Measurements in Sewer Systems

Transcription

1 16 th European Junior Scientist Workshop Real Time Control of Urban Drainage Systems Milo, Etna Mountain, Italy 7-10 November 2002 Measurements in Sewer Systems Jan Koch Darmstadt University of Technology, Institute for Hydraulics and Water Engineering, Section for Hydrology and Water Management, Petersenstr. 13, D Darmstadt, Germany Abstract The task of carrying out measurements in sewer systems is very versatile and an indispensable basis for RTC. More and more requirements are made on these measurements in the future, with respect to reliability and precision. One should think that it is easy to record the parameters water-level and discharge correctly. However, the daily experience in the inspection authority has shown, that it sometimes still comes to severely erroneous measurements. Erroneous measurements can arise due to different situations. On the one hand, manufacturers of gauges suggest that their gauges can be used by anyone in every place without having to get an idea about boundary conditions in the individual measuring site. On the other hand, simple parameter setting errors as well as transmission errors are encountered. Comprising, it can be noticed that the mounting of a measuring site requires special knowledge of the used technology and particular care. All boundary conditions appearing at the measuring site should be taken into account regarding the choice of the measurement method. While the measuring technique to determine water levels is considered to be technically sophisticated, the portable automated discharge measurement in the channel partly still causes considerable difficulties. Improvement possibilities are still existent here. Advanced technical methods open up new possibilities. Despite these improvements, the used approaches to calculate discharges are oftentimes very much simplified, and can no longer be regarded as state of the art. Hence, there is a need for development. Keywords: measurement, water level, discharge, RTC, sewer system, 1. The task of measuring In order to evaluate a sewer system and/or its buildings, it is an indispensable prerequisite to carry out measurements within the system. These measurements can then be used as a basis for further development of the drainage system, for the calibration of a numerical model or for the control of the system. The further explanations will focus on the classical parameters water level and discharge. 1

2 The type of measuring site differ depending on their utilization time: - continuous measuring sites - long time measuring sites - short time measuring sites continuous measuring sites One finds continuous measuring sites primarily at overflow structures to continuously determine water levels. The chosen measuring method and the location for the mounting of the measuring equipment should already be taken into account when planning the overflow structure. Continuous discharge measurements in a canal often serve as a clearinghouse. The choice of a suitable measuring method must be carried out with particular care, since it demands for a high degree of data security in a big measurement range. Dependent on the local conditions, structural measures have to be carried out when boundary conditions are not suitable for the mounting of the equipment for a measuring site. The measuring site should be well accessible and should have power supply. long time measuring sites When talking about a long time measuring site, the operation time of the sites usually ranges within a period of several weeks or months. With the results of the measurements, representative data shall be gained for typical operating states (dry weather flow, discharge peaks...). As a rule, greater structural measures for the mounting of the measurement equipment cannot be carried out. The choice of a suitable measuring site is therefore very difficult since the measurements must take place automated and a power supply oftentimes isn't assured. Dependent on the failure safety of such a measuring site, the maintenance intervals for inspection of the measuring site, for exchange of the accumulator unit and for collecting the data must then be defined. These boundary conditions strongly limit the choice of the measuring method, especially for discharge measurements. Hence, there exists a high potential for further development in this section. short time measuring sites One talks about short time measuring sites when the measurements take place for several hours or for a day at maximum. As a rule, such measuring places serve for the purposes of checking existing measuring facilities or discharge limiters. Since staff is at the spot during the complete measuring period, one isn't so limited regarding the choice of the measuring method. Also non-automated methods are applicable here. In principle, all data from all type of measuring sites must be checked for plausibility and reproducibility. This means that every measuring site should undergo a check in regular time intervals. In this way, errors can be recognized and removed quite early. 2

