Fictitious force makes quant um leap forward
New functions in Coriolis mass flow rate measurement

A well-established physical (measurement) principle, suitably in-depth market penetration and technical sophistication deliver the KO for technical progress, they say. Far from it, however: even in a market that is as hard fought as the mass flow rate measurement market—or perhaps for that very reason—significant technical progress is still possible. The following article describes the extension of Coriolis measurement to multiphase flows and a new form of sensor control.

The flow measurement market is tough. Coriolis, differential pressure and ultrasound have all found their niche in the three billion dollar segment. As with other automation technologies, there is little scope for significant movement here. Only for mass flow measurement on the Coriolis principle do market researchers see a gentle rise in the medium term. Such assessments, nevertheless, are based on rather vague predictions and stated intentions, while concrete technical innovations, or at least the potential for them, have less of a role to play. Even so, there are at long last reports of technical innovations that are tending to confirm the market success of the Coriolis principle and could even exceed the already positive expectations.

In June this year, Micro Motion, a subsidiary of Emerson Process Management and a manufacturer that has been working with Coriolis measuring devices since the seventies, announced two major new products. As so often, the trigger for development came in a survey of customers, which revealed that gas or air fractions in the mass flow frequently caused problems in operation. In addition, inline assembly, normally an advantage of the measuring principle, proves to be a disadvantage when it comes to the ability to inspect the sensors, because the measuring instruments have to be removed, examined and calibrated at intervals of time that were difficult to define.
Two phases, two questions
One of the hurdles to use of the Coriolis principle is two-phase flow, as user Jim Reizner of Procter and Gamble explains: “If Coriolis systems could measure two-phase flows better, the possibilities of using this technology for liquids would broaden considerably, significantly reducing wastage in filling processes.” Reizner is not alone: the chemical industry, represented by NAMUR, is demanding something similar, as stated in NAMUR Recommendation 107 (see also Info-Click). To resolve the problem, Micro Motion's engineers identified a variety of approaches: small fractions of gas or air (bubbles only) are combated by changes in the sensor. Slug flows are countered by a modification in the electronics and, in the case of partially filled meter tubes, by means of sensors and electronics.
In general the patented improvements offer a faster processing speed, with the device also delivering correct measurements even under rapidly changing process conditions, e.g. at the start of a foam phase. In addition, special algorithms process the change in the signals which are recorded under unstable flow conditions. And, since the behavior of the sensor was further insulated from external influences, the new technology can still detect the measuring signal even in the case of high photoelectric noise levels, e.g. caused by gas in the liquid. The result is that the measuring instruments no longer interrupt the measurement but instead deliver values within the measuring range, i.e. useful values.
No need for removal
It was a search for a clever solution that also led to the demand from users in the oil and gas industry: “We have to remove the system so that we can test it on external test benches. That is cumbersome, takes up almost a full day and costs 2000 to 3000 euros per system per year.” So what “unencumbered” value could information about the complete sensor system deliver in order to avoid or at least delay this need for dismantling? What value is not a function of the installation location, ambient conditions or process variables such as temperature or pressure? The answer is attractively simple: the structural integrity of the measuring instrument, whose measuring principle exploits the vibratory reaction of the mass flow (see also text box). To utilize the sensor dynamics to record the sensor status, therefore, the experts imitated a mass-spring system into which several defined pulses are sent. In the low-frequency range under 50 Hz— in normal operation the instrument measures at about 100 Hz—the reading is virtually independent of the mass and other parameters and is merely affected by the rigidity of the system. This enables all events that bring about changes on this system to be determined. They include erosion as well as coatings or corrosion damage.
From the reaction of the pulses, the electronic system permits the following conclusions, for instance:
-There is an error, the measuring instrument is working outside specifications.
-The error is X% of the reading.
-The change began at point Y.
It takes about three minutes to inspect a measuring instrument; this can be initiated by Emerson’s AMS Suite software package for predictive maintenance or from the local user interface. System engineers can run the test without any special training. The process does not need to be paused, nor do external references have to be procured and installed. And because it gives users the ability to predict when a flow meter needs to be repaired, recalibrated or replaced, they can actively plan their maintenance activities instead of simply reacting to faults.
Accuracy also improved
This ability to inspect flow measuring devices is, says manufacturer Micro Motion, only available for flow meters of the Elite series, the high-end measuring instrument with MVD (Multi Variable Digital) technology of the next generation.
The development of MVD technology and the improved sensor design bring a further welcome innovation: mass flow and the density of liquids can be determined more accurately than before regardless of variations in operating conditions. The accuracy of density measurement actually doubles and the long-term stability is increased so much that there is no need to adjust the zero point in the field. With an accuracy of -0.10% over a measuring range of 20:1, Elite was previously the most accurate Coriolis device on the market. Today the manufacturer offers an accuracy of +/-0.05% in mass and volume flow rate over a measuring range of 20:1 and a density measurement accuracy of +/-0.0002 g/cm3.
This new product is attracting attention for applications where high accuracy is important, e.g. the filling of reactors, the dosing of additives or catalysts or the transfer of weights and measures. According to Tom Moser, President of Micro Motion, the accuracy of the new measuring instrument has already persuaded users to apply it for “other” purposes: an analyzer for pure density determination. “In the past Coriolis devices were known for their precision, but they were also regarded as being very sensitive to varying ambient conditions, e.g. rapid changes in the temperature of the process medium and environment or stresses in the meter tube”, adds Moser. These objections from users have probably been overcome with the news on areas of application and accuracy. It remains to be seen, then, whether analysts’ expectations of the market will be met or even exceeded—but Coriolis’ market leadership appears to be secure.