Clamp-On Measurements of Fluid Flow in Small-Diameter Metal Pipes Using Ultrasonic Guided Waves

Abstract

Clamp-on ultrasonic transit time difference is used extensively to calculate the volumetric flow rate of a fluid through a pipe. The operating principle is that waves travelling along a path that is generally against the flow direction take longer to travel the same path than waves travelling along the same path in the opposite direction. The transit time difference between the waves travelling in opposite directions can be used to calculate the flow rate through the pipe, by applying suitable mathematical correction factors. The approach is non-disruptive and noninvasive and can be retrospectively fitted to pipes and easily relocated to different positions. When ultrasonic clamp-on transducers are attached to pipes with diameters of less than 30mm and a wall thickness of less than a few millimetres, the resulting guided waves can appear confusing, and produce very different signals to those observed on larger diameter pipes. The experimentally observed behaviour of these guided waves in fluid-filled, small diameter pipes is analysed, modelled and explained. Experiments are performed in copper pipes of sizes that are commonly used in buildings, and accurate measurements of water flow rates are taken down to a few millilitres per second . This technique presents new possibilities for smart metering of water supplies, where the positioning of the small clamp-on transducers is not sensitive to variations in water temperature, and low power electronics can be used.

Transducer Design for Clamp-on Guided Wave Flow Measurement in Thin-Walled Pipes

“…In the thin-walled pipes considered in this research, the received signal is complicated by the presence of guided waves in the pipe wall -liquid system [7]. Fig.…”

“…This process arises along the length of the fluid filled pipe between the transducers, and an ultrasonic wave is eventually detected by the receiving transducer. The corresponding pulse that is seen at the receiving transducer can be viewed as arising from all of the possible paths through the system that take the same amount of time [7]. This mode will henceforth be referred to as the "1V" mode, since the ultrasound travels once downwards and once upwards through the fluid.…”

“…The same is true for larger diameter, thickwalled pipes, but in that case waves that do not travel along the pipe diameter do not make a significant contribution to the signal detected by the receiving transducer, so this effect is often safely neglected. In the small diameter, thin-walled metal pipes, it is expected that ultrasonic energy that has taken a chordal path across the pipe will contribute to the received ultrasonic signal [7].…”

“…those commonly used in domestic and commercial water supplies. The generation of guided waves in the combined pipe wall-liquid system complicates the received signal, typically resulting in an inability to reliably measure flows less than 32 ml•s −1 , commonly with an accuracy less than ±50% [7].…”

“…In previous work, a technique for performing clamp-on TTD measurements with miniature sensors has been demonstrated. Using this technique, flow rate measurements down to 1.25 ml• s −1 have been achieved with an error of only 1% [7]. In the work documented in this paper, the effect of features including scattering surfaces and curved contact faces are evaluated via Huygens modelling and experiment.…”