IS 15646-1: Measurements of free surface flow in closed conduits, Part 1: Methods

2006·1.2 MB·English

by  Bureau of Indian Standards

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IS 15646-1 (2006): Measurements of free surface flow in
closed conduits, Part 1: Methods [WRD 1: Hydrometry]
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IS 15646 (Patt 1) :2006
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Indian Standard
MEASUREMENT OF FREE SURFACE FLOW
IN CLOSED CONDUITS
PART 1 METHODS
Ics 17.120.20

0 BIS2006
B-U REAU OF INDIAN ST AN DA-RDS
MANAK 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 the
Hydrometry Sectional Committee had been approved bythe Water Resources Division Council.
The measurement of fluid flow and level inpartially filled closed conduits poses particularly difficult problems
and isnotfilly documented. An attempt hasbeen made inthis standard to give guidance to users onthe existing
methods employed and on recent developments inthis field.
The measurement of flee surface flow inclosed conduits isanalogous to normal gauging inopen channels, and
thus open channel gauging techniques may be applied to fi-eesurface flows inclosed conduits. Closed conduits
may be classified as:
a) Sanitary, where only domestic and industrial waste are conveyed inthe conduit;
b) Storm, where, following a rainstorm, run-off from impermeable areas is conveyed to the nearest
watercourse;
c) Combined, where both domestic and industrial waste together with storm run-off are contained inone
conduit; and
d) Culvert, where the water course isconveyed under aroad, railway, etc.
The purpose of closed conduit systems types (a), (b) and (c) isto remove waste water tlom urban areas to asite
where treatment (mechanical, chemical and/or biological) can be undertaken. The cheapest way of conveying
effluents inaconduit isby layingthe conduit sothat itfollows thenatural topography, while providing sufficient
gradient sothat effluents willnot stagnate butwillflow under gravity. Invery flatareas, conduits mayhave to be
laid at greater depths to attain a sufficient hydraulic gradient to avoid frequent pumping.
At times of intense rainfall, systems oftype (b) mayrun full, andunder such circumstances, overflow system be
constructed sothat excesswater isconveyedtothenearestwatercourse orstorage areastoavoid surface flooding.
Conduits maybeconstructed asclosed oropen,andmaybe madeofvarious materials, suchas,vitreous clayware,
concrete, asbestos cement, cast iron, brick and, more recently, plastic and resin-bonded materials. They may

range in diameter from 1-50mm onwards, and itisrare to have diameter longer than 3.mfor the conduits.

Effluents carry floating andsuspended solids,andmay,attimes becorrosive innature. Inaddition, theatmosphere
within the closed conduit system may contain both inflammable and corrosive gases. Thus the environment
within the closed conduit system may be described ashostile.
Aflow-meter will have to operate inthese hostile conditions over awide range of flows, from free surface open
channel flow to conduit full pressure flow. The nature of urban drainage issuch that steady flow conditions are
rare, except near the outfall from largecatchment areas, where flow attenuation has-occurred.Non-uniform flow
also occurs as aresult 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 is
manholes). Manholes are constructed atregular intervals andatpoints where the conduit changes indirection or
gradient or at ajunction. The manhole points are hydraulically not suitable for discharge measurement. Within
the channel at amanhole, an additional difficulty may beencountered when the depth of flow ismore than half
the diameter, as working space is required to be provided for workmen to stand within the manhole. It is
recommended that in deep conduits, a platform or gallery be constructed at vertical intervals of 10m from the
poin[ of view of safe climbing up or going down inthe manhole. The sensing and recording elements must be
capable of withstanding inundation.
(Continued on third cover)
M 15646 (Part 1) :2006
Indian Standard
MEASUREMENT OF FREE SURFACE FLOW
IN CLOSED CONDUITS
PART 1 METHODS
1 SCOPE 4 METHODS OF-FLOW MEASUREMENT
This standard (Part 1) provides the methods of flow Thereare.twobasictypesofflowmeasurements, known
measurements that canbeempl yedinclosed conduits asdirect measurement and indirect measurement.
flowing under partial and fulTflow conditions and
4.1 Direct Measurement
detailstheadvantages anddisadvantages ofthevarious
methods. A direct measurement is one in which the flow is
determined from measurements of various flow
2 REFERENCE
components, that is, it is not inferred. The methods
The following standard contains provisions, which available for direct measurement -arevolumetric and
through reference in this text, constitutes provisions dilution gauging.
ofthis standard. Atthetime ofpublication, the edition
indicated was valid. All standards are subject to 4.1.1 Volumetric Method
revision and parties to agreements based on this 4.1.1.1 A tank of known volume istaken and all the
standard is encouraged to investigate the possibility flow isdirected intothis tank. Thetime forthe tank to
of applying the most recent edition of the standard fill is recorded and the average flow is calculated
indicated below:
accordingly. This method isusually notpractical inan
1SNo. Title underground system.
1191:2003 Hydrometric determinations — 4.1.1.2 Inasystemwhereasumporwet-well has,been
Vocabulary and symbols constructed and apump isavailable to draw from the
sump, the flow can be measured by:
3 DEFINITIONS
a) calculating the depthholume relationship of
For the purposes ofthis standard, definitions given in
the sump, and
IS 1191 and the following shall apply.
b) measuring the water level (using a level

