First introduced in 1977, Coriolis mass flowmeters have grown to become one of the largest markets in terms of worldwide revenue, second only to electromagnetic flowmeters. Their unrivalled ability to directly measure mass flow and density (and, indirectly, volume flow) with high accuracy and repeatability, low maintenance requirements, and suitability for use with a wide range of fluids, both liquids and gases, makes this a fast growing flow measurement technology.
With such fast moving technology it can be difficult to forecast the ‘next big thing’, but it is possible to focus on two recent developments that look set to influence the market in the near future: high capacity flowmeters; and the ability of Coriolis meters to measure liquids with entrained gas.
Sometimes bigger is better
For many years, the vast majority of Coriolis flowmeters supplied were less than 300 mm. As the technology matured, the demand for higher capacity meters steadily increased. Significantly, the rise in the price of oil drove demand for larger meters, particularly for applications in the upstream sector where high volume custody transfer is common. While the fall in oil prices in 2014 resulted in many oil and gas projects being cancelled, or postponed, the last few years have seen oil prices (and investment) on the rise again, and large line size Coriolis meters are firmly back on the order books.
Various designs have been developed to deliver high flow rates with accuracies to 0,05%. The majority of high capacity meters are based on the enlargement of existing designs so a medium capacity bent or omega tube meter is scaled-up to give two larger measuring tubes to accommodate larger flows. In another design, two twin bent tube meters mounted back-to-back give four measuring tubes that split the flow evenly. While these meters offer flow rates of up to 4 100 000 kg/h, both designs mean that the overall size of the meter envelope increases substantially.
The release of the twin straight tube flowmeter by Emerson in 1987 followed by the launch of the single straight tube meter by Krohne in 1994, were milestones in Coriolis evolution. Straight measuring tubes offer a number of advantages over bent and other shaped tubes, including lower pressure drop and better performance in abrasive and corrosive applications, especially with titanium tubes.
Krohne focused on the development of straight tube technology releasing a number of meters in this format and resulting in the 2008 launch of a range of twin straight tube high capacity meters. In 2017, to meet the demand from the oil and gas market, the range was augmented with the launch of a 400 mm meter featuring four straight measuring tubes with a maximum flowrate of 4 600 000 kg/h.
Another advantage of large straight tube meters is their compact installation envelope. In many projects, the diameter of the meter body is almost identical to that of the pipe, meaning that they can be fitted as part of the pipe run with minimum adaptations and lower installation cost. Their bent tube cousins with long belly drops often require complex pipe layouts to accommodate them, which adds to installation costs and increases pressure drop across the process. Straight tube meters can also be installed in tight spaces where other models simply would not fit and are particularly suited to offshore and other applications where space is at a premium such as metering skids.
Whether the size of Coriolis meters will continue to increase remains an open question. The technology to deliver larger meters already exists but currently the demand does not. Having said that, it is worth considering that not so many years ago 400 mm Coriolis flowmeters only existed as R&D projects.
Entrained gas and the challenge for Coriolis flowmeters
Entrained gas in liquid (two-phase flow) is present in many processes. Sometimes it is unwanted, having been introduced by mixers, leaking seals or by top filling tanks; while with some liquids, such as crude oil, it is present naturally. Many systems are designed to prevent or remove entrained gas, but this is not always successful, or even practical. In other applications, the addition of gas is a necessary part of the process. Air is added to many foodstuffs such as yoghurt and mayonnaise to give it a light texture. Accurate measurement is therefore important.
Two-phase flow causes problems for most flow measurement technologies. Some are unable to measure at all when entrained gas is present, while others will measure the gas as liquid giving a measurement error proportional to the gas volume fraction (GVF). Some will even stop measuring when the GVF reaches a certain level. Adding a further complication, changes in process conditions can cause the flow regime and GVF to shift making it difficult to predict and manage.
Entrained gas presents a particular challenge for Coriolis flowmeters that calculate fluid density from the frequency of the measuring tube as the fluid passes through it: the lower the frequency of the tubes the greater the density, and vice versa. During two-phase flow, the gas and liquid ‘decouple’ and move at different speeds through the flow tubes, which dampens the tube vibration. Varying process conditions also cause the flow regime, GVF and tube frequency to change rapidly.
In the past, this rapid change in frequency caused Coriolis flowmeters to ‘lose’ the signal from the sensors mounted on the measuring tubes. As well as giving wildly erratic and non-repeatable measurement, the meters would often freeze at the last confirmed reading, or go into reset mode having assumed that an internal error had occurred resulting in no measurement of the process. The leading Coriolis manufacturers have developed technologies that enable their meters to work with two-phase flow (albeit sometimes with limitations), but have come at the problem from very different angles.
In 2012, Krohne first provided a two-phase flow solution with the inclusion of ‘Entrained Gas Management’ (EGM) as a standard feature on its new MFC 400 transmitter that is fitted to all Optimass flowmeters. Using high speed digital signal processing and software-based algorithms, EGM makes constant and precise corrections to the tube driver level based on real-time frequency information received from the sensor and driver feedback. This allows the flowmeter to provide continuous and repeatable mass flow and density measurement with gas volume fractions from 0 to 100%. Although Krohne does not specify accuracy (the shifting nature of the flow regime and GVF make each application unique), it does point to the fact that EGM technology has been successfully proven in use for many years.
From this, it can be seen that when it comes to entrained gas there are solutions, but they cannot yet be considered perfect. However, with the market demanding highly accurate and repeatable measurement of liquids with gas volume fractions from 0% to 100% across the full spectrum of flow regimes this is one Coriolis technology area that is worth watching.
So what will be the next big thing in Coriolis mass flowmeters? Is it bigger meters, accurate two-phase (or multi-phase) flow measurement, diagnostics, communication, or Big Data and the IIoT? Watch this space.
For more information contact Deon Rampathi, Krohne SA, +27 11 314 1391, [email protected], www.za.krohne.com
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