Measurement of Wall Thickness in Heat Exchanger Tubes by Simultaneous Use of Low- and Highcoherence Fiber-Optic Interferometry

Measuring wall thickness (WT) in industrial tubes, pipes, and fittings, such as elbows and tees, is vital for ensuring safety in chemical plants, nuclear facilities, and gas or liquid systems. Wall thinning due to flow-accelerated corrosion and erosion can lead to structural failures, causing leaks of hazardous substances. Current non-destructive testing methods, such as ultrasound and eddy current, require plant shutdowns and manual operation. To address this, we propose a fiber-optic sensing technique for WT measurement in running plant operations, which uses lowand high-coherence interferometry applied simultaneously. The measuring set-up consists of a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$3 \times 3$</tex> single-mode fiberoptic coupler, with the sensing and the reference arms as multi-turn coils bonded to a monitored tube. A dummy stainless-steel tube (120 mm length, 18 mm OD) with three welded sections of different WT (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{0. 5} \boldsymbol{,} \mathbf{1. 0} \boldsymbol{,} \mathbf{1. 5 ~ m m}$</tex>) was tested under 0-8 bar air pressure. WT was determined from the strain derived from the low- and high-coherence interference signals. The measured circumferential strain ranges up to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$50 \mu \varepsilon$</tex> for a 5 -turn fiber coil.