The influence upon Coriolis mass flow meters of external vibrations at selected frequencies

Coriolis flow meters are vibrating tube devices and they are, therefore, potentially susceptible to the influence of external vibrations transmitted from the installation environment. This paper presents the results of an experimental investigation of the effects of external vibrations carried out on eight Coriolis flow meters, (from five manufacturers) covering a range of meter tube geometries and sizes. Results published by others working in this area have been very incomplete and have not revealed any insight into the underlying processes by which metering errors may occur. The present work was guided by the identification of discrete frequencies which merit experimental investigation, from our theoretical work on vibration effects (straight tube meters) and from finite element simulations of complex geometry meters. Of particular importance are vibrations at the drive frequency, at the Coriolis frequency and, for non-sinusoidal waveforms, at their respective sub-multiple frequencies. Meters were mounted in a horizontal pipe-run, using the manufacturer's recommended orientation, and were vibrated vertically. The vibration tests were repeated following a 90degrees rotation (of the meter) about the axis of the pipe-run. Both uniform and non-uniform vibrations were applied. All meters displayed errors when vibrated at precisely the drive frequency; the magnitude of these errors was dependent on vibration amplitude, but appeared largely independent of flow rate. For some meters, the drive frequency induced errors appear to be due to vibration induced meter-zero offset. From current knowledge it is difficult to identify a procedure to overcome meter errors induced by energy added at the drive frequency by external vibration. Errors induced by vibrations at the Coriolis frequency and at other non-drive frequencies were found to be largely due to the nature of the algorithm implemented (by the manufacturer) for the determination of the phase difference between the two internal sensor signals. Some manufacturers using later generation algorithms have successfully overcome the problem of errors due to the addition of vibration energy at the Coriolis frequency. As predicted by the results of our theoretical and simulation work, the amplitude and sign of meters' errors were, in general, strongly influenced by whether the vibration was applied to the meter uniformly or asymmetrically. The amplitude and, in some cases, the algebraic sign, of meter errors were different for the two planes of vibration which were tested. (C) 2003 Elsevier Science Ltd. All fights reserved.