Ultrasonic flow meter

Ultrasonic flow meter ( USD) measure the speed of a flowing medium ( gas, liquid) using acoustic waves. This flow meter is according to the basic norm DIN 1319 of two parts: the sensor ( ultrasonic sensor ) and an evaluation and feed portion (transmitter or transmitter).

The acoustic flow measurement offers some advantages over other methods of measurement. The measurement is substantially independent of the properties of the medium used, such as electrical conductivity, density, temperature and viscosity. The absence of moving mechanical parts, less maintenance and a loss of pressure by cross-sectional narrowing does not arise. A large measurement range is one of the more positive features of this process. For the acoustic flow measurement using ultrasound used in industrial plants two essential principles of measurement used: The ultrasonic Doppler method and transit time method. The measurement method of ultrasonic flow measurement can be divided into the following four different measurement principles:

  • 5.1 multi-channel method 5.1.1 Principle multichannel
  • 7.1 In-line measurement paths
  • 7.2 In-line installation measurements 7.2.1 applications
  • 8.1 Standards
  • 8.2 applications

Doppler method

Wherein Doppler ultrasound measurement, the frequency shift of the transmitted signal based on the flow rate of the ( non-uniform ) particles is detected in the medium. To this end ( pollution, air bubbles ) are needed in the medium reflection points. In partially filled pipes in addition to the flow rate, the liquid level must be determined in order to calculate the flow can.

Stroboscopic method

The stroboscopic measurement method is similar to the Doppler method by the reflected acoustic signals from the moving particles. It is in this case, unlike the Doppler method is not analyzed, the frequency shift of the sound signal, but measures the time it takes for a particle to traverse a defined path in a sound cone. The ultrasonic pulses are successively emitted in a short and quick succession (similar to stroboscopic light ).

Drift method

The so-called drift process a continuous ultrasonic signal is emitted perpendicular to the flow of the medium to be measured. It is the intensity distribution is deflected by said medium in accordance with the direction of flow. From the relative intensity distribution of the ultrasonic signal in the receiver, the opposite relative flow velocity can be determined.

Time difference measurement

For this measurement method, the media must be as homogeneous as possible and occupied only with a low solids content.

In simplified terms, one considers two boats that cross a river on the same line diagonally opposite the one in the flow direction and the other. The boat, which moves in the direction of flow, it has a significantly shorter time to reach the opposite shore. Just behave the ultrasonic waves. A sound wave propagates in the direction of flow of the medium from faster than the sound wave in the opposite direction.

The running times are measured continuously. The transit time difference of the two ultrasonic waves is thus directly proportional to the mean flow velocity. The flow volume per unit time is the product of the average flow velocity multiplied by the pipe cross-section of the sensor.

The identification of a measured substance can be determined directly using the time of flight measurement of ultrasonic waves. The sound propagation time, for example in water is shorter than in heating oil.

Basis of calculation

Calculating the flow rate according to the delay process is carried out according to the following equation:

This includes:

  • V - average flow velocity of the medium
  • T1 - term of the ultrasonic signal with the flow
  • T2 - duration of the ultrasonic signal against the flow
  • L - length of the ultrasonic path
  • α - angle of the ultrasonic signal to the flow

Data acquisition

In modern evaluators, the arrival times and the running time differences are determined today with digital signal processors (DSP). Common methods are the edge evaluations First - negatives and the cross-correlation method.

Multi-channel method

The evaluation is mostly operated in the multi-channel method, which can thus capture the actual flow conditions safely and accurately even under difficult conditions.

Principle of multi-channel

An ultrasonic flow meter measures the flow velocity in the measuring tube by means of two opposing sensors arrays. These are arranged at an angle so that a sensor is mounted slightly further downstream than the other. The flow signal is determined by alternately measuring the transit time of an acoustic signal from one sensor to another, wherein the effect is used such that sound is transmitted faster than the flow direction against the direction of flow. The volume flow is determined by sequentially measuring between all pairs of sensors in the array. This arrangement ensures that for typical flow disabilities, such as through a pipe bend in one or two planes a short straight run pipe is required upstream of the flowmeter. The digital signal processing guarantees the constant evaluation of flow measurement and reduces the sensitivity to multi-phase flow conditions, thus increasing the reliability of the measurement significantly.

Clamp-on flow meter

As a special form of ultrasonic flow measurement, there is a measuring device which is inserted from the outside of the pipe, requires no intervention in the tube itself, and thus the quantity or the flow rate can be determined. For operation in nominal sizes DN 5 to DN 6000. Especially with large diameters, these clamp-on method is a cost-effective solution.

