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Hydraulics

Prof. B.S. Thandaveswara

Parshall flumes

Parshall flumes are devices for the measurement of flow of water in open channels. They were developed by Parshall in 1922 after whom the devices have been recognized. The flume consists of a converging section with a level floor, a throat section with a downward sloping floor, and a diverging section with an upward sloping floor.

B L G

Flow

E

Water surface, Submerged

Level floor Y

Crest

N x

Water surface, Free flow

K

Section along X-X

M B L G

hb

hc a

X P D A

Crest

X b C

hb Alternate 45 wing wall

0

Converging section

Throat section

Diverging section

Dimensions of Parshall flume (refer to Table)

PLAN

In deviation from the general rule long throated flumes, where the upstream head is measured in the approach channel, Parshall flumes are calibrated against a piezometric head, ha, measured at a prescribed location in the converging section. The "downstream" piezometric head hb is measured in the throat. In practice this is also used in the cut-throat and H flumes.

Indian Institute of Technology Madras

Hydraulics

Prof. B.S. Thandaveswara

Parshall flumes were developed in various sizes. Care must be exercised while constructing the flumes exactly in accordance with structural dimensions (See table) as the flumes are not hydraulic scale models of each other. Since throat length and throat bottom slope remain constant for series of flumes while other dimensions are varied, each of the 22 flumes is an entirely different device. This factor becomes more important as the size becomes smaller and smaller. For example, it cannot be assumed that a dimension in the 3.657 m flume will be three times the corresponding dimension in 1.219 m flume. The discharge can be measured within 3 to 5% if the flume is constructed properly as per recommended standard dimensions. On the basis of the throat width, Parshall flumes have been classified into three main groups. (i) Very small - 25.4 mm to 76.2 mm. (ii) Small 152.40 mm to 2438.4 mm. (iii) Large 3048 mm to 15240 mm.

b 1" 2" 3" 6" 9" 1' 1'6" 2' 3' 4' 5' 6' 7' 8' 10' 12' 15' 20' 25' 30' 40' 50' 25.4 50.8 76.2 152.4 228.6 304.8 457.2 609.6 914.4 1219.2 1524.0 1828.8 2133.6 2438.4 3048 3658 4527 6096 7620 9144 12192 15240 A 363 414 467 621 879 1372 1448 1524 1676 1829 1981 2134 2286 2438 a 242 276 311 414 587 914 965 1016 1118 1219 1321 1422 1524 1626 1829 2032 2337 2845 3353 3861 4877 5893 B 356 406 457 610 864 1343 1419 1495 1645 1794 1943 2092 2242 2391 4267 4877 7620 7620 7620 7925 8230 8230 C 93 135 178 394 381 610 762 914 1219 1524 1829 2134 2438 2743 3658 4470 5588 7315 8941 10566 13818 17272 Dimensions in mm (Refer to the figure above) D E L G H K VERY SMALL 167 229 76 203 206 19 214 254 114 254 257 22 259 457 152 305 309 25 SMALL 397 610 305 610 76 575 762 305 457 76 845 914 610 914 76 1026 914 610 914 76 1206 914 610 914 76 1572 914 610 914 76 1937 914 610 914 76 2302 914 610 914 76 2667 914 610 914 76 3032 914 610 914 76 3397 914 610 914 76 LARGE 4756 1219 914 1829 152 5607 1524 914 2438 152 7620 1829 1219 3048 229 9144 2134 1829 3658 305 10668 2134 1829 3962 305 12313 2134 1829 4267 305 15481 2134 1829 4877 305 18529 2134 1829 6096 305 M 305 305 381 381 381 381 457 457 457 457 457 N 29 43 57 114 114 229 229 229 229 229 229 229 229 229 343 343 457 686 686 686 686 686 P 900 1080 1492 1676 1854 2222 2711 3080 3442 3810 4172 R 406 406 508 508 508 508 610 610 610 610 610 X 8 16 25 51 51 51 51 51 51 51 51 51 51 51 305 305 305 305 305 305 305 305 Y 13 25 38 76 76 76 76 76 76 76 76 76 76 76 229 229 229 229 229 229 229 229 Z 3 6 13 -

Indian Institute of Technology Madras

Hydraulics

Prof. B.S. Thandaveswara

Very small flumes

The discharge capacity of the very small flumes ranges from 0.09 l/s to 32 l/s. The capacity of each flume overlaps that of the next size by about one-half the discharge range. The flumes must be carefully constructed. The exact dimensions of each flume are listed in Table. The maximum tolerance on the throat width b equals ± 0.0005 m. The relatively deep and narrow throat section causes turbulence and makes the hb gauge difficult to read in the very small flumes. Consequently, an additional gauge hc, located near the downstream end of the diverging section of the flume is introduced. Under submerged flow conditions, this gauge may be read instead of the hb gauge. The hc readings are converted to hb readings by using a graph.

