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है”ह”ह

IS 15646-1 (2006): Measurements of free surface flow inclosed conduits, Part 1: Methods [WRD 1: Hydrometry]

IS 15646 (Patt 1) :2006

wF?rMmi’m-

T@MfaditiymmmTmTwl-mm 1 wfifi

Indian Standard

MEASUREMENT OF FREE SURFACE FLOWIN CLOSED CONDUITS

PART 1 METHODS

Ics 17.120.20

0 BIS 2006

B-U REAU OF INDIAN ST AN DA-RDSMANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEW DELHI 110002

March 2006 Price Group 2

Hydrometry Sectional Committee, WRD 01

FOREWORD

This Indian Standard (Part 1) was adopted by the Bureau of Indian Standards, afler the draft finalized by theHydrometry Sectional Committee had been approved by the Water Resources Division Council.

The measurement of fluid flow and level in partially filled closed conduits poses particularly difficult problemsand is not filly documented. An attempt has been made in this standard to give guidance to users on the existingmethods employed and on recent developments in this field.

The measurement of flee surface flow in closed conduits is analogous to normal gauging in open channels, andthus open channel gauging techniques may be applied to fi-eesurface flows in closed conduits. Closed conduitsmay be classified as:

a)

b)

c)

d)

Sanitary, where only domestic and industrial waste are conveyed in the conduit;

Storm, where, following a rainstorm, run-off from impermeable areas is conveyed to the nearestwatercourse;

Combined, where both domestic and industrial waste together with storm run-off are contained in oneconduit; and

Culvert, where the water course is conveyed under a road, railway, etc.

The purpose of closed conduit systems types (a), (b) and (c) is to remove waste water tlom urban areas to a sitewhere treatment (mechanical, chemical and/or biological) can be undertaken. The cheapest way of conveyingeffluents in a conduit is by laying the conduit so that it follows the natural topography, while providing sufficientgradient so that effluents will not stagnate but will flow under gravity. In very flat areas, conduits may have to belaid at greater depths to attain a sufficient hydraulic gradient to avoid frequent pumping.

At times of intense rainfall, systems of type (b) may run full, and under such circumstances, overflow system beconstructed so that excess water is conveyed to the nearest water course or storage areas to avoid surface flooding.

Conduits may be constructed as closed or open, and maybe made of various materials, such as, vitreous clayware,concrete, asbestos cement, cast iron, brick and, more recently, plastic and resin-bonded materials. They mayrange in diameter from 1-50mm onwards, and it is rare to have diameter longer than 3.m for the conduits.

Effluents carry floating and suspended solids, and may, at times be corrosive in nature. In addition, the atmospherewithin the closed conduit system may contain both inflammable and corrosive gases. Thus the environmentwithin the closed conduit system may be described as hostile.

A flow-meter will have to operate in these hostile conditions over a wide range of flows, from free surface openchannel flow to conduit full pressure flow. The nature of urban drainage is such that steady flow conditions arerare, except near the outfall from large catchment areas, where flow attenuation has -occurred. Non-uniform flowalso occurs as a result of bends, junctions and displaced joints.

The access to any closed conduit system is limited -to-the outfall or to specially built access points (that ismanholes). Manholes are constructed at regular intervals and at points where the conduit changes in direction orgradient or at a junction. The manhole points are hydraulically not suitable for discharge measurement. Withinthe channel at a manhole, an additional difficulty may be encountered when the depth of flow is more than halfthe diameter, as working space is required to be provided for workmen to stand within the manhole. It isrecommended that in deep conduits, a platform or gallery be constructed at vertical intervals of 10 m from thepoin[ of view of safe climbing up or going down in the manhole. The sensing and recording elements must becapable of withstanding inundation.

(Continued on third cover)

Indian

MEASUREMENT OF

M 15646 (Part 1) :2006

Standard

FREE SURFACE FLOWIN CLOSED CONDUITS

PART 1 METHODS

1 SCOPE

This standard (Part 1) provides the methods of flowmeasurements that can be empl yed in closed conduits

Tflowing under partial and ful flow conditions anddetails the advantages and disadvantages of the variousmethods.

2 REFERENCE

The following standard contains provisions, whichthrough reference in this text, constitutes provisionsof this standard. At the time of publication, the editionindicated was valid. All standards are subject torevision and parties to agreements based on thisstandard is encouraged to investigate the possibilityof applying the most recent edition of the standardindicated below:

1SNo. Title

1191:2003 Hydrometric determinations —Vocabulary and symbols

3 DEFINITIONS

For the purposes of this standard, definitions given inIS 1191 and the following shall apply.

