Sensing in Pressure Vessels: Fiber Optics Open Doors for an Industry

Matt Reaves
Optical Scientist
Lightwave Division

We’ve already shown you some of the benefits of fiber optic sensors in our previous blog post “So, what is so compelling about distributed fiber optic sensing?!”.  In that article, we showed that fiber optic sensors show excellent agreement with traditional foil strain gages while also exhibiting superior fatigue performance, as demonstrated in a fatigue test of a 9-meter wind turbine blade.  Here we would like to show you how distributed fiber optic sensing can open doors (or rather create them!) in well-rooted, mature industries.

Figure 1: Luna’s fiber optic sensing technology for composite-overwrapped pressure vessels

Composite-overwrapped pressure vessels (COPVs) have been around since the early 1970s and can be found providing fuel for natural gas buses, oxygen for scuba divers and firefighters, and propellants for satellites.  (For more information on COPVs, see the NASA document referenced below.[i])  Because of the safety concerns associated with pressure vessels, manufacturers are subjected to strict qualification and certification requirements, including large factors-of-safety for operational conditions and service lives.  In fact, many COPVs certified by the DOT are required to be requalified every five years![ii]  Often, recertification requires cylinders to be taken out of service and sent to a third-party for testing, including a volumetric expansion test.  In such a test, the cylinder is pressurized up to a specified test pressure; if the permanent expansion of the cylinder (after depressurization) exceeds a pre-determined
fraction of the total expansion, the cylinder is condemned!  In other words, a COPV can be taken out of service if the cylinder exhibits a large amount of residual strain.  This sounds like a great opportunity for fiber optic sensing!

Embedded fiber optic sensors may provide an in situ measure of volumetric expansion, as well as damage and impact detection capabilities, which could save the industry time and help ensure safer operation and longer service lives of COPVs.  In addition, the embedded sensors can be also used to monitor each step of the manufacturing process, which can aid in the design process and serve as a process control tool on the manufacturing floor.

Figure 2: Fiber optic strain data mapped to COPV geometry following drop test.

Over the past few years, Luna has worked with Structural Composites Industries (SCI, a Worthington Cylinders Company) and the United States Special Operations Command (SOCOM) to develop a structural health monitoring tool for COPVs.  Over the course of the program, Luna and SCI manufactured more than 30 COPVs with embedded fiber optic sensors.  An updated sensing platform and software was also produced that allows the user to see a color map of the measured strain profile on a 3D model of the cylinder.  The COPVs were subjected to a variety of tests including drop tests, burst tests, and cycling, all of which were monitored using the embedded sensors.  In addition, Luna demonstrated the ability to monitor residual strain during the manufacturing process.  This exact measurement could potentially provide a means to recertify cylinders in place! 

Will you be at SAMPE 2013?

We’ll be presenting some of our results at SAMPE 2013 in Long Beach, California on May 7th.  If you’re attending SAMPE 2013, you can find us in booth Q41 with several demonstrations on hand.  Be sure to visit and try out our ODiSI B high-speed distributed sensing system or just stop by and talk with an engineer – we’re happy to discuss your application and provide more information on the capabilities of fiber optic sensors.  See you in Long Beach!

This material is based upon work supported by the United States Special Operations Command under Contract No H92222-10-C-0046.  Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the United States Special Operations Command.

[i] McLaughlan, P. B., Grimes-Ledesma, L. R., “Composite Overwrapped Pressure Vessels, A Primer,” NASA/SP-2011-573

[ii] 49 CFR 180.205 (2010)

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