Methods of Flow Measurement

for Water and Wastewater

New ordinances (for billing, control and supervision etc.) as well as construction of structures

and relief structures, demand in peak time for quantity measurement

of raw sewage.

Riyaz Jiwani, M.Sc., P.E. Steffen Lucas, Dip. Eng.

NIVUS GmbH Im Täle 2 D - 75031 Eppingen Tel. 0 72 62 / 91 91 - 0 Fax 0 72 62 / 91 91 - 29 E-mail: info@nivus.de Internet: www.nivus.de NIVUS AG Hauptstrasse 49 CH - 8750 Glarus Tel. +41 (0)55 / 645 20 66 Fax +41 (0)55 / 645 20 14 E-mail: swiss@nivus.de NIVUS Sp. z o. o Długie Ogrody 8 PL - 80 765 Gdańsk Tel.: +48 (0) 58 / 344 25 25 Fax: +48 (0) 58 / 344 25 25 E-mail: poland@nivus.de NIVUS France 14, rue de la Paix F - 67770 Sessenheim Tel. +33 (0)388071696 Fax +33 (0)388071697 E-mail: france@nivus.de NIVUS U.K. P.O. Box 342 Egerton, Bolton Lancs. BL7 9WD Tel. +44 (0)1204 591559 Fax: +44 (0)1204 592686 email: bg@nivus.de NIVUS (America) Inc. 10120 Yonge St., Unit 35B Suite 212 Richmond Hill, Ontario L4C 3C7 Canada Tel. +1 (905) 833-0885 Fax +1 (905) 833-0823 E-mail: america@nivus.de

Contents 1. Definitions ................................................................................................... 4 2. Introduction ................................................................................................ 6 3. Measurement Methods................................................................................ 7

3.1 Measurement in Full-Filled Lines ....................................................................... 7 3.1.1 Throttle Devices ................................................................................................. 7 3.1.2 Measurement with Volume Meters .................................................................... 8 3.1.3 Magnetic-inductive Flow Measurement ............................................................. 8 3.1.4 Ultrasonic Flow Measurement.......................................................................... 10 3.1.4.1 Ultrasonic Transit-Time Method ..................................................................... 11 3.1.4.2 Ultrasonic Phase Difference Measurement ...................................................... 12 3.1.4.3 Ultrasonic Doppler Method.............................................................................. 12 3.1.4.4 Ultrasonic Pulse Doppler Method .................................................................... 13 3.2 Measurement Partially Filled Pipes................................................................... 14 3.2.1 Measurement with Q/h-characteristics without mechanical Bracing .............. 14 3.2.2 Flume Measurement ......................................................................................... 17 3.2.3. Weir Measurement ........................................................................................... 19 3.2.3.1 Overfall Weirs .................................................................................................. 20 3.2.3.2 V-Notch Weir................................................................................................... 24 3.2.3.3 Special Measurement Methods for Overfall Weirs .......................................... 25 3.2.4. Magnetic-Inductive Flow Measurement in Partially Filled Pipes ..................... 27 3.2.5 Ultrasonic Doppler Method.............................................................................. 28 3.2.5.1 Fan-like Beams with statistical average Value plotting .................................... 29 3.2.5.2 Fan-like Beams with Vmax Evaluation ............................................................... 31 3.2.5.3 Selective Flow Velocity Evaluation .................................................................. 32 3.2.5.4. Measurement Technique Installation............................................................... 34 3.2.6 New Measurement Methods – Profiler............................................................. 36

4. Bibliography............................................................................................... 43 5. Selecting Measurement Devices for Flow Measurement in Wastewater.. 44 6. Flow Measurement Applications............................................................... 45

3rd Edition - 21.02.2002

3

1. Definitions

Flow Q is the volume flowing through a mathematically precise determined cross- section over a certain time unit.

Q = =Volume

TimeVt

The most common measurement units are:

l/s ; m³/s and m³/h (metric) cfs ; gpm and Mgd (imperial)

The same can be determined by the multiplication of the average flow velocity, v, with the wetted cross-section area, A, vertical to the flow direction, known as the continuity equation:

Q = v • A Volume V is the flow volume within a certain time interval.

V = Q • t

Measurement units are most commonly l; m3 (metric) or ft3; gal (imperial). The average flow velocity v is the velocity determined within cross section A.

According to physics a laminar flow represents a layered flow. The separate water layers are gliding one above the other unless they are getting mixed. Based on frictional influences (wall roughness, medium viscosity) the flow velocity at the pipe wall is 0. It reaches its maximum at various points within the cross-section, depending on the level and the channel profile (round, oval, square).

4

In a full-filled pipe the velocity profile is rotationally-symmetrical and its maximum is situated in the tube axis (see Fig. 1 and 2).

Fig. 1; laminar Flow Fig. 2; Velocity Profile Within a turbulent flow an intermixing between the water layers occurs, and is not influenced by the pipe roughness (see Fig. 3). Thus, the velocity profile is more uniform (see Fig. 4).

Fig. 3; turbulent Flow Fig. 4; Velocity Profile Transition flows are intermixings between laminar and turbulent flows. This form of the hydraulic flow is unstable and sways. No defined, stable flow profile is developed. The flow conditions aren’t predictable. (see Fig. 5)

Fig. 5; Transition Flow

5

2. Introduction

Flow measurements are a critical element for water resources monitoring for various applications. This booklet only gives a basic understanding in a short and concise form the various problems encountered in flow measurement techniques. It doesn't claim to be complete with reference to the described methods as well as the hydraulic boundary conditions. For more extensive information, the bibliography listed at the end must be referred to. For any questions, unsolved technical problems and unclear application solutions, our applications engineers from our "Flow Team" at NIVUS is at your disposal. They have extensive flow and hydraulic monitoring experience. 2.1 Potential Problems Flow is usually monitored in open channels (gravity flow) and closed conduits (pressurised flow in completely closed pipes). Flow measurement in the wastewater field is always carried out under harsh sewer conditions. Wastewater is polluted with sludge, solids, fibers, grease and oil, etc. as well as sewer films and overlays. This causes the measurement methods often inaccurate or even to fail. According to structural conditions we often encounter difficult hydraulic flow conditions. Not understanding the interrelation between the flow condition at the desired measurement point and the choice of the proper measuring device sometimes leads to a wrong determination of measuring point and method. Test points where the best measurement techniques are used may deliver faulty or useless measurement results. The required measuring ranges represent a further problem. The range varies from extreme low levels at night an in dry periods to extremely high levels in rain periods with full-filled pipes and high flow velocities. Other circumstances, like high levels at low flow velocities also occur often during backwater conditions.

