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(Image: Methods for structural investigations of drains and sewers (the methods of engineering measurements for application in accessible sewers are not shown)) The determination of the actual structural condition of sewers is carried out qualitatively with the aid of optical processes and quantitatively by the use of suitable measurement and testing processes. A review of the processes used in practice for this purpose, and that are not as a rule … |
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Nov 19, 2004 Depending on the respective aims of a flow measurement, very different hydraulic situations will occur in the cross section to be measured for which not all measuring methods and certainly not all measuring instruments are equally suitable. If, for instance, it is meant to gather data on the hydraulic loading and capabilities of the sewer, then a large bandwidth of flow conditions up to possible complete filling under a backwater must be acquired. |
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Feb 16, 2011
The selection of the site for measurement is decisive for the results of a flow measurement. Measuring methods and devices, which, because of the system, have a guaranteed rate of error of no more than a few percentage points, can generate operating errors of more than 20 percent when the required hydraulic … |
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Feb 16, 2011 The time of the measurements, also, are determined largely by the purpose. The measurement must take place primarily at a time in which the flow section of the sewer of interest can be viewed as isolated as possible from other part flows. Investigations for determining the extraneous water share through infiltration of groundwater are, for example, carried out at about 2 am on a dry night. At this time the flow of domestic sewage is about zero so … |
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Feb 16, 2011 Basically, flow measurements can be separated into short-time and continuous measurements. In the short time measurements, one deals mostly with individual measurements, control or calibration measurements [DIN19559-1:1983]. If a particular event is to be measured with the help of the flow measurements, such as for instance, a strong downpour event, then a particular single measurement is undertaken. These measurements can be extended to multiple … |
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Feb 16, 2011 Rehabilitation and Maintenance of Drains and Sewers Flow Measurement Procedure Without Cross-Sectional Restriction The simplest but also least accurate method of determining the volume flow for a free flow is the calculation of the median flow velocity by means of partially empirically developed flow formulae. In these, the friction forces at the sewer walls are accounted for by means of coefficients. An example of this is the Manning-Strickler formula: v = K ยท I1/2 ยท Rh2/3Where K = Coefficient of friction in [m1/3s-1]I = Energy line gradient [-] and Rh = Hydraulic … |
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Feb 16, 2011 Rehabilitation and Maintenance of Drains and Sewers Flow Measurement Procedure Without Cross-Sectional Restriction (Image: Volumetric measurement with a pivot counter [Bonfi87]) Volumetric procedures can be used for the determination of very small volume flows, for short-term applications or for calibration. In these methods, the volume (by means of calibrated containers) and the time can be measured at the same time. In order to automate the method for long-term measurements, tipping counters are often used (Bild 4.4.1.5.1.2). The two-chamber container rotates … |
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(Image: Velocity distribution in a partly filled circular pipe [Valen94]) For circular pipes filled one-third to two-thirds full it is known that the median flow velocity vm required for the calculation of the volume flow, is in a constant relationship to the measurable maximum velocity vmax. Under ideal conditions this factor k is equal to 0.82 [Valen89]. Under actual conditions, which are determined by the relatively large velocity sensors employed … |
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(Image: Cross sectional profile of a sewage collector of the City of Frankfurt [Kölli]) With the help of a numerical flow model that has been developed at the TU Munich [Kölli95], it is now possible to determine the k factor for almost any desired open channel section (also natural) and a large bandwidth of grades of filling with which a single point measured maximum velocity must be multiplied in order to determine the median flow velocity vm required … |
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(Image: Principle of the network measurement [VDIVDE2640]) A further measuring method that manages without cross sectional restrictions is network measurement. In this method, two sizes for determining the volume flow are also necessary: one is the flow depth over which the surface of flow is to be calculated and the other is the median flow velocity in the open channel. To find the velocity profile the individual velocities are measured within a … |
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Nov 19, 2004 Rehabilitation and Maintenance of Drains and Sewers Flow Measurement Procedure Without Cross-Sectional Restriction All the methods mentioned below have in common that two variables must be found for calculating the volume flow, one is the cross section A and the other is the median flow velocity vm. For known geometrical dimensions of the open channel, the cross section is easily found with all methods by means of flow depth measurement. For determining the median flow velocity, however, a different measurement and long calculation method must be used that is … |
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Nov 19, 2004 Rehabilitation and Maintenance of Drains and Sewers Flow Measurement Procedure Without Cross-Sectional Restriction The calculation of the flow with Dilution or Tracer Methods is based on a measurement of the concentration of a marker substance. A tracer (marker substance) is added to the sewage upstream of the measuring point at a known concentration and at a constant dosing rate. After traversing a mixing section, its thinning is measured in the measuring cross section. The continuity equation: QTracer · CTracer = Q · aTracer where QTracer = Constant dosing quantity … |