3.1 Permanent Flow-Meter — F low-meter installed recorder) in the wet-well at discrete time

for along period of time (more than 12months) and intervals.
used to measure flow continuously or at discrete time
4.1.1.3 [n a drainage systemwhere, during a storm,
intervals.
thepumpsareworking continuously, thedischarge can
NOTES be measured by:
1 The high costs incurred in the installation of these flow-
meters may be accepted as they are to operate over a long a) monitoring the level inthe wet-well, and
periodoftime. b) monitoring the discharge of pumps under
.2 Themeasurements mademaybe storedinanarchivesystem operation.
and used to examine present trends to forecast future trends
andtodetermine daily operational requirements. 4.-1.1.4 Discharge through a pump or a combination
ofpumps shallbecalibrated by independent fieldtests
3.2 Temporary Flow-Meter — Flow-meter installed
that is by monitoring the time it takes to empty the
for aspecific period oftime (lessthan 12months) and
sump. The volumetric method has the following
used to measure flow continuously or atdiscrete time
advantages and disadvantages:
intervals.
a) Advantages:
NOTE—Theinstallation ofthemeterneedstobesimplewith
minimal orrToassociated civil work. 1) Flow ismeasured directly,
3.3 Portable Flow-Meter — Flow-meters that are 2) Instrumentation can be powered by an
used to obtain instantaneous measurements of flow, electrical supply main, and
or the velocity and depth components thereof. 3) Sensing can be by remote methods.
1
IS 15646 .(Part 1) :2006
b) Disadvantages: doesnotexcessivelyobstructtheflow.Inthiscasealso,
1) Measurements may be erroneous in theratingcurveisdepended ontheslopeandroughness
drainage systems with large sediment oftheconduit. Thetrapezoidal weircanmeasurelower
andtor sludge contents, asthe volume of depths and smaller flows more accurately than the
the sump gets reduced as they get vertical slot weir. This type of weir ismost suited for
deposited atthe bottom. useinlargediameterconduitswherethereisnosurgingl
surcharging.
2) Sediment can also reduce the rate of
discharge of the pumps.
4.1.2 Dilution Gauging
Standard dilution gauging techniques, either constant-
rate injection or integration method canbe employed.
Dilution gaugingisbestsuitedtomeasureinstantaneous
floworatbestaverage flowoverashortperiod(usually L---J
less than 1 h), although it is possible to take
measurements over alongperiod oftime (that isdays) AHdimensions inmetres.
if suitable devices for injection and analysis are
FIG. 1 VERTICALSLOTWEIR
available (for example, radioactive tracers).
The following points shall be kept in view while
employing dilution gauging techniques:
a) itisnecessary to obtain asut%cientdegree of
mixing atthe sampling site, and
b) itisimportant that atracer ischosen which is
not absorbed and which does not react with
the liquid-that isbeing measured.
4.2 Endirect Measurement
Alldimensions inmetres.
An indirect measurement is one in which flow
FIG.2 TRAPEZOIDALWEIR
is derived from measurements of various flow
components. Slope-area and area-velocity techniques 4.2,1.3 USgeological survey meter (USGS meter) (see
are generally employed for indirect measurements of Fig. 3).
discharge. Hydraulic structures like weirs and flumes
Sincedrainage systemsfrequently become surcharged,
are also used for this purpose.

