Sewer Flow Monitoring

Vladimir Patrčević, Ph.D.Civ.Eng.a; Kristijan Boda, B.Sc.Civ.Eng.b;

Jürgen Skripalle, Ph.D.Civ.Eng. c

a Faculty of Geotechnical Engineering Varaždin, University of Zagreb, b Faculty of Civil Engineering, University J.J. Strossmayer in Osijek,

c Hydro Vision GmbH, Kaufbeuren, Germany.

Abstract. Measuring of discharge in the sewage system of town in fact are measures the flow in particular canals of system, i.e. in suitable points of network. Based on such measurements was able reconstruct the individually charge of parts of systems. Such suitable points find mostly on collection supply channels or pipes from some area and downstream of couplings of larger canals, as well as on outlet canals, and on network components where has changed the section of canal and the where possible approach shaft. How it is about the liquid, the more or less burdened suspension and floating substances, often very aggressive, measuring instruments as well as the monitoring must is adapted such conditions. Generally, instruments have been conceived so that measure level and average velocity of water in the profile in programmed time intervals, then based on geometry of profile calculate the flow or discharge of water passed main sewer. But exists and another methods of monitoring depending on hydraulic conditions of channel network. Key words: Flow measurement, hydraulics condition, methods of sewer flow monitoring

1 Introduction

This paper wants to give a brief outline about hydraulics as well as the most common flow measurement techniques and what should be taken into account. Flow measurement in full-filled pressure pipes normally is a routine task that is easily performed. The difficulties begin in:

- Increasing accuracy requirements - Large pipes/channels with irregular profiles, low flow levels and flow velocities - Partial filling with backwater

The paper will present and explain basic approaches measuring the discharge in sewage or sewer canals. It will be describe necessary requests and explanations of concept of particular measurement method like:

- Measurement Principles tracer and with propeller - Weirs and Flumes - Magnetic Inductive Flow Measurement - Radar method - Ultrasonic Pulse-Doppler method

Will be present also comparative analysis of advantage and disadvantage of particular measurement method.

2 Requirements for Flow Measurements in Channel Systems

Flow Measurements in sewer systems must be suitable for specific flow hydraulics and geometric boundary conditions. Channels often are dimensioned to convey off high water volumes at heavy rainfall, while the water volume by night or in dry weather periods may be very low. Measurement devices for such applications must yield usable measurement values under conditions of high water volumes, backwater, low water levels and low flow velocities. Complex channel profiles, stream turbulences caused by feed openings, manholes and arcades must be taken into account. This task is highly demanding for the used measurement technique.

3 Background - History

The water and waste management practices of the olden times required great skills of the engineers for its performances. The old Romans had already built such plants and recognized that: The Flow Q cannot be measured by only measuring the fluid height.

Fig. 1. Romans aqueduct

The simplest measurement method for investigating flow consists of measuring the fluid height in a channel with defined geometry. Flow Q is a function of Q/h, the slope J and the roughness coefficient k. The roughness coefficient is determined experimentally and is dependent on the material types and the age of the material. For example, a concrete channel will have a different roughness from the original due to coating of the channel by grease and slime. This will cause a smoother surface and thus a different measurement value.

4 Free Flow - Manning Principle

Robert Manning (1816 – 1897) was born in Normandy. but he has lived in Ireland. He wrote many papers on hydraulics. During living period, Manning devoted considerable effort to the development of a simple, dimensionally homogeneous formula for open-channel flow. In the and, his paper “On the Flow of Water in Open Channels and Pipes” (1891), became the primary reference for his work and the source of Manning’s monomial equation:

vkn

nR J=

2

3

1

2

where: v - cross-sectional average velocity R - hydraulic radius J - energy slope Kn - 1 for SI units and 1,486 for English units N - Manning resistance coefficient

Similar conditions are given for pipes with the exception that, unlike in channels, Q might be smaller in full filled pipes than in partially filled pipes. Fig. 2. Partially filled pipes flow When calculating full filled pipes generally the flow velocity v (m/s) or the possible flow Q (l/s) is investigated at a certain slope (J) and a known diameter (DN). Besides the slope (1:x or ‰) and other dissipation coefficients the values Q and v are influenced especially by the resistance coefficient according to Prandtl-Colebrook. The trade-specific, computer-based calculation systems as well as calculation by tables cannot handle dissipation coefficients of single components like e.g. manholes. Thus, in practical applications we calculate with the operational roughness value kb. Fig. 3. Graph for correction flow in circular partially filled pipe (according to Manning)

Hydraulic Radius

W.P.

