Coriolis flowmeter for measuring two-phase fluid flow

All flow meter technologies have their limitations, especially for two-phase flow measurement, such as air mixed in liquid gas and liquid two-phase flow. The dynamic characteristics of liquid and air mixed gas and liquid two-phase flow are particularly complex, and even with the use of modern fluid dynamic models, their dynamic characteristics cannot be clearly described to a large extent. Therefore, the flow measurement of such unpredictable two-phase fluids is challenging for any flow meter, including Coriolis flowmeters. The

outstanding performance of Coriolis flowmeters

Coriolis flowmeters are widely promoted and used in various industrial fields due to their extremely high measurement accuracy, reliability and very low maintenance costs. This development trend makes it common for Coriolis flowmeters to be used in the flow measurement of gas and liquid two-phase fluids.

When other types of flowmeters are helpless in the face of two-phase flow problems, Coriolis flowmeters provide users with a feasible solution, which is very fortunate for users. For example, mechanical turbine flowmeters or volumetric flowmeters cannot successfully measure the flow of two-phase fluids. Through continuous research and efforts over the past years, Micro Motion Coriolis flowmeters have achieved better performance in two-phase fluid flow measurement (such as the unique patented technology instantaneous bubble correction function).

3-inch Micro Motion Elite flow meter is used for flow measurement of truck loading

Room for Further Improvement

Although Coriolis performs better than other flow meter technologies in challenging two-phase flow measurement, this performance still does not meet Micro Motion's standards or meet the requirements of most customers. The challenges lie in two areas: air entrainment patterns and flow meter sizing.

Air entrainment patterns

■ "Purge first, start batch later"

Air entrainment often occurs in "purge first, start batch later" fluid delivery processes. This phenomenon occurs when a process, such as loading a truck or train, requires that the pipeline be completely emptied after the previous loading process is completed, especially if some fluids remain in the pipe and crystallize, which is a very common requirement. When the next loading is completed, the line must be measured with the pipe empty because the line has been emptied after the previous loading.

Micro Motion Coriolis flowmeters have a successful history of "purge first, start batch later" operations. There are more than 5,000 Micro Motion flowmeters successfully used in this mode of flow, with flow sensor sizes ranging from 2 inches to 6 inches, and fluids such as glucose, phenolphthalein, petroleum products and liquid fertilizers. It is a common operation to vent the line after loading, and these Micro Motion Coriolis flowmeters work well throughout the entire loading process, including the short initial charging period and the process of venting the line with air (or nitrogen) after loading.

■Continuous entrained air

Another way is that air may be continuously entrained in the flow, for example, if a pump seal leaks, or air is sucked into the container due to low liquid level, or gas is continuously sucked into the line by other means. Usually this entrained air mode entrained air content in the fluid will be in the low range of 0-5%. Micro Motion believes, and most users agree, that no Coriolis flowmeter supplier can provide perfect flow measurement accuracy for this entrained mode of flow.

Micro Motion is constantly working to improve flow meter performance through adjustments to signal processing, sensor design and more advanced drive design. To properly evaluate the flowmeter, Micro Motion has customized a test facility that can test a wide variety of fluids. So far, the fluids tested have ranged from beer wort to soap (viscosity greater than 5 Pa·s). Over the past year, many of the flowmeter's features have been further improved, and in the near future, the next generation of Micro Motion Coriolis flowmeters will be able to provide more reliable flow measurement for two-phase fluids.

There are also a few users who may repeatedly encounter the problem of continuous entrained air in the fluid at 30% or more. Although Coriolis manufacturers are working hard to develop their technology to address this problem, it is not the primary consideration for most users who face two-phase flow problems. From our discussions with customers, we believe that we need to focus on improving the performance of the flowmeter to provide more reliable flow measurement for the most common two-phase fluids faced by customers, such as "purge the pipeline before starting the batch process" and "0-5% continuous entrainment of air". [page]

Test equipment for fluids with aeration (up to 30%) and high viscosity (up to 10 Pa·s)

Calculating Flow Meter Size

We understand the aeration patterns that users face and the limitations of current flow technology. What can users do to improve the performance of their Coriolis flow meters? A simple and immediate solution to mitigate aeration problems is to select the correct flow meter size.

Many aeration problems occur with high viscosity fluids, and most high viscosity fluids are non-Newtonian. In addition, most non-Newtonian fluids measured by Coriolis flow meters are thixotropic (thixotropic fluids have a viscosity that decreases as flow rate increases). Most flow meters are sized using the viscosity of a Newtonian fluid, which can be thousands of times higher than the viscosity of a thixotropic fluid when flowing. This results in the selection of a flow meter that is too large and the flow rate through the sensor is too low to cause air and liquid phase separation.

