Andrew Mangell examines the strengths and limitations of current gas flowmeter technology. He concludes that new technology flowmeters are offering increased levels of accuracy and competitive lifecycle costs, which make them worthy of serious consideration.

Flow measurement is recognized as one of the need-to-know process parameters, alongside temperature,pressure and level, and accurate measurement of gas flow is critical in the operation and control of many industrial and laboratory processes.

In the food and beverage sector, the chemical industry and semiconductor fabrication, flowmeter accuracy is often the determining factor between optimum quality and reject products, while in areas like laboratory research, pilot plants and custody transfer,precise and repeatable measurement is equally critical. Elsewhere, high levels of accuracy are not so crucial and flowmeters are used to give an indication of the rate at which a gas is flowing
through a pipeline.

In this article, I will attempt to review the main categories of gas flowmeter
available on today’s market and consider their relative strengths. Mass flow
measurement and control specialist Bronkhorst, maintains an intelligent overview of the whole flowmeter sector,as well as its main customer groups, and
I have endeavoured to offer a balanced appraisal of the current scene.

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of gas flow is critical in the operation and control of many industrial and laboratory processes.

Installed base

With regard to the global installed base,Differential Pressure (DP) is the dominant means of measuring both gas and liquid flow, although there are clear signs that emerging or evolving technologies like Coriolis, ultrasonic, vortex and thermal are growing strongly, as considerations like accuracy, reliability and lifecycle cost ascend the customer’s agenda. Not surprisingly, DP flowmeters are currently maintaining their market lead, partly because users keep replacement instruments in stock and partly since retaining the same type of flowmeter is often regarded as the most risk-free solution.

But reliability and performance problems reportedly arising with these instruments mean that Coriolis flowmeters in particular are being increasingly specified for new plant and new processes, in addition to being integrated into existing schemes.

Indeed, flow control experts now tend to distinguish between ‘new technologies’
and ‘traditional technologies’, grouping Coriolis, magnetic, ultrasonic and vortex flowmeters under the ‘new’ category, with methods such as DP, turbine, positive displacement (PD) and variable area (VA) under the ‘old’. It’s a useful classification.

Flow measurement techniques
TypeStrengthsLimitations
Coriolis

These use the Coriolis effect, which causes a vibrating tube to distort, for measuring the flow rate directly, eliminating the need to compensate for temperature, pressure and density. They can also measure a mixture of gases, unknown gases, and fluids moving between gaseous and liquid states. Coriolis meters offer high measuring accuracy, unaffected by flow profile, even down to very low flow rates as used in nanochemistry. Line size was considered a limitation, but the largest instruments can be fitted to 12 in diameter pipes.

• Universal measuring principle

• Direct measurement of mass

• Very high accuracy

• Low flow rates

• Low cost of ownership

• No major moving parts

• Higher set-up costs

• More limited line size

• High flow rates

Differential pressure

The most common type of flowmeter, the DP, measures the flow of gases inferentially, employing the Bernoulli equation that interprets the relationship between pressure and flow rate. These flowmeters introduce a constriction or obstruction in the pipeline, creating a pressure drop from which velocity and volumetric flow can be calculated.

Various types of DP flowmeters are used, the most popular being orifice plate, which can be subject to wear. They are typically less accurate than Coriolis flowmeters and are best used with clean, non-corrosive gases.

• Large installed base

• Comparatively inexpensive

• Simple, straightforward

• Extensive pipe diameters

• Subject to wear

• Medium accuracy

• Complex mass calculations

• Possible regular maintenance/inspection

• Poor turn-down

Positive displacement

PD flowmeters measure volumes of fluid by repeatedly filling and discharging compartments of known volume, with gas from the flowstream. There are various types of PD meter, using vanes, gears, pistons, paddles or diaphragms to separate the fluid. They provide high accuracy, but cannot handle dirty fluids and incorporate moving parts that are subject to wear.

• High accuracy

• Unaffected by flow profile

• High rangeability

• Clean gases only

• Subject to wear

• Pressure drop issues

• Higher maintenance costs

• Low flow rates

Thermal

Thermal flowmeters measure the mass flow of gases, employing a combination of heated elements and temperature sensors, with thermo-dynamic principles used to derive actual mass flow. They do not require correction for changes in gas temperature, pressure or density and are extremely accurate, especially when measuring low and very low flow rates, and no longer regarded as high cost. Thermal meters are often limited to clean gas, but Bronkhorst’s non-bypass principle means there are no narrow pipelines to clog with particulate, so can handle most gases.

• Measure mass flow

• Highly accurate and repeatable

• No moving parts

• Minimal maintenance

• Low to very low flow rates

• Clean gases (bypass

types only)

• High flow rates (above

11,000 m3/h)

Turbine

Fluid passing through a turbine flowmeter spins an axial rotor, the rotation speed of which indicates flow velocity. They have a wide flow range and offer a reasonable level of accuracy at an affordable price, although they are restricted in use to clean, non-corrosive fluids and measure only volumetric flow. Similar comments apply to paddle wheel and pin wheel flowmeters, which translate the mechanical action of paddles/wheels into volumetric flow.