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3. Measurement Methods

Various techniques are available for measuring flow. Each one has its advantages and disadvantages, and selecting a proper technique depends on its specific applications. The following methods describe the most common methods and techniques used for flow monitoring.

3.1 Measurement in Full-Filled Lines

For cross-sections of mostly full pipes, the wetted area is defined by its geometry and dimensions and thus can be considered a constant. This means that the area standardized flow velocity V must only be determined.

3.1.1 Throttle Devices These measurings are classical measurement peocedures based on the differential pressure principle. That is: a defined reduction creates a positive pressure before and a negative pressure after the throttle device. The difference is a measurement of the flow (see Fig. 6 and 7) . Examples of these devices are nozzles, blank faceplates and Venturi pipes. Due to its high failure vulnerability, these do not find any use for sewage measurement techniques.

Fig. 6; Flow pattern at a Measurement faceplate

Fig. 7; Pressure distribution at a Measurement faceplate 1. Differential pressure 2. Loss of pressure

7

3.1.2 Measurement with Volume Meters Motor meters are used for the volume measurement by a volume count, where the wall of a room, either locked or not locked is replaced by the current or the pressure of the medium. The covered distance or angle for piston, a drum or the wing of a winged wheel corresponds to the volume to be measured. Typical measurement devices: rotary-piston meter, oval wheel meter, winged wheel meter, etc. Example:

Fig. 8; Oval wheel meter Oval wheel meter: Consisting of a chamber (3) with influx (1) and drain (4) as well as two oval gear wheels fitted in, (2 and 5). A sickle-shaped space arises between the wheels and the outer chamber walls, by which the liquid reaches the run (4) under simultaneous drive of the wheels of the finish (1) . The counter is set in movement by the wheels. Due to the vulnerability to dirt this kinds of measurement are unsuitable for sewage applications.

3.1.3 Magnetic-inductive Flow Measurement The magnetically inductive flow measurement has a very strong position in sewage applications for full filled pipes. The measurement principle is based on Faraday’s Law of Magnetic Induction: a homogeneous magnetic field is built up. An electrically conducting liquid flows through this magnetic field . By the movement of the electrical conductor (liquid) a current gets induced which is proportional to the average flow velocity and the magnet field strength. It is given by:

Ue = B • L • v

8

Ue is the induced electromotive force. It is picked up by two electrodes contacting the medium. The induced voltage B is the strength of the induction of the magnetic field . This is produced by magnet field coils which are operated by alternating voltage or pulsating direct voltage. L represents the conductor’s length. By using the legitimacies of the magnetic-inductive measuring principle this length is determined by the gap between the both measuring electrodes. v is the average flow velocity.

1. Magnetic Field Coils

2. Pipe Boundary

3. Measuring Electrodes

dV Volume Element

B Magnetic Induction

v Flow Velocity

E Resulting Field Intensity

Fig. 9; Measurement Principle: Magnetic-Inductive Flow Measurement

The magnet field strength as well as the electrode distance are fixed values. This means that the induced current is directly proportional to the flow velocity. Prerequisites for Measuring • minimum conductivity (oil, grease and similar cannot be measured) • defined hydraulic conditions (5 - 10 x Diameter influx stage, 3 - 5 x Diameter

outflow stage) • minimum flow velocities (usually 1m/s (3.28 fps), high-end devices require 30 -

50cm/s (0.98 – 1.64 fps) minimum flow velocity) • full filling (special types of construction will be mentioned later). Advantages • high precision and consistency of measurement values at exact mounting conditions • calibrateability • wide range of diameters available (25 - 1200 mm; 1 - 48 in) • high pressure resistance (up to 600 bar) • no cross-section reduction • independent of the medium’s pressure or temperature

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Disadvantages • minimum conductivity necessary (> 1mS) • electrodes sensitive to contamination by oil, grease and sewer films (zero point error

& measurement failure) • high structural costs / siphon and bypass line necessary when sewers need to be

blocked and/or cleaned • device costs increasing proportional to diameters to be measured • minimum flow velocity required (1m/s; 3.28 fps, in exceptional cases 0,3 - 0,5m/s;

0.98 – 1.64 fps, below these limits, high increase in measurement errors)

3.1.4 Ultrasonic Flow Measurement

Ultrasonic flow measurement devices, like magnetic-inductive ones have a free passage; this means, there are no mechanical or moving parts within the pipe. Principally we distinguish between clamp-on technology (see Fig. 11) (transducers fastened on top of the pipe with no medium contact ) and insertion technology (see Fig. 10) (transducers fitted to the pipe with direct medium contact).

