Modern hydropower utilization would be quite unthinkable without high-precision flow metering systems. Data collected this way provides the basis for a variety of processes in areas from control technology to safety, ecology, and operational optimisation. Ultrasonic flow metering technology, in particular, has advanced immensely in recent years. Especially the use of multiple acoustic paths and related self-diagnostic options has convinced numerous users of the many benefits, such as high-level precision, wear resistance, low operating and maintenance costs, and long-term stability.

In terms of both quality and quantity, the availability of water is a crucial economic factor in every part of the world. Aside from its importance for domestic use, water it is also essential for agricultural irrigation, as a medium for industrial processes, and as a fluid used for energy production and serving as a coolant. Considering that the globally available water volume remains constant, with irrigation consuming 69 % and industry (including energy production) using 18 % of the overall amount available for human use, the rising demand for freshwater resulting from population growth is set to further take its toll on existing water resources. Studies predict a 40 % deficit between available worldwide water resources and human consumption alone by 2030 (source: The 2030 Water Resources Group). Apart from irrigation, a significant increase in the demand for water is also being predicted for industrial purposes and energy production. The accelerated rate of urbanisation, as well as the expansion of municipal water supply and sewage systems, contribute further to the rising demand. As a result, the competition for water resources is likely to intensify between agricultural irrigation and industrial use, as irrigation utilises freshwater with only around 45 % efficiency.

In high demand for many applications
High-precision flow metering is set to rise in importance across the board: not just in agricultural irrigation but also in industrial areas, for wastewater disposal billing, in energy production for the use of cooling water, in hydropower generation, for effluent water processing, sewage storage management, water processing for urban landscaping, street cleaning, and water purification for domestic use.

• Wherever it is used, high-precision flow metering helps to provide understanding of existing systems, localise points of water loss in channels or pipes, optimise processes, control supply networks, and ensure fair, transparent billing.

• Flow metering systems that have only been tested under laboratory conditions are no longer up to the job. What is needed today are metering instruments that deliver a high level of precision under local conditions without the need for on-site calibration.

• Every drop counts when it comes to ensuring a sufficient water supply for everyone, both now and in future.

Acoustic flow metering
Acoustic (ultrasonic) flow metering technology has advanced by leaps and bounds over the last few years. The introduction of acoustic multi-path methods and self-diagnostic capabilities have especially boosted market acceptance. Multiple acoustic paths are used to measure the cross-sectional flow velocity in several locations, providing highly accurate flow rate readings. In case of flow disturbances, this method eliminates the need for the upstream/downstream straight lines that are typically required by traditional technologies. This is a key factor to be considered when retrofitting flow metering systems in existing buildings when space is rather limited. By using the acoustic method, adaptations and extensions to existing building structures can be avoided, which drastically reduces the overall investment in a flow metering solution.

Acoustic multi-path systems are well established in the world of hydropower as an in-situ measuring method for monitoring the effi-
ciency of turbines (with an error margin of
≤ ±1 %). This standard is defined by two norms/guidelines:

• ASME PTC 18-2020, hydraulic turbines and pump-turbines, Performance Test Codes

• CEI/IEC 41:1991, Filed acceptance tests to determine the hydraulic performance of hydraulic turbines, storage pumps and pump-turbines

Current high-precision acoustic flow metering systems come in two versions: inline systems and insertion systems. Owing to their wide scope of application and high metering precision, the annual market share of acoustic systems (besides coriolis flow metering devices) is growing more strongly than that of other physical methods such as differential pressure, electromagnetic, turbine impeller, turbulences, and others.

Insertion systems with multiple acoustic paths
Insertion systems are used primarily for large-scale pipes (> DN400). Depending on the number of acoustic paths, if the pipe wall is thick enough, the sensors are screwed in directly at specific points. In thinner-walled pipes, additional welding studs are used for stability. Compared to a traditional in-line flow meter this approach does not rely on the calibration of the overall system on a testing station. Instead, the level of accuracy is determined by the quality of the installation and on project-specific parameters that have to be measured on-site and fed to the system. For example, the acoustic path lengths, angles and sensor positions have to be measured to millimetre precision and provided to the system. In addition, the pipe diameter has to be measured to a high degree of accuracy at differing cross-sectional points along the pipe. The penetration deviation of the sensors must be quantified by the manufacturer and has to be taken into account when calculating the volumetric flow. Using such an approach, experience from numerous installations shows that using 8 acoustic paths and a straight pipe stretch of 5*D upstream and 1*D downstream of the metered cross-section point will keep the margin of inaccuracy at or below ±0.5%. For longer straight pipe stretches or when using additional metering paths, the error margin may be as low as ≤ ±0,15 %.

Ultrasonic flow metering devices excel in many respects:

• The underlying metering principle is equally applicable to fluids, gas and steam flows

• The accuracy and reproducibility of measurements is independent of fluid-specific characteristics, such as viscosity, temperature, density, and electric conductivity

• There are no moving parts extending into the metered pipe

• The systems don’t wear out

• Operating and maintenance costs are low

• The method provides excellent long-term stability

• The need for re-calibration is eliminated

• Redundant metering paths ensure highly reliable results

• The devices cover a very wide dynamic metering bandwidth

• Very low and very high flow rates can be measured accurately without the need for introducing constrictions in the pipe cross section, which otherwise reduces the loss of pressure and therefore the amount of energy expended on pumping

Enabling self-diagnostics
The most crucial benefit, however, consists in the fact that when using multiple acoustic paths, the flow velocity is measured at different points of the cross section, which provides valuable information on the existence (or non-existence) of distortions in the velocity profile. Information like this can then be used to calculate the volumetric flow with the help of integration methods. In addition to flow velocity, ultrasonic flow metering systems can also be used to measure other parameters, such as the speed of sound in the fluid, enabling self-diagnostic. This way, differentials between the individual metering paths can be detected independent of the flow velocity, and possible preventive maintenance measures can be suggested as a result. Where certified metering precision is concerned, customers have traditionally put their trust in a ready-made pipe section with a test certificate. However, there is a growing awareness among users that the metering precision determined this way may deviate considerably from actual on-site metering values. Whilst there is undoubtedly some further persuasion to be done, the growing adoption of acoustic flow metering using sensors that are installed and calibrated in-situ brings significant advantages in terms of accuracy and cost benefit.