3.1 Permanent Flow-Meter — Flow-meter installedfor along period of time (more than 12 months) andused to measure flow continuously or at discrete timeintervals.

NOTES

1 The high costs incurred in the installation of these flow-meters may be accepted as they are to operate over a longperiod of time.

.2 The measurements made maybe stored in an archive systemand used to examine present trends to forecast future trendsand to determine daily operational requirements.

3.2 Temporary Flow-Meter — Flow-meter installedfor a specific period of time (less than 12 months) andused to measure flow continuously or at discrete timeintervals.

NOTE — The installation of the meter needs to be simple withminimal or rToassociated civil work.

3.3 Portable Flow-Meter — Flow-meters that areused to obtain instantaneous measurements of flow,or the velocity and depth components thereof.

4 METHODS OF-FLOW MEASUREMENT

There are.two basic types of flow measurements, knownas direct measurement and indirect measurement.

4.1 Direct Measurement

A direct measurement is one in which the flow isdetermined from measurements of various flowcomponents, that is, it is not inferred. The methodsavailable for direct measurement -are volumetric anddilution gauging.

4.1.1 Volumetric Method

4.1.1.1 A tank of known volume is taken and all theflow is directed into this tank. The time for the tank tofill is recorded and the average flow is calculatedaccordingly. This method is usually not practical in anunderground system.

4.1.1.2 In a system where a sump or wet-well has,beenconstructed and a pump is available to draw from thesump, the flow can be measured by:

a)

b)

4.1.1.3

calculating the depthholume relationship ofthe sump, and

measuring the water level (using a levelrecorder) in the wet-well at discrete timeintervals.

[n a drainage system where, during a storm,the pumps are working continuously, the discharge canbe measured by:

a) monitoring the level in the wet-well, and

b) monitoring the discharge of pumps underoperation.

4.-1.1.4 Discharge through a pump or a combinationof pumps shall be calibrated by independent fieldteststhat is by monitoring the time it takes to empty thesump. The volumetric method has the followingadvantages and disadvantages:

a) Advantages:

1) Flow is measured directly,

2) Instrumentation can be powered by anelectrical supply main, and

3) Sensing can be by remote methods.

1

IS 15646 .(Part 1) :2006

b) Disadvantages:

1)

2)

Measurements may be erroneous indrainage systems with large sedimentandtor sludge contents, as the volume ofthe sump gets reduced as they getdeposited at the bottom.

Sediment can also reduce the rate ofdischarge of the pumps.

4.1.2 Dilution Gauging

Standard dilution gauging techniques, either constant-rate injection or integration method can be employed.

Dilution gauging is best suited to measure instantaneousflow or at best average flow over a short period (usuallyless than 1 h), although it is possible to takemeasurements over a long period of time (that is days)if suitable devices for injection and analysis areavailable (for example, radioactive tracers).

The following points shall be kept in view whileemploying dilution gauging techniques:

a) it is necessary to obtain a sut%cient degree ofmixing at the sampling site, and

b) it is important that a tracer is chosen which isnot absorbed and which does not react withthe liquid-that is being measured.

4.2 Endirect Measurement

An indirect measurement is one in which flowis derived from measurements of various flowcomponents. Slope-area and area-velocity techniquesare generally employed for indirect measurements ofdischarge. Hydraulic structures like weirs and flumesare also used for this purpose.

4.2.1 Flumes and Weirs

In closed conduits where the liquid contains a highproportion of solid materials and where pressure aswell as fi-eesurface flow conditions occur, conventionalflumes and weirs cannot be used. Therefore, specialdevices, designed and built to work under suchconditions, have to be employed. These are describedin 4.2.1.1 to 4.2.1.6.

4.2.1.1 Vertical slot weir

This weir as shown in Fig. 1, allows solid materials topass through the installation. The rating curve of thevertical slot weir depends on the slope and roughnessof the conduit. This will be all the more relevant duringlow flows throwgh weirs with wide slots. Verticalslot weir is most suited for use in large diameterconduits,when there is free surface flow only.

4.2.1.2 Trapezoidal weir (see Fig. 2)

This weir timctions in the same way as the vertical slotweir in that it allows the passage of solid materials and

does not excessively obstruct the flow. In this case also,the rating curve is depended on the slope and roughnessof the conduit. The trapezoidal weir can measure lowerdepths and smaller flows more accurately than thevertical slot weir. This type of weir is most suited foruse in large diameter conduits where there is no surginglsurcharging.