Fig. 10; Ultrasonic Insertion Technology

Fig. 11; Ultrasonic Clamp-On Technology

10

3.1.4.1 Ultrasonic Transit-Time Method The transit-time method is based on the physical legitimacy that a sound wave’s propagation velocity within a medium in motion depends on its velocity. 1) 2)

Fig. 12; Ultrasonic Transit-Time Principle VR = Sound propagation velocity in medium at rest VO = local medium velocity V1 = sound propagation velocity in flow direction V2 = sound propagation velocity counter to flow direction α = mounting angle between acoustic path and flow direction L = gap between electroacoustic transducers Result Vaverage

2 • L •cos αV = • (t - t )m

VR2

2 1

Prerequisites for measurement • acoustic attachment/sound transmission within the medium necessary • pipes must be filled up (or additionally a level measurement is required) • acoustic path must be situated within the medium Advantages • easy mounting / demounting (clamp-on technology) • hydraulic disturbances within the acoustic path are taken into account in the calculation • insensitive to grease and oils • no minimum conductivity necessary Disadvantages • incorrect measurement or failure in partially filled pipes • measurement failure in liquids with high solid matter contents • incorrect measurement / failure in incrusted pipes (clamp-on technology) • no clamp-on technology measurement possible in pipes with inhomogeneous outer

jacket (concrete or similar)

11

3.1.4.2 Ultrasonic Phase Difference Measurement

The measurement is similar to transit-time measurement . Here, instead of the transit time of the sound, the phase difference angle is used for the determination of the average flow velocity. This is determined by the phase relationship between transmitted and received signals against the flow direction. The determined phase difference angle is proportional to the transit-time difference and is evaluated further as listed above.

3.1.4.3 Ultrasonic Doppler Method

The principle of the Doppler effect is based on transmitting a bundled ultrasound beam with a defined frequency and a well-known angle into a liquid. A part of the ultrasound energy is reflected by the solid particles or gas bubbles carried in the liquid. Due to the movement of the particles a frequency distortion ∆F occurs. This distortion is directly proportional to the particle velocity . It is given by:

C∆ ƒ= 2ƒ • • Vcos α

0p

ƒ = Transmission frequency C0 = Sound velocity within medium to be measured VP = Particle velocity α = Transmission angle between ultrasonics and flow direction

Fig. 13; Doppler Method Principle At a constant transmission frequency, transmission angle and sound velocity one can get the particle velocity from:

VP = K • ∆ ƒ Due to the flow profile arising and a multitude of reflective particles of various flow velocity a frequency spectrum results.

12

The various possibilities of the evaluation of this frequency mixture are explained in the section ultrasonic measurements.

3.1.4.4 Ultrasonic Pulse Doppler Method

The ultrasonic Pulse Doppler Method represents a further development of the known Doppler method as a new measurement technique. Unlike the common Doppler method, using a continuous transmission frequency, the Pulse Doppler transmits a short frequency bundle with a defined length. Due to the defined transmission angle and the known sound velocity within the medium the transducer after transmitting switches to receiving standby mode within a time t1. The change of the sound velocity depending on temperature is considered and compensated by an additional transducer-integrated temperature measurement. Thus, the scan slot’s allocation for a received signal is possible. The frequency distortion of the transmitted ultrasonic signal in a defined measurement window is a measure of the flow velocity corresponding to that measurement window. Reflections of particles in other areas don't have any influence on the velocity measurement.

Fig. 14; Pulse Doppler Method Principle

Putting this window onto Vmax (prerequisite: axially symmetrical flow profile), Vaverage can be determined. From Vaverage and the known diameter the flow Q is determined.

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3.2 Measurement Partially Filled Pipes

There are basically two ways of measuring partially filled pipes. One, the detection of level and average flow velocity, the other, a simple level detection and based on mechanical/hydraulic conditions like roughness, slope, diameter variations etc.

3.2.1 Measurement with Q/h-characteristics without mechanical Bracing This method represents the most simple way of volume measurement: Measurement principle:

Fig. 15; Nonpressure Channel Measurement The MANNING Equation This equation gives a rough estimation of the flow: Q = 1.486/n . (A.e.1.667 S.e0.5)/P.e0.667 Where: Q = Flow A = Cross-sectional Area P = Perimeter n = Roughness Coefficient S = Slope e = exponent Based on the above, many flow tables have been developed for various pipe diameters, level, slope and the roughness coefficient. This equation is not very accurate (10 - 30 %), but is acceptable when only an approximation of flow measurement is allowed. The flow Q is represented as a function of Q/h. The allocation of the level h to the flow Q is based on channel geometry und channel dimensions (known and invariable), the slope α (mostly known) as well as the roughness k between pipe walls and medium. The roughness has been experimentally determined for various materials and may vary according to different types of construction and material stress. It may further vary during the measuring process (rinsed concrete = rougher, sewer film and grease

14

sedimentation = smoother) or it may vary depending on different grades of roughness at different levels. Depending on the water level these conditions will change. Thus, roughness represents a big instability factor ( Table 1). Further sedimentation and backwater cause huge measurement errors.

Fig. 16; Measurement Error caused by Sedimentation

Fig. 17; Measurement Error caused by Backwater

Measurement Prerequisites • no backwater • no deposition/sedimentation • channels in good structural condition • constant slope Advantages • cheap • easy mounting Disadvantages • extreme uncertanity in measurement • not suitable for many measurement points • preclarification of hydraulic conditions absolutely necessary • high calibration and maintenance expenses (recalibration)

15

Flume Wall Qualities M in m 1/3 /s k in mmGlass, Piacrylics, polished Metal Surfaces > 100 0...0,003Plastic (PVC,PE) new Steel Plate with Protective Coating, smoothened C tPlaster

≥ 100 0,05

0,03...0,06

Asphalt coated Steel Plate Concrete from steel or Vacuum Formwork, no joints,

f llsmoothened; planed Wood, joint-free, new; Asbestos Cement, new

90...100 0,1...0,3

smoothened Concrete, smooth Finish planed Wood, good joints

85...90 0,40,6

Concrete, good Formwork, high Cement contents 80 0,8non-planed Wood; Concrete Tubes 75 1,5hard-burned Brick; carefully joint well-manufactured Ashlar Facing; Concrete from joint-free Wood Formwork

