Absolute all-rounder
Dosing, filling and mixing with Coriolis mass flowmeters

Whereas at the beginning of the 1980s acceptance of Coriolis mass flowmeters was very limited and experts had to explain the technology and function intensively, now the focus of consultation is on what the meters can do and where the limits of their application lie. This article gives answers.

Coriolis meters have since become number two in the field of state-of-the-art flowmeter technology and continue to enjoy increasing popularity. They are true multipurpose appliances, measuring not only mass flow but also density, volume flow, temperature and derived scales such as Brix, Plato and Baumé. And this occurs irrespective of inlet and outlet runs, pressure, temperature, viscosity and conductivity. As a result, and due to the approval for custody transfer, there are more and more possible applications for Coriolis meters.

When Krohne entered the market in 1994 as the first manufacturer to produce a sensor consisting of a single straight tube, such a solution was not considered suitable for the industry. Problems such as decoupling the measuring system from the adjacent pipeline, measuring the extremely small Coriolis signals and overcoming the thermal voltage due to increases in temperature were seen as obstacles too large to overcome. However, the success of this first Coriolis mass flowmeter with a single, straight measuring tube led to a change in attitude amongst manufacturers and there are now several straight tube systems on the market.
Measuring principle
From a design perspective, however, it is far more difficult to develop a one tube measuring system with the same or better performance data because the Coriolis force affecting the system is ten times smaller than that of a loop system. The graph at top illustrates the coriolis principle and the setup of the Optimass 7000, the third generation of Krohne Coriolis straight tube meters. The figure shows the basic structure of a Coriolis straight tube meter consisting of a single straight measuring tube without flow splitter. The meter consists of a straight measuring tube with a stable cylinder mounted inside. The field coils which cause the measuring tube to vibrate and the two sensor coils which record the Coriolis effect are fitted to this cylinder. The outer cylinder surrounds this entire system while simultaneously serving as secondary containment. To minimize both the impact of external interference on the measuring tube and the transfer of energy from the measuring tube to the surroundings, the system consisting of the inner cylinder/measuring tube is connected to the process connector (flange) via a connecting tube. The connecting tubes were designed to allow the meter to adjust precisely to all conditions. The inner cylinder/measuring tube system thus functions as a “floating basis” through which external interference is filtered out.
To compensate for external stresses and thermal expansion, the temperature of the vibrating measuring tube is precisely recorded using a Pt 500 measuring device and the stress condition of the measuring tube as well as that of the inner cylinder are determined using a strain gage for each.
Whereas previous models used a permanent absorber mass as a counter-balance, the Optimass 7000 uses dynamic compensation. The patented design, known as AST, consists of a stable inner cylinder attached to the measuring tube and to which drivers and sensors are attached. The masses of the measuring tube and the inner cylinder vibrate simultaneously using the connecting tube as a spring. Thus, for example, if the density of the process liquid changes, so too does the mass of the inner cylinder. When the masses and connecting tube (spring) are ideally designed, frequency tuning is fully preserved. Measuring performance is not affected by the lack of brackets or poor brackets on the housing or external vibrations from pumps. The user thus receives a completely balanced measuring system with good zero stability. This is particularly important as these meters are frequently used in the lower measuring range (see applications below).
Technology and applications
The Optimass 7000 comes in seven sizes and three different materials. It is the only straight-tube mass flowmeter to feature components in titanium, stainless steel or Hastelloy, thus ensuring optimum corrosion resistance. The surfaces coming into contact with the process liquid have a roughness of 0.8 µm; all materials are FDA-compliant and are produced in agreement with Atex and FM. Krohne Coriolis flowmeters are suitable for CIP and SIP up to 150°C. The sensor does not require a flow splitter, is self-draining and can be sterilized. In addition to the most common connector flanges as per DIN, ANSI and JIS, numerous other hygienic process connectors are also available. A fully-welded aseptic flange without internal seals is also available. A crucial advantage of this connection is its sophisticated and patented sealing system for the aseptic process connections as per DIN 11864-2. This ensures compliance with even the most stringent hygiene regulations. The design fulfils a multitude of standards, including ASME Bioprocessing Equipment, EHEDG and 3A. The Optimass can measure flows from 10 kg/h up to 430,000kg/h.
It is a highly accurate measuring system which is also approved for custody transfer to OIML R 117 accuracy class 0.3. It is the only Coriolis meter to enable the highest accuracy class for custody or so-called domestic transfer. Specific applications for which the Optimass is particularly well-suited on account of its sensor technology are described below.
Dosing of hops extract
Although in the early days the hops were processed in the form of pellets in beer production, breweries are now tending to use liquid hops. This improves the brewing process and ensures a more stable beer quality. Furthermore, the automatic addition of hops makes it much easier to log and document the process. In the following example the hops (density approx. 1.04 kg/l, viscosity up to 10 000 mPas) are heated to 45 °C in a heating chamber, making the hops fluid enough to be pumped. Very precise dosing of the hops extract in the wort kettle minimizes the amount of extract required and thus lowers the cost. Through the Profibus PA interface, other parameters such as the current product temperature can also be measured.
The customer tested the Optimass 7000 by comparing the flowmeter’s measuring values to those on a calibrated scale. The results of the trials were impressive as the deviation between the two methods was negligible. On average the results were only 60.1% lower for a total dosed mass of 8 kg compared to the scale. What is interesting in this application is that the dosing pump is installed directly at the front-end, in front of the meter. The ability of the Optimass to measure the highly viscose hops even at low flow rates (approx. 0.2 m/s) was also important. Adaptive Sensor Technology is what makes the difference in this case. Outstanding accuracy is possible, even at low flow rates that may be well below 10% of the rated flow. The tests confirm the outstanding meter characteristics in terms of accuracy and zero-point stability.
Slurry density at smeltery
A lot of blast furnace gas accumulates when producing raw iron and steel. These emissions are at a high temperature and contain extremely fine dust particles up to concentrations of 50 g/m3. Water injected into the gas washing system is used to cool and clean the blast furnace gas. The waste water contains 5 to 10 kg/m3 of very abrasive particulate matter and is conducted into settling basins. The clean water on the surface is pumped into the steel works via the cooling towers. The slurry collects at the bottom of the settling basin with a density of approx. 1050 to 1200 kg/m3 (depending on the type of slurry) and is transported into the thickening basins using the circulating pump. Water is once again removed and the slurry density increases to 1300 kg/m3 to 1500 kg/m3. The thick slurry is then thickened in the centrifuge or slurry press.
A Czech client had ten Coriolis meters installed to measure slurry density and mass flow. This customer replaced the radiometric density meters it had been using. These radiometric density meters required regular and extensive calibration and the measuring sections had to remain filled with the process liquid at all times (running empty posed a danger to the scintillators). The customer also had to deal with the extensive safety and organizational measures related to the use of radiometric density meters.
Optimass DN 50 were installed under the settling basins to measure slurry density and slurry flow. Each mass flowmeter is installed on the pressure side of the circulation pump and has two independent current outputs. The density output is used to optimize slurry conveyance, the mass flow output helps the customer monitor the condition of the pump and compile the slurry data.
Additional Krohne mass flowmeters are installed in front of the centrifuges. For the centrifuges to operate efficiently, a slurry density of approx. 1200 kg/m3 is necessary. Each flowmeter is installed on the pressure side of the pump and has two independent current outputs. The added water is steered via the density output in order to keep the density within the desired range for the centrifuges. The mass flow is important for the slurry production result and monitoring the condition of the pump.