Small flumes

The discharge capacity of the small flumes ranges from 0.0015 m3/s to 3.95 m3/s. The capacity of each size of flume considerably overlaps that of the next size. The length of the side wall of the converging section, A of the small flumes with 304.8 mm upto 2438.4 mm size flume has a throat width 'A' in meter given by A = is the throat width in meter.

b + 1.219 in which b 2

Large flumes

The discharge capacity of the large flumes ranges from 0.16 m3/s to 93.04 m3/s. The capacity of each size of flume considerably overlaps that of the next size. The axial length of the converging section is considerably longer than it is in the small flumes to obtain an adequately smooth flow pattern in the upstream part of the structure. All flumes must be carefully constructed to the dimensions listed, and careful leveling is necessary in both longitudinal and transverse directions. When gauge zeroes are established, they should be set so that the ha-, hb-, and hc- gauges give the depth of water above the level crest - not the depths above pressure taps. The Parshall flume is not to be operated above the 0.60 submergence limit, there is no need to construct the portion downstream of the throat. The truncated Parshall flume (without diverging section) has the same modular flow. The truncated flume is sometimes referred to as the "Montana flume". As with the cut - throat flume, the

Indian Institute of Technology Madras

Hydraulics

Prof. B.S. Thandaveswara

Parsahall flume may be used in both the modular and non-modular flow ranges and the modes of operation are similar. A second head measurement is again required in the non-modular range of flows and it is usually taken towards the downstream end of the throat. Parshall flumes are, however, predominantly used in the modular flow range (Skogerboe et al,.1967). Recently, Parshall flumes have gone out of favor due to their construction complexity and likelihood to trap sediment compared to newer flume designs. Partial flume is not recommended for measurement of submerged flow as long throated flumes can be designed for 90% of submergence, as the drop in water surface level required is less in long throated flume particularly modified Broad crested weir types. If the submergence is not expected in the downstream converging section in the downstream need not be constructed. Imprecision of head measurement increases discharge error by 4 to 20 % over the primary free flow accuracy of 3 to 5 %. The coefficient C and exponent n ranges between 0.338 to 186.88 in FPS units and 'n' varies from 1.55 to 1.60 in general for the range of Parshall flumes.

Indian Institute of Technology Madras

Hydraulics

Prof. B.S. Thandaveswara

4 3 2 1.5 1 0.8 0.6 0.5 0.4 0.3 0.2 0.15 0.10 0.08 0.06 0.05 0.04 0.03 0.02 0.015 0.01

1' to 8' Parshall Flumes Small Parshall Flumes ( 621 to 2438 mm )

8' 7' 6' 5' 4' 3' 2' 1'

95

90

80

60

0.005

0.01

0.02

0.05

0.10

0.20

0.40

Percentage of submergence

Head Loss

H through Flume in metre

Head-loss through Parshall flumes.

( 621 to 2438 mm )

(after Bos)

Indian Institute of Technology Madras

Hydraulics

Prof. B.S. Thandaveswara

60 50 40 30 20 15 10 8 6 5 4 3 2 1.5 1 0.8 0.6 0.5 0.4 0.3

Large Parshall Flumes ( 10' to 50' Parshall Flumes ) ( 3048 to 15240 mm )

50'

40'

30' 25' 20'

15' 12' 10'

95

90

80

60

0.02

0.05

0.10

0.20

0.50

1

Percentage of submergence Head Loss

H through Flume in metres

Head-loss through Parshall flumes

( 3048 to 15240 mm wide )

Problem Design a Parshall flume for 15 m3/s, given the bed width of the approach channel being 6 m. The cross section of the channel is rectangular. Comment on the error.

Indian Institute of Technology Madras

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