L---JAHdimensions in metres.

FIG. 1 VERTICAL SLOTWEIR

Alldimensions in metres.

FIG.2 TRAPEZOIDALWEIR

4.2,1.3 USgeological survey meter (USGS meter) (see

Fig. 3).

Since drainage systems frequently become surcharged,a device wlich can measure flows under both freesurface and pressure flow conditions is necessary; theUS geological survey meter is such a device. Underfree surface flow conditions, the meter acts as a.flumeand offers the reliability and accuracy of a flume. Theaccuracy, however, has to be traded off againstconstriction of the conduit, that is the greater theconstriction, the better the accuracy. For pressure flowconditions, the meter operates as a nozzle and flow isdetermined by measuring the pressure upstream anddownstream of the nozzle through piezometers.

‘ThmetweePienmeter

m

teps

Q

b.lm

-do

sideview “ThroetWOee-mction

FIG. 3 US GEOLOGICALSURVEY.SEWXRFLOW-METER

2

4.2.1.4 University of illinois meter (UI) (see Fig. 4)

This meter is similar in performance to the USGS meterand measures flow under both tiee surface and pressureflow conditions in an identical way. The transition fromfree surface to pressure flow, however, is not as smoothfor this meter as for the USGS meter owing to the shapeof the constriction in the conduit sofflt.

oCrii”mlxbn Thmm crc.ss-section

FIG. 4. UNIVERSITYOFILLINOISSEWERFLOW-METER

4.2.1.5 Parshallflume (see Fig. 5)

The critical-depth or venturi flume has been developedin many different forms. One of the more populardesigns is the Parshall flume, originally developed in1920 for irrigation channels. For this device the head/discharge relationship independent on the throat width.Again, this device is most suited for large diameterconduits.

COnvwging Throat DlwrglngMaiOn section

m

m.a.

tl,3b I 0,61 0,91

iWmwWrface

All dimensions in metres

FIG. 5 PARSHALLFLUME

4.2.1.6 Palmer-Bowlusflume (see Fig. 6)

Another critical-depth flume, the Palmer-Bowlusflume, has recently been tested for sloping conduits. Itexhibits a successful head/discharge calibration forboth sub-critical and supercritical free surface now.Modifications to the flume design are recommendedto improve the head/discharge relationship in thetransition zone horn free surface to pressure flow.

Cdtiill’hfwt

[’-” i aedul

Wme7‘:=%F)q tThroat

---- mm -*m

J=FIG. 6 PALMER-B• WLUSFLUME

IS 15646 (Part .1) :2006

4.2.2 Slope Area Method

In the slope area method, the flow level is measuredusing constriction devices such as flumes and weirs but,in addition, a level sensor which measures the depth offlow can be used to calculate the discharge inconjunction with the conduit roughness and gradient.The two most commonly used formulae are given below.

4.2.2.1 Manning formula

where~ = ma~ing ~gosity coefficient,

R~= hydraulic radius,

S = energy gradient, and

~ = mean velocity.

The level measurement is best achieved by using smallnon-mechanical devices such as bubblers, pressuretransducers, resistancelcapacitance and acousticsensors. The environment within the conduit may betoo corrosive for the use of mechanical devices. Theenergy gradient is assumed to be equivalent to theconduit gradient, but two level sensors may be used tomeasure both depth and energy gradient; the unsteadystate conditions for free surface flow, however, makethe measurement of the energy gradient difficult. Flowis calculated by multiplying the cross-sectional areaof the conduit by the mean velocity.

4.2.2.2 Colebrook-white formula

( k 1.255i+~ —

14.8R~+ R~y32gR#)

where

g = acceleration due to gravity,

k = roughness height,

R~ = hydraulic radius,

S = energy gradient,

v = mean velocity, and

y = kinematic viscosity of the fluid.

The main advantage in using this formula is that onlythe fluid level has to be measured. However, severaldisadvantages exist as follows:

a)

b)

c)

Hydraulic conditions must be ideal (that is nobends or junctions) and the energy andconduit gradient must be identical, unless twolevel sensors are employed in upstream anddownstream reaches;

Roughness value has to be assumed; and

It is not possibie to measure surcharge flowunless two level monitors in adjacent manholesare employed to determine the energy gradient.

3

IS 15646 (Part 1) :2006

Until now, the slope area method has been the methodmost widely used but the velocity area method isbecoming increasingly popular.

