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 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.

LiquidGasSteamSlurry
Variable AreaVariable AreaVariable AreaVariable Differential Pressure (Wedge, eccentric, segmental, Venturi.)
Variable Differential PressureVariable Differential PressureVariable Differential PressureElectromagnetic
Positive DisplacementPositive DisplacementTurbineUltrasonic
TurbineTurbineOscillatoryCoriolis
ElectromagneticThermal
UltrasonicOscillatory
ThermalCoriolis
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.

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