Coriolis mass flowmeters under entrained gas conditions

Coriolis mass flowmeters are usually only used for single-phase fluids, i.e. either liquids or gases, since it has been found that their accuracy can be affected by two-phase flows, e.g. the existence of entrained gas in a liquid flow. It is now known that there are various sources of error, among which the significantly increased compressibility due to gas entrainment brings the most difficulty when Coriolis meters are used in the field.

The authors of this white paper have developed a new Coriolis sensor together with an innovative technology, Multi-Frequency Technology (MFT), which can compensate the measurement errors introduced by the increased compressibility of an entrained gas flow. The new Endress+Hauser Coriolis sensor provides a novel hardware platform so that a higher natural mode of the measuring tube can be reliably excited in addition to the basic working mode. Being driven at different frequencies, the same two phase fluid in the measuring tube can have different influences on the primary mode (i.e. the basic working mode) and the auxiliary mode (i.e. the higher tube mode).

By analyzing the corresponding vibrational response of the two phase fluid at the two frequencies, the unique resonance property of this mixture can be obtained and the induced measurement errors can be compensated. In this paper, the theoretical background of Coriolis entrained gas metering is given. Afterwards, the basic principle of the MFT is explained in detail. Measurement data obtained at two independent laboratories are provided to prove the improvement brought about by this technology.