4.2.3 Velocity Area Method

The need to assess the hydraulic performance of adrainage system has led-to the use of the velucity areamethod, where the velocity and the depth of flow aremeasured. This enables the.flow to be calculated whensurcharge occurs without having to use two levelsensors some distance apart.

4.2.3.1 Electromagnetic method

This method as shown in Fig. 7, allows directmeasurement of the mean velocity. The principleof measurement is based on Faraday’s law ofelectromagnetic induction where the average velocityof the liquid moving through a magnetic field isproportional to the electromotive force generatedbetween two electrodes.

Normally, the coil is mounted above the conduit wherethere is easy access, that is in the manhole. Theelectrodes which sense the electromotive force shouldbe housed in a preformed insulated pipe section thatcan be positioned in the manhole/pipe configuration.The depth of fluid is measured using a bubbler, anacoustic sensor or a pressure transducer sensor. Theadvantage of this method is that it gives a directmeasurement of the mean velocity.

An alternative arrangement is to place the coil insideand the electrodes on the surface of a small streamlinedsensor. The sensor is fastened to a metal band or ringwhich is in turn positioned in the flow, normally withthe sensor in the invert of the conduit. The disadvantageof this arrangement is that the measured velocity hasto be related to the mean velocity for different depthsof flow. Again, depth measurement is carried out inorder to calculate the cross-sectional area of flow andhence the flow.

and downstream is recorded. The velocity is calculatedusing the formula:

L

()

11~. — ——2 cOse tBA tAB

where

L = acoustic path length from A to B,

t~A= downstream night path time,t*B= upstream flight path time,

~ = line velocity, and

O = angle between the direction of flow and theacoustic path.

In this method the mean velocity is inferred ftom aline velocity this is a disadvantage, but the uncertaintyis reduced, when more than one acoustic path is used.

4.2.3.2,2 The second method uses a Doppler velocitymeter to measure the velocity. The meter is mountedin a streamlined housing on a metal band or ring andpositioned in the conduit invert.

The acoustic signal transmitted by the meter is reflectedby air in suspension and particulate matter and returnedto the sensor at a different frequency (Doppler effect).The shitl in frequency between the signal transmittedand that received is proportional to the liquid velocity.The advantage of this method is that velocity ismeasured in the flow away from the sensor head, andthus, the effect of the size and shape of the sensor on,,.the measured velocity is minimized. The disadvantagesare that it does not measure mean velocity and it isdependent on the presence of air and particulate matterfor its correct operation.

4.2.4 Other Methods

Other flow measuring techniques tested in conduitsbut found to be either too expensive or impractical arethe following:

a) Optical flow-meter,

b) Vibrating-mass flow-meter,

Fieldcoil c) Thermal-wave flow-meter,

d) Steam-pulse flow-meter,

e) ‘Passive’ flow-meter,

f) Rotating-type flow-meter, and

g) Hot-wire and hot-film flow-meters.

5 UNCERTAINTIES

FIG. 7 ELECTROMAGNETICFLOW-METER Sensors can measure level and velocity to a betteraccuracy but the inaccuracies present in measuring the

4.2.3.2 Acoustic method cross-sectional areas of old brick or irregular-sha~ed

Two methods are available. In both these methods, the conduits may be large. In addition, steep gradients, non-

fluid level ismeasured to enable the flowto be calculated. uniform flow and unsteady flow conditions increasethe uncertainty in the calculation of discharge.

4.2.3.2.1 In the time-of-flight method, the time it takes However, it is necessary that measurement uncertain~esfor sound pulses to be transmitted diagonally upstream are kept within +10 percent.

4

(Continuedfrom second cover)

In the formulation of this standard, considerable assistance has been derived from ISO/TR 9824-1:1990‘Measurement of free surface flow in closed conduits — Part 1: Methods’.

The other standard in this series is IS 15646 (Part 2): 2006 ‘Measurement of free surface flow in closed conduits:Part 2 Equipment’.

The Committee responsible for the formulation of this standard is given at Annex A.

For the purpose of deciding whether a particular requirement of this standard is complied with, the final value,observed or calculated, expressing the result of a test or analysis, shall be rounded off in accordance withIS 2:1960 ‘Rules for rounding off numerical values (revised)’. The”number of significant places retained in therounded off value should be the same as that of the specified value in this standard.

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This Indian Standard has been developed from Doc : No. WRD 1 (356).

Amendments Issued Since Publication

Amend No. Date of Issue Text Affected

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