Flow tutorial: Turbine flowmeters
In a recent Product Research article on flowmeters , Control Engineering found that two technologies tied for first place among end users: turbine and magnetic. Turbine flowmeters represent an old style technology, but are still very popular in many applications and are available from a variety of manufacturers. In the right situations, they offer a useful combination of simplicity, accuracy, and economy.
The technical concept of a turbine flowmeter is very simple: a turbine that is on a shaft coaxial with the pipe is placed in the liquid stream where passing process fluid strikes it. Since higher flow means higher velocity, the shaft spins faster when there is more liquid. A proximity sensor is mounted in the pipe wall where it can detect passing turbine blades. This creates electrical pulses that translate into liquid velocity which translate into flow.
Advantages typically include:
Wide range of sizes and flow rates;
Wide range of pressure and temperature limits;
Adaptable to gas and steam measurement as well as liquid;
Good tradeoff of accuracy and cost compared to other flowmeter technologies;
High stability and repeatability;
Low pressure drop;
Some can measure bi-directionally;
Availability of specialty materials; and,
Many choices of configurations, connections, etc.
The fact that there is a mechanical device that has to spin inside the pipe, fully immersed in the process liquid, creates the main constraints for applications:
The liquid needs to be relatively low viscosity. The specific value varies between manufacturers, so make sure you check this. Units can be recalibrated to some extent for higher viscosities, but for those applications there are better approaches.
Some conditions are not suitable: solids or fibers in the liquid, entrained air, partially filled pipes, any of these can interfere with the mechanism and reduce accuracy.
Like any velocity based approach, turbulence in the stream has to be minimized. That means long sections of straight pipe (10 diameters, typically) up- and downstream of the turbine.
Pressure drop is relatively low, but if it has to be zero, you should choose something more along the lines of magnetic or ultrasonic technology where there is no intrusion into the pipe at all.
Turbine flowmeters are available from:
You can also use the Control Engineering Supplier Search online.
—Peter Welander, process industries editor, PWelander@cfemedia.com , Control Engineering Weekly News
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