FLOW RATE MEASUREMENT IN FULL PIPES USING THE TRANSIT TIME METHOD
Many processes such as cooling processes in chemical industry or energy generation with hydro- electric plants are dependent on the water supply. Handling water as a capital good or in order to maintain processes becomes increasingly important. Particularly in large diameters the relevance of water volume measurement is growing. The challenge with cooling water or fresh water processes is to generate accurate fl ow recordings or to document and to control individual consumptions and withdrawal quantities. Flow detection, however, particularly in large diameters is highly demanding. In such cases the transit time measurement thanks to its high fl exibility as a reliable and cost-effi cient measurement system is literally made for permanent fl ow metering.
Securing the Water Supply
The requirement for many processes is to feed as little fresh water as possible. Water feed and withdrawal volumes need to be monitored constantly as well. All these tasks require to permanently investigate and to verify fl ow rates. Integration into higher systems (such as SCADA systems) is indispensable since the systems are generally used within large areas.
Conception and Selecting the Measurement System
To ensure constant fl ow recording it is necessary to use a measurement system capable of determining the medium velocity covering the entire wetted area. This is important particularly with fl uctuating fl ow conditions. Many measurement systems commonly used either feature spot velocity measurement only or do provide the required penetration depth. Quite simply, some measurement systems cause too much costs or require too many employees when it comes to installation. Measuring high medium velocities to many systems is an impossible task, too. A cost-effi cient method to obtain reliable information on the prevailing discharge / fl ow is the measurement using the ultrasonic transit time difference principle. Such systems stand out for low maintenance expenses and high operational safety. They can be fl exibly used with all needed sizes and media. Compared to other methods, the measurement system moreover has the advantage to be largely independent of the properties of the media to measure such as electrical conductivity, fl uctuating temperatures or viscosity.
This measurement principle is based on directly measuring the transit time of an acoustic signal between two ultrasonic sensors. Such sensors are also described as hydro-acoustic converters (A and B in the illustration below). Two sonic impulses are transmitted successively after each other and the different transit times
Figure: Schematic illustration of transit time difference principle t1
t2
= Impulse time against fl ow direction = Impulse time in fl ow direction
L= Transit time / distance between sensors IET Annual Buyers’ Guide 2016/17
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between transmitter and receiver are measured. The impulse heading downstream (t2) reaches the receiver sooner than the impulse heading upstream (t1).
The required times are measured by utilising highly accurate time measurements as well as a signal correlation. This signal correlation compares the transmitted signal with the signal received by the opposite converter. The comparison therefore enables to determine the accurate moments of transmission and reception of the measurement signal. The difference between both determined times is proportional to the average fl ow velocity within the measurement path.
Formula: Average transit time difference in a measurement path
By using this formula it is possible to determine the average cross- sectional velocity and hence the fl ow rate from the measured average velocities within the individual layers related to the according velocity coeffi cients.
Q= fl ow rate k= measurement place-specifi c correction factor A= wetted area vg
= average velocity Formula: General fl ow calculation
The more measurement paths are used, the more information on the fl ow profi le prevailing at the measurement spot can be gained. The total fl ow rate in this case is the total of the individual fl ow rates. Using multiple measurement paths hence will increase the accuracy of the fl ow rate determination. Arranging the sensors of a multi-path measurement crosswise reduces the effects of disturbing fl ows crossing the main fl ow direction. Cross fl ows may cause measurement errors. Using a multi-path measurement setup may also reduce the length of intake and discharge sections required to calm down the fl ow profi le at the measurement point.
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