Article for ‘Process Products’
Flow Measurement Issue.
ABB-KT/557 1,660 words
CHOOSING THE RIGHT FLOWMETER
by
Bryan Franklin: Flow Products Specialist
ABB Instrumentation Limited
With more than 100 different designs of flowmeters available to address just 12 basic principles of operation, selecting the optimum instrument for a particular application can be a daunting task. Each flowmeter type has its strengths and weaknesses and there are many design variations within the type specifications. Add to this the fact that previously impossible areas of application are now being opened up through modern manufacturing processes and materials and it is hardly surprising that many users fall short of securing the optimum systems for their specific requirements.
Since much of a plant’s profitability depends upon accurate and reliable flow measurement, it is of paramount importance that selection is based on knowledge and skills which are broadly based and under constant review. Selecting a meter on cost alone maybe short-term expediency but it can also be the root cause of a highly problematic installation in the medium to long term. Although cost is clearly an important factor, it should, nevertheless, be viewed in the wider context of a supplier’s capability credentials. Good technical back-up, the provision of independently traceable test facilities, a long established track record in the field and a reputation for high-reliability products based on sound research and development ultimately provide for the most cost-effective investment.
Global Business
More than 100 different ways of measuring flow using a dozen different technology principles form a two billion dollar plus global business. While millions of working flowmeters measure every conceivable fluid and powder flow in the chemical industries using older technologies such as differential pressure and level sensing, an increasing number of applications are requiring the newer meter types such as electro-magnetic, Coriolis mass meters, correlation devices or lasers.
More than 1,000 different suppliers world-wide compete to meet these needs. They range from multi-national corporations, such as ABB with its product portfolio encompassing seven flow technologies backed by nationally approved calibration facilities in the UK, Germany, the USA and Australia, to small companies with special designs serving niche markets and special applications. The market is fragmented and is becoming more so each year, with bulk sales moving toward the larger corporations. The USA tops the league for sales into the process sectors with 39% (280,000 units a year), while Western Europe purchases 27% (190,000 units), the UK 6% (45,000 units) and the rest of the world 28% (200,000 units).
With so many technologies, designs, suppliers and applications needs, the task of choosing the right flowmeter to meet particular application, installation, environmental and economic criteria is becoming increasingly more difficult. However, before the would-be purchaser becomes embroiled in vast quantities of meter information on which to base his selection it could be worthwhile to ask himself: “Do I need a flowmeter at all?”
In many industrial applications the user may often merely wish to know whether the fluid in the line is moving slowly, rapidly or not at all. This being the case, he would only require a flow indicator readily available from flowmeter vendors at a fraction of the cost of the simplest flowmeter. If alarm limits for ‘high’ or ‘low’ are required, indicators can be fitted with micro switches and, even for something more sophisticated, such as an indication of flow to within 10%, it may still be unnecessary to purchase a flowmeter. Many installations have changes of section or bends somewhere in the system and, by placing pressure tappings at convenient points, the purchase of a differential pressure transmitter will turn the pipework into a crude Venturi or elbow meter. If calibration can be performed under these conditions, a reasonable accuracy of around 5% could be achieved.
Selection Standard
However, if better accuracy is required or the signal is to be used to control the process, then the difficult task of meter selection needs to be undertaken. A starting point in this process is an appreciation of the various operating principles - an aspect which has been embodied in British Standard 7405 to form a simple and convenient basis for classifying the many types of flowmeter. Indeed, BS 7405 is the only meter selection standard in the world. It classifies closed pipe flowmeters into 10 major groups, with two additional groups for solids meters and open channel measurement. The table below shows the basic flowmeter classification from BS 7405.
Table 1
Group / Description / Examples / Category1 / Conventional DP types / Oriface, Venturi, nozzle / extractive
2 / Other DP types / VA meters, Wedge, Dall Tube / extractive
3 / Displacement types / Piston, Vane, diaphragm meter / extractive
4 / Rotary Inferential / Turbine, helix, propeller / extractive
5 / Fluid Oscillatory / Vortex, fluidic / extractive
6 / Electromagnetic / DC, AC, probe types / additive
7 / Ultrasonic / Doppler, transit time / extractive
8 / Mass meters / Coriolis, momentum / extractive/additive
9 / Thermal types / Hot wire, bridge, profile / additive
10 / Miscellaneous types / Lasers, tracers, Weighers, NMR / extractive/additive
11 / Solids meters / Momentum, capacitance / extractive
12 / Open Channel types / Weirs, flumes, level devices / extractive.