Slope

5 Explanation of Concepts

The modern electronic methods determining flow Q (m3/s) measure the average flow velocity v (m/s) depending on the wetted cross section A (m2). Q = v average A (m3 /s) Average Flow Velocity The velocity in flowing water isn’t evenly distributed. Therefore the average flow velocity is used for calculation of Q. This is the product of the cross-sectional area A and the average flow velocity. Flow can be determined by the product of the average flow velocity and the wetted cross-sectional area of the flow. For cross-sectional area, one must know the exact geometrical data of the channel. For partially-full channel, the level must be determined additionally. If there are going to be deposits or silting in the channel, then this must be taken into account too. Fig. 4. Flow velocity profile Flow Profiles Sufficiently low flow velocities give rise to laminar flow. This is represented as stratified flow in a physical condition. The single water layers glide over each other without any mixing. By means of the frictional strength (roughness of the walls, viscosity of the medium), the flow velocity at the walls is 0. For full-filed pipes, for example, the maximum flow velocity is in the middle of the pipe. Transition flows are intermixing of the laminar and turbulent flows. These forms of hydraulic flows are unstable and swinging. It cannot develop a defined, stable flow profile. The flow profiles cannot be assessed. Increase in flow velocities lead to turbulent flows. In turbulent flows, one finds an intermixing of the single water layers. The wall roughness has little influence, and the velocity profile is uniformly shaped. A characteristic value to describe streaming conditions is the dimensionless Reynolds value “Re” (Re = v • d / cinematic viscosity). The critical Reynolds value for liquids is around 2320. Beyond this value, the streaming condition may switch from laminar to turbulent. Fig. 4. Flow velocity profile

v1

v2 vL

v

vnchan

nel h

eigh

t

channel bottom

v

??

The ideal shapes of the different flow profiles from flow measurement equipment. General problems of the velocity measurements in liquids.

1. Flow profile (section) of an ideal pipe " without " roughness: 1. Flow profile (section) of an ideal pipe " without " roughness:

VmiddleVmiddle

Vmiddle

2. Flow profile (section) of a pipe with usual roughness of concrete :

Vmiddle

Vmiddle

2. Flow profile (section) of a pipe with usual roughness of concrete :

Vmiddle

Fig. 5. Different flow velocity profile

middle roughness middle roughness

flow hight hpart < 1/3 DN

large roughness

flow hight hpart < 1/3 DN

large roughness

flow hight hpart < 1/3 DN

small roughness

flow hight hpart < 1/3 DN

3. profile of a pipe with usual rough

flow hight hpart < 1/3 DN

small roughness

flow hight hpart < 1/3 DN

3. profile of a pipe with usual roughness of concrete with different and(cross section):

levels velocities

only swirls

flow hight hpart <<< 1/3 DN

no development

ness of concrete with different and(cross section):

levels velocities

only swirls

flow hight hpart <<< 1/3 DN

no development

flow hight h << 1/3 DNpart

large vortex sheetsmall, initial development

flow hight h << 1/3 DNpart

large vortex sheetsmall, initial development

flow hight h < 1/3 DNpart

decreasing vortex sheetmore strongly becoming development

flow hight h < 1/3 DNpart

decreasing vortex sheetmore strongly becoming development

flow hight h > 1/3 DNpart

minimum vortex sheetgood flow profile development

4. Flow profile development depending on pipe roughness (cross section):

flow profile development vortex sheet/thickness depending on pipe roughness

Flow

Fig. 6. Different development of the flow profile

6 Tracer Dilution Methods Basically we distinguish between splash input and continuous input. One of the methods is the dilution method. Here a dissolved marker medium (dyes; salt; radioactive tracers) in a defined amount and concentration is brought in. Above and below the point of insertion from a measured magnitude (light absorption; electric conductivity; activity) to the resulting background or overall concentration of the tracer is concluded. From that known magnitudes the flow is calculated.