70...75 1,5...2,0

rolling-cast Asphalt Finish 70 2 well-manufactured Ashlar Masonry; Steel Pipes moderately incrusted; non-finished Concrete, Wood Formwork; Stones, squared Wood, old and swelled, Cement Walls

65...70 3

non-finished Concrete, old Wood Formwork, Brickwork, no joints, finished; Soil Material, smooth (fine-grained)

60 6

Concrete from Wood Formwork, old, corroded 55 10rough Ashlar Masonry; Paved Slopes, Sand or Gravel Bottom; Concrete Plates; old Concrete with open Joints

45...50 20

Regular Earth Channels, no Shingle; moderate Gravel 40 Fine Gravel; sandy Gravel Fine Gravel up to moderate GravelModerate Gravel, broken Stone

30 50 75

Moderate to coarse Gravel; Earth Channels with slight Weed Growth, moderate Bed Load and Potholes

35 90

Natural River Beds with coarse Shingle; Rivers with heavy Bed Load; Earth Channels with plaiced Loam;River Environs with Vegetation

30

Soil Material with moderate Bed Load, Coarse Gravel to coarse broken Stone

...200

Mountain Stream with coarse Shingle, Earth Channels with heavy Weed Growth, Soil Material plaiced and thrown up

25 ...400

Coarse Rock Filling; refinished Rock Excavation ≤ 20 ...500

(max. 0,4 R)Mountain Streams with heavy Bed Load; Random Rubble Stone

< 20 ...650(max. 0,4 R)

Collapsed Bed < 20 ...900(max. 1,0 R)

Rock Excavation, medium coarse < 20 ...1 500(max. 0,4 R)

Collapsed Bed with heavy Bed Load; Earth Channels with very heavy Weed Growth

<20 ...1 500(max. 1,0 R)

raw Rock Excavation, extremely coarse <20 ...3 000(max. 0,8 R)

* The absolute roughness values for these surfaces are currently incomplete.

smoo

th

mod

erat

ely

roug

h ro

ugh

very

roug

h

extre

mel

y ro

ugh*

Table 1: Roughness Types

16

3.2.2 Flume Measurement Flume measurements are flow measurements in a specially shaped channel with defined lateral narrowings. Within these narrowings, the flow velocity changes from streaming to shooting. Most commonly used, they are stationary channels made of concrete as well as prefabricated pieces. According to the well-known measurement principle, the geometric shapes vary over a wide range (such as Parshall channel, Palmer-Bowlus channel and many special constructions by different manufacturers). In Germany, two kinds of flumes are used: the Khafagi channel as well as the classical flume channel. Both have been included in the DIN 19559 (German Industrial Standard). Based on this standard, reference for mounting conditions, measurement ranges, accuracies etc. will be made. In North America, the most commen types of flumes are the Parshall and Palmer-Bowlus flumes. Parshall: Commonly used in concrete-lined channels, for permanent monitoring applications. The size of the flume is determined by its throat (narrowings) width. Palmer-Bowlus: Used in round channels, they measure flow over a narrow flow range. They are sized according to the pipe diameter. Basic Functional Principle: Caused by the narrowing, a banking-up of the medium to be measured occurs in front of the flume. It is here where the flow velocity’s alternation from streaming to shooting happens (point of alternation). This defined hydraulic state allows the flume’s roughness and slope to be ignored. Thus, the banking-up level in front of the flume represents a proportional value for the flow volume.

Fig. 18; Section View of a Flume Channel

17

Fig. 19; Plan View of a Flume Channel

B = Channel width b = Narrowing ho = Upper water level hu = Lower water level L1 = Transducer’s distance to beginning of flume When using flume measurements, the following should be taken into account: • The smallest measurable wastewater flow is 5l/s (79.26 gpm) approximately. • The ralation between Qmax and Qmin usually is 10:1; in specially shaped flumes, its maximum is 20:1 • It is absolutely necessary to avoid backwater when measuring. This means, that the upper water level h0 will not be influenced by the lower water level hu. • The transducer must be installed 1,5 - 2 x h0 max above the begin of the flume’s narrowing. If it is necessary to measure within the maximum possible flow volume range, the distance should be increased up to 3 – 4 x h0. The transducer’s zero point always is related to the flume’s zero point, not to the measuring point’s!!!

Fig. 20; Transducer Installation

Special attention must be paid to the inflow stage. It is absolutely necessary for the head flow to be in laminar condition. If in turbulent condition, the transition to laminar condition must take place at least 20 x B before the narrowing’s begin. Within the range 10 x B before the flume’s beginning the following points should be taken into account:

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• constant head of the whole inflow stage • rectilinear run of the flume’s and the inflow channel’s axial center • constant channel cross-section • no additional delivery pipes or drains • no installations, salient or rebound unevennesses in pipe walls or channel bottom Prerequisites for measuring • no backwater • no deposition/sedimentation • sufficient banking distances • high minimum water flow Advantages • no moving parts • very suitable for polluted wastewater measurement • good consistency of measurement values and measurement control • easy to clean Disadvantages • high costs of construction • large space required • high cleaning expenses • low measurement dynamics between Qmin and Qmax , usually factor 10 - 20 • minimum determinable wastewater flow Qmin ≥5 l/s (79.26 gpm)

3.2.3. Weir Measurement Measurement weirs, also called spillways or overflow weirs, are backwater constructions (dam-like structures) that the water flows over. These constructions change the channel bottom’s run in such a way as to cause backwater in the upper stream. Based on the defined hydraulic conditions and respective the construction’s dimensions and design, an interrelation between head water and overfall volume = flow volume can be made.

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hü = Head on weir b = Overfall width

Fig. 21; Overfall Weir Measurement Principle

Q = ƒ (h ) • kü

k depends on threshold shape, threshold width, viscosity and more.

