Improved Ultrasonic Flow Measurements of Coal Slurries

The measurement of volume flows, using portable ultrasonic flow meters, has been a major tool used in the data collection phase during plant audits in the coal industry. The accuracy of these ultrasonic flow measurements have been inconsistent due to various influences, generally related to fluid turbulence and non-full pipe flows. Additionally, more recently, the proliferation of wear resistant lined pipes, eg: ceramic and basalt linings, has resulted in a significant decrease in the accuracy and reliability of these portable ultrasonic flow meters, usually reflected in reduced unit signal strengths.

This project investigated the principal causes of the decreased signal strength and any associated "Non-Doppler" frequency altering effects of the ultrasonic signal as it passes through the steel/lining/coal slurry medium. The project testing involved the measurement of Doppler-Effect flow signals under a range of piping mediums and flow conditions. The specific objective of the project was to investigate and optimise the variables involved with attaining reliable and accurate flow rate measurements.

Current technologies basically address the resolution of the "true" signal from the sampled signal and not signal attenuation issues, caused by the transmission path. The proposed approach in this project is to alter the emitted signal to enhance unit performance rather than waiting until signal capture and then to try and resolve the Doppler spectrum. The proposed technique seeks to improve the accuracy and precision of the resultant flow signal by enhancing the emitted signal instead of the captured waveform.

Key conclusions for the project:

1. The difficulty in achieving an adequate Doppler-Effect flow meter signal on lined pipes is directly related to the attenuation of the transmitted signal resulting in an inability to discern its specific frequency spectrum from that of the background noise.
2. Lined pipes may cause high signal attenuation in two (2) ways
1. Bonding material degradation may cause an air gap to form in the wall cross section. (Resulting in a high attenuation solid/air interface).
2. Material used to create the pipe wall may cause high attenuation due to its high sonic impedance.
3. Due to the attenuation for a given transmission path remains constant with variation in emitter signal strength, an increase in the magnitude of the emitted signal (Voltage peak-to-peak) is believed to be the optimum mechanism to generate improved signal to noise ratios on lined pipes, or mediums of high sonic impedance.
4. Basalt lined pipes gave the worst return signal frequency spectrums due to the low signal to Noise Ratio SNR. Both Poly and steel piping mediums displayed adequate SNR's, sufficient to determine viable flow readings.
5. It is believed that as flow rate increases there is a larger differential between the laminar wall flow rate and that of the centre flow rate. Should the emitted signal from the flow measurement device fail to penetrate into the centre flow stream, then increasing errors in the observed flow rate are likely to result. This was reflected in a divergence of certain flow meter readings with increasing flow rate.
6. Using an exposed lining technique and mounting the transducers directly to the lining material significantly improved the ability to achieve a reliable flow measurement.
7. The impact on flow measurement accuracy on transducer position around the circumference of the pipe is critical. The mounting of the transducers on the pipe at 90 degrees was found to deliver the most reliable flow readings under the conditions tested. However, it is believed that 180 degrees should deliver the optimum measurement of average particle flow in the pipe due to the likelihood that return frequency spectrum would be generated from reflection from the full stream flow profile.
8. The test unit with the greatest available transmitter voltage peak-to-peak consistently delivered the best flow rate measurements for all of the test run conditions and piping mediums when compared to a reference MagFlo meter.

Key outcomes from this project:

1. Development of an exposed lining technique for measurement on lined pipes, in particular basalt lined pipes.
2. The detailing of a proposed conceptual Doppler flow meter inclusive of a range of functionality believed to make such a device capable of generating viable flow measurements on basalt lined pipes and/or high sonic impedance piping mediums.