Fig. 7. Method and device for measurement Splash Input: A small amount of salt is brought into the water above the point of measurement. At this point the salt concentration is constantly detected as the salt cloud passes the sensor. As soon as the measurement device detects the end of the cloud the user is asked to finish the measurement. The flow rate is displayed immediately. The flow rate is calculated with following formula: Q = A/C (l/s) where Q is the flow rate in l/s, A is the known salt quantity, C is corresponding measured concentration of the salt cloud. Advantages: no measurement of channel geometry Disadvantages: a complete mixing is necessary 7 Velocity measurement with propeller Flow velocity measurement can be approximated with floats. The velocity of the water can be determined with the time of the float over a defined distance. Often used for the measurement of the velocity is the hydrometric propeller. From the rotations of the propellers at different points and in different heights of the cross section of the flow, the average velocity can be determined from the rating curve. The point measurements have to cover sufficiently the cross section of the flow. The flow is again determined with the average current velocities of the cross section. Flow measurements are used to calculate the volume of the flow at the time of the measurement.The relationship between water level (H) and flow (Q) can be determined by measuring various times. With this various values a H/Q rating curve can be developed.

c mg l( / )

t (s)

t0 t n

Q=A/C (l/s)

C =Ic dt

Rating Curve H/Q

H

Fig. 8. Device for measurement and rating curve Advantages: Very simply determining the flowing Disadvantages: Measure with the hydrometric propeller are possible only in relatively clean water, without of the bigger suspensions.

8 Weirs Weirs are hydraulic object for the measurement of discharge in open canals. It is possible use a different types and forms of a weir, depending of the hydraulic characteristic of measuring point. In everyone is the necessary measure the level of the water H (cm) on weir.

Fig. 9. Different forms of the weir

Flow quantity for low flows, very sensitive to sedimentation.

High accuracy

Flow quantity for medium flows, less sensitive to sedimentation

Average accuracy

Flow quantity for higher flows, less sensitive to sedimentation

Average accuracy

a b c

9 Flumes

Flumes are flow measurement devices that are specially formed for channels and have a defined lateral narrowing. A flow change of the current takes place in this narrowing from free flow to fast flow. Relative flume measurements are done on stationary, pre-fabricated forms, but also concrete flumes can be constructed in the channel. The geometric shapes varies depending on the measurement method (e.g. Parshall Flume, Palmer-Bowlus Flume, and many other forms manufactured).

Fig. 10. Principles of measurement More detailed information for the installation conditions, stilling well section, measurement range, preferred accuracy etc. must be known. Basic Operation: An accumulation of the medium to be measured occurs due to the restriction in front of the flume. A defined transitional flow takes place at the throat of the flume and the medium shoots through (hydraulic jump). Due to this defined hydraulic condition, it is

B = Channel width L1 = Distance from transducer

Ho = High water level Hu = Sub-water level

possible to ignore the roughness and slope of the flume. The impounded height in front of the flume is directly related to the flow measurement in the flume. The following must be noted when using a flume for measurements: The least measurable quantity of raw sewage is about 5 l/s. The ratio Qmax to Qmin lies at 10:1; for special flumes, it is maximum 20:1. It is important that the measurements are made without any back-flow condition. This means that higher water level ho must not be lower than the sub-water level h. The measurement transducer must be 1.5 - 2 ho max above the start of the installed throat (L). The flow quantity should be in the maximum possible range, and the water level raised 3-4ho. The zero point measurement of the transducer is always the zero point of the flume, not the zero point of the measurement place. Advantages: measurement relative exact, easy to control, suitable for each medium Disadvantages: no measurement under backwater condition, low measurement dynamics.

10 Magnetic Inductive Flow Measurement (MID) The magnetic inductive flow measurement is based on Faraday‘s Law of Induction. A homogeneous magnetic field is built up in the pipe. The flowing liquid induces an electric current coupled with the magnetic field in the pipe, which acts like an electric wire (e.g. copper wire). This current measured by the electrode, is directly proportional to the average flow velocity.

Fig. 11. Principles and device of measurement Partially full MID´s operate on the similar principle as the full filled MID. In contrast to full filled MID, the induced current over several electrode pairs differs with differing heights. The magnetic field structure becomes the actual flow height evaluated. The characteristics of the magnetic fields at various heights are stored in the measurement transmitter. By means of correction methods depending on the actual flow height, a signal is produced that is proportional to the flow. The Magnetic Inductive Single sensor, generally shaped like a „Channel Mouse“ can be used in wastewater channels and pipes. Similar to the traditional MID, the current is induced on the electrodes of the sensor. The level is measured by a pressure transducer at the bottom of the channel, or by an external level measurement device. The instrument can be used in both, temporary and permanent applications.

Advantages: No disruptive fittings, measures on surfaces, Requires no reflective particles or gas bubbles in water, easy mounting Disadvantages: No measurement under 10% of filling, not very accurate, measurement errors with decreasing flow heights, measures only conductive fluids >10mS, isolating coatings on electrodes introduces errors, high power requirement, expensive with increasing diameter of a pipe.

