FLOWMETERS MASS OR VOLUME?

This is just one choice – there are 40 others! Make sure your selection matches the full application BY DICK FURNESS

Flowmeter use is on the increase, due to lower ownership costs, increased reliability and improved interfaces. One area where debate has raged is whether we should use mass or volume flow rate devices.

Suppliers have undertaken expensive marketing campaigns, both to raise awareness and change perceptions on mass meters. However, sometimes flowmeters are `oversold’, resulting in expectations just not being met.

The current vogue approach to flow measurement is to use no moving parts, have electronically-based wide rangeability, and to aim for low cost of ownership.

British Standard BS 7405 (1991) was the first standard to classify and discuss all types of closed pipe flowmeters. Flowmeters operate on a wide range of principles. Each type has its strengths and weaknesses, and it is taking an increased amount of skill to select the optimum meter to be used for a particular application.

The principle of operation forms a convenient way of classifying meters, and the table on the right shows the 12 major groupings. The first 10 are the closed pipe methods covered in BS 7405, and some of these can be adapted to the solids meter and open channel flows, which are the last two groups listed.

Only those meters contained in groups eight and nine, and some in 10 and 11, are mass meters. The majority of devices available are the volumetric types. If they extract energy, then there is an associated pressure drop.

Taking volumetric meters first, located in groups 1-7, and some of the technologies listed in groups 10-12, these meters measure either volume flow rate, totalised volume flow, point velocity or mean fluid velocity. All are influenced by fluid temperature, pressure, viscosity, density and many other parameters, which are listed in BS 7405.

Some meters offer wide rangeability (modern magnetic meters), clamp-on technology (ultrasonic types), tried and tested standards (differential pressure types), high accuracy (displacement and turbine meters), low cost (domestic turbine type meters), or steam applications successes (vortex meters), to name but a few features and benefits. Volumetric meters are the most trusted and proven.

Mass devices

Mass flow measurement, by direct or indirect means, has been one of the major growth areas. Mass meters came of age when the semiconductor, fine chemical and pharmaceutical industries in particular, required meters which would enable material balances, small flow demands (for example, in gas diffusion), plus tight process control, to be implemented simultaneously.

Both thermal and Coriolis mass meters have found successful applications, but in common with volumetric types, there have been reports of application failures.

Fluid volume, flow properties and system pressure drop can all change markedly as process temperature, process pressure and ambient temperature change.

In theory, therefore, it may be better to measure mass flow rate to minimise these external influences. In those processes where chemicals are mixed, where fluid properties can vary, or where fluids are bought and sold on a weight, a mass meter may be more appropriate.

Some of the mass devices, although sold as being insensitive to some of these parameters listed, are in fact good densitometers, good thermometers or good pressure transducers.

Mass is not affected by any fluid property variable. However, there are many examples of such effects being reported, and it is difficult to determine the true sensitivity to fluid properties, as Coriolis meter designs vary greatly. The thermal types are known to be dependent on fluid properties, such as specific heat, thermal conductivity and fluid density, to name but three.

Mass meters offer wide versatility in choice of materials, a wide variety of electronic functions (such as batching and PID control), easy interfacing capability with controllers and computers, and an acceptance that has been faster than most other types of metering technologies.

Coriolis types have been shown to work effectively on a variety of liquid processes, including slurries, and have had limited applications successes on gases. Thermal meters have had success in the automotive industry, in the medical and analytical fields, and in semiconductor manufacture. The economics of applying mass meters does, however, requires consideration, especially as the line size increases.

Matching features to applications is the important issue. Five broad areas for selection criteria in BS 7405 are shown in the table above. These factors also provide a useful checklist for buying and applying any field instrumentation, not just flowmeters.

If you are making a mass balance, then a mass meter could be the first choice, but a volumetric meter with ancillary instruments that compute the mass flow rate may be acceptable and may be cheaper.

The application should be specified in full detail, including consultation with the supplier and plant operations, and then evaluated against the data tables in BS 7405. It is not merely a straight choice between mass or volume types.

BS 7405 highlights over 40 other factors which affect the choice of meter, including cost of installation, calibration and maintenance, which are rarely considered.

Process conditions, fluid properties and environmental influences can alter the performance obtained quite considerably. That’s why BS 7405 was written.

It is the complex interplay between all these factors that is the challenge for specialists to understand. In fact, the true cost of using any meter is hardly ever known at the time of selection.

Selection pitfalls

If an inadequate meter is selected, performance requirements may not be met, process efficiency may suffer and product quality is almost always affected.

Selection economics are difficult, as old ideas and specifications continue to exist. One area to be wary of is not to call for unnecessary extras. Users should ask –

* Do we need wide flow range, high repeatability and low maintenance?

* Do fluid properties have an effect?

* What happens if I put pumps too close to measurement points etc?

However, instruments are always blamed, and if the flow is not as high as the design says, then the meter is automatically incorrect.

Price is the element that has the most focus. `What is the cheapest device I can get away with within a reasonable period of time?’, seems to be the common theme. However, installation, operation, maintenance and efficiency costs are the true lifecycle cost of ownership – which are many times cost price!

Mass meters, though the current first choice in many industries, are not always the perfect answer, and can be over-applied and then blamed when failures occur.

When any new flow measurement technology comes along, it is usually over-sold and frequently misapplied in the early days.

So it is not a question of mass or volume, but, in a complex plant, maybe mass and volume, each metering technology matched to the specification needs and characteristics of the application.

* The Author is with ABBInstrumentation (USA). This feature is based on a paper from the `96 C&I Show.