26 Water / Wastewater
Flow Rate Measurement in full Pipes using the Transit Time Method
t1= Impulse time against fl ow direction t2= Impulse time in fl ow direction L= Transit time / distance between sensors
Formula: Average transit time difference in a measurement path
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 record- ings or to document and to control individual consumptions and withdrawal quantities. Flow detection, however, particu- larly 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 indispensa- ble 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 measure- ment 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 main- tenance 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 uctu- ating 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 im- pulses are transmitted successively after each other and the different transit times 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
Figure:
Schematic illustration of transit time difference principle
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
Photo: Schematic illustration of multiple measurement paths
transmission and reception of the meas- urement signal. The difference between both determined times is proportional to the average fl ow velocity within the measurement path. The more measure- ment 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 multi- ple 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 meas- urement setup may also reduce the length of intake and discharge sections required to calm down the fl ow profi le at the measurement point.
Thanks to novel CFD models (Computational Fluid Dynam- ics) and comprehensive testing at renowned institutes, infl u- ence and behaviour of fl ow profi les downstream of standard disturbances could be examined. Based on the results it is now possible to integrate fl ow profi les downstream of el- bows and other disturbances into calculation models directly in the transmitter of the measurement system. Only the type of disturbance and the distance to the measurement spot need to be specifi ed. From these specifi - cations the measure- ment automatically determines the cor- rection factors to use. The result of the fl ow measurement is there- fore highly accurate and can be even used together with shorter calming and intake sections.
Photo: CFD-model of a disturbance (elbow)
The new NIVUS GmbH device types allow using the transit time method both as invasive measurement and clamp-on system. Here the type of sensor used must be selected depending on the situation on the measurement place. Highest measurement accuracy can be achieved by using a multi-path system with wetted sensors in a defi ned ar- rangement. If it is not possible to insert the sensor into the process (abrasive, corrosive or other problematic media), the sensors can be installed on the pipeline from the out- side without process interruption (clamp-on measurement system).
Implementing a Measurement
Tasks of the example depict- ed: long-term recording of fl ow rate and fl ow velocity for archiving in a distribution sys- tem operator’s drinking water supply pipeline. Accuracy requirements in this case are very high since the measure- ment place is to be used for billing purpos- es. A NivuFlow 600 system with invasive sensors by NIVUS GmbH was used. The sensors were inserted into the pipe by using tapping nozzles. This is how the readings could be provided to the follow- ing SCADA system with the required level
Photos: Transmitter and various sensor types
of accuracy via data connection. The variety of sensors and installation material allows picking up readings at various measurement spots. A very minimalistic approach can be followed in terms of spare parts stock: no need to stock diameter-specifi c parts, one measurement system for almost all pipe diameters and measurement places.
Summary
Flow measurements based on the ultrasonic transit time difference principle have not only undergone many years of extensive testing. They have also proven successful in practical use and stand out for a high level of accuracy and fl exibility in terms of applicability in various measurement places. Thanks to robustness and ease of maintenance the ultrasonic transit time principle is perfectly suitable for both measuring in pipes with smaller diameters (such as process water or cooling water) as well as for permanent measure- ments on demanding measurement sites (such as large pipe diameters, hydro-electric plants, high process water volumes and varying media). With new-generation devices, however, the benefi ts of the method have been signifi cantly extended. Among other things, measurement ranges and accuracies of fl ow measurements have been increased considerably.
AET Annual Buyers Guide 2017
www.envirotech-online.com
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