3.2.3.1 Overfall Weirs Overfall weirs are mostly used in rain basins and separation constructions for the removal of high rain water loads. The formula for the overflow volume calculation is:

23

232 hgbQ •••••= µ

µ represents the overfall coefficient. This coefficient is dimensionless and depends on threshold shape and partially on the head on weir (see Tab. 2). b is the weir width (overfall width). h is the head on weir, also indicated as hu. The zero point of this height is situated exactly on the overfall beginning point. g is the acceleration due to gravity (9.81 m/s²)

20

The most commonly used weir crest shapes in rainwater treatment plants are: round crest

or broad crest

or or sharp-edged

21

sharp edged Weirwith aerated jet

Weir Crest Type µ Overfall Coefficient

µ = 0,64

µ = 0,75

µ = 0,65 .... 0,73

µ = 0,79

µ for = 0,40 ( =0,06m)hü

µ = 0,55 (for =0,45m)hü

µ = 0,49a)

µ = 0,52b)

(valid for >0,06m; >0,01m; <0,8 )w h h wü ü

( , in )h wü m

Log Weir

round Crest Weir( =r Weir CrestRadius)

cylindrical Weir

Broad Crest Weir

broad Overfall, completely rounded off

such as a completely reversed Fishbelly Flap

roof-shaped, Crest rounded off

overflown Dike

Lawn

alternative Radius when Crest is elliptical r b =

r

+ - 0,573( ;(4,752 +

ba b

ab20

h üh ü

h ü

b1

a) b)

h üh ü

h üw

ww

ww

d

1:0...1.:1

µ = 0,605 + + 0,0811000 hü

hü

w

µ = 0,49 (for >3)a)

µ for = 0,50 ( =0,10)b)

µ for = 0,55 ( =0,55)b)

b1

hü

hüwhüw

The value valid for the sharp edged Weir is to be multiplicated with

µe

d / hü ≤ 2/3 1,0

0,88

1,5

0,82

2,0

0,79

2,5

0,77

3,0

0,76

1,0eb

a

within the boundaries 6> >0,5ab

µ = 0,312 +√ 0,3-0,01(5- )²+0,09 hü hü

r w

( > >0,02m;w rvalid for )20rhü< 6-r w r+3

µ = 0,55+0,22 hüw

hüw(valid for 0,1< < 0,8)

90°

1:15

approximate exact

Table 2: Overfall Coefficients

22

Due to its very exact overflow coefficient, it is easy to gauge and make supplementary installation in existing, inexact round crest concrete weirs with sharp-edged weir crest and is recommended as standard measurement weir according to ATV A111. The weir must be dimensioned as follows:

Fig. 22; Dimensions sharp-edged Overfall Weir

The weir has to be adjusted horizontally and must be secured in order to avoid unintentional moving. The water must flow vertically against the weir and on the other side, no backwater is allowed to occur. In case of an oncoming flow tending to backwater formation a single head water measuring is not sufficient! Prerequisites for Measurement • no backwater • precise horizontal overfall edges with defined overfall coefficient • no additional coarse rakes (or similar) • high flow volumes • vertically oncoming flow Advantages • no moving parts • existing basins uncomplicated to refit • cheap measurement technology Disadvantages • suitable only for high flow volumes • relatively inaccurate

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• measurement errors due to wind pressure • unsuitable for tangential oncoming flow and backwater in the ditch

3.2.3.2 V-Notch Weir One more special weir shape is the V-Notch weir, also called Thomson Weir or Gourley Weir. This weir features a weir plate standing vertical to the flow direction with a sharp-edged triangular cutout. The backwater level in front of the weir is directly proportional to the flow volume. Due to its special cutout the V-Notch weir is especially suitable for small volume measurement (0.05 l/s - 30 l/s; 0.79 - 475.59 gpm). A polluted weir edge very strongly adulterates the measurement values. Thus, it is primarily suitable for the evaluation of clean media like spring water, small sewage plant’s drains or partially even for volume measuring of percolating waters in dumps (in the latter case the prerequisite is that the dissolved components do not tend to sedimentation, precipitation or incrustation at air access on the weir edge). The following conditions should be payed attention to:

Fig. 23; V-Notch Weir Principle

• hü min = 3 cm (1.18 in)! The jet is „glued“ below this, measuring impossible. • hü max should not be more than 30 cm (11.81 in). • The oncoming flow velocity should not be higher than 5 cm/s (0.16 fps). Eventually

baffles must be installated in order to avoid a direct oncoming flow. • No backwater (absolutely necessary)!

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Prerequisites • no backwater • low oncoming flow velocity • no sedimentation • clean media without slubs, fibers or similar Advantages • low costs for the needed measurement technique • accurate measurement results (even at low volumes) • easy to verify Disadvantages • partially high mechanical expenses needed to realize the necessary hydraulic

conditions • no measurement in polluted or sediment carrying media • no measurement of very high volumes

3.2.3.3 Special Measurement Methods for Overfall Weirs The level measurement before the weir as well as the measurement results are influenced by the ditch backwater (refer to ATV A 111). For an accurate measurement under backwater conditions, therefore, either 2 components measurements (flow velocity and level) in the main channel should be made; or the lower water level also must be included at least in addition to the upper water level.

Fig. 24: Backwater Measurement

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The relation between the lower water level and the upper water level results in the reduction coefficient c.

nhhc

o

u )(1−=

n = 2 for sharp edged weirs n = 3 for round crest weirs n = 4 for rectangular weirs Due to the need for better overfall value, always a sharp edged weir should be assumed in reduction measurements!

The reduction coefficient c depending on the current backwater must be calculated and fixed into the Poleni formula for rectangular overfalls.

