Une technique ultrasonore pour la supervision des canaux d’irrigation ouverts Ultrasonic technique applied to flow measurements in open channel irrigation

Résumé

Cet article présente un débitmètre à ultrasons conçu en tant que capteur intelligent pour la mesure des écoulements en surface libre ou en conduites fermées. Ce type de débitmètre peut être utilisé pour des applications industrielles ou de gestion de l’eau. Il répond à la forte demande des autorités de supervision des ressources en eau dans les régions semi-arides d’agriculture irriguée. Un état de l’art et les principes généraux des mesures de débits de liquide par ultrasons sont donnés. Le débitmètre original proposé utilise le principe de la mesure des délais entre les impulsions envoyées vers l’amont et vers l’aval dans le fluide. L’approche repose sur la mesure de la différence du temps de transit des ondes acoustiques sur une ou plusieurs cordes de mesure, réparties sur un intervalle suffisant, de manière à assurer la précision requise. Cette méthode permet une véritable intégration du profil des vitesses, ce qui assure une meilleure précision. La vitesse du son dans le fluide est éliminée des calculs grâce à l’utilisation des temps de transmission réciproques. Les calculs de débit sont assurés par un micro-contrôleur. Les calculs théoriques et les technologies proposées ont donné lieu à la réalisation d’un prototype expérimental. Une première application de ce débitmètre à ultrason est réalisée en laboratoire en conduite fermée dans une installation d’essai comportant une seule paire de transducteurs à ultrason. Une seconde application expérimentale est menée sur un banc d’essai comprenant un canal artificiel de forme trapézoïdale de dimensions géométriques bien déterminées avec une pente variable. Ces tests ont permis de vérifier la précision de l’appareil et de valider les techniques et les principes mis en oeuvre dans la conception du débitmètre.

Mots-clés:

• Modèles d’écoulement,
• Mesure de débit à ultrason,
• Irrigation,
• Gestion des ressources en eau,
• Capteur intelligent

Abstract

Measuring fluid flow is one of the most important measurements in the industrial fields of process control and water management. In fact, it may well be the most frequently measured process variable. Other measurements are water temperature, pressure and level. There are two types of ultrasonic flow meters used for discharge measurement: the Transit Time and Doppler flow meters. The first type is widely applied to fluid flows in pipes and open channels and is sensitive to suspended solids or air bubbles in the fluid. The second type is more popular and less expensive, but is not considered as accurate as the transit time flow meter. In this work, the Transit Time flow meter is discussed and applied, for the first time, to discharge measurements in pressurized conduits in a laboratory setting, and to open channel irrigation.

We used the Transit Time method, which is based on the dependence of the velocity of an ultrasonic signal in a given medium on the velocity at which this medium is itself flowing. Therefore, pressure disturbances of small magnitude are propagated through a fluid at velocity that is the sound velocity relative to the fluid. If the fluid also has a velocity, the absolute velocity of the pressure disturbance propagation is the algebraic sum of the two. This ultrasonic method uses measurements of the transit time of high frequency energy pulses between one or more pairs of transducers to determine the flow rate of the liquid. The relationship between the measured transit time of an ultrasonic pulse and the average velocity along the pulse path is well described in this paper.

This paper presents the state of the art of the general principle of liquid flow measurements by ultrasonic methods and the problems associated with such measurements. Using the ultrasonic transit time approach, we designed an ultrasonic flow meter according to a smart sensors concept, for the measurement of irrigation water flowing through pipelines or open channels. The new flow meter works on the principle of measuring time delay differences between sound pulses transmitted upstream and downstream in the flowing liquid. The speed of sound in the flowing medium is eliminated as a variable because the flow rate calculations are based on the reciprocals of the transmission times. The transit time difference is digitally measured by means of suitable microprocessor controlled logic.

We initially carried out experimental tests in closed conduits. The installation consisted of three tanks (storage tank, volumetric tank or etalon tank, and supply tank), a pipeline with the primary intelligent flow meter devices being tested, an ultrasonic level-meter and a volumetric flow meter for calibration. The secondary devices included a pump, valves for controlling the apparatus and a level gauge. This apparatus enabled the calibration of flow meters for various volumetric flows. The data can be gathered with a fixed sampling time and are stored and/or transmitted to the computer system, which enables not only data storage, but also the calculation of the time of measurement at a specified level of accuracy and volumetric flow rate.

The laboratory experiment was carried out to facilitate hydraulic studies with a flow rate ranging from 1.25 L/s to 50 L/s. The steel pipeline had a diameter of 100 mm. The flow rate is calculated and the characteristics of calibrated flow meter are plotted. The report from each measurement is shown on the monitor and may be printed. The flow meter is connected to a straight section of pipe that is run at least 15 pipe diameters upstream of the sensor without obstructions, and a minimum of 10 pipes diameters downstream without obstructions (locating a flow meter too close to a flow disturbance may cause a systematic error).

In addition to laboratory tests, the flow meter has also been evaluated in open channels.

Open channel flow is defined as flow in any channel where the liquid flows on a free surface, is not under pressure, gravity is the only force causing the flow and a continuous and progressive decline in water surface elevation occurs as the flow moves downstream. Examples of open channel flows are rivers, streams, creeks, and other uncovered conduits.

Open channel flow measurement is used in many applications: water supply networks, allocation of water for irrigation and agriculture, sewage treatment plants, etc. The discharge measurements in open channels may be computed by means of a rating curve, which is used to convert records of water level readings into flow rates. Because many phenomena can cause the rating curve to change, it must be checked periodically. In this context, the ultrasonic transit time technique is extremely useful in water management. By means of this technique, the flow rate is determined using single and multipath configurations. However, special attention must be paid to define the vertical velocity distribution and the suitable integration method.

The experiment was conducted in liquid (water) using a single path with two transducers (i.e., the velocity profile is sampled along a single line only), and a double path with four transducers; the liquid velocity was varied from 0.075 m/s to 2.4 m/s. The channel has a trapezoidal form (length: 4.2 m, width: 0.45 m, depth varying from 0.05 to 0.38 m) with channel slopes manually adjusted from 0.001 to 0.02, so as to produce variable flows.

In conclusion, the ultrasonic flow meters have the potential to significantly reduce the installation and operating costs of meter stations while providing accuracy levels consistent with, or better than, other traditional metering methods (vortex shedding, differential pressure, orifices, nozzles and venturi meters, Pitot-static-flow meters, magnetic flux, etc.). There may also be a substantial reduction in maintenance cost as the device utilizes digital measurement techniques and provides a wealth of diagnostic information on the status of the electronic measurement system.

The suggested ultrasonic flow meter designed according to a smart sensor concept constitutes a necessary link for the optimal water resources management of an irrigation system. It can be used with confidence and offers an economical and accurate means of measuring the flow of water in open channels. Typical uncertainties are of the order of 2% for single path and 1% for two paths and are attainable with careful application. In closed conduits, the results of the experiments demonstrate the validity and the performance of the laboratory ultrasonic flow meters. The application of the flow meters under favourable conditions produces a relative error that does not exceed 1%. This preliminary result shows the usefulness of these ultrasonic flow meters. Further investigations, however, are still needed in order to determine the overall performances of this instrument in terms of repeatability, linearity, reliability, and response time.

Keywords:

• Liquid flow models,
• Ultrasonic Transit Time Flow meters,
• Open channel irrigation,
• Water management,
• Intelligent Sensor