Remote Identification of Impact Forces on Loosely Supported Tubes: Analysis of Multi-Supported Systems

Abstract

Impact forces are useful information in field monitoring of many industrial components, such as heat exchangers, condensers, etc. In two previous papers we presented techniques—based on vibratory measurements remote from the actual impact locations—for the experimental identification of isolated impacts (Araújo et al., 1996) and complex rattling forces (Antunes et al., 1997). In both papers a single gap support was assumed. Those results concern systems which are simpler than the actual multi-supported tube bundles found in heat exchangers. Impact force identification is a difficult problem for such systems, because 1) when sensed by the remote motion transducers, the traveling waves generated at several impact supports are mixed, and there is no obvious way to isolate the contribution of each support; 2) multi-supported tubes may be quite long, with significant dissipative effects (by interacting flows or by frictional phenomena at the clearance supports), leading to some loss of the information carried by the traveling waves; 3) in multi-supported systems, some of the supports are often in permanent contact, leading to nonimpulsive forces which are difficult to identify. In this paper, we move closer towards force identification under realistic conditions. Only the first problem of wave isolation is addressed, assuming that damping effects are small and also that all clearance supports are impacting. An iterative multiple-identification method is introduced, which operates in an alternate fashion between the time and frequency domains. This technique proved to be effective in isolating the impact forces generated at each gap support. Experiments were performed on a long beam with three clearance supports, excited by random forces. Beam motions were planar, with complex rattling at the supports. Experimental results are quite satisfactory, as the identified impact forces compare favorably with the direct measurements.