Parshall flumes - E- courses@VTU
Parshall flumes - E- courses@VTU
Parshall flumes - E- courses@VTU
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Hydraulics Prof. B.S. Thandaveswara<br />
<strong>Parshall</strong> <strong>flumes</strong><br />
<strong>Parshall</strong> <strong>flumes</strong> are devices for the measurement of flow of water in open channels.<br />
They were developed by <strong>Parshall</strong> in 1922 after whom the devices have been<br />
recognized.<br />
The flume consists of a converging section with a level floor, a throat section with a<br />
downward sloping floor, and a diverging section with an upward sloping floor.<br />
Flow<br />
Indian Institute of Technology Madras<br />
X<br />
P D<br />
M B L G<br />
A<br />
Converging<br />
section<br />
B L G<br />
E Crest<br />
Level floor N<br />
hb<br />
a<br />
Crest<br />
0<br />
Alternate 45<br />
wing wall<br />
Y<br />
b C<br />
Throat<br />
section<br />
Diverging<br />
section<br />
PLAN<br />
Dimensions of <strong>Parshall</strong> flume (refer to Table)<br />
x<br />
Section along X-X<br />
hb<br />
hc<br />
K<br />
Water surface, Submerged<br />
Water surface, Free flow<br />
In deviation from the general rule long throated <strong>flumes</strong>, where the upstream head is<br />
measured in the approach channel, <strong>Parshall</strong> <strong>flumes</strong> are calibrated against a piezometric<br />
head, ha, measured at a prescribed location in the converging section. The<br />
"downstream" piezometric head hb is measured in the throat. In practice this is also<br />
used in the cut-throat and H <strong>flumes</strong>.<br />
X
Hydraulics Prof. B.S. Thandaveswara<br />
<strong>Parshall</strong> <strong>flumes</strong> were developed in various sizes. Care must be exercised while<br />
constructing the <strong>flumes</strong> exactly in accordance with structural dimensions (See table) as<br />
the <strong>flumes</strong> are not hydraulic scale models of each other. Since throat length and throat<br />
bottom slope remain constant for series of <strong>flumes</strong> while other dimensions are varied,<br />
each of the 22 <strong>flumes</strong> is an entirely different device. This factor becomes more<br />
important as the size becomes smaller and smaller. For example, it cannot be assumed<br />
that a dimension in the 3.657 m flume will be three times the corresponding dimension<br />
in 1.219 m flume. The discharge can be measured within 3 to 5% if the flume is<br />
constructed properly as per recommended standard dimensions.<br />
On the basis of the throat width, <strong>Parshall</strong> <strong>flumes</strong> have been classified into three main<br />
groups.<br />
(i) Very small - 25.4 mm to 76.2 mm.<br />
(ii) Small 152.40 mm to 2438.4 mm.<br />
(iii) Large 3048 mm to 15240 mm.<br />
Dimensions in mm (Refer to the figure above)<br />
b A a B C D E L G H K M N P R X Y Z<br />
VERY SMALL<br />
1" 25.4 363 242 356 93 167 229 76 203 206 19 - 29 - - 8 13 3<br />
2" 50.8 414 276 406 135 214 254 114 254 257 22 - 43 - - 16 25 6<br />
3" 76.2 467 311 457 178 259 457 152 305 309 25 - 57 - - 25 38 13<br />
SMALL<br />
6" 152.4 621 414 610 394 397 610 305 610 - 76 305 114 900 406 51 76 -<br />
9" 228.6 879 587 864 381 575 762 305 457 - 76 305 114 1080 406 51 76 -<br />
1' 304.8 1372 914 1343 610 845 914 610 914 - 76 381 229 1492 508 51 76 -<br />
1'6" 457.2 1448 965 1419 762 1026 914 610 914 - 76 381 229 1676 508 51 76 -<br />
