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Flow Metering Challenges: Removing Air and Gas

Mon, 03/13/2006 - 1:40pm
Figure 1: Working with Oxford University researchers, engineers at Invensys Process Systems, Measurements & Instruments Division developed digital technology to accurately measure flow even when air is entrained in the flow tube. The resulting patented product is the Foxboro CFT50 digital Coriolis flow meter.
By Wade Mattar New digital Coriolis flow meters can be used for process measurement of substances as diverse as whipped butter, tomato paste, paint and even ground beef. Equally important, this technology can solve problems that are going undetected. Introduction Coriolis technology, which offers unprecedented accuracy and reliability for measuring material flow, is often hailed as one of the most superior flow measurement technologies. However, conventional Coriolis meters have displayed several significant limitations. They invariably have problems — often serious ones — in batch measurements starting or ending with an empty flow tube. They haven’t been up to the task when truly rapid response is required such as when a number of small batches must be measured quickly. In addition, conventional Coriolis meters have generally not performed well in almost any measurement of two-phase flow, where a combination of gas and liquid process material is involved. This includes aerated liquid in which small bubbles of air are thoroughly intermixed in the process stream as well as slug flow, a variation that sees larger slugs of liquid and gas in rapid succession.In processing plants worldwide, inaccurate Coriolis flow measurement has been responsible for untold profitability drains including the following:
Figure 2A - Foxboro CFT50 Density click to enlarge
• Lost production of expensive process streams • Inaccurate pricing in custody transfer applications • Excess downtime due to meter stalls or even shutdowns and restarts This article examines the Coriolis approach, the challenges of measuring problematic flows and the recently improved technology that promises untroubled performance in even traditionally difficult or impossible applications. Trends and Challenges
Figure 2B - Foxboro CFT50 Mass Flow click to enlarge
With worldwide revenues of more than $400 million and growth expected to exceed $600 million, Coriolis meters are among the fastest growing flow measurement technologies. They measure flow by analyzing changes in the Coriolis force of a flowing substance. This force is generated in a mass moving within a rotating frame of reference. The rotation produces an angular, outward acceleration, which is factored with linear velocity to define the Coriolis force. With a fluid mass, the Coriolis force is proportional to the mass flow rate of that fluid. To use Coriolis force for measurement, a Coriolis meter has two main components: an oscillating flow tube, which is equipped with sensors and drivers, and an electronic transmitter that controls oscillations, analyzes results and transmits information. Consistent oscillation is vital for reliable Coriolis meter performance. Two-phase flow in just about any form, however, compromises oscillation — dependent on process liquid density, tube balance, dampening caused by the flow stream and physical isolation of flow tubes from the environment. That’s why applica-tions involving as little as 2 percent volume of entrained gas have been poor candidates for Coriolis force measurement.
Figure 2C - Other Flow Meters Mass Flow click to enlarge
Of course, Coriolis technology is highly accurate in single-phase flow, with perhaps a ±0.1 percent error level. However, two-phase flow boosts an error rate of 20 percent and higher. Making matters worse, entrained air may not emerge as the culprit until a frazzled process engineer has invested many hours trying to figure out why he or she can’t get the required results. Some analysis reports show that up to 92 percent of all Coriolis measurement difficulties are due to entrained air or gas. Yet in most cases, two-phase flow is not even recognized as the cause. It’s a critical problem that may go unnoticed. Improved Technology To crack the Coriolis conundrum, digital technology has been developed that provides accurate measurement of flow, even when air is entrained in the flow tube. The resulting digital Coriolis flow meter incorporates new signal processing techniques that provide useful measurements of mass flow and density and helps to keep the meter stable in single- or two-phase flow conditions. One of the many patents the meter has received involves an advanced control and measurement system with high-speed digital signal processing. It responds to changing flow conditions many times faster than standard Coriolis flow meters. Another patent relates to detecting and compensating for two-phase flow conditions and generating a validated mass flow measurement. The flow meter, which can perform in batch applications starting with empty flow tube conditions, can be used in any fluid metering application including difficult-to-handle materials such as slurries, non-homogenous fluids and problematic fluids that foam or flash. When used with a 3-A approved flow tube, it is also suitable for sanitary applications. Even with the flow tube empty, it responds faster than traditional Coriolis transmitters, reducing startup time and increasing production throughput.
Figure 3A - Foxboro CFT50 Density click to enlarge
Real-World Example Batching processes must either start or finish with an empty or partially full flow tube, particularly when process requirements include cleaning or purging between batches. It is generally not possible to keep a meter and the associated pipe work full. As a result, many users accept the fact that the first batch of the day, for instance, may be somewhat “off” and require reprocessing. Additionally, many batch users continue to draw from emptying supply tanks until they are practically dry. This introduces large amounts of gas as levels approach bottom. As a result, Coriolis meters have serious measurement problems at the start and the end of the batch.
Figure 3B - Foxboro CFT50 Mass Flow click to enlarge
Figures 2A and 2B and Figures 3A and 3B show the capabilities of new digital Coriolis meter technology in one such batching application at Great Lakes Chemicals in Manchester, UK. For comparison, Figures 2C and 3C show the capabilities of traditional Coriolis meter technology, which Great Lakes Chemicals had been using and which had proved unsatisfactory. A new digital Coriolis meter was installed in series with the traditional meter, enabling performance comparisons in real time. Figures 2A, 2B and 2C illustrate startup. Values have been normalized so that density and mass flow can be shown on the same graph. Densities have been normalized to the maximum value of 1,156 kg per cubic meter, and flows have been normalized to the maximum value of 14.4 metric ton per hour. At startup, the meter is partially full, as signified by the indicated normalized density of about 0.1. During this period, both Coriolis meters correctly indicate zero flow. At about 18 seconds, as indicated by the increase in density, flow begins. Although the meters are not full until about 30 seconds into the batch, there is flow that needs to be metered immediately. Notice in Figure 2C how the traditional Coriolis meter is unable to deal with the partially full condition. Not only does it miss the process fluid passing through, but it also produces a substantially negative indication, which will obviously affect the overall batch total.
Figure 3C - Other Flow Meter Mass Flow click to enlarge
Even after the meters have reached a full condition, at about 30 seconds, the traditional Coriolis meter exhibits an additional five-second lag before indicating full flow. The digital Coriolis meter, however, not only indicates full flow immediately when full but also gives a flow indication while partially full. The new technology measures roughly 15 seconds’ worth of flow at the beginning of the batch that otherwise would have been lost. Over a series of short batches, this lost measurement can produce substantial losses. Figures 3A, 3B and 3C depict the end of the same batch, which is when operators want to pump in as much material as possible to minimize waste or additional manual labor that would be needed to utilize the bottom of the supply tank. As soon as the density reading indicates a partially filled condition, the traditional Coriolis meter shuts down. It stays offline for several minutes, missing the final blow-through of product, where the flow is never entirely free of air. By contrast, the digital Coriolis meter keeps operating, providing an accurate indication of true mass flow. About the Author: Wade Mattar is a flow specialist at Invensys Process Systems, Measurements & Instruments Division, 33 Commercial St., Foxboro, MA 02035. Questions about this article can be addressed to him at wade.mattar@ips.invensys.com or 508-549-2067. Additional information is available at www.foxboro.com/instrumentation.
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