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Introduction To Measuring Pipe Flow With An Ultrasonic Flow Meter

Posted on: Tuesday, September 25, 2012

If you have a need to measure volumetric flow for surveys, troubleshooting or monitoring, using the proper type of flow meter is important. Issues such as leaks, solid buildup, shrinkage, and obstructions can cause problems with final output. There are a variety of methods available to measure fluid flow, one being the ultrasonic flow meter. Sound waves are longer when the source is further away, and shorter when they are closer; causing a frequency shift. The shift is what is used to provide this type of flow meter measurement.

Ultrasonic flow meters most commonly come in two forms; transit-time and doppler-shift. Transit-time meters indicate differences in fluid travel time. This is done by measuring the difference in the travel time between the upstream and downstream beams generated by the transducers, which are generally attached to the outside of the pipe. The Doppler-shift meter detects particle movement through frequency shifts by reflecting off any solids, bubbles, or other discontinuities in the fluid. This is done by calculating the flow rate of the velocity of the particle, rather than the liquid. 

Transit-time meters are generally used when the liquid or gas is very clean, as particles in the fluids can interrupt the beams. They use higher frequencies and work best in smaller pipe applications. Doppler-shift meters work best when used with liquids that have particles or bubbles, and are in larger pipes.

Both types come as either clamp-on or installed in the pipe (wetted). Clamp-on meters are non-intrusive, and easy to use. They are installed on the outside of the pipe so there is no cutting or drilling needed for installation, and most have easy to read handheld indicators. The clamp-on meter is best used when the pipe cannot be broken or if the liquid cannot be contaminated in any way. 

Wetted meters can be either inserted or welded into the pipe, and are low maintenance with no moving parts to replace. Inserted meters are stable with good signal quality. They are generally screwed into the pipe giving them the ability to be taken off and put back on without difficulty. Welded meters are permanent and most suited for piping in rugged environments.

For best results, the pipes need to be full and have as much straight pipe runs as possible. Pipes that have two or more out of plane elbows can induce swirl and cause disruptions to your measurements. Certain types of pipe such as ones that are concrete-lined may absorb the sonic energy and cause refraction angle changes, so be sure your pipes are made out of compatible material. Proper installation is critical and is best done by following the manufacturer's instructions. Maintenance and calibration is important as well, and each meter should be inspected often.

It can be a challenge to decide which is the right flow meter for your application, and an incorrect choice can lead to unreliable or inaccurate flow measurements. It makes sense for you spend some time to evaluate your criteria by understanding what's in the pipe, knowing the flow rates, and determining the operating conditions and constraints. Once this is done, you should have an easier time choosing the most efficient product to suit your needs.

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