A flume in an open or closed conduit is similar to a venturi in a fully charged pipeline. The channel is constructed to cause a decrease in pressure and an increase in velocity. A common characteristic of flumes is the formation of a standing wave close to the outlet of the constricted section which is why they are sometimes called standing wave flumes. There is no “standard” flume; each is individually designed.
1. Meter Designs: The Parshall flume and the Palmer-Bowlus flume are two designs currently in common use.
OPERATING PRINCIPLES: In flumes, the measuring section can be designed with a contraction of the sidewalls, by a raised section, or hump, of the channel bed, or by a combination of both contraction and hump. These changes cause a measurable difference in head pressure between the converging section and the throat. The difference in these pressures is used to calculate flow rate.
1. Parshall Flume: The Parshall flume is a measuring flume that is well-adapted to use in irrigation canals and ditches. This flume consists of an entrance section with converging walls and level floor, a throat section with parallel walls and a downstream sloping floor, and an outlet section with diverging walls and rising floor (Figure-7-4). The crest is the line where the level floor of the converging entrance section joins the inclined floor of the throat section. Still wells may be installed alongside a flume to view water level if flume surface is frozen over or water surface is thick with debris.
Parshall Flume
FIGURE 7-4. Parshall Flume
2. Palmer-Bowlus Flume: The Palmer-Bowlus flume is used primarily in circular conduits such as storm and sanitary sewers. Both sidewall and bottom contraction are used (Figure 7-5).
LIMITATIONS: The major limitation of flumes is that they are designed for individual locations, such as general information metering of storm and sanitary sewers or irrigation purposes.
INSTALLATION: Since flumes are not off-the-shelf meters, installation is governed by design. Upstream and downstream cross-sections and lengths are in the design specifications for each location. Follow design specifications for grade height and zeroing of still wells, if used.
MAINTENANCE: All types of head and area meters are used for open flow measurement. Their operation depends on the absence of interference at the discharge opening.
Palmer-Bowlus Flume
FIGURE 7-5. Palmer-Bowlus Flume
1. Monthly Maintenance: Make sure the throat section is unobstructed and clean. Remove algae growth.
2. Quarterly Maintenance: Clean the stilling well and make sure connecting pipe is clear.
ACCURACY AND RELIABILITY: Accuracy of the Palmer-Bowlus flume is +3%; this error coupled with the level measuring device error gives a combined accuracy of approximately ±10%. The reliability of flumes is not a quantitative characteristic since it must be kept free of debris to function correctly. Flumes are less accurate than a weir.
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