2' 609.6 1524 1016 1495 914 1206 914 610 914 - 76 381 229 1854 508 51 76 -<br />
3' 914.4 1676 1118 1645 1219 1572 914 610 914 - 76 381 229 2222 508 51 76 -<br />
4' 1219.2 1829 1219 1794 1524 1937 914 610 914 - 76 457 229 2711 610 51 76 -<br />
5' 1524.0 1981 1321 1943 1829 2302 914 610 914 - 76 457 229 3080 610 51 76 -<br />
6' 1828.8 2134 1422 2092 2134 2667 914 610 914 - 76 457 229 3442 610 51 76 -<br />
7' 2133.6 2286 1524 2242 2438 3032 914 610 914 - 76 457 229 3810 610 51 76 -<br />
8' 2438.4 2438 1626 2391 2743 3397 914 610 914 - 76 457 229 4172 610 51 76 -<br />
LARGE<br />
10' 3048 - 1829 4267 3658 4756 1219 914 1829 - 152 - 343 - - 305 229 -<br />
12' 3658 - 2032 4877 4470 5607 1524 914 2438 - 152 - 343 - - 305 229 -<br />
15' 4527 - 2337 7620 5588 7620 1829 1219 3048 - 229 - 457 - - 305 229 -<br />
20' 6096 - 2845 7620 7315 9144 2134 1829 3658 - 305 - 686 - - 305 229 -<br />
25' 7620 - 3353 7620 8941 10668 2134 1829 3962 - 305 - 686 - - 305 229 -<br />
30' 9144 - 3861 7925 10566 12313 2134 1829 4267 - 305 - 686 - - 305 229 -<br />
40' 12192 - 4877 8230 13818 15481 2134 1829 4877 - 305 - 686 - - 305 229 -<br />
50' 15240 - 5893 8230 17272 18529 2134 1829 6096 - 305 - 686 - - 305 229 -<br />
Indian Institute of Technology Madras
Hydraulics Prof. B.S. Thandaveswara<br />
Very small <strong>flumes</strong><br />
The discharge capacity of the very small <strong>flumes</strong> ranges from 0.09 l/s to 32 l/s. The<br />
capacity of each flume overlaps that of the next size by about one-half the discharge<br />
range. The <strong>flumes</strong> must be carefully constructed. The exact dimensions of each flume<br />
are listed in Table. The maximum tolerance on the throat width b equals ± 0.0005 m.<br />
The relatively deep and narrow throat section causes turbulence and makes the hb<br />
gauge difficult to read in the very small <strong>flumes</strong>. Consequently, an additional gauge hc,<br />
located near the downstream end of the diverging section of the flume is introduced.<br />
Under submerged flow conditions, this gauge may be read instead of the hb gauge. The<br />
hc readings are converted to hb readings by using a graph.<br />
Small <strong>flumes</strong><br />
The discharge capacity of the small <strong>flumes</strong> ranges from 0.0015 m 3 /s to 3.95 m 3 /s. The<br />
capacity of each size of flume considerably overlaps that of the next size. The length of<br />
the side wall of the converging section, A of the small <strong>flumes</strong> with 304.8 mm upto<br />
2438.4 mm size flume has a throat width 'A' in meter given by<br />
is the throat width in meter.<br />
Large <strong>flumes</strong><br />
Indian Institute of Technology Madras<br />
b<br />
A = + 1. 219 in which b<br />
2<br />
The discharge capacity of the large <strong>flumes</strong> ranges from 0.16 m 3 /s to 93.04 m 3 /s. The<br />
capacity of each size of flume considerably overlaps that of the next size. The axial<br />
length of the converging section is considerably longer than it is in the small <strong>flumes</strong> to<br />
obtain an adequately smooth flow pattern in the upstream part of the structure. All<br />
<strong>flumes</strong> must be carefully constructed to the dimensions listed, and careful leveling is<br />
necessary in both longitudinal and transverse directions. When gauge zeroes are<br />
established, they should be set so that the ha - , hb-, and hc - gauges give the depth of<br />
