Mesure des débits à l'aide de diaphragmes lors des essais de réception des turbines à vapeur

Ingénieur à la Direction des Etudes et Recherches, E.D.F.

Résumé

Until 1945 thermal plant power production equipment used to operate with turbo-alternators with comparatively simple cycles, no normal reheat system or of the type whose steam, drawn from the turbine to feed them, returned to the condenser after condensation. As no turbine output exceeded 60 MW in those days, the cold water at the condenser outlet could be weighed or gauged. A new generation equipment made its appearance after 1945. Due to the fact that unit outputs were steadily increasing (100 MW, 125MW, 250 MW, and 600 MW) and that much of the water from water reheater steam condensate was no longer returning to the condenser, alternative measurement methods were required. For various reasons, the choice fell on discharge measurement by sharp-edged orifice plates. Attempts were initially made to calibrate these under operating conditions, but this was found to require costly equipement which it was sometimes difficult to manufacture. It was found after a certain amount of experimentation that the calibration accuracy was not up to that laid down by the standard specifications for equipment built, installed and used in accordance with their recommendations. This article states the reasons for the choice of the vena contracta tappings system which was adopted, and the precautions required in machining the orifice plates and installing them in the pipes, quality requirements to ensure accurate discharge measurement, and a brief description of instruments for measurement of the differential pressure across the orifice plate. Discharge measurement control methods for complex circuits are mentioned, followed by a summary on a statistical study by Stoltz of a large number of flow orifice discharge data, showing the high accuracy this measurement method may be expected to provide (± 0.2 %), provided those using it are aware of the fact and exercise all the necessary care in its application.