A parameter identification technique for detection of spacer locations in an assembly of two coaxial flexible tubes

Assemblies of two horizontal coaxial flexible tubes with loosely held spacers to maintain the annular gap between the coaxial tubes, are generally used in nuclear reactor for carrying hot fluid inside the inner tube with an insulating gas filled annulus between the outer and inner tubes to reduce heat losses. The appropriate location of these spacers is important for maintaining coaxiality and preventing contact between inner and outer tubes due to bending creep of inner tube. Determination of spacer locations is therefore an important task. The conventional method of inspection may be costly and time consuming. This paper presents a non-intrusive technique based on vibration measurement, developed for the detection of such spacer spring locations in the assembly of the two coaxial tubes. The technique is based on a parameter identification approach. It involves the identification of spacer locations by updating the position parameters of the spacer in a Finite Element (FE) structural model through the optimization of an error criterion based on the difference between measured and computed natural frequencies of the assembly of the two coaxial tubes. A gradient-based method is used for optimization in the FE model updating problem. The proposed technique has been validated by numerical simulation as well as on a laboratory scale experimental setup.