Stennis Space Center has the need to measure the inner wall thickness of cryogenic storage tanks without entering the inner tank of the double-walled vessels. It was proposed that an ultrasonic probe be inserted through access points in the outer wall of the vessel and delivered to the inner wall of the vessel, which would provide for at least a discrete set of inner wall thickness measurements where access points were available.
The process would require that a tube (the length would be determined by the width of the annular region of the tank under inspection) be inserted through an access point [typically an access point with a ¾-inch (≈1.9-cm) threaded connection]. The tube will be pushed through the insulation between the inner and outer walls of the vessel until it comes in contact with the inner vessel wall. The tube would then be secured in place with an adapter to the threaded connection on the outer tank wall. Any insulation contained in the tube would be vacuumed out, and borescope inspections of the inner tank wall surface could be performed if necessary to determine if any surface preparation is required prior to inserting the ultrasonic probe.
An ultrasonic probe is mounted at the end of ½-inch (≈1.3 cm) outer diameter tube that will slide through the access tube that was previously inserted through the outer tank wall. For the prototype version of this tool, a 5-MHz transducer with a 0.242-inch (≈0.615-cm) case diameter was secured at the end of the tube. Coupling gel should be applied to the tip of the transducer prior to inserting into the access tube. Alternatively, liquid couplant (water or alcohol) could be delivered through the ½-inch tube and to the transducer face through grooves in the mount used to secure the transducer to the end of the tube. This would require the use of a waterproof transducer and an integrated cable to prevent water intrusion. Once in place, wall thickness measurements can be performed.