Sound Wave Principles

By David W. Spitzer

The same is true for ultrasonic flow measurement. Wouldnt it be great if you could clamp a sensor onto the pipe and measure the flow through the pipe? Benefits would aboundincluding not having to shut down the flow or cut the pipe. Installation would be faster and easier. Unfortunately complications tend to get in the way of this smooth-flowing scenario.

Ultrasonic flowmeter technology falls into two general categoriesDoppler and transit-time. Early ultrasonic flowmeters used the Doppler effect to measure flow because Doppler signal processing techniques were more readily adaptable to the analog technology of the time. The development of transit-time ultrasonic flowmeters accelerated as microprocessor technology became more prevalent and cost-effective. Now only a handful of ultrasonic flowmeter designs currently use Doppler technology exclusively.

The Doppler Effect

The Doppler effect is characterized by a frequency shift that occurs when an object is moving towards or away from an observer. For example, an ambulance siren will sound differently when the ambulance is approaching an observer from when it is moving away. When the ambulance is moving closer, the waves emitted by the siren will tend to be closer together and, therefore, have a higher pitch (frequency). The frequency shift is proportional to the speed with which the ambulance is moving closer.

Figure 1. A small ultrasonic flowmeter with two traverses.
(Courtesy Siemens)

Doppler ultrasonic flowmeters use this principle to determine the velocity of the fluid in the pipe. Ultrasonic energy is emitted into the pipe. In no-flow conditions, particles in the fluid will reflect some of the ultrasonic energy, but the frequency of the reflected signal will be the same as that of the ultrasonic energy emitted into the pipe. With flow in the pipe, particles in the moving fluid will also reflect some of the ultrasonic energy. However, the frequency of the reflected signal will be different from the frequency of the ultrasonic energy emitted into the pipe. The velocity in the pipe can be determined by comparing the reflected frequency to the emitted frequency.

Transit Time

Transit-time ultrasonic flowmeters measure the amount of time the ultrasonic signal takes to travel upstream and downstream in the fluid. With no flow, these times are the same. In a flowing fluid, the ultrasonic signal will travel faster in the downstream direction. The fluid velocity is determined by analyzing the measured upstream and downstream transit times. Transit-time ultrasonic flowmeters mitigate the need for particles in the fluid. However, these flowmeters will not function when the ultrasonic path is broken because a fluid is excessively opaque to ultrasonic energy.

Some correlation flowmeters could be considered ultrasonic flowmeters because they use ultrasonic energy to sense the flow stream at two or more locations in the pipe. The distance between the locations and signatures are correlated to determine the fluid velocity.

Figure 2. A transit-time ultrasonic flowmeter.
(Courtesy Emerson Process Management)

Complications

Ultrasonic flow measurement may sound straightforward, but there can be a number of complications. For example, how many and what type are the particles in the fluid? If there are too few particles, then the reflected signal may not be strong enough to process. If there are too many particles, then the measured velocity may measure the fluid velocity at the pipe wall and not represent the average velocity in the pipe. A distorted velocity profile also can adversely affect the measurement.

Ultrasonic flowmeters measure fluid velocitynot flow. Users want to know the flow ratenot the velocity. This may appear to be a subtle point because the flow can be calculated by multiplying the measured velocity times the cross-sectional area of the pipe. However, flow measurement accuracy can be affected by the uncertainty associated with the cross-sectional area. Some ultrasonic flowmeters have spool pieces that are tested in a flow laboratory to ensure a known relationship between the measured velocity and flow. Other ultrasonic flowmeters consist of a pair of sensors that the user attaches to the outside of pipe where the inside diameter and internal condition of the pipe may not be known accurately. In such an installation, the accuracy of the flow rate is questionable, even if the sensors can measure velocity with no error.

Ultrasonic flowmeters are available for pipe sizes range between approximately 6 mm and 10 m in diameter. Fluids include liquids, gases and multi-phase fluids, such as slurries. Ultrasonic flow measurement techniques can be used to measure the velocity of stack gas and flare gas. In general, sensors can be wetted or non-wetted (clamp-on). Wetted sensors tend to exhibit better ultrasonic connections with the fluid while clamp-on sensors are easier to install.

Figure 3. Clamp-on ultrasonic flowmeter
(Courtesy GE Sensing)

Both Doppler and transit time ultrasonic flowmeters have had difficulty measuring in stainless-steel tubing. In these installations, ultrasonic energy can travel in the pipe and create strong return signals from upstream and downstream fittings. Sometimes this can be resolved by wrapping the associated piping with electric tape or other deadening material. In other applications, a sensor with a different beam angle can resolve the issue.

Be wary of Doppler installations located near radio antennas because their frequency of operation is approximately the same as that of AM radio stations. In one such installation, it was possible to listen to a local radio station on the test leads. That installation was a failure because there was no practical way to shield the instrument.

Doppler ultrasonic flowmeter applications include fluids with a solids concentrations between 100 mg/l and 30%. Solids size should be greater than approximately 75 microns, and the solids specific gravity should be at least three times greater than the carriers specific gravity. That is, the specific gravity of the solids alone (not the bulk density of the slurry) should be over three when flowing in water. Avoid stainless-steel tubing and high-density polyethylene pipe.

Velocity Matters

Regardless of the flowmeter technology, you should ensure that there is a good velocity profile in the flowmeter. Distorted velocity profiles can significantly increase measurement errors. Multi-path transit-time ultrasonic flowmeters are available to measure flows that exhibit non-uniform velocity profiles. Increasing the number of paths generally increases the accuracy with which the flow can be measured. It is advisable to use straight runs or flow conditioners to develop a good velocity profile for the flowmeter rather than increase the number of paths (and the cost). That said, in critical applications, such as for custody transfer, it is not uncommon to install a multi-path spool-piece ultrasonic flowmeter with lots of straight run in conjunction with a flow conditioner to achieve superior accuracy.

Figure 4. Clamp-on Doppler ultrasonic flowmeter
(Courtesy Siemens)

Small pipe sizes can present a measurement constraint on transit-time ultrasonic flowmeters. Because the transit time is small in small pipes, the difference between the upstream and downstream transit times is smaller yet. This can be remedied by using multiple traverses that direct the ultrasonic energy to bounce off the pipe wall one or more times as it travels between the transducers. Increasing the transit time increases the difference between the upstream and downstream transit times, making the difference easier to measure. Transit-time ultrasonic flowmeter installations that exhibit internal pipe coating and/or reflectivity issues can adversely affect the effective path length and, hence, the transit-time and flow measurements. For example, the path length of a transit-time ultrasonic flowmeter with four traverses (three reflections) in a 25-mm pipe with 5 mm of coating will be reduced by over 25 mm.

Be sure that the fluid itself is uniform in the flowmeter. Some fluids can separate when flowing in horizontal piping, so the top and bottom phases may travel at different velocities and cause measurement problems. Relocating the sensors in a vertical pipe section often resolves this problem.
Life would be simpler if there were fewer complications. Wouldnt it be great if you could clamp a sensor onto the pipe and measure the flow through the pipe? Ultrasonic flowmeters solve this problem in some, but alas, not all cases.

David W. Spitzer is a principal in Spitzer and Boyes, LLC, and can be reached at 845.623.1830 or www.spitzerandboyes.com.

For more information on flow technology go to www.controlglobal.com/flowmeter.html.