3 2. Water level measurements With the help of water level measurements at overflow structures, statements can be made about the actual operating behavior. Normally, a calculation or simulation is used as a basis for the dimensioning of the overflow structure. However, only with measurements one can find out if the structure behaves as predicted by the simulation. The essential components of interest when measuring the water level are - the determination of the duration and frequency of backwater events, - the determination of the duration and frequency of weir overflow events, and - the calculation of the relieved water quantity. If both the backwater time and the relieved water quantity shall be determined, special requirements must be made on the water level measuring equipment. In most cases the use of two different measuring devices is recommended, due to different measurement ranges; one measuring device for the backwater-behavior, and one for the overflow behavior. A partial overlapping of the two measurement ranges permits a check of the two facilities under each other. Unlike measuring the water level in overflow structures, which is quite straight forward, the calculation of the relieved water quantity is considerably complicated to calculate, and furthermore its calculation is also bound to a high level of uncertainty. Reason for this uncertainty is the oftentimes very shallow overfall height in combination with a long overfall length, and again the different hydraulic boundary conditions. Regarding the topic of water level dependent ascertainment of the overflow quantities a doctoral thesis is carried out at our institute at this moment. The results of this work will be presented soon. Some common methods to the regulation of the water-level are explained in the following Measuring techniques Diving probes Measuring the water pressure is conducted by transforming the movement or deformation of a membrane in an electrical signal. Different physical effects can be used. Inductive or capacitive taps as well as stretching measurements are common. Pressure gauges are to be distinguished in absolute-, relative- and difference pressure gauges. At all gauge types, the sensitive pressure sensor (primary sensor) is separated from the measuring medium (sewage) by thin, corrosion-resistant membranes. The deformation of the membrane then causes a pressure on the primary sensor, which is transmitted by means of a liquid. Particularly the temperature dependence and the long-time drift are disadvantages of this measurement procedure, which have to be taken into account carefully when choosing this method for the concrete field of application Ultrasound probe The water level measurement procedure most frequently found at the moment in sewage technology is the echo sounder, also called ultrasound measuring. The measuring principle consists in measuring the running time of a directional sound impulse along the measuring section which is to be determined. The sound impulse is sent by a sensor, reflected at the 3

4 liquid surface and then registered by the sensor that switched it's functionality to act as a receiver. The fast sequence of sound impulses makes an almost continuous measuring possible. The time which the sound impulse covers during the changeover between sending and receiving the impulse is called block distance. Since no measurements can be conducted within the block distance of the sensor, one has to make sure during the mounting process of the measurement device, that the maximal water level to be measured doesn't lie within the area of the block distance. Measuring of the water levels by means of ultrasound has the great advantage that it works contactlessly. This means that no components of the measuring device come into direct contact with the sewage Bubble technique A thin, not flexible tube with a little opening at the lower end is brought in and pressurized with compressed air. Then, the pressure within the tube and the corresponding pressure according to the water level can be determined. Following aspects are to be considered, when using this measuring technique. - The possible maximum pressure in the tube must be greater than the maximum water pressure. - The rate of air escaping from the tube must be adjusted according to the change of water level elevation - The tube must be placed with a steady descent without creases and bends, so that blockages and water collections are avoided. - The tube must be scoured out regularly with high pressure and as automatically as possible. Furthermore the tube must be absolutely leak-proved. - The opening must be cleaned regularly Other equipment Gage The water level can be measured directly with a vertically moving swimming body, situated at the water surface. The vertical movement of the swimmer is changed to electronic data by means of a potentiometer. Load cells with wall mounting. These kind of measurement devices determine the pressure directly by means of the pressure head. As in the case of the diving probes, the measuring sensor includes a pressure measuring cell. 4

5 3. Discharge measurements Whereas the recording of the water evel still represents a relatively simple task, the exact discharge measurement causes substantially more difficulties. 3.1 Principles of measurement Hydraulic methods The flow is determined from the measuring of a term (e.g. water level, pressure) under the use of a known relation between this measured term and the discharge. All kinds of "Venturichannels" and measuring weirs are part of the hydraulic methods. "Venturi-channels" can be installed as either completely prefabricated unit, or as half-shells for measurements in open channels. In fact, measuring weirs are offered standardized, however, in most cases they are manufactured after measure and also used mostly in open channels. As measurement recorder ultrasonic probes are mostly used. Modern ultrasonic apparatuses with an integrated measurement converter are easily programmable and can be checked easily. Their advantage is particularly the contactless measuring of the water level which excludes an impairment by sewerage. Moving and therefor delicate parts, as they are used on swimmers, are no longer necessary. The relation between the measurand (e.g. water level) and the discharge is given by the manufacturer, in case of prefabricated building units. Individual structures require the determination of this relation either by calculation or by experiments. For Venturi-channels with rectangle cross-section DIN part 2 can be used. For measuring weirs appropriate literature [1] is referred to. Important for all hydraulic methods is the proof that the measurement site is not impaired by backwater effects. However, if this is the case, the measured values can be severely altered Flow velocity methods The velocity of flow of the medium is recorded in profiles, in parts of cross sections or averaged over the complete cross section. Integrating the velocity profile over the cross section area results in the flow discharge. Physical rules are used which clearly relate the measured term to the velocity. Devices to measure flow velocity comprehend electromagnetic flowmeters, ultrasound (U.S.) running time difference as well as ultrasound Doppler gauges. These type of gauges have to be differed into gauges that measure in completely filled pipes, in partly filled pipes and in open channels. Electromagnetic flowmeters are used in completely filled pipes in most cases; however, ultrasound running time gauges come into operation as well. Electromagnetic flowmeters take into account the velocity profile in the complete flowing cross section while ultrasound running time difference gauges record a single velocity only, which is averaged over one or several lines which are at a certain angle to the flow. This single velocity is then used to make conclusion about the discharge. For partly filled pipes and open channels electromagnetic flowmeters are offered recently, ultrasonic apparatuses, however, are of greater importance. They must be distinguished in running time difference and Doppler gauges. 5

