Understanding Ultrasonic Flowmeters

The Doppler effect, discovered in 1842 by Christian Doppler, is familiar to most of us; stationary observers of waves such as sound waves will see wavelengths that depend on the relative motion between the observer and the source; if the source approaches them, the wavelength will appear to be shorter (compressed) – if the source recedes, the wavelength will seem to be stretched.

This explains the rising and falling pitches of cars as they pass us by; when the car recedes, the pitch seems to drop as the wavelength is “stretched” for the observer.

This frequency shift is used for measurements across a range of different industries, and ultrasonic doppler flowmeters can employ this to measure flow rates in so-called dirty liquids. These are those that contain acoustical discontinuities such suspended particles, entrained gas bubbles or turbulence vortexes, which can make measuring sound waves ordinarily difficult.

Ultrasonic pulses or beams can be transmitted into pipes containing these acoustically dirty liquids; they will reflect with a Doppler change in frequency that depends on the flow of the liquid. Observing the frequency shift allows the flow rate to be calculated from this, rather than the velocity of the fluid which can be harder to measure; the method is therefore based on the velocity of the discontinuities, and can be used for liquids such as certain wastewaters, slurries, sludges, crude oils, phosphates and pulp stocks.

Mining slurries often use high-density polyethylene (HDPE) pipes, but these can throw off the ultrasonic-doppler flowmeter method because they can flex as fluids flow through them, which changes the relationship between the frequency shift and flow rate; if the flexure is great enough the pipe’s surface-transducer may no longer measure the flow rate within. How “dirty” is acoustically “dirty”? Typically, to reflect the signal properly, the discontinuities – solids or bubbles – must be 100 microns or larger and at concentrations of 100 parts per million or higher.

Ultrasonic Doppler Flow Sensor

However, it won’t work if the discontinuities are greater than 45% or if the bubbles are very fine and in high concentrations; discontinuities at these extremes attenuate the reflected signal making it resemble the background noise of the pipe. The same is true for acoustically absorbent slurries, such as lime and kaolin.

A 100 micron/100 parts per million liquid equates to a 1 MHz transducer frequency. A 6Hz shift is typical for every foot per second of velocity; so ultrasonic measurements aren’t practical for flow velocities less than ½ a foot per second. No one yet knows how high the velocities this system can measure are, although the upper limit is greater than 50 feet per second.

The constituent parts for the flowmeter are a transmitter/indicator/totalizer and a transducer. The user chooses configurations depending on the liquid, the size and concentration of solids or bubbles, and the dimensions and lining of the pipe; typically they will also filter out noise due to the pipe.

Ultrasonic Doppler Flow Sensor

A high-frequency oscillator in the transmitter drives the transducer, which can be clamped onto the pipe exterior. The transducer’s ultrasonic signal that passes through the pipe wall into the flowing liquid. The transmitter then converts the difference between the input and output frequencies into electrical pulses, which can be processed and converted into the flow rate signal.

Advantages of clamping the ultrasonic doppler flow sensor on the outside include that it can operate non-invasively, without moving parts. It requires no maintenance, involves no pressure drop, and can have ±1% accuracy. However, interference can arise from the pipe wall and any air space between the wall and the liquid; stainless steel pipes might conduct the signal and reflect it along which can result in a measurable shift. This can be alleviated with concrete-lined, plastic-lined and fiberglass-reinforced pipes, but then there are acoustical discontinuities in the pipe itself that can scatter or attenuate the signals as they are transmitted or reflected. For this reason, they are unsuitable for use with lined pipes; many simply won’t work, and those that do have an accuracy of ±20% at best.

You can get around this using in-line, or wetted transducers; these transit-time flowmeter measures a signal traveling between two transducers, one upstream and one downstream. Then the difference in time elapsed depending on whether the signal follows or goes against the flow determines the liquid velocity; the advantage of this is that you’re using the speed of sound in that specific liquid. However, if the sonic velocity in that liquid refracts the signal through a large enough angle, it can miss the downstream transducer altogether, which is a failure mode known as walk-away.

Single-path and multi-path designs are available which use various combinations of transducer pairs, depending on whether you want to measure a small section of fairly uniform flow or a longer section of non-uniform flow – for example, in a large-diameter pipe. Multi-path designs are used in raw wastewater and storm water applications, and to measure stack gas flows in power plant scrubbers.

FDT100 Series In-Line Ultrasonic Flowmeter

The new in-line, single-path FDT100 Series from Omega Engineering has many of the advantages that have led to transit-time flowmeters becoming the standard for clean-liquid use (with no acoustical discontinuities.) FDT100 models are battery-operated, have no moving parts, can take measurements across a wide range and don’t need filtration.

They are available in two flange styles (150-pound ANSI and DIN) and in various sizes. You can choose integrated or remote electronic displays which show you the flow rate in the fluid.

Specialist equipment can be purchased that can handle liquids that are very hot, such as molten metals, or very cold – liquid nitrogen and other cryogenic liquids as cold as -300C. For low-flow applications, specialist transit-time flowmeters can also be used. Axial and coaxial transit-time flowmeters, for in-line use with pipes as small as 0.5 inches are particularly sensitive and can measure flows that are many times smaller than the pipe diameter.

Transit-time flowmeters can be used across a range of viscosities, providing Reynolds number at minimum flow is under 4,000 (laminar flow) or over 10,000 (turbulent flow). However, in the transition regime between laminar and turbulent flow, there are serious non-linearities which mean they can’t easily be used.

Microprocessors can be used that automatically switch the mode of flowmeter operation between “clean” and “dirty” modes depending on the signal that is received. These are called cross-correlation hybrid flowmeters, and you could measure both activated or digested sludge with the same device. If carefully engineered, accuracy levels of up to 0.5% can be obtained.

Until recently, clean liquids and Doppler flowmeters were incompatible. You could try aerating the flow and introducing bubbles for clean water, for example – this adds acoustical discontinuities, but is tedious and the bubbles need to be large and present in a high enough concentration. However, now multi-liquid Doppler flowmeters avoid this problem. Omega’s FD-7000 can measure dirty liquids in the ordinary way, but with clean liquids it senses ultrasonic waves reflecting off turbulent swirls.

As previously mentioned, this process can be automated, with flow analyzers determining which of the two modes to use for a given fluid by analyzing the flow. If you are in the clean regime with too few particle or bubble reflectors, the FD-7000’s mating-flow transducers should be mounted on the pipe at a point where non-symmetrical hydraulic turbulence exists.

FD7000 Series Ultrasonic Flowmeter

Typically a good location for this is 1-3 pipe diameters downstream from a sharp elbow in the pipe; a digital filtering system and signal recognition circuitry transforms the turbulence reflections into linear data. The FD-7000 therefore doesn’t require the pipe run upstream to be straight, as it actually operates using the turbulence due to kinks in the pipe.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.