(Courtesy Invensys)Jerry Stevens, senior marketing manager for flow at Endress+Hauser adds, approximately 90% of two-phase Coriolis mass flowmeter problems we encounter involve accuracy degradation during a process that involves filling an empty flowmeter, flowing for a period of time and then emptying the flowmeter.
Blake Howe, process engineer at Nestle Nutrition, Eau Claire, Wis., uses a nitrogen blanket to transfer oil out of a pressurized vessel. The amount of oil is totalized using a Coriolis mass flowmeter where the slug-flow feature detects oil flow and starts/stops totalization.
This was a standard practice the time that the instruments were installed. However, a new transmitter with two-phase flow algorithms can measure liquid flow directly and more accurately compensate for partially full pipe conditions that are present at the start and stop of the transfer process.
Stevens remaining applications, largely include high-solid slurries or higher-viscosity liquids that tend to trap gases. Some such products require air or gas in their formulation. Many of these applications can be improved by judicious flowmeter orientation and backpressure elements that reduce the effect of entrained gases.
Matt Watson is a senior process engineer at BASF, Louisville, Ky. He installed some Coriolis mass flowmeters in the late 1990s to charge batch reactors. In that case, Each material used one flowmeter that could feed several reactors. The original hydraulic design had design flaws that made the flowmeters subject to transient slug flow, entrained air and finely dispersed bubbles.
Watson says the sources of the problems included low levels and vortexing in the storage tanks and the inability of the flowmeter piping to remain full of liquid. The downstream control and shutoff valve piping created additional opportunities for the introduction and collection of air that adversely affected the flow measurement.
Even after remedying some hydraulic issues, the flow measurement system still occasionally faulted due to two-phase flow conditions, says Watson. Many flow tests were performed on the flowmeters that exhibited zero shifts of between 5% and 15% percent after two-phase hydraulic events. This was so problematic that flowmeter fault alarms were interlocked to flag events and stopped the filling operation. The net result was a system that could overcharge or undercharge the reactors because the flow measurement system was not tolerant of process faults that caused reliability, safety and quality issues. The introduction of Coriolis mass flowmeters that could measure with entrained gases provided a solution to our problemsalbeit at a high price. During the last year or so, we were able to justify, purchase and install three Coriolis mass flowmeters in this service. All have performed well in handling intermittent two-phase flow conditions.
By way of advice, Watson says to, pay close attention to the hydraulic design in two-phase or potentially two-phase flow applications because seemingly small details are important and can render an installation useless. In particular, try to keep the flowmeter full of liquid. If you cannot do this, install a flowmeter that is tolerant to two-phase flow conditions. Do not forget to consider carefully the hydraulics of the feed lines and valves to ensure that the flowmeter accurately measures the flow of material into the vessel and not the flow of material that fills or empties the pipes. In short, hydraulic design is critical but somewhat less critical now that flowmeters are available that can tolerate two-phase flow conditions. The availability of these flowmeters is not an excuse for poor hydraulic design.
Correlation flowmeters can measure the flow rate and gas void fraction for some continuous liquid-gas mixtures.
Wade Mattar, consulting engineer at Invensys Foxboro agrees that traditional Coriolis mass flowmeters have their issues. In practice, measurement errors approaching 10% occur in Coriolis mass flowmeters under two-phase flow conditions with only 5% gas (by volume). Increasing gas volume in single-phase Coriolis mass flowmeters causes the energy required to operate the tubes to increase excessively and stall the flowmeter so that it no longer functions properly. Accurate measurement of two-phase flow under these conditions is not possible and can cause major process problems.
The Two-Phase Solution
Invensys researched this problem for almost a decade before launching its two-phase Coriolis mass flowmeter. Matter explains that, the key to measuring two-phase flow with a Coriolis mass flowmeter is to sense when two-phase flow is present and to what extent it exists. Doing so allows the electronics to adjust the energy to the tubes so that the flowmeter continues to measure flow and density without stalling. When the flowmeter does not stall, the phase distribution can be identified, and the electronics can compensate the raw flow signal to measure the mass flow and flowing density more accurately.