2003-01-21 - Dr Bryan Franklin, Flow Products Manager at ABB Limited, explains the main points that need to be considered when selecting a flowmeter for an application - Since much of a plant's profitability depends upon accurate and reliable flow measurement, it is essential that selection is based on broadly based, constantly reviewed knowledge and skills. Selecting a meter on cost alone may be short-term expediency but it can also lead to problems in the longer term. Although cost is clearly important, it should be viewed in the wider context of a supplier’s capability credentials. Good technical back-up, the provision of independently traceable test facilities, an established track record and a reputation for high-reliability products based on sound research and development ultimately provide for the most cost-effective development.
With over 100 different designs of flowmeters available to address just 12 principles of operation, selecting the optimum instrument for a particular application can be daunting. 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 practices and it is hardly surprising that many users fail to get the best system for their requirements.
While millions of working flowmeters measure every conceivable fluid and powder flow using older technologies such as differential pressure and level sensing, an increasing number of applications are requiring the newer meter types such as electromagnetic, Coriolis mass, correlation devices or lasers.
Over 1,000 different suppliers world-wide compete to meet these needs. They range from multi-national corporations, such as ABB, to small companies serving niche markets and special applications.
With so many technologies, designs, suppliers and application needs, choosing the right flowmeter to meet particular application, installation, environmental and economic criteria is becoming increasingly difficult. However, before the would-be purchaser becomes embroiled in vast quantities of information on which to base his selection, he should perhaps ask himself "Do I need a flowmeter at all?"
In many industrial applications, it may be desirable just to know whether the fluid in the line is moving slowly, rapidly or not at all. For these applications, flow indicators are readily available 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 that can be turned into a crude Venturi or flow meter by purchasing a differential pressure transmitter and placing pressure tappings at convenient points. If calibration can be performed under those conditions, a reasonable accuracy of around 5% could be achieved.
Where greater accuracy is required or the signal is to be used to control the process, then the more difficult task of meter selection needs to be undertaken. A starting point is an appreciation of the various operating principles - an aspect embodied in British Standard 7405 to enable simple and convenient classification of 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.
It is important to appreciate that flowmeter performance under process conditions will not necessarily reflect the data from calibrating the device under reference conditions. All flowmeters are affected by the fluid they are metering and the way they are installed.
Achieving a total measurement uncertainty of better than 1% requires careful 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 0.1%.
The various flowmeter types offer numerous techniques for measuring flow. Each, however, has its strengths and weaknesses. For the lowest uncertainty of measurement, displacement meters are generally the best option. Electromagnetic meters provide for the widest flow range, while turbine meters are usually the best choice for the highest short-term repeatability. In terms of sheer numbers, the orifice plate is the most commonly used metering device in the history of flow measurement.
Consideration must also be given to flowmeter installation, environmental location and to the economies of 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 principal installation considerations.
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 hazardous area operation.
The economics of flowmetering extend beyond the purchase price of the instrument. 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 recalibration can also have significant effects on long term cost effectiveness.