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Nov 19, 2004 The following methods belong to the flow measurement procedures without cross-sectional restrictions:
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Nov 19, 2004 Rehabilitation and Maintenance of Drains and Sewers Flow Measurement Procedure with Cross-Sectional Restrictions
In hydraulic measuring methods with cross sectional reductions, the installation of measuring weirs or venturi sections forces a change of flow from streaming to jetting. The flow depth upstream of the measuring cross section, because of the change of flow, has a direct relationship to the flow quantity [Preis92] [Hager95]. … |
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Nov 19, 2004 Rehabilitation and Maintenance of Drains and Sewers Flow Measurement Procedure with Cross-Sectional Restrictions
If hydraulic measuring methods that are other than those described in the above section are to be utilised, then, besides the depth of flow h, also the median flow velocity in the cross section must be determined either by means of approximations or with considerable measuring and calculation effort … |
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Nov 19, 2004 The measuring methods described in the following differ from the methods in the previous section in two aspects. For one, structures are always required in the open channel profile, which sometimes substantially reduce the hydraulic capability of the section of the sewer and can thus create backwaters. Thus, these methods cannot be used to solve all the problems. Besides this rather practical point of view, there also occurs a common feature: The … |
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Nov 19, 2004 Flow measurements in the sewage field are only seldom direct measurements. The volume flow through a particular cross section is much easier deduced indirectly from a metrologically easier acquired size that is proportional to the volume flow. For measurements in a gravity open channel with known dimensions, two parameters are normally necessary to calculate the volume flow: the depth of flow h and the median flow velocity vm. From these measurement … |
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Nov 19, 2004 The planning of a flow measurement requires a knowledge of the subject and experience in the field of hydrometry and must always be individually adapted to the respective case requirements as well as the structural and hydraulic conditions at the measuring site. The catalogues of numerous portable flow measurement devices promise ease of handling and universal application possibilities for every hydraulic problem. In fact, portable measuring devices … |
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Nov 19, 2004 For flow measurements in the sewage sector, direct or indirect acting contact transducers with pressure sensors are mainly used. In the direct pressure measurement, a pressure box is installed in the open channel invert, which measures the hydrostatic water pressure as a size proportional to the flow depth. Sensors that operate according to the bubbling-through method, measure the hydrostatic water pressure in that, in a hosepipe that extends to … |
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Feb 16, 2011 The standard transducer for contact-less flow depth measurement is the ultrasonic sonar. The principle of the measurement is based on the reflexion of pulsed ultrasonic signals from the surface of the water (Bild 4.4.2.1.2). Depending on the depth of flow, the signals require different time periods until they are again received by the sonar. This period is inversely proportional to the depth of flow in the open channel. Each sonar device must be positioned … |
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Nov 19, 2004 For most of the measuring methods in open channels, one of the measurements, or the only measurement necessary, is the flow depth. The transducers required for this purpose are divided into two large groups: sensors that are positioned in the medium and transducers that acquire the flow depth without contact. In some borderline cases (extraneous water measurement), contact sensors cannot be used due to the fact that the sensor with its large size … |
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Feb 16, 2011 (Image: Schematic of a magnetic inductive throughflow sensor [Hager95]) MID transducers are based on the electrical conductivity of the sewage. The basis of the principle of measurement is Faraday's law of induction: a voltage is induced in a conductor that is positioned at right angle to the lines of force of a magnetic alternating field. The same effect occurs when the conductor (in this case the flow of sewage) is moved through a static magnetic … |
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Feb 16, 2011 Mechanical measuring propellers (Bild 4.4.2.2.2.1) (Bild 4.4.2.2.2.1) are precision instruments with friction resistant bearing mounted propeller blades whose speeds of revolution are directly linearly proportional to the median flow velocity in the pipe to be measured. The propeller blades trigger a measuring impulse with each complete rotation that is acquired by a mechanical or electrical counter and that is included with the results of a time … |
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Feb 16, 2011 (Image: MID velocity sensor for measuring local flow velocities) Magnetic inductive velocity sensors, when correspondingly designed, can also be used to measure quasi-point velocities in a flow cross section (Bild 4.4.2.2.2.2), i.e. they represent a kind of "MID measuring propeller". The magnetic field is created only at the face of the magnetic coil of the open measuring transducer in a tightly limited space in the inflowing water. The voltage induced … |
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