Thermography has been used in the past to monitor active, large, cryogenic storage tanks. This approach proposes to use thermography to monitor new or refurbished tanks, prior to filling with cryogenic liquid, to look for insulation voids. Thermography may provide significant cost and schedule savings if voids can be detected early before a tank is returned to service.
The Launch Complex 39 Pad B liquid hydrogen storage tank at Kennedy Space Center has had performance issues since it was put into service in 1965. The loss rate from the Pad B tank was two to three times more than the Pad A tank, which has resulted in a significant cumulative loss of hydrogen over more than 40 years of service. It has been theorized for years that the performance degradation was due to an insulation void; however, because of the cost and schedule disruption that would be required to fix the problem, it remained in service until Pad B was turned over after its support for the Shuttle program was finished. With the tank taken out of active service, it was confirmed that a major insulation void was present.
Because of the large thermal mass of the inner and outer spheres, heat transfer between surfaces to equalize temperatures can be relatively slow, even when the temperature differences between the spheres themselves is small. Therefore, thermography has been suggested as an aid in acceptance testing of the tanks before cryogen is introduced to any tank, new or refurbished.
Models suggest that areas without insulation will heat less rapidly under solar illumination than areas with insulation, due to better thermal contact with the inner storage sphere. The resulting temperature difference across the outer shell of the tank should be a few degrees Celsius, which can be easily visualized by off-the-shelf long-wave and mid-wave cameras.
The opportunity to test this theory presented itself over the last year as the Launch Complex 39 hydrogen tank was taken out of service in order to complete weld repairs.
The ability to detect insulation voids prior to filling with cryogen will save money and time, eliminating the expense of losing cryogen and the months required for chilling down and warming up the tank if a void is discovered after the cryogen is introduced into the system. Potential savings could be in the millions if large voids are detected early.
This work was done by Ellen Arens and Robert Youngquist of Kennedy Space Center. KSC-13575