Method for Detecting Perlite Compaction in Large Cryogenic Tanks

April 1, 2010

John F. Kennedy Space Center, Florida

Perlite is the most typical insulating powder used to separate the inner and outer shells of cryogenic tanks. The inner tank holds the low-temperature commodity, while the outer shell is exposed to the ambient temperature. Perlite minimizes radiative energy transfer between the two tanks. Being a powder, perlite will settle over time, leading to the danger of transferring any loads from the inner shell to the outer shell. This can cause deformation of the outer shell, leading to damaged internal fittings.

The method proposed is to place strain or displacement sensors on several locations of the outer shell. Loads induced on the shell by the expanding inner shell and perlite would be monitored, providing an indication of the location and degree of compaction.

Testing involved a small, metal Dewar tank composed of an inner and outer shell. The annular region was filled with perlite. Displacement sensors were connected at two locations on the outside of the outer shell. With the perlite not compacted, the inner tank was thermally cycled and the difference in the two displacements was measured as the inner tank warmed and pressed on the perlite.

The perlite was then compacted by hand in two areas while the inner tank was cold in order to mechanically couple the inner and outer shells. When the inner tank was allowed to warm and expand, it deformed the outer tank into an elliptical shape, and the displacement sensors detected different motions for the fluffy and compacted perlite. In any location where the perlite was still fluffy and not compacted, there was no deformation. In areas where the perlite was packed more solidly, the sensors detected a slight deflection. By running these checks between cycles, it becomes a simple matter to identify areas of perlite compaction, and replace it before it can cause damage to the outer shell.

This work was done by Robert Youngquist of Kennedy Space Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. KSC-13214

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