Coriolis flowmeter selection

Familiarity with the principles makes it easy to find the right instrument.

Q: We have certain questions regarding Coriolis mass flowmeters, which are:

  1. Tube material of construction?
  2. How is tube diameter defined? Should it be line size or something else?
  3. Frequency of tube?

Ashwin Patel, senior engineer, Garden Industries, Surat, India / heetashwin79@gmail.com

A: Based on your questions, it appears that you know very little about Coriolis flowmeters, so before answering your questions, I will start with briefly explaining how this device works and what characteristics it has. Naturally, you will find full details in my handbook.

When flow passes through an oscillating tube, which is held at both ends and is oscillated at its natural frequency from the middle, it will vibrate in a symmetrical manner as shown in Figure 1. Coriolis tubes have a wall thickness of only about 1.5 mm and are usually oscillated at at around 80 cycles per second, with an amplitude of 0.1 inch (2.5 mm).

The bottom half of Figure 1 shows the shape of the tube when fluid flows through it: Fluid entering while the tube is moving upward will be accelerated vertically up, and will apply a downward force to the tube. Having gained an upward velocity, when the fluid approaches the exit, it applies an upward force to the tube, which is moving downward. The result is a slight deformation (twisting) of the tube (solid line bottom part of Figure 1) that is proportional to the mass flow of the fluid. In other words, Coriolis mass flowmeters measure the force resulting from the acceleration caused by mass moving toward (or away from) a center of rotation.

Straight-tube Coriolis flowmeter designs are used only rarely, (when split-flow designs are likely to plug) because they have the disadvantages of relatively low accuracy and being subject to interference from external vibration. The more common tube forms are the U (Figure 2) and S shapes.

Because a single U tubes can still be affected by external vibration, two (or less frequently, four) U tubes are used. These multiple-tube designs are more accurate because their reference is not the housing, but the other tube. This tends to stabilize and isolate them from external vibration. Figure 2 shows single U-tube designs where the twist angle references are to the housing. Figure 3 illustrates a two-tube design, which is more accurate because the reference is the other tube.

The use of Coriolis flowmeters has increased over the years and today, they represent about 21% of the flowmeter market. Standard sizes range from 0.1 in to 6 in. (3-150 mm), but larger sizes are also available (8, 10, 12 or even 16 in. from some vendors for custody-transfer applications), with maximum flows of up to about 500 tons/hr. High-pressure designs can handle up to 5,000 psig (345 bars). The standard temperature range is -100 °C to 300 °C, but cryogenic and high-temperature designs go beyond these limits.

Coriolis flowmeter advantages are that it measures mass instead of volme flow; can also measure density, temperature or bi-directional flow; provides good accuracy (up to 0.1% of actual flow) and high rangeability (usually > 10:1); has no moving parts, hence needs little maintenance; is unaffected by density or turbulance (Reynolds number) variations, hence requires no straight pipe run; and the sensor tubes are available in corrosion-resistant materials including stanless steel, Hastelloy, titanium, tantalum and zirconium.

Coriolis flowmeter disadvantages include high initial cost (but full life cost can be competitive because of low maintenence). The wall thickness of the tubes is thin, therefore reliable and detailed data must be provided to the suppliers concerning solid contaminants and corrosive components because the corrosion allowance of the tubes is zero. They are not available in very large sizes or for low-pressure gas flows. Because the pipeline flow is split as the fluid enters the two smaller diameter U-tubes, plugging, high pressure drop, flashing or somtimes even cavitation are possibilities. Preferred installation is in vertical upward flows and in locations where there is no vibration or pipe forces due to tension, compression or shear.

2017 State of Technology Report: Flow measurement

Now, as to your specific questions: Concering tube materials of construction (MOC), provide very detailed composition and flow information in your bid invitation to the vendors, because the tube material must stand up to corrosion and erosion. The vendor will recommend the tube material and if you are interested, will give you the oscillation frequency, which is either the natural frequency or some multiple of it. The tube diameters are much smaller than pipe size and should be selected so that the maximum normal flow falls into in the upper quarter of the meter's flow range.

Béla Lipták / liptakbela@aol.com

A: You should specify the following:

1. Fluid composition, so the vendor can assess corrosion/erosion properties;

2. Piping line class, as a starting point for material selection;

3. Fluid behavior with respect to entrained solids and gases, dissolved gases and flashing, since Coriolis flowmeters have specific limitations on two-phase flows. These limitations depend on the vendor design;

4. Pressure drop limitation from the process hydraulic calculations, since there will be a higher pressure drop across the Coriolis tube; and

5. The usual fluid flow conditions/properties you need for any flowmeter sizing.

Coriolis meters are normally sized to match the pipeline size (e.g., 4-in. meter in 4-in. pipe). The vendor will then confirm meter selection. It may be possible to increase the flowmeter size if the pressure drop is too high, but this makes the piping design a bit more awkward. The fact that the meters are specified to match the piping size does not mean that the tube size the same size as the piping. It just means that the meter will match the pipe size. Most often, there are two parallel tubes inside the meter.

You need not be interested in tube frequency. It is available from the meter manual but it is not something you can specify or change. It is what it is.

On tube material selection, you should work very closely with the vendors by giving them the exact process fluid composition, and you should leave the final selection to the vendor. This is because the Coriolis tubes are very thin and are not able to follow the typical piping line class specifications. They tend to use much higher-grade materials because the corrosion allowance in the tubes is practically zero.
If you have corrosive service, you have to be extra careful with accurately stating the composition of the process fluid. The contaminants will influence the selection of the tube material. These contaminants can be in very small quantities and may not be of concern to the process, but they are important to meter selection. This is because piping corrosion allowance can be 6 mm, while the Coriolis tubes are only about 1.5 mm thick.

For example, titanium will be dissolved by HCl acid. However, if there are iron impurities in the acid, forming ferrous chlorides, then titanium is perfectly fine. In this example, the impurities help to prevent corrosion. In other examples, trace amounts of impurities accelerate the corrosion.

Simon Lucchini, CFSE, MIEAust CPEng (Australia)
Chief controls specialist, Fluor / Simon.Lucchini@fluor.com

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