DISCHARGE COEFFICIENT CHANGES IN CRITICAL FLOW VENTURI NOZZLES AFTER SEVERE FIELD SERVICE

摘要

This paper presents calibration accuracy results of fivernsonic nozzles after three hundred hours of field service. Inrn1995, the sonic nozzles were removed from the system forrninspection. Upon removing the nozzles, a thin layer ofrnelemental sulfur, compressor oil, and glycol was found on thernsurface of the nozzles. The nozzles were then re-calibrated "asrnfound" at the Colorado Engineering Experiment Station, Inc.,rn(CEESI) to determine any drift in the discharge coefficient duernto the surface contamination. K-Lab in Norway also foundrnsulfur deposits on their nozzle system in 1989 and laterrninstalled zinc oxide scrubber to remove low level H_2S in therngas stream.rnSonic nozzles were chosen as on-site transfer provers tornverify and calibrate natural gas custody transfer turbine metersrnin Chevron’s Venice flow metering station. The facility,rnlocated in Venice, Louisiana, USA, is owned and operated byrnChevron U.S.A., Inc., since 1987 to meter dry processedrnnatural gas downstream of a gas plant. The sales capacity isrn14.16 × 10~6 std m3 (500 MMSCFD) at a nominal operatingrnpressure of 6.9 MPa (1,000 psig). The system includes a bankrnof five sonic nozzles and a master turbine meter, all connectedrnin series with three 30.48-cm (12-in.) sales turbine meters.rnThe sales meters were tested monthly against the master meterrnfor accuracy. The accuracy of the master meter was thenrnverified with sonic nozzles every three months. If any turbinernmeters did not meet the accuracy requirement, on-siterncalibration was allowed using sonic nozzles to develop a newrnk-factor for the meter. The sonic nozzles were constructed,rncalibrated, and operated under the guidelines of ANSI/ASMErnMFC-7M-1987 standard.rnThe "as found" discharge coefficient of all fiverncontaminated nozzles were in the range of 0.1% to 0.3% lowerrnthan the original calibration data. However, the shift was stillrnwithin the measurement uncertainty of the CEESI calibrationrnfacility. The results clearly indicated that a sonic nozzle is arnrobust flow proving device and that measurement accuracyrnwas affected slightly when operating in a hostile environment.