5,1•2••••32 hgbcQ µ=

All functions described above are already implemented in the NIVUS-HydraulicCalculator. The two level measurements must be only established as analog inputs. The tangential oncoming flow of thresholds represents another special form. This application is frequently found primarily in separation buildings in front of storm water reservoirs. The tangential oncoming flow of the overfall threshold results in shaping a parabolic mirror line which is not parallel to the cast-off threshold. Determining a measurement point for the average overfall volume is difficult, because this point is situated at various places according to the water volume flowing through. In this case, two measurements for the overfall threshold and a determination of the average overfall level is necessary as well.

26

Fig. 25; Tangential Threshold Oncoming Flow

The average overfall level hü-m must be determined as follows:

)(32

ouomü hhhh −+=−

The resulting value hü-m must be integrated into the Poleni formula mentioned earlier. This function also is already implemented in the NIVUS-HydraulicCalculator.

3.2.4. Magnetic-Inductive Flow Measurement in Partially Filled Pipes Magnetic-inductive flow measurement in partially filled pipes is based on the same principle like in fully filled pipes (see chapter No. 3.1.3). Unlike in fully filled pipes the induced voltage is registered by multiple pairs of electrodes which are attached at different levels. UE = Measurement Voltage B = magnetic Induction D = Distance between Electrodes v = average Flow Velocity qv = Volume Flow k = Correction Factor UE ~ B • D• v

qv = 4

2πD • v • k

UE • k ~qv

Fig. 26; Principle magnetic-inductive Flow Measurement in partially filled Pipes

27

By transversing the magnetic field the current flow level is evaluated. The various magnetic field characteristics at various levels are saved in the transmitter’s memory. Depending on the current flow level, a signal proportional to the flow is produced by a correcting method. Prerequisites for Measurement • good conductivity of the medium to be measured • defined pipe cross-section with inflow and outflow stage • minimum filling level more than 10% of the diameter Advantages • high accuracy at higher filling levels and not/low polluted liquids • low sensitivity against asymmetrical velocity dissipation Disadvantages • high initial costs, especially from diameter > 300 (12 in), increasing proportional to

diameter • minimum flow velocity 50 - 100 cm/s (1.64 – 3.28 fps) • considerable higher conductivity necessary than magnetic-inductive flow

measurement in fully filled pipes (factor 10) • no measuring possible below 10 % filling level • measuring error increases excessively with falling level • long-time drift caused by dirty electrodes in the sewage area (sewer films) may result

in total measurement failure after a certain time (obvious through constant discharge indpendent of day or nighttime) = high cleaning and maintenance expenses in wastewater

3.2.5 Ultrasonic Doppler Method

This method represents a two-component-measurement. The current flow level and thus the wetted hydraulic radius is evaluated by ultrasonic or pressure probes. In this case ultrasonic measurement represents the more accurate, maintenance- and drift-free, non-contact gauging method. The flow velocity is evaluated by the Doppler method (see chapter 3.1.4.1).

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Fig. 27; Measurement Installation Ultrasonic Doppler Method

Evaluating the Doppler signal and the determination of the average flow velocity can be achieved in different ways.

3.2.5.1 Fan-like Beams with statistical average Value plotting

Using this method, constant beams are transmitted into the liquid. The receiving crystal, placed at the same angle and parallel, receiveses continuously all Doppler signals reflected from the transmitted signal, spreading like a fan .

Fig. 28; Evaluation by average Value plotting

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It is characteristic for this measuring method that, a spatial allocation of the received signals is not possible. In clean liquids the transmitting signal is able to penetrate the water surface. In highly polluted liquids the penetration depth is limited and not defined. The water surface represents an ideal plane of reflection whose propagation velocity, under certain circumstances, is higher when waving than the flow velocity to be measured. Since the statistic average value determined by all signals cannot represent the average flow velocity it must be corrected with a calibration factor. A general mathematical derivation of the calibration factor from other channel parameters isn't known to the authors. Depending on the applications this factor (see VDE 2640 ) must be determined empirically by comparative measurements. For long time stationary measurements, a single, high calibration and putting into operation, a single value acceptable. The calibration often isn't possible or outrageously expensive for various channel operations.

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3.2.5.2 Fan-like Beams with Vmax Evaluation

Transmitting beam and signal reception correspond to the method described under 3.1.4.1. The received Doppler signals are evaluated by frequency analysis (Fast Fourier Transformation for example). The maximum flow velocity is evaluated in the frequency spectrum consisting of many single velocities.

Fig. 29; Performance with vmax

The detected maximum velocity in the channel is multiplied with the reduction factor (c <1) to get the average flow velocity.

This reduction factor within fully filled pipes (discharge pipes) is known. a) laminary flow: c= 0,5 (BLASIUS) b) turbulent flow (ideally smooth pipe and Re<45000): c=0,817 (ECK) c) turbulent flow in rough pipes (practically always presesent): C = 1 + 1,326 •

= 0,316/Re • (für Re<10000) = 0,0032 + 0,221 / Re

λλ λ

(PRANDTL)(BLASIUS)(NIKURADSE)

1/2

4

0,237

The extent of how far the reduction factor’s calculation can be applied to other channel types or partially filled pipes is subject to further examination. The measured maximum flow velocity cannot be allocated to a certain point in space even by this method.This is why a plausibility verification only by using the frequency spectrum is difficult. As mentioned in 3.2.5.1., influences like signal penetration depth as well as surface and waving reflections may cause false informations.

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3.2.5.3 Selective Flow Velocity Evaluation

By having cross-shaped order of the transmitter and receiver crystals an intersection point is created close to the transmitting and receiving direction. At this intersection point the highest reflection intensity occurs. The size of the intersection point is defined by the crystal size (normally approx. 10 mm of diameter) as well as an inexact determinable marginal zone. Therefore a "virtually" dot-like flow velocity must be spoken of. The frequency moving measured in this area sways with the velocity gradient appearing at the intersection point.

Fig. 30; selective Flow Velocity Evaluation

Via this local flow velocity, determined in the local area of the probe, using a coherence formula, tha average flow velocity can be calculated.