Most flowmeters are volumetric in nature - they measure fluid flowrate, velocity or quantity passed, although the more recent measure mass flow directly. In the above table, the volumetric designs are Groups 1-7, mass meters are Groups 8 and 9 and Group 10 represents a miscellaneous selection of mixed operating principles.
In the right-hand column of Table 1 the 12 groups are categorised into those that extract energy from the flow and those which add energy. Those which extract energy have an associated pressure drop, whereas meters which add energy to the flow possess the feature of virtually zero pressure across them. In the extractive approach, an obstruction is used to infer flow. This could be a change in section in, for example, an orifice meter, a rotating wheel in the case of a turbine meter, or a specially shaped body to induce oscillations with the vortex meter. Energy, on the other hand, may be added in the form of sound (ultrasonic meters), heat (thermal meters) or light with laser or other optical meters. The flow acts upon this addition and a change from the known input value is used to infer flow.
It will be noted from the Table that the lower group numbers are extractive devices and represent the older technologies. The higher numbers are mainly additive and infer that the newer types of meters have less of an influence on the flow they are designed to measure.
Strengths & Weaknesses
It is important to appreciate that flowmeter performance under actual process conditions will not necessarily reflect the data resulting from calibrating the device under reference conditions. All flowmeters are affected to some degree by the fluid they are metering and by the way they are installed. Achieving a total measurement uncertainty of better than 1% takes a great deal of care in selection and application. The best that manufacturers can achieve, even under stable reference conditions is no better than 0.1%. Beware, therefore, of accuracy claims of less than 0.25%.
For the lowest uncertainty of measurement, displacement meters are generally the best option. Magnetic meters provide for the widest flow range, turbine meters are usually the best choice for the highest short-term repeatability while, in terms of sheer numbers, the orifice plate is the most commonly used metering device in the history of flow measurement.
The various flowmeter types offer a number of different techniques for measuring flow. Each, however, has its strengths and weaknesses and, when making a selection, it is necessary to appreciate those types which offer good or poor suitability for the application under consideration. The following Table 2 groups the various types of flowmeter according to their suitability in the main areas of liquid, gas, steam and slurry applications.
Table 2.
Flowmeters Categorised by ApplicationLiquid / Gas / Steam / Slurry
Variable Area / Variable Area / Variable Area / Magnetic
Variable Differential Pressure / Variable Differential Pressure / Variable Differential Pressure / Coriolis
Positive Displacement / Positive Displacement / Turbine / Ultrasonic
(Doppler)
Turbine / Turbine / Oscillatory / Variable Differential
Magnetic / Thermal / Pressure (Eccentric,
Ultrasonic / Oscillatory / Segmental, Venturi)
Thermal / Coriolis
Oscillatory
Coriolis
Plant Considerations
As well as selecting a flowmeter on its merits as an instrument, due consideration must also be given to its installation, environmental location and to the economics of its operation. Will the meter orientation be horizontal, vertical or inclined? Will the flow be uni or bi-directional? What disturbances, such as bends and valves, will be encountered upstream and downstream? Where are the power sources and will the power supply be ac, dc, battery or solar? These, together with space availability for the chosen size of meter and its service accessibility, are among the principle installation considerations when pursuing the selection process.
In terms of the meter’s environmental location one has to consider the instrument’s protection rating (IP65/IP68) in corrosive-type atmospheres and whether it needs to be sanitary specified or buriable. Appropriate intrinsic safety (IS) or explosion-proof certification will be required for operations in hazardous areas. If electrical interference or humidity are likely to cause problems, then these should be addressed in the instrument’s specifications.
The economics of flowmetering, of course, extends beyond the purchase price of the instrument. Its installation may involve mechanical and electrical supports, chambers, filters, reducers and up/down stream pipework. In operation there are pumping and power consumption costs, as well as the question of maintenance, to consider. The reliability of the meter in terms of its operational life, downtime and frequency of re-calibration can have significant effects on costs - or cost-savings in the case of sound and well planned investments.
Clearly, selecting a flowmeter which is right for both the application and its operating environment involves careful consideration of various inter-related factors. British Standard 7405, referred to previously, offers sound guidance for those undertaking the process and there is a wealth of experience to be tapped from reputable manufacturers such as ABB which was heavily involved in the drafting of the British Standard and is committed to ensuring customers’ long-term satisfaction from their flowmeter investments.
END
Prepared by
Richardson Public Relations. Jan 2000.
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