a device wlich can measure flows under both free
4.2.1 Flumes and Weirs surface andpressure flow conditions isnecessary; the
In closed conduits where the liquid contains a high US geological survey meter is such a device. Under
proportion of solid materials and where pressure as free surface flow conditions, the meter actsasa.flume
wellasfi-eesurfaceflowconditions occur,conventional and offers the reliability and accuracy of aflume. The
flumes and weirs cannot be used. Therefore, special accuracy, however, has to be traded off against
devices, designed and built to work under such constriction of the conduit, that is the greater the
conditions, have tobe employed. These are described constriction, thebetter the accuracy. Forpressure flow
in4.2.1.1 to 4.2.1.6. conditions, the meter operates asanozzle and flow is
determined by measuring the pressure upstream and
4.2.1.1 Vertical slot weir
downstream of the nozzle through piezometers.
Thisweir asshown inFig. 1,allows solid materials to
‘Thmetwee
Pienmeter
pass through the installation. The rating curve of the
teps
vertical slot weir depends onthe slope and roughness
oftheconduit. Thiswillbeallthemorerelevant during
low flows throwgh weirs with wide slots. Vertical Q
slot weir is most suited for use in large diameter b.lm
conduits,when there is free surface flow only.
m -d
4.2.1.2 Trapezoidal weir (see Fig. 2) o
sideview “ThroeWtOee-mction
This weir timctions inthe samewayasthevertical slot
weir inthat itallows thepassage ofsolidmaterials and FIG.3 US GEOLOGICASLURVEY.SEWXRFLOW-METER
2
IS 15646 (Part .1):2006
4.2.1.4 University of illinois meter (UI) (see Fig. 4) 4.2.2 Slope Area Method
Thismeter issimilar inperformance totheUSGSmeter In the slope area method, the flow level is measured
andmeasures flowunder bothtiee surfaceandpressure usingconstriction devicessuchasflumesandweirsbut,
flow conditions inanidenticalway.Thetransition from inaddition, alevelsensorwhichmeasures the depth of
freesurfacetopressure flow,however, isnotassmooth flow can be used to calculate the discharge in
forthismeter asfortheUSGS meterowingtotheshape conjunction with the conduit roughness and gradient.
ofthe constriction inthe conduit sofflt. Thetwomostcommonlyusedformulaearegivenbelow.
o
4.2.2.1 Manning formula
where
~ =ma~ing ~gosity coefficient,
Crii”xmbln Thmmcrc.ss-section
R~=hydraulic radius,
FIG.4. UNIVERSITYOFILLINOISSEWERFLOW-METER
S =energy gradient, and
4.2.1.5 Parshallflume (see Fig. 5)
~ =mean velocity.
Thecritical-depth orventuri flumehasbeendeveloped Thelevelmeasurement isbestachieved byusing small
in many different forms. One of the more popular non-mechanical devices such as bubblers, pressure
designs isthe Parshall flume, originally developed in
transducers, resistancelcapacitance and acoustic
1920for irrigation channels. For this device the head/
sensors. The environment within the conduit may be
discharge relationship independent onthethroatwidth.
too corrosive for the use of mechanical devices. The
Again, this device is most suited for large diameter
energy gradient is assumed to be equivalent to the
conduits.
conduit gradient, buttwo level sensors maybeused to
COnvwging Throat Dlwrglng measure both depth andenergy gradient; the unsteady
MaiOn section
state conditions for free surface flow, however, make
themeasurement oftheenergy gradient difficult. Flow
m is calculated by multiplying the cross-sectional area
.a.
ofthe conduit by the mean velocity.
4.2.2.2 Colebrook-white formula
m
l,3b I0,61 0,91
t i ( k 1.255
i+~ —
Wmw
Wrface 14.8R~+ R~y32gR# )