V = N • g ½ • M • In [N • h • ( )]1,49 • R • V

P60,031

n

P1/16

(PARR, JUDKINS, JONES)

Assuming fully filled pipes according to NIKURADSE:

N = 29.7 M = 2.5

N = roughness according to MANNING Vp = point flow velocity (ft/m) hp = point flow velocity level (ft) Since the MANNING coefficient varies from one partially filling to the other, the equation above was extended by the Maryland University in order to consider the conditions within partially filled pipes too. Further the coefficients “M” and “N” were determined anew. Basis for the change of the MANNING coefficient at partially filled pipes is the relationship diagram according to CAMP.

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Fig. 31; Relationship Diagram

To make this diagram, it was expressed as the following equation:

nt/n = 0.8194 + 2.0355 (h/D) – 6.1305 (h/D)² + 7.074 (h/D)³ - 2.991 (h/D) 4

nt = roughness coefficient according to MANNING in partially filled pipes For „M“ and „N“ the following values were found: pipes: M = 1.55 N = 3953.224 e (-0 , 44xD) rectangular channels: M = 1.3 N = 3300 Inserted into the equation above, it is possible to determine the average flow velocity from a dot-like flow velocity measurement in the vertical axial center.

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3.2.5.4. Measurement Technique Installation

Measurement Installation of the Pipe Measurement Section

Fig. 32; OCM Measurement Installation at Pipe Measuring Section A typical measurement and controlling section consists of: - OCM with PID-Controller - Level Measurement (NivuMaster) - Power Supply - Ex-Preamplifier DS4 (only necessary in Ex-Zone 1) - Level Sensor P-06 (Ex) - Insertion Velocity Sensor DER (Ex) - Safety Barrier (only necessary in Ex-Zone 1) - Pipe Measurement Section Flanges, fixing pieces, slide valves and control valves are supplied by the plant operator.

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Measurement Installation in open Channels

Fig. 33; Open Channel Measurement Recapitulation Ultrasonic Doppler devices need a certain minimum amount and size of reflecting particles within the medium to be measured. Without these particles no reflection of the ultrasonic signal occurs. For NIVUS measurement technology even gas or air bubbles are sufficient. Applications like sewage works effluents are measurable as well as return-sludge, wastewater in pump stations and more. On the other hand the Doppler method fails in clear water, boiler feed water or similar. Prerequisites for Measurement • minimum particle size and volume within the medium to be measured • defined pipe cross sections with feeding and outlet stages • correct building construction without setoffs or additional hydraulic disturbances

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Advantages • no channel constriction • additional installation easily possible • measurement can be carried out even at lowest possible levels (from 30 mm, 1.18 in)

and lowest possible velocities (3cm/s, 0.098 fps) (refer to: NIVUS device technique) • drift free • easy measurement even in grease and sewer film formation on the transducer • maintenance free Disadvantages: • careful selection of the measurement point indispensable • individual calibration for higher accuracy requirements or unfavourable hydraulic

conditions necessary • no measurement possible in clean media

3.2.6 New Measurement Methods – Profiler

The use of measurement principles which were regarded as unthinkable and not possible a few years ago, can now be used because of the development of faster and more efficient electronics and processors, as well as research in the area of the sensor technique today. NIVUS follows the path of consistent research and development and perfecting the flow measurement techniques and actively promotes the use of new measurement principles. A result of this this is the technical transition from theory into practice of a method which offers completely new techniques of flow measurements, even under difficult hydraulic conditions. NIVUS has brought this new method to the market through the newly developed, user friendly and high-accurate OCM product line in the year 2000. At the editorial deadline more than 100 measurement units were in use over a wide range of hydraulic conditions. Measurement Method The NIVUS OCM Pro works with a completely new multi-purpose transducer (combination sensor) which simultaneously determines flow velocity and level. This is achieved by using two especially manufactured piezoelectric crystals which work independent of each other as transmitter and receiver, and thus determine flow velocity and level simultaneously.

Fig. 34; Schematic Diagram Combination Sensor Type „Pro“ for Ground Level Installation

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These particular piezoelectric crystals (Composite-Crystals) consist of a high amount of parallel, small rod-shaped crystals which can be stimulated to oscillate simultaneously as well as being switched to receiving mode for the reflected signals.

Fig. 35; Schematic Diagram Transmitter/Receiver Crystals

The special form of these piezoelectric crystals offers the advantage of precise orientation and directivity of the ultrasonic signal (sonic beam lobe with 3° angular aperture). Thus, very exact spatial positioning and high precision as well as a very short die-down time (mechanical baseline overshoot of the crystal after transmitting pulses. A certain time, t, must pass before the crystal is able to receive reflected signals) can be achieved. The very short die-down behaviour makes a dead-zone (the distance before the transducer where measurement is not possible) of 20 mm possible. These small distances are necessary when it comes to measuring low levels. The horizontal transducer works as a level meter with the known ultrasonic transit-time method. The time elapsed from transmitting a pulse and receiving it, reflected by the boundary layer between water and air, is measured. Since the pulse must cover the distance h2 twice (up to the water surface and back to the transducer) the result is:

2•

2ltc

h =

c = Sound Travel Time tl= Time Lapse between transmitting and receiving the Signal

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Fig. 36; Measurement Principle Level Evaluation in Channels

Fig. 37; Speed of Sound Determination

The speed of sound in water is exact 1480m/s (4854 fps) at 20°C (68°F). It depends on the temperature with a tolerance of 0.23% per Kelvin. To achieve a millimetre-accuracy for level measurement, the medium temperature is continuously monitored and the speed of sound is corrected accordingly. The constant level value h1 determined by the transducer crystal installation is added to the evaluated value h2. The result is the overall level h. The advantage of ultrasonic measurement from below is that surface foam does not influence the measurement results (for conventional air-ultrasonic measurements, foam can distort the measurement results or lead to the failure of the measurement). Installing the transducer in partially filled pipes can be easily made by welding a socket into the pipe vertically from below and fixing the transducer by a pipe double nipple with an O-ring.

