A Fiber-optic Technique for the Wall Thickness Measurement of the Industrial Tubes under Harsh Environment

In industrial tubes used in harsh environments like high pressure or temperature in heat exchangers, the issue of wall thinning due to flow accelerated corrosion can end in structural failure. The conventional method for the wall-thickness (WT) monitoring is offline, involving non-destructive testing (NDT) of the tubes, only when the plant is shut down. Currently, there is no NDT technique available on the market for online WT measurement, primarily due to challenges posed by high-temperature, high-pressure, steam atmosphere, and confined spaces. We propose a sensor system capable of overcoming these limitations, associated with common NDT techniques. It is a system based on a Michelson interferometer made around a 3x3 single-mode fiber optic coupler, where the sensing element is a fiber-optic coil firmly attached to the subject tube. During the normal operation, the tubes vibrate, and wall material loss alters its resonant frequency and stiffness, which can be detected via change of strain across the tube wall. The resonant frequency is calculated through FFT analysis of the strain signals acquired by the optoelectronic unit. The wall-thickness is determined from the resonant frequency of the tube. Experimental trials were conducted in laboratory on stainless-steel tubes with different lengths, outside diameters, and wall-thicknesses, by subjecting the tubes to vibrations induced by a shaker, or hammer impact.