Fluid flow meter selection guide.

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Top Tips for flowmeter selection
The huge array of flow technology options on offer can make selecting the correct
flowmeter for an application a bewildering task. A broad range of factors can
influence flowmeter selection, of which cost is just one. Dr Bryan Franklin, Flow
Products Manager, ABB Limited recommends a list of top tips for selecting the best
all round flow system for an application.
1. Do you even need a flowmeter?
Many users often just want to know the rate at which a liquid or gas is moving
through a pipeline. In such cases, a simple flow indicator, available from any
flowmeter vendor at a fraction of the cost of the simplest flowmeter, will usually
suffice. Simple and easy to install and requiring no external power, these instruments
can be used to provide local indication of flow. Even where there is a demand for
something more sophisticated, such as an indication of flow to within 10%, there may
still be no need to purchase a flowmeter. Many installations typically feature bends or
joints that can be readily converted into a crude flowmeter by purchasing a
differential pressure transmitter and installing sensors to measure the difference in
pressure between two or more points. Provided that calibration can be correctly
achieved, these simplified flowmeters can achieve an accuracy of around 5%.
2. Don’t choose on cost alone
When it comes to selecting a flowmeter, cheapest is by no means best. Although it
might seem the best way to save money in the short term, opting for the lowest cost
solution may potentially result in problems later down the line. Be particularly
careful where reductions in the purchase price have been achieved by cuts in supplier
back-up and expertise. Ultimately, the most cost-effective installation will be the one
where the supplier can offer good technical back-up, independently traceable test
facilities, an established track record and a reputation for high-reliability products
based on sound research and development.
3. Know your flow A key thing to remember when selecting a flowmeter is that
every fluid or gas behaves differently when flowing through the pipeline. The main
cause of this is viscosity - how much the fluid resists flow, which in turn affects the
velocity of flow through the pipeline. By profiling the flow of a fluid or gas
through the pipeline, it is possible to find out how it behaves and from there to narrow
down the choice of flowmeters to those best able to cope with the conditions of the
application. The flow profile of a fluid will vary according to whether it is
Newtonian or non-Newtonian. Newtonian fluids include milk, water, sugar solutions
and mineral oils and have a tendency to ‘stick’ to the pipe walls, resulting in the liquid
moving more slowly at the sides of the pipe than in the middle. Newtonian liquids
have a directly proportional relationship between the pressure of the liquid flowing
through and the resistance, or shear force, caused by the fluid sticking to the pipe
walls. The behaviour of Non-Newtonian fluids, such as paints, shampoos and
yogurt is harder to predict, as there is no relationship between pressure and resistance.
Instead, the flow of these fluids tends to vary as viscosity changes either with time or
due to increased resistance caused by the collision of two different velocities as the
fluid sticks to the pipe walls. To select the best flowmeter, it is necessary to

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calculate the Reynolds number of the application. This figure is basically the ratio of
momentum against viscosity and can be calculated by using the minimum and
maximum fluid flow and viscosity figures of the application. Once the Reynolds
values are known, they can then be matched against a flowmeter’s Reynolds range to
help pick the one that is best able to meet the demands of the application.
4. Opt for the widest turndown Put simply, turndown is the ratio of the maximum
and minimum flow rates a flowmeter can measure within its specified accuracy range.
The turndown of a flowmeter is particularly important because it is virtually
impossible to know in advance the exact range of flows to be measured. Selecting a
flowmeter that offers the widest possible turndown will ensure that it can cover all
anticipated flow variations.
5. Pay attention to installation When selecting a flowmeter, it is important to
consider exactly where and how the device will be installed, as this can significantly
affect both accuracy and efficiency. Obstructions in the pipeline such as joints,
bends or valves in close proximity to the meter can all cause distortions in flow,
affecting flowmeter accuracy and repeatability. To ensure best results, flowmeters
should be installed in locations where there are several straight-lengths of
unobstructed pipeline both upstream and downstream of the meter. It is therefore
important to find out the manufacturer’s installation recommendations before buying
a flowmeter, particularly where installation space is limited.
6. Pick the flowmeter that will offer the best accuracy for the application When
selecting a flowmeter, it is important to find out which types are most suited to the
application. For the lowest uncertainty of measurement, positive displacement meters
are generally the best option. Electromagnetic meters provide for the widest flow
range, turbine meters are usually the best choice for the highest short-term
repeatability while orifice plate meters are the most commonly used metering device.
Despite their high initial cost, Coriolis mass flowmeters are ideal for measuring
particularly viscous substances and anywhere that the measurement of mass rather
than volume is required. The following table groups the various types of
flowmeter according to their suitability for liquid, gas, steam and slurry applications.

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Liquid Gas Steam Slurry
Variable Area Variable Area Variable Area Variable Differential
Pressure (Wedge,
eccentric, segmental,
Venturi.)
Variable
Differential
Pressure
Variable
Differential
Pressure
Variable
Differential
Pressure
Electromagnetic
Positive Displacement Positive
Displacement
Turbine Ultrasonic
Turbine Turbine Oscillatory Coriolis
Electromagnetic Thermal
Ultrasonic Oscillatory
Thermal Coriolis
Oscillatory
Coriolis
Where accuracy is concerned, it is important to remember that all flowmeters are
affected to some extent by the medium they are metering and by the way they are
installed. Consequently, flowmeter performance in real life conditions will often be
different from the reference conditions under which the flowmeter was calibrated.
It is also important to beware of manufacturers’ calibration accuracy claims. Even
under stable reference conditions, the best accuracy that manufacturers can hope to
achieve is 0.1%.
7. Use the same supplier for all your flowmetering equipment
A flowmeter is often only as good as the equipment that sits alongside it. For
example, a flow computer or other form of display is needed to process data from the
meter and show flow rates.
Although there are many suppliers offering ancillary flow equipment, the best way to
ensure a completely matched system where all components are completely compatible
is to specify everything from a single reputable supplier. This will guarantee that all
equipment has been manufactured to the same standards, and will also ensure that
back-up is available from the same supplier for your whole flowmetering system.