11 Radar Flow Measurement Alexander Watson-Watt (1892-1973) was the Scottish physicist who developed the radar locating of aircraft in England. He was born in Brechin, Angus, Scotland, educated at St Andrews University in Scotland, and taught at Dundee University. Radar was patented in April, 1935. A new measurement principle is the application of radar waves. These are preferable in the industry for level measurements. The radar waves through the antenna are transmitted to the surface of the medium and reflected. For flow measurement, a similar principle is used. It is similar to the evaluation of frequency shift by the ultrasonic Doppler method. At a defined angle the radar waves are transmitted and reflected. The flow height is measured by a separate level sensor. Advantages: Non-contact measurement, no fittings in the medium, low mounting costs, low mobilization, no particles required. Disadvantages: A minimum dielectric constant of the medium is required, measures point velocity on the fluid surface, flow profile is calculated or empirically determined., on a smooth flow surface the signal will not be reflected, waves will be refracted and not reflected back. Fig. 12. Radar sensor 12 Ultrasonic Doppler Method Doppler radar is named after Christian Andreas Doppler. Doppler was an Austrian physicist who first described in 1842, how the observed frequency of light and sound waves was affected by the relative motion of the source and the detector. This phenomenon became known as the Doppler effect. HydroVision uses the Doppler effect for flow measurement. Unlike the origin, transmitters and receivers are in fixed positions with a moving medium and reflecting particles. (CW Doppler principle) The measurement principle is based on the fact that a bundled ultrasonic beam (f1) is continuously insolated into a liquid in a defined angle and a known frequency. A part of the ultrasound energy is reflected by the solids or gas bubbles contained in the liquid. Caused by the particles‘ movement a frequency diversion (rf) occurs. This diversion is direct proportional to the particle velocity.

Radar

Vo

deflectedsignal h

sensor

sedimentation

At a constant transmitting frequency, insolation angle and sound velocity the particle velocity Vp can be determined, where: f – transmitter frequency, C0 – medium sound velocity, Vp – particle velocity, αααα - beam angle.

Fig. 13. Principles and device of measurement

At a constant transmitting frequency, insolation angle and sound velocity the particle velocity can be determined. A particular frequency spectrum results from the numerous reflecting particles and the flow profile. This Doppler frequency spectrum must be evaluated with the help of analytical methods to get a characteristic velocity within the measurement distance. From that the average flow velocity v (m/s) in the wetted cross section can be calculated. The Ultrasonic Pulse Doppler Principle is a new development that supersedes all the older Doppler principles. In contrast to CW Doppler principle, the transmitting frequency is continuously adjusted with the pulse Doppler and a shorter ultrasonic frequency bundle of defined length is transmitted.

Fig. 14. Mounting of sensor Q-Eye in the sewer and principles of measurement Based on defined beam angle, the acoustic velocity and the medium temperature (compensated), the sensor transmits ultrasonic frequency bundles at time t1, and receives the signal at time t2. This makes it possible to assign a defined measurement window for received signals. The frequency shift of the transmitted ultrasonic signal into a defined measurement window is the measurement of the flow velocity in that measurement window. Reflections of particles in other areas do not have any influence on the velocity measurement. The measurement window is defined on Vmax (rotationally

symmetrical flow profile). From this and a known weighting Vaverage is determined. With the Vaverage and the known diameter, the flow Q can be calculated.

Discharge in sewage system of town Osijek

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Fig. 15. Monitoring flow in very non stationary conditions. (Sewage system of Osijek) Device Q-Eye is a major improvement in open channel flow measurement. This compact system uses HydroVision’s proven Acoustic Pulse-Spectral-Correlation Technology. The system is designed to measure both, the water level and the velocity in a single transducer assembly. Q-Eye directly reads mean velocity with superior accuracy, using up to 10 scan cells. Gone is the time wasted in figuring roughness factors and velocity corrections. A sub-merged ultrasonic level sensor is combined in the velocity sensor.

13 Conclusion Measurement in the sewage system is extremely important for getting to know and the system management. Which of mentioned measurement method choose, depends of numerous local conditions of measuring point. In the non stationary conditions flow in sewage, has to necessary choose the ultrasonic Doppler method.

References

1. Vladimir Patrčević: Hidrološka studija slivnog područja kanalizacijskog sustava grada Osijeka, Faculty of Civil Engineering, Osijek, 2002.

2. Kristijan Boda: Oborinske vode gradske kanalizacije, diplomski rad, Faculty of Civil Engineering, Osijek, 2004.

3. Jürgen Skripalle: Total flow monitoring system, Kaufbeuren, 2005.