where

Alldimensions inmetres g = acceleration due to gravity,
FIG.5 PARSHALFLLUME k =roughness height,
R~=hydraulic radius,
4.2.1.6 Palmer-Bowlusflume (see Fig. 6)
S =energy gradient,
Another critical-depth flume, the Palmer-Bowlus
v = mean velocity, and
flume, hasrecently beentested for sloping conduits. It
y = kinematic viscosity of the fluid.
exhibits a successful head/discharge calibration for
both sub-critical and supercritical free surface now. The main advantage inusing this formula isthat only
Modifications to the flume design are recommended the fluid level has to be measured. However, several
to improve the head/dischtarge relationship in the disadvantages exist as follows:
transition zone horn free surface to pressure flow.
a) Hydraulic conditions mustbeideal (that isno
Cdtiil
bends or junctions) and the energy and
‘:=%l’hfwt
[’-” i aedul conduit gradient mustbe identical, unlesstwo
Wme7 level sensors are employed in upstream and
F)q downstream reaches;
Throat
b) Roughness value has to be assumed; and
---- mm -*m
c) It is not possibie to measure surcharge flow
=
J unlesstwolevelmonitorsinadjacentmanholes
FIG.6 PALMER-B•WLUFSLUME areemployedtodeterminetheenergygradient.
3
IS 15646 (Part 1) :2006
Until now, the slope area method hasbeen themethod anddownstream isrecorded. Thevelocity iscalculated
most widely used but the velocity area method is using the formula:
becoming increasingly popular.
L 11
~. — ——
4.2.3 Velocity Area Method 2cOse tBAtAB
()
The need to assess the hydraulic performance of a where
drainage system has led-tothe use ofthe velucity area L =acoustic path length from Ato B,
method, where the velocity and the depth of flow are
t~A=downstream night path time,
measured. This enables the.flow tobecalculated when
t*B=upstream flight path time,
surcharge occurs without having to use two level
~ =line velocity, and
sensors some distance apart.
O =angle between the direction of flow and the
4.2.3.1 Electromagnetic method
acoustic path.
This method as shown in Fig. 7, allows direct
In this method the mean velocity is inferred ftom a
measurement of the mean velocity. The principle
linevelocity thisisadisadvantage, buttheuncertainty
of measurement is based on Faraday’s law of
isreduced, whenmore than one acoustic path isused.
electromagnetic induction where the average velocity
of the liquid moving through a magnetic field is 4.2.3.2,2 The second method uses aDoppler velocity
proportional to the electromotive force generated meter to measure the velocity. The meter is mounted
between two electrodes. ina streamlined housing on ametal band or ring and
positioned inthe conduit invert.
Normally, thecoil ismounted abovetheconduit where
there is easy access, that is in the manhole. The Theacousticsignaltransmitted bythemeter isreflected
electrodes which sensethe electromotive force should byairinsuspensionandparticulate matter andreturned
be housed in a preformed insulated pipe section that tothe sensor atadifferent frequency (Doppler effect).
can be positioned in the manhole/pipe configuration. The shitl infrequency between the signal transmitted
The depth of fluid is measured using a bubbler, an andthatreceived isproportional tothe liquid velocity.
The advantage of this method is that velocity is
acoustic sensor or a pressure transducer sensor. The
advantage of this method is that it gives a direct measured inthe flow away from the sensor head, and
thus, the effect of the size andshape ofthe sensor on
measurement of the mean velocity. ,,.
themeasuredvelocity isminimized. Thedisadvantages
An alternative arrangement isto place the coil inside are that it does not measure mean velocity and it is
andtheelectrodes onthesurfaceofasmallstreamlined dependent onthepresence ofairandparticulate matter
sensor. The sensor isfastened to ametal band or ring for itscorrect operation.
which is inturn positioned inthe flow, normally with
4.2.4 Other Methods
thesensor intheinvertoftheconduit.Thedisadvantage

of this arrangement isthat the mea sured velocity has Other flow measuring techniques tested in conduits

to be related to the mean velocity for different depths butfoundtobeeither too expensive or impractical are
of flow. Again, depth measurement is carried out in the following:
order to calculate the cross-sectional area of flow and
a) Optical flow-meter,
hence the flow.
b) Vibrating-mass flow-meter,
c) Thermal-wave flow-meter,
Fieldcoil
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 ELECTROMAGNETFILCOW-METER Sensors can measure level and velocity to a better
accuracy butthe inaccuracies present inmeasuring the
4.2.3.2 Acoustic method cross-sectional areas of old brick or irregular-sha~ed
conduitsmaybelarge.Inaddition,steepgradients, non-
Two methods are available. In both these methods,the
uniform flow and unsteady flow conditions increase
fluidlevelismeasuredtoenabletheflowtobecalculated.
the uncertainty in the calculation of discharge.

4.2.3.2.1 Inthetime-of-flight method, thetimeittakes However,itisnecessarythatmeasurement uncertain~es
forsound pulsestobetransmitted 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 inclosed conduits — Part 1: Methods’.
The other standard inthisseries isIS 15646(Part 2): 2006 ‘Measurement offreesurface flowinclosed conduits:
Part 2Equipment’.
The Committee responsible for the formulation ofthis standard isgiven atAnnex A.
For the purpose of deciding whether aparticular 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 with
IS2:1960 ‘Rulesfor rounding off numerical values (revised)’. The”numberofsignificant places retained inthe
rounded off value should be the same asthat ofthe specified value inthis standard.
,,,,