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Flow measurements.

Flow in Open Channels

Measuring the flow of liquids is done in open channels and involves the use of a weir or Parshall flume or various modifications of these devices.

Weirs and Instruments. The Cipolletti weir is trapezoidal in shape. The sides are inclined so that they compensate for the effect of contraction; when the inclination of the ends is 1 horizontal to 4 vertical, the contraction effect is almost negated. The primary application of these devices is that it aids velocity regulation required in grit channels.

Instruments to measure, indicate, and record the height of water on the weir are manufactured by American Sigma Inc.; Badger Meter, Inc., Utility Div.; Bailey Controls Co.; Bristol Babcock, Inc.; Fischer & Porter Co.; Instrumentation Northwest Inc.; Leeds & Northrup Unit of General Signal; F.B. Leopold Co.; Magnetoelastic Devices Inc.; Milltronics, Inc.; N-Con Systems, Inc.; Solinst Canada Ltd.; and Stevens Water Monitoring Systems, Div. of Leupold & Stevens, Inc. In these, either the movements of a float rotates a drum carrying a chart across which a pen, driven by a chronometer travels at a uniform rate; or the drum is rotated by the chronometer with the movement of the pen controlled by the float. The recorded data may also be electrically, electronically, or pneumatically transmitted to a distant point, such as a central recording or control panel. Measuring weirs are available from Environmental Services & Products, Inc.; and Thel-Mar Co.

Electromagnetic and ultrasonic gauges for measuring flow in open channels are available from American Sigma Inc.; Isco, Inc., Environmental Div.; Magnetoelastic Devices Inc.; Marsh-McBirney, Inc.; Micro-Trak Systems, Inc.; Milltronics, Inc.; Quality Control Equip. Co.

Flumes and Instruments. The Parshall flume is constructed with ends to correspond to the width and height of the channel in which the flow is measured, but it varies in cross-section to produce a contraction near the influent end. The flow is gauged by the head differential caused by the contraction.

For measuring this, American Sigma; Bailey Controls Co.; Drexelbrook Engineering Co.; Environmental Services & Products, Inc.; Fischer & Porter Co.; Isco, Inc., Environmental Div.; Leeds & Northrup Unit of General Signal; F.B. Leopold Co.; Milltronics, Inc.; J.L. Rochester Co.; Raven Industries, Inc.; Stevens Water Monitoring Systems, Div. of Leupold & Stevens, Inc. and others offer an indicating recording and integrating meter, which can be placed at the site or in a remote location by use of a telemetering system.

Parshall flumes in a wide range of throat dimensions are prefabricated for insertion in a channel section and are made by Badger Meter, Inc., Utility Div.; Fischer & Porter Co.; Kenco Plastics; Leeds & Northrup Unit of General Signal; F.B. Leopold Co.; TN Technologies; Plasti-Fab Inc.; and Warminster Fiberglass.

Variations of the Parshall flume as differential producers include the parabolic flume and Kennison nozzle, in that they are open channel measuring devices.

Prepackaged Systems. For installations where flow data is to be recorded over a long period of time or for a permanent installation, a packaged manhole system can be considered. The manholes are usually composed of corrosion-resistant materials (such as fiberglass) and are designed to be incorporated in the collection system. Measuring flumes are built into the structure as are access ladders and other accessories. The types of flow meters can be specified and the entire package can be custom fabricated if necessary. Firms supplying this type of package include Plasti-Fab Inc.

Flow in Pipes

If the flow of wastewater is through a pipe under pressure, a common and accurate method of flow measurement is by determining the velocity head lost by passage through a constriction or throat in the pipe. The head lost bears a precise and calculable relation to the rate of flow. Such constrictions may take the form of the venturi tube, or of a nozzle or "thin plate" orifice. These are differential producers, which, with a secondary instrument, comprise a "flow meter."

With use of such devices with wastewater, particularly untreated flows, problems occur in that ports employed for measurement of pressure differential or the constricted area become fouled or clogged.

For that reason, measuring instruments not relying on ports or constrictions have been developed. Among them are propeller meters in which a propeller or turbine is turned by the flow in the line. They can be integrated with registers for rate of flow indication and volume totalizing. Another is the magnetic flow meter, consisting of non-magnetic tubes through which the liquid flows, electrical insulating liners, and electromagnets, which induce a megnetic field through the tubes. As the magnetic field alternates, a voltage is produced across the conductor (the liquid in the pipe) which is proportional to the velocity of the conductor.

This voltage is measured by two electrodes within the tube and is transmitted for direct reading. Since the inside of the tube is unobstructed there is little pressure drop and solids will not cause clogging. The ultrasonic flow meter has a similar advantage in that they are not subject to clogging and transducers and electronic circuitry can be arranged to minimize influence by stray or otherwise unwanted sortie signals.

Flow meters of these types are further discussed in Section C-9, along with various manufacturers. Rate indicators and flow totalizers with gauges or other read-out devices are discussed in Section C-9.

Bourdon tube gauges suitable for use on sludge lines are furnished by the Bristol Babcock Inc.

Manhole Meters. With emphasis on infiltration/inflow detection and control, portable readout meters with sensors designed to operate in manholes or insert sewer pipe sections have been made available. Manufacturers include ADS Environmental Services, Inc.; American Sigma Inc.; Badger Meter, Inc., Utility Div.; Isco, Inc., Environmental Div.; Marsh-McBirney, Inc.; Milltronics, Inc.; Ramsey Lake Industrial Ltd.; Stevens Water Monitoring Systems, Div. of Leupold & Stevens, Inc.; and TN Technologies.
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Title Annotation:Water Pollution Control
Publication:Public Works
Date:Apr 15, 1995
Words:955
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