Automatized Scaling Monitoring in Pipelines with Resonance Testing

Abstract

Scaling is a significant cause of efficiency loss, downtime and maintenance costs in almost all industries in which pipelines play an integral role. In this work, a non-invasive, cost-effective and automatized monitoring approach to detect the onset and evolution of scaling growth was developed. The designated field of interest was geothermal plants, which particularly struggle with calcitic scaling in Bavaria. An additional criterion that had to be met was therefore applicability at high temperatures. A resonance testing setup, consisting of automatized excitation using a solenoid-based impactor, piezoelectric acceleration sensors coupled to the pipe, an oscilloscope, a signal conditioner and mounting gear, serves to collect a number of amplitude-time signals. These are Fourier transformed, averaged and evaluated in the frequency domain. By coupling the sensors (standard operating temperature limit ~ 120 °C) to the pipe with a high-temperature couplant and samarium-cobalt magnets, the setup is robust in an environment with elevated temperatures. Such an environment is presented by the injection borehole at a geothermal power plant where in-situ measurements are currently being carried out (60 – 80 °C). It can be observed that the frequency peak positions shift with scaling thickness. In order to analyze this shift in frequency, an experiment was conducted in which a heavily scaled piece of pipeline from the production well of a geothermal power plant in Bavaria was inserted in a specially designed descaling test rig. An acidic solution was pumped through the pipe, gradually etching away the calcitic scaling. By carrying out resonance tests at regular time intervals over the course of the descaling process, a continuous reduction in peak frequencies could be determined with decreasing scaling thickness. With an adequately chosen time interval and appropriate measurement parameters (sampling rate, signal length, gain), the onset- and growth of scaling are measurable in the submillimeter regime.