VELOCITY MEASUREMENT IN BOILER TUBES USING A NOVEL ULTRASONIC FLOW MEASUREMENT TECHNIQUE |
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| M. L. Sanderson, R. H. Al-Rabeh |
- Abstract:
- A method is reported here for the ultrasonic measurement of flow in boiler super-heater tubes. The tubes are in a tube bank formation running at high temperature (typically 300°C) and pressure (typically 100bar). To enhance heat transfer, the tubes in this application are ribbed internally with thick helical ribs. Further, all measurements have to be made on one side of the tube bank to avoid direct radiation on the fire side. Conventional ultrasound methods are difficult to apply in these conditions. The combination of thick steel walls, internal ribbing and the substantial temperature and density gradients do not provide for a well defined sound beam path. Instead a new method is proposed to measure the flow rate using the properties of the frequency spectrum of the noise imposed on the sound beam by random fluctuations in the velocity and density of the fluid caused by the nature of the flow in such tubes.
The measurement system consists of an ultrasonic beam generated using a conventional piezoelectric disc transducer/buffered from the hot boiler tubes by long threaded steel stalks which are bolted to a bracket welded to the tube bank. A second similar probe is fitted to receive the reflected sound beam signal. A resonant cavity is formed composed of the tubes, the water and the buffer rods. Envelop detection and frequency analysis is then applied to the received signal and the flow rate is deduced from the frequency contents of the noise spectrum. Such noise is caused by small changes to the effective path length of the sound beam caused by the temperature and density variations accompanying the flow. The received spectrum shows random components and its frequency content correlates well with the flow velocity.
A simple theoretical model in which the sound beam is assumed to be deflected by velocity and density variations is used to model the events leading to the observed results. Both laboratory and site measurements made during commissioning of a 300MW boiler are reported showing very similar behaviour when the frequency spectrum of the demodulated signal is considered. A similar observation applies to the frequency spectrum obtained from the theoretical model. A linear relationship is observed in all these cases between the flow velocity and the frequency content of the received signal- within the range of velocities considered. Further experimental and theoretical work is needed to cover a wider range of velocity and include effects of the internal ribbing and other factors not taken into the theoretical or the experimental treatment of the present work. - Download:
- IMEKO-TC9-2003-060.pdf
- DOI:
- -
- Event details
- IMEKO TC:
- TC9
- Event name:
- FLOMEKO 2003
- Title:
- 11th Conference on Flow Measurement
- Place:
- Groningen, NETHERLANDS
- Time:
- 12 May 2003 - 14 May 2003