Research on Nonlinearity in In situ Calibration of Permanent Magnet Sodium Flowmeter Without Bluff Body

Abstract

The permanent magnet sodium flowmeter (PMSF) without the bluff body installed on the large-diameter pipe in the fast reactors to measure the sodium coolant flow rate has to be calibrated in situ owing to the degradation of the magnetic field. Since the length of the magnetic field of the PMSF is limited in practical engineering applications, the characteristic between the cross correlation flow rate (CCFR) and the standard flow rate is nonlinear in in situ calibration based on the cross correlation method. Moreover, the nonlinear characteristic has a large repeatability error, which is difficult to apply to in situ calibration. In order to realize the in situ calibration, an “M-type” mean velocity profile in the pipe is obtained by analyzing the influence of the magnetic field on the fluid field. The distribution of the CCFR of the signal at each frequency is analyzed by using the phase-frequency characteristic of the cross-power spectral density (CPSD) of the two signals, and a physical model of the turbulence distribution in the pipe cross section is obtained combining with the “M-type” mean velocity profile. A nonlinearity correction method based on signal frequency band selection is proposed according to the physical model. The in situ calibration experiments of DN150 PMSF without the bluff body are carried out at the sodium temperatures of 400 °C. The signals within 11–20 and 18–35 Hz are used to calculate the CCFR for correcting the nonlinear characteristic, and sodium flow rate measurement accuracy is ±3.0%.