6 Running time difference gauges measure averaged velocity in one or several depths within the cross section. These averaged velocities run parallel to the channel sole. Together with the recorded flow depth and an assumption about a velocity profile, the equipment calculates the discharge. The US-Doppler probes, mounted at the sole of a channel, generally send out ultrasound impulses of a known frequency in the opposite of the flow direction, which are then reflected on particles or little air bubbles. Because of the movement the sound undergoes a frequency displacement which is a measure for the velocity of the flow. Together with the flow depth, which is either measured by the pressure sensor within the US-Doppler, or by an external device, and a calibration factor, the discharge is calculated. The calibration factor is necessary, because the position of the reflective particles cannot be derived from the frequency displacement alone and because the recorded velocity is representative only of a not obviously definable part of the flowing cross section. This factor only can be won by a calibration at the installation place at all occurring water levels and discharges. The complexity of such a calibration increases by far if the measuring cross section can come under backwater influence at times. A new method for discharge measurements is working according to the radar principle. A sensor is attached above a water level and measures an average surface velocity. This velocity is then converted to an average cross section velocity with the help of empirical algorithms. An additional sensor measures the filling level of the pipe. Since the both sensors do not measure in the same range, a prerequisite for this measuring technique is a straight water level and a prismatic channel. An advantage at this measuring technique is that it is contactless. Erroneous measurement due to defilement of the sensor is therefore impossible. Additionally, manual flow measurement procedures are available, where local velocities are measured within a measuring grid in the cross section area with either propeller meters, a U.S. probe or an electromagnetic probe. After integrating over the cross section one gets the average discharge Tracer dilution gauging An additional group of measurement procedures represents the group of the dilution gauging measurements. A so called tracer is introduced into the channel in a certain concentration or quantity. Then, at another place downstream, the concentration of this tracer is measured over time. From the distribution of the concentration in time, the discharge can be calculated. In the field of dilution gauging one distinguishes methods of continuous tracer injection, and impulse injections. 6

7 4. Oncoming Research Project None of the presented methods is generally applicable in sewers. Due to inherent boundary conditions in sewerage networks, every site needs an adjusted solution. For this reason it would be desirable to have a measurement device which is cheap, easy to handle and does not need calibration. Newer bottom fixed velocity sensors are able to measure a complete velocity profile in a perpendicular line. With these sensors many more information can be obtained than with simple sensors, which measure only the peak velocity. Nevertheless, the velocity profile isn't inevitably sufficient to determine an average velocity over the complete cross section. A future task will be the compilation of a collection of velocity profiles for partly filled crosscuts, dependent on cross section type, bottom slope, roughness and partial filling degree. This can be accomplished e.g. with the help of a numerical model and by practical tests in the hydraulic engineering laboratory. With those results it can be tried to find a relationship between the perpendicularly measured velocity profiles and the average flow velocity. It has further to be shown, that using this relationship leads to better results than using simple conversion techniques as described above. Furthermore, it would be interesting to investigate if results could even be improved more by coupling several sensors which cover a broader range of the cross sectional area and how they should be arranged. The above-mentioned work shall partly be dealt with in the context of my doctoral thesis. The main objectives of this work can be listed as follows. - comprehensive description of the field of responsibility for measuring in sewer systems - presentation of available measuring devices and measuring techniques - explanation of the physical basis of the measuring principles and comparison with the boundary conditions found in the channel - practical tests with the different measuring devices - Construction of a qualification profile for an "ideal gauge" References Bos, M.G., Discharge measurement structures, International Institute for Land Reclamation and Improvement/ILRI, Wageningen (NL), 1978 Chow, Ven Te OPEN-CHANNEL HYDRAULICS, International Edition, McGraw-Hill, 1973 DIN , Durchflußmessung von Abwasser in offenen Gerinnen und Freispiegelleitungen; Venturi-Kanäle, Ausgabe: Kölling, K., SIMK Kalibrierung von Durchflussmessanlagen, in Durchflussmessung an Regenüberlaufbecken, ATV-DVWK Schriftenreihe Band 19, Hennef,