water above the level crest - not the depths above pressure taps.<br />
The <strong>Parshall</strong> flume is not to be operated above the 0.60 submergence limit, there is no<br />
need to construct the portion downstream of the throat. The truncated <strong>Parshall</strong> flume<br />
(without diverging section) has the same modular flow. The truncated flume is<br />
sometimes referred to as the "Montana flume". As with the cut - throat flume, the
Hydraulics Prof. B.S. Thandaveswara<br />
Parsahall flume may be used in both the modular and non-modular flow ranges and the<br />
modes of operation are similar. A second head measurement is again required in the<br />
non-modular range of flows and it is usually taken towards the downstream end of the<br />
throat. <strong>Parshall</strong> <strong>flumes</strong> are, however, predominantly used in the modular flow range<br />
(Skogerboe et al,.1967). Recently, <strong>Parshall</strong> <strong>flumes</strong> have gone out of favor due to their<br />
construction complexity and likelihood to trap sediment compared to newer flume<br />
designs. Partial flume is not recommended for measurement of submerged flow as long<br />
throated <strong>flumes</strong> can be designed for 90% of submergence, as the drop in water surface<br />
level required is less in long throated flume particularly modified Broad crested weir<br />
types. If the submergence is not expected in the downstream converging section in the<br />
downstream need not be constructed.<br />
Imprecision of head measurement increases discharge error by 4 to 20 % over the<br />
primary free flow accuracy of 3 to 5 %. The coefficient C and exponent n ranges<br />
between 0.338 to 186.88 in FPS units and 'n' varies from 1.55 to 1.60 in general for the<br />
range of <strong>Parshall</strong> <strong>flumes</strong>.<br />
Indian Institute of Technology Madras
Hydraulics Prof. B.S. Thandaveswara<br />
Indian Institute of Technology Madras<br />
4<br />
3<br />
2<br />
1.5<br />
1<br />
0.8<br />
0.6<br />
0.5<br />
0.4<br />
0.3<br />
0.2<br />
0.15<br />
0.10<br />
0.08<br />
0.06<br />
0.05<br />
0.04<br />
0.03<br />
0.02<br />
0.015<br />
0.01<br />
1' to 8' <strong>Parshall</strong> Flumes<br />
Small <strong>Parshall</strong> Flumes ( 621 to 2438 mm )<br />
95 90 80 60 0.005 0.01 0.02 0.05 0.10 0.20 0.40<br />
Percentage of submergence<br />
Head Loss H through Flume in metre<br />
Head-loss through <strong>Parshall</strong> <strong>flumes</strong>. ( 621 to 2438 mm ) (after Bos)<br />
6'<br />
5'<br />
4'<br />
3'<br />
7'<br />
8'<br />
2'<br />
1'
Hydraulics Prof. B.S. Thandaveswara<br />
Indian Institute of Technology Madras<br />
4<br />
3<br />
2<br />
1.5<br />
1<br />
0.8<br />
0.6<br />
0.5<br />
0.4<br />
0.3<br />
( 10' to 50' <strong>Parshall</strong> Flumes )<br />
95 90 80 60 0.02 0.05 0.10 0.20 0.50 1<br />
Problem<br />
60<br />
50<br />
40<br />
30<br />
20<br />
15<br />
10<br />
8<br />
6<br />
5<br />
Large <strong>Parshall</strong> Flumes<br />
( 3048 to 15240 mm )<br />
Percentage of submergence Head Loss H through Flume in metres<br />
Head-loss through <strong>Parshall</strong> <strong>flumes</strong> ( 3048 to 15240 mm wide )<br />
50'<br />
15'<br />
12'<br />
10'<br />
40'<br />
30' 25'20'<br />
Design a <strong>Parshall</strong> flume for 15 m 3 /s, given the bed width of the approach channel being<br />
6 m. The cross section of the channel is rectangular. Comment on the error.