The main advantages of clamp-on:

  • Used for all homogeneous media in sound-permeable tubes, even with lining;
  • Chemical or process applications;
  • Large medium temperature range -40 ... 170 ° C;
  • Ideal for retrofits;
  • Installation without interrupting the process.

Clamp-on probes are also available for measuring the flow of hoses. This method is non- invasive and can be optionally combined with bubble detection. In order to achieve optimum measurement accuracy, the probe is usually calibrated to the specific hose material. This clamp-on probes are used primarily in the medical sector. Thus, the precise blood flow in the tubing system of the extracorporeal circuit, for example, the use of the heart -lung machine that takes over the function of the heart and lungs during open-heart surgery, determined. Also in the ECMO application ( Extracorporeal membrane oxygenation ) of the blood flow is monitored by default with a clamp-on sensor, usually in combination with bubble detection.

In -line flow meter

In-line measurement paths

This type of ultrasound flow meter is installed in the pipe. The devices can be calibrated at the factory and therefore achieve a much higher accuracy, which can still be reached in the field. By mehrpfadige sensor array, the accuracy can be further increased and the influence can be minimized by an asymmetric flow profile.

The sensors are suitable for many tasks of process control and power applications in almost all industries. The advantages of the inline design are:

  • Calibrated flow meter for conductive and non -conductive liquids, such as solvents and hydrocarbon;
  • Diagnostic ability and data back for increased process quality;
  • Permanent self- monitoring of transmitter and sensor;
  • Fewer demands on upstream pipelines in the design with four beams only inlet sections ( pipe diameter ≤ 5 ) is required;
  • No pressure loss;
  • Maintenance-free, no moving parts.

Typical applications are in the area of water and wastewater, energy and heat, and in the field of chemistry.

In -line installation measurements

In addition to the design of the in-line measuring sections, there are also in-line installation measurements, in which the sensors are installed in existing pipelines. Advantages here are the upgradeability and ease of maintainability of the measurements. Such measurements are particularly appropriate for larger pipes, as in-line measuring lengths are difficult to produce and untransportabel then. Also the use in open channels, canals and rivers is thus possible, the ultrasonic transducers are then mounted on the Gerinnewand

Applications

  • Turbine acceptance tests in hydro power plants according to EN ISO 60041 or ASME PTC 18
  • Pipe breakage monitoring of storage power plants
  • Cooling water measurements in thermal power plants
  • And discharge measurements in wastewater treatment plants by EKVO
  • Hydrological monitoring and flood forecasting in rivers and streams
  • Monitoring of irrigation canals

Flowmeter for open channels and partially filled pipes

In addition to the above-described ultrasonic flowmeters for filled cables also they exist for partially filled pipes and open channels. These also work on the transit time principle with one or more horizontal measurement paths. Ultrasonic velocity measurement is usually combined for such applications having a level or filling level meter and an evaluation unit which calculates the flow cross -section. The evaluation then calculated average velocity and flow through the cross section, the flow volume.

Standardize

The flow measurement in open channels is described in ISO 6416. Known methods for calculating the flow rates should traverse the mid- section and the Mean - Section.

Applications

  • Supply of hydroelectric power plants
  • Water hydrology / flood monitoring in rivers and streams
  • Urban Hydrology / sewer system investigations
  • Wastewater discharge to rivers
  • Cooling water intake of power plants

Benefits

  • Virtually complete independence of the Reynolds number, ie the same measurement accuracy in laminar and turbulent flow profiles;
  • Accurate measurement over wide spans possible;
  • Displays the flow in operating units;
  • Insensitive to pressure and flow shocks, vibrations; Dirt particles in the gas stream;
  • Suitable for custody transfer;
  • High temperature versions to Messtofftemperaturen of 500 degrees Celsius depending on the manufacturer and design

Disadvantages

Sound waves are pressure waves, which generate mechanical waves due to the compressibility of the fluid. Extremely high viscosity damps this movement, and thus the propagation of the sound waves. Therefore, there are limits for viscosity. Typically, this limit is much higher than the values ​​occurring in practice, it plays a role only in a very small number of individual cases. High gas fractions lead to an increased compressibility of the fluid and extremely low sound velocity. Both effects can lead to failure of the measurement.

Not all fluids can be measured:

  • Gas bubbles: reflections / attenuation, max. 1 vol %
  • Solid particles (reflections ) standard value < 5 vol %
  • Viscosity ( damping ) Indicative amount: max 100 cps / DN [ m]
Stroboscopic effect
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