Modern Flow Meters: Determining fluid velocity through sound waves

Determining fluid velocity through sound waves

The pattern of technology change witnessed in flow measurements is similar to that of mobile phones, which started with big and bulky designs and have reduced in size to become “smart” gadgets with multiple applications. Similarly, flow meters started with differential pressure measurements, then used rotary turbines and now we have modern ultrasonic measurements.

The technology

Ultrasonic flow meters use sound waves to determine the velocity of a fluid flowing in a pipe. Under no-flow conditions, the frequency of an ultrasonic wave transmitted into a pipe and its reflections from the fluid is the same. Under flowing conditions, the frequency of the reflected wave is different due to the Doppler effect. When the fluid flows faster, the frequency increases linearly. The transmitter processes signals from the reflected wave to determine the flow rate.

Transit time ultrasonic flow meters send and receive ultrasonic waves between transducers. Under no-flow conditions, it takes the same time to travel upstream and downstream between the transducers. Under flowing conditions, the upstream wave travels slower and takes more time than the faster downstream wave. When the fluid moves faster, the difference between upstream and downstream travel time increases. The transmitter processes upstream and downstream time to determine the flow rate. Transit time ultrasonic flow meters represent about 12 per cent of all flow meters sold globally.

Advantages and disadvantages

This technology can be very accurate and is therefore used for transferring expensive fluids like natural gas and petroleum liquids. Ultrasonic flow meters have a high turndown, can handle high pressures, are consistent and can function under extreme temperatures. Since they can be clamped on the outside of a pipe, penetration into pipes is not required. They have a low maintenance cost, and are highly reliable and self-diagnosing.

Ultrasonic flow meters do not obstruct the flow of the liquid so they can be used for sanitary, corrosive and abrasive liquids. Temporary flow measurements can be made using portable ultrasonic flow meters with clamp-on transducers. Clamp-on transducers that can be mounted externally on the pipe are particularly useful when piping cannot be disturbed, such as in power and nuclear industry applications. In addition, they can be used to measure the flow without facing issues pertaining to materials of construction, corrosion and abrasion. However attractive they seem, the use of clamp-on transducers introduces additional ultrasonic interfaces that can affect the reliability and performance of these flow meters. Moreover, if not applied and maintained properly, the attenuation of the ultrasonic signal can occur at the interface between clamp-on transducers and outside pipe walls, and between inside pipe walls and the fluid.

Disadvantages include high costs, sensitivity to stray process vibrations, problems related to pipe diameter change, and lower accuracy of clamp-on units.

Utility areas

Ultrasonic flow meters are commonly deployed to measure the velocity of liquids that allow ultrasonic waves to pass. These include water, molten sulphur, cryogenic liquids and chemicals. Transit time designs are also available to measure the gas and vapour flow. Fluids that do not pass ultrasonic energy, such as many types of slurry, limit the penetration of ultrasonic waves into the fluid. In Doppler ultrasonic flow meters, opaque fluids can limit ultrasonic wave penetration near the pipe wall, which can reduce accuracy and cause the flow meter to fail. Industries in which such meters are installed include oil and gas, water and wastewater, power, chemical, food and beverage, pharmaceutical, metals and mining, and paper.

Flowsic 500

SICK Industries Limited has launched a special model of the flow meter for the Indian market, Flowsic 500. It is the first ultrasonic flow meter for natural gas distribution. It can operate at a high pressure of 20 bar, and at temperatures ranging from -40 °C to +70 °C. It is available in a range of sizes. It also has a flange-to-flange dimension compatible with rotary and turbine meters. Flowsic 500 comes with a battery power supply of more than five years. Certified in gas metering, volume correction as well as against combustion, the Flowsic 500 provides some unique benefits for natural gas measurement in India. These benefits include the measurement of time differential; measurement independent of the gas temperature, pressure, density and contamination of transducers; and a high measuring accuracy even at near-zero gas velocity. It can be installed easily, has no moving parts, and no loss of pressure and hence low maintenance costs.

In the Indian market, Flowsic 500 can be used for municipal and regional natural gas distribution, for industrial and commercial measuring stations, and for any application where continuous gas supply needs to be ensured. n

With inputs from a presentation by Gerard Klooster, Product Manager, Flow Solutions, Sick India, at an India Infrastructure conference on City Gas Distribution in India