First, users should identify the characteristics of the fluid viscosity and its relationship to flow rate, especially considering the variability of the viscosity of thixotropic fluids. A good example of a thixotropic fluid is ketchup. Ketchup in a static state is so viscous that it will not flow out of the bottle even if the bottle is turned upside down. But once it starts to flow out of the bottle, it flows very fast. Viscous fluids in pipes often behave this way. So if the user tells the Coriolis flowmeter salesperson that the viscosity is the data of the fluid in a static state, then the flowmeter size cannot be correctly calculated based on the viscosity of the fluid in a flowing state.

The selection of flowmeter size depends largely on the correct data of fluid viscosity. There are two specific examples: calculating the size of the flowmeter based on the viscosity of Newtonian fluids; calculating the size of the flowmeter based on the characteristics of thixotropic fluids.

Depending on the different process equipment, many different sizes of flowmeters may be selected. For simplicity, we only compare 1-inch and 3-inch flowmeters.

■ Calculating the size of the flowmeter based on the viscosity of Newtonian fluids

This calculation example meets the following assumed process conditions: flow rate = 500 pounds/minute; density = 1.01g/ml; room temperature and atmospheric pressure; Newtonian fluid viscosity = 4.4Pa·s. The pressure drop
caused by the two different sizes of flowmeters: 1-inch flowmeter causes a pressure drop of 760psid; 3-inch flowmeter causes a pressure drop of 21psid.

Pressure drop caused by two different size flowmeters under different flow conditions

The pressure drop caused by the 1-inch flowmeter exceeds the normal acceptable range. In this case, the 3-inch flowmeter will naturally be selected, which also buries potential risks. When air is present in the fluid, the performance of the flowmeter will be greatly affected. This is mainly because the flow rate of 500 pounds per minute (10:1 range adjustment ratio) is very low relative to the range of the 3-inch flowmeter. The air will tend to gather into large bubbles in the sensor, making flow measurement completely impossible.

■Calculate the size of the flowmeter based on the characteristics of thixotropic fluids

. Knowing the viscosity of your fluid in the flowing state will not only have a great impact on the performance of the Coriolis flowmeter, but also help you make the entire operating system more efficient and save more costs.

At the same time, you can also choose smaller pipes, different valves, pumps and other system components including Coriolis flowmeters.

In this case, the small size flowmeter has three main advantages:

1. Since the given operating flow rate will operate in a higher flow range for the small size flowmeter, the small size flowmeter has better measurement accuracy.
2. The higher flow rate will make the air better mixed in the liquid, making the gas-liquid mixture more uniform and single.
3. Low cost.

A good calculation, like the one used by Micro Motion salespeople, allows you to input the properties of a thixotropic fluid usually in the form of "K&N" values, or "viscosity index" and "flow characteristic index." These are the coefficients of a simple dynamic law equation.

A test fluid with viscosity parameters K = 4.4 Pa·s and n = 0.2 was used to estimate the pressure drop caused by a 1-inch and 3-inch flowmeter under the process conditions given previously. The fluid has a viscosity of up to 4.4 Pa·s in the absence of flow, but the viscosity drops to only a few centipoise at high flow rates. The graph below compares the pressure drop caused by two different flowmeter sizes at different flow rates.

When calculated using a Newtonian viscosity of 4.4 Pa·s, the 3-inch flowmeter causes a pressure drop of less than 21 Psid. However, given the thixotropic fluid properties, it is possible to select a 1-inch flowmeter. The

most important benefit of sizing a flowmeter based on the properties of a non-Newtonian fluid is that the flowmeter performs better, especially when gas is present in the fluid. Higher flow rates allow for better mixing of different fluids; a more homogeneous, homogeneous flow naturally results in more reliable flow measurements. Comparing the relative flow rates of Coriolis flowmeters of different sizes can be used to estimate how much air content is acceptable. For example, at the same flow rate, the flow rate in a 1-inch flowmeter will be about 6 times higher than that in a 3-inch flowmeter, so a 1-inch flowmeter will be about 6 times more tolerant of air than a 3-inch flowmeter. Different flowmeters have different flow cross-sectional areas; consult with the flowmeter manufacturer to calculate the specific data required to size the flowmeter.

Summary

Measuring two-phase flow will continue to present challenges for flow technology. Coriolis flowmeters have proven to be very good in "line emptying first-then start batch processing" processes. Although Coriolis flowmeter manufacturers are still working hard to measure the flow of continuously aerated fluids, we believe that better solutions will eventually be found.

In addition, the difficulty of measuring the flow of aerated fluids can be alleviated by properly selecting the flowmeter size. When the fluid has the possibility of aeration, always try to use a smaller Coriolis flowmeter. In addition to the normal process parameters (flow, temperature, pressure, etc.), the characteristics of non-Newtonian fluids must also be considered. To ensure that you get a correctly sized flowmeter, be prepared to tell your flowmeter salesperson:

■ The K&n value, or "viscosity index" and "flow characteristic index."
■ The maximum acceptable pressure drop.

Calculate the smallest flowmeter possible so that the flowmeter still performs well when aerated. As technology develops and the flowmeter is sized correctly, Coriolis flowmeters will more easily cope with the flow measurement problem of fluid aeration in the coming years.