• Familiar technology

• Medium purchase price

• Reasonable accuracy

• Easy to install and maintain

• Wide-flow rangeability

• Clean gases only

• Need consistent elocity

• Require straight pipe runs

• Require straight pipe runs

• Need enough flow to spin

Ultrasonic

Ultrasonic flowmeters use sound waves to determine the velocity of a gas, employing either transit-time sound travel or doppler frequency shift techniques. Volumetric flow is determined by multiplying velocity with the pipe area. Available in both inline and clamp-on formats, ultrasonic meters are non-obstructive and very accurate, although this may require multipath meters, where flow rate is determined by averaging the values.

• Non-invasive (clamp-on)

• No moving parts

• Detect zero flow

• High life expectancy

• Wide range of larger pipe sizes

• Higher set-up costs

• Pipe-wall interference

(clamp-on)

• Clean gases only (transit)

• Need particles(doppler)

• Known gas profiles only

Variable area

Also known as rotameters, VA flowmeters typically comprise a tapered glass or plastic tube and an internal metering float, with the volumetric flow rate proportional to the displacement of the float. Among the oldest flow technologies, it is inexpensive and easy to install, although historically had to be fitted vertically, and is sensitive to changes in temperature, pressure and density. Interestingly, Bronkhorst recently introduced a digital alternative, the mass-view, which offers greatly improved accuracy, electronic output signals and no fragile glass components in the flow path, eliminating some of the limitations of VA flowmeters.

• Ease of installation

• Large installed base

• Low maintenance

• Simple technology

• No data output (glass/

plastic types)

• Glass breakage issues

• Sensitive to condition

changes

• Moderate accuracy

• Float sticking problems

Vortex

Vortex flowmeters measure the frequency of vortices created by an obstacle placed in the fluid stream, which are proportional to the flow velocity. Velocity multiplied by pipe area again gives volumetric flow. The measuring signal is not subject to drift, so vortex meters do not need frequent recalibration, although they are best used with swirl-free, medium-to-high speed flow rates, which generate sufficient vortices.

• Long term stability

• Low sensitivity to process

variations

• Moderate set-up costs

• No moving parts

• Minimal maintenance

• Good accuracy

• Not for low flow rates

• Straight pipe run needed

• Some pressure drop

• Pipe vibration/noise

issues

• Limited turn-down

that I will adopt in part, especially since it underlines the advanced computer
processing capabilities of newer instruments;

Available today is a wide range.jpg


of instruments designed for mass flowmeter and mass flow controller technology for gases and liquids, pressure controllers and evaporation systems.

although, I would unequivocally place thermal flowmeters in the ‘new’ category, since they are very much at the forefront of digital technology and upcoming products from manufacturers like Bronkhorst put thermal mass monitoring firmly at the cutting edge of flowmeter design. For the purposes of this article, I have also had to ignore magnetic flowmeters, since they are restricted to monitoring conductive liquids, so cannot measure gases.

Measuring flow rates

Flowmeters can also be distinguished by whether they measure flow rates in terms of volume, expressed in units like mL/min, or mass, in units such as kg/h or lbs/min. Strictly speaking, PD flowmeters are the only ones that directly measure volumetric flow, although techniques liketurbine, ultrasonic and vortex measure the velocity of the gas stream, to determine volumetric flow.

Inferential flowmeters, such as DP and VA sensors,measure neither volumetric nor mass flow, but infer its rate from other parameters, like a drop in pressure or the displacement of a float. Coriolis and thermal instruments are the only ones that measure the mass flow of the gases,albeit using rather different techniques.

All this would be somewhat immaterial if mass and volume were much the same,
but measured volumetric flow rates do vary dramatically with temperature and
pressure changes. Moreover, whilst volumetric and mass measurements can be converted between one another if the fluid density is known, the density of
gases is equally sensitive to pressure and temperature, unlike liquids which are less susceptible to changing conditions.

So, while volumetric flow measurement undoubtedly still has its place in many
industry processes, it is widely accepted that current ‘best practice’ is to measure mass flow in gases and steam, thereby reducing process variables and leading to more consistent quality.

Accurate,repeatable mass flow measurement improves chemical reactions, leads to
more precise dosing and custody transfer,facilitates laboratory analysis and helps to eliminate wastage. This also explains why the Coriolis technique in particular, which continuously and directly monitors mass flow, is often described as the nearperfect measuring principle.

“It is widely accepted that current ‘best practice’ is to measure mass
flow in gases and steam.”

Having said that, various specifying criteria like compatibility, repeatability, reliability, simplicity and, of course, purchase price,
dictate that the different types of new and old technology flowmeter each have their own advocates and optimum applications.

So, in the accompanying table,I have looked at the main flow measurement
techniques, in purely alphabetical order, and examined their strengths and
limitations.

Conclusion

In this article, I have tried to be balanced in my appraisal, although doubtless there will be debate from some quarters and new product developments may well have addressed some of the perceived technical shortcomings.

Nevertheless, I hope to have convinced you that compatibility with existing meters, familiarity with a traditional technology and low purchase price are not necessarily the most valid specifying criteria for many applications. New technology flowmeters are offering increased levels of accuracy, fewer maintenance issues, impressive digital output capabilities and competitive lifecycle costs, which surely make them worthy of serious consideration.

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