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Fig. 38; Measurement Principle Level Determination in partially filled Pipes For velocity measurement, the piezoelectric crystal is inclined in a defined angle against the flow direction and works as a flow velocity transducer. For the first time worldwide in the field of wastewater measurement technique, the digital pattern recognition of reflections is used, which allows, in conjunction with the correlation method an exact spatial allocation of various scan window velocities by a defined acoustic path. This measurement method is based on the following physical principles: A short ultrasonic signal bundle is insolated into the medium to be measured in a defined angle. All particles within the measurement path (air, dirt etc.) reflect a small amount of the ultrasonic signal. This signal is received by the flow velocity crystal and is transformed to a time-dependent voltage signal. The result is a particular reflection signal, dependent on size an shape of each particle. The multitude of the reflected signals result in a reflection pattern (see Fig. 39a), which the crystal receives again and transforms it to voltage signal equivalent to this pattern. This signal pattern is fed into an extremely powerful digital signal processor (DSP) and stored.

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Fig. 39a; Situation at first Signal Detection After a defined period of time another ultrasonic impulse is insolated into the medium. There are different hydraulic flow velocities in various level layers. The reflective particles therefore have moved further depending on height since the previous measurement. The result is a distorted image of the reflection pattern (see Fig. 39b). Simultaneously slightly different reflections appear: some particles have been turned around showing a different plane of reflection; some particles have been moved out of the measurement range and some new ones have been moving into the measurement range.

Fig. 39b; Situation at second Signal Detection These two received reflection samples are checked mathematically in the digital signal processor by means of the correlation method for their similarities. All available signal differences are rejected in order to have two signal patterns similar to each other with a temporal offset left to enable a velocity evaluation. A large number of measurement windows are put over these samples depending on the flow levels. Then in each window the evaluation of the time offset ∆t of each pattern (see Fig. 40) is carried out.

Fig. 40; Echo Signal Images + Evaluation

40

Each scan window’s position in vertical sense and size are known due to the flow level measurement carried out simultaneously. As a result of insolation angle and the ultrasonic signal’s speed of sound, each window’s exact beginning and ending time point is determined. The accompanying flow velocity therefore can be determined based on the insolation angle, the temporal distance of the two transmitted signals succeeding one another and the difference of the signal pattern in each measuring window. Mathematically stringing together the single calculated flow velocities yields the velocity profile of the acoustic path.

Fig. 41; Evaluated Flow Profile within the Measurement Path If the wave trap at the measurement point is large enough, a three-dimensional image of the flow density spread can be generated based on the available geometric data of the channel.

41

Fig. 42; Generated 3-dimensional Flow Profile The transducer is constantly testing the received signal’s quality and level. In case of a decreasing signal level (if contamination should occur for example) the transmitter output will automatically be equalized. This results in very low sensitivity in case of transducer contamination. The depicted combination transducer has not been cleaned for more than 40 weeks and thus documents this statement impressively.

Photo 01; dirty multi-purpose Transducer

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Recapitulation The Profiler Method opens new doors in the field of wastewater technique. The method described above and the available measument technology captures with high accuracy, exact verifiableness, easy installation in available profiles, extreme easy and menu-driven operation, a high number of additional information (7-day-totalizer, error-free memory, graphical representation of velocity values, control and flush functions) measurement value memory, it is absolutely drift-free etc.

4. Bibliography

- ATV A 111 - DIN 19559 Teil 1 und 2 - Fachwissen des Ingenieurs, Band 5, FBV Leipzig - Technische Hydromechanik, Band 1, Preisler / Bollrich, Verlag für Bauwesen Berlin - Abwasserhydraulik, W. H. Hager, Springer- Verlag Berlin & Heidelberg - Durchflussmesstechnik für die Wasser- und Abwasserwirtschaft, Heinz G. Erb, Vulkan Verlag Essen - Hydraulik im Wasserbau, Prof. Dr. -Ing. R. Rössert, R. Oldenburg Verlag München/Wien

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44

6. Flow Measurement Applications

Flow measurements are necessary for planning, water treatment, pollution control, storm water monitoring and control, and billing purposes. The following are common users of flow monitors for measurement purposes: Cities and Municipalities Water works engineers and municipal managers understand the hydraulic loadings on their sewer systems and manage them. It helps them understand the capacity of their sewers in wet weather conditions; why overflows and backup occur; where is the worst I/I in the sewer system; amount flow entering lift stations; what part of the sewer system needs to be fixed and what is their effectiveness after rehabilitation. Wastewater Treatment Plants For control of aeration, chemical feed, chlorination etc. based on amount of flow entering the plant. Storm water collection systems RÜB controllers are used to monitor rainfall as well as flow and water quality in wet weather conditions. The following applications are briefly described: Infiltration & Inflow (I/I) Analysis Infiltration is the total extraneous flow entering the sanitary sewer eystem. It is usually groundwater migration into a sanitary sewer. Inflow is the extraneous flow entering a sanitary sewer during rainfall event, from roof leaders, basement drains, manhole covers and storm sewer cross connections. Flow monitoring allows the sewer system to be identified for sources of I/I and remediation suggested. Capacity Analysis In areas where municipal and industrial growth is planned, capacity analysis studies determine how much additional flows during dry and wet weather the sewer system can handle due to growth. Pre- and Post Remediation Studies Most remediation work is done to eliminate I/I. Flow monitoring allows to know the effectiveness of the remediation. Billing Many industries and municipalities have their wastewater treated by other treatment plants and pay for these services based on portable usage, flat fees etc. These do not take into account increased flows during wet weather, or illegal dumping. Direct flow measurement is the only true way to bill according to wastewater treated. This eliminates ambiguities etc.

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