SHare: How to select a flow meter from flow products manager of ABB

Top Tips for flow meter 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 



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