Problems in Measuring Steam Flow at 1250 Psia and 950 F With Nozzles and Orifices

Abstract

Abstract Measurement of steam flow at 1250 psia and 950 F with nozzles and orifices is discussed (a) during their calibration and (b) during performance tests on a turbine. All primary elements were constructed to ASME specifications, the nozzles being of long-radius, low-ratio design with throat taps, the orifices employing radius taps. The calibrations for the flow nozzles indicated that reliable repeatable results could be obtained although one nozzle exhibited an abrupt increase in discharge coefficient of about 0.5 per cent at about 2 per cent pressure differential. The measured coefficients of discharge were within Power Test Code “probable tolerances” of ±1.25 per cent. The calibrations of the orifices were of little value because of permanent distortion of the orifice plates as a result of difference in expansion between the plates and their holding flanges. For measuring steam flows to the turbine, additional orifices were made up of different material, reducing distortion of the plates to the point of being negligible. The results of the turbine test with orifices and nozzles in series indicated excellent agreement between the flows based on orifice data using calculated coefficients and those determined from nozzle data using coefficients obtained from the calibration. Some trouble was experienced from deposits found around the nozzle throat taps. While this paper presents data indicating excellent agreement of the two methods of flow measurement during the turbine-performance tests, it also points out the need for more experimentation and knowledge of steam-flow measurement at elevated temperatures and pressures.