sporadically on your piping is representative of the thousands of feet in your plant? The truth is you can’t, and without that knowledge you won’t be able to make a proper, well-informed decision regarding the future of your FRP equipment.
I want there to be a tool that can give me the local or global health of an FRP laminate including the secondary bonds and the corrosion barrier surfaces, just by attaching a couple of probes to the exterior of a piece of FRP equipment. I really do. You need to use all of the available inspection tools that are available to you, together with a review of that data by your company’s Subject Matter Expert when making decisions regarding maintaining your fiberglass equipment. Gather all the available data from your inspection sources, which have been performed during the recent life-cycle of the equipment and make confident maintenance and replacement plans, in order to maximize the safe service life of your FRP equipment. Don’t just blindly trust a single source of inspection information because it is the most convenient when it comes to making important decisions that could impact the lives of several of your coworkers.
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