Going with the flow

Enhancements in flow meter design bring improvements in process efficiency.

Flow meters are an integral part of any process industry. They act as a window onto the deepest recesses of a plant or network and enable operators to exercise greater control over their assets.

Flow meters provide measurement points for gases or liquids that can flag up inefficiencies and system leakages or simply provide custody transfer records for billing purposes.

Their application in the Middle East is growing, given the huge investment in the hydrocarbon and utility sectors, which are prime end-users of the technology.

"In this region the oil and gas industry is making great investment in areas where they can improve the production of the basic hydrocarbons, also we are seeing across the board investments in the downstream petrochemical processes in order to monetise that core hydrocarbon, be it a crude oil or a gas product," observes Keven Dunphy, director of Emerson Process Management's flow division for the Middle East and Africa.

"On top of that, there is infrastructure development for power and water as the region grows more hotels and as we build the economic diversity outside the hydrocarbon industry. As a result, we are seeing growth greater than 20% per year on both revenue and flow meter units sold in the region."

Within the utility sector, the main applications for flow meters would be to measure the power source into a power plant, such as gas or fuel oil; to control the processes in water, steam and electricity production; to regulate the distribution of steam, water, wastewater and treated water around networks; and for fiscal transfer records.

A desalination facility for instance might have 100-200 devices installed. The district cooling industry also employs flow meters to track the movement of chilled water around networks.

A choice selection

The wide variety of meters on the market take flow readings using different methods, among them turbine, positive displacement, ultrasonic, electromagnetic and vortex meters. The choice of meter is generally determined not merely by price but also by the level of accuracy required and the specific application.

"Depending on the process, you might want an indication of your flow with plus or minus 10% of accuracy, so you would take a cheap meter," says GE Sensing's Hilko den Hollander.

"But when you look at the oil and gas industry for custody transfer meters, where somebody is going to pay for the amount of oil or gas delivered, you want a very accurate measurement so you would go for high-end solutions.

With gas, obviously you want to measure for control purposes and to check there are no leaks in your pipelines, but then in the end you are going to sell your gas to somebody so you also want to make a custody transfer measurement, with water it is much the same thing."

Maintenance needs

A further consideration in flow meter selection is the simplicity of use and the device's maintenance needs. "If you use the old-style positive displacement meter you have to do a lot of maintenance," notes den Hollander.

"There is a wheel with small chambers in, and if you draw water the wheel is going to turn a number of rotations to determine how much water you have taken.

The problem there is that there are moving mechanical parts that wear out, so your accuracy is changing because of mechanical wear.

With ultrasonic devices there is nothing moving, you just have transducers that send acoustic pulses, so there is no wear in the flow meter, which means the measurement result is valid even after ten years and the meter doesn't need to be recalibrated."

In ultrasonic meters, each transducer functions as a transmitter, generating a certain number of acoustic pulses. They then act as a receiver for an identical number of pulses and the time interval between transmission and reception of the ultrasonic signals is measured in both directions.

The difference between downstream and upstream transit times is proportional to the velocity of the liquid and indicates the direction of the flow.

"There is also the issue of pressure drop," den Hollander continues. "If you have a pipeline of course you have a pump somewhere in your factory, pumping something.

You want the pump to be as small as possible and to consume as little electricity as possible, so you don't want pressure drop in your pipeline and therefore you don't want any moving parts or obstructions. An ultrasonic flowmeter is full bore so there is very low pressure drop."

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