Ensemble pour la mesure instantanée du débit en conduite au moyen de radio-éléments

Equipment for instantaneous flow measurment by radioactive tracer

Electricité de France, Centrale de Recherche et d'Essais de Chatou.

Résumé

This equipment features a radio-nuclide (barium 137) generator directly connected to a pneumatic injector, and two similar detectors on the outside of the pipe connected to an electronic counter giving a direct reading of the values from which the rate of flow is derived. In developing these measurement methods, use was made mainly of such medium-period radio-nuclides as iodine 131, bromine 82 and sodium 24, With the following main purposes in view : (i) To find applications for short pipes (i.e. up to 80 diameters) ; (ii) Experimental confirmation of mathematical approaches ; (iii) Design and development of equipment for simple use. A barium 137 generator was developed, which has added considerable interest to the measurement of pipe flows by injection of radio-active salts. It relies on the natural caesium → barium 137 relationship, with the "parent" caesium 137 being fixed to a highly-selective ion- exchange resin. Washing this resin in water carries the barium 137 bred by the disintegrating caesium away, but without moving the latter. As barium 137 has a period of about 2.5 minutes, much higher radio-activity can now be fed into the pipe, for 100 minutes later finds it reduced to only 2-40 times its original value. This even makes repetitive closed-circuit tests feasible. In view of these promising results, the author's team developed an injector fitting the generator jointly with Société Saint-Gobain-Techniques Nouvelles-who had supplied the generator-in order to climinate any need to manipulate the radioactive solution between preparing it and feeding it into the pipe. A brief description of the method now follows, in order to seriate the principal questions considered in the study : In order to determine the rate of flow in a pipe at least 50 diameters long and whose cross-sections are known, a radio-active solution is injected and its times of passing two specific cross-sections A and 13 are detected from outside the pipe. The most important points considered in describing the method are the following : (i) Injection ; (ii) Distance of point of injection from first detection section ; (iii) Distance between the two detection sections ; (iv) Detection. The mathematical approaches are specifically dealt with in a three-part Appendix, as follows: The tirst part shows that, knowing the abscissa of the temporal centre of gravity of the curve corresponding to the passage of the radio-active solution, the rate of flow in the pipe can be asceriained by outside observation. The second part discusses deformation due to the detector filters. The third part leads to the estimation of discharge mor where interpretation is based on one particular geometrical point only. The heading "injection" ((i) above) covers the injector and its operation. Heading (ii) refers to a study of turbulent diffusion phenomena and the "efficient mixing" length where detection is outside the pipe, and especially the determination of I∞ from observed I2 and I2 values. The distance between detection sections (heading (iii)) does not enter into the method error calculations. It determines the accuracy of radio-active solution passage time measurement at the two sections. Heading (iv) comprises the analysis of the two following data abstraction methods : (a) Plotting of curves and determination of a point with about the same characteristics as the centre of gravity as regards measurement. (b) Bringing the centre of gravity of the radio-active "cloud" into evidence by direct processing of observed data. The discussion of (a) shows the difficulties of using a single geometrical point and defines one of them, and Table 1 lists the results obtained from 110 injections in successive groups of 10. Under (b), the calculator associated with the detectors is described, sub-groups are listed, and the development tests involved are reviewed. Finally, the advantages, applications and potential development scope of the method are discussed.