Basics of Open Channel Flow Measurement Webinar starts

Basics of Open Channel Flow Measurement • Webinar starts at 9: 00 am • Attendee audio will be muted. Please send any questions via chat request. • If you need a PDH certificate, please send chat request, along with your PE# Jon Boykan, PE. Sales Engineer Gilson Engineering Sales

Open Channel Flowmeter Topics • Primary and secondary devices. • Installation considerations. • Sources of error.

Primary Device The flume or weir is referred to as the primary device. The function of the primary device is to BACK UP THE FLOW in the channel. Every primary device has a known relationship between the level of the fluid (upstream of the device) and the flow rate. This allows users to measure level, then compute flow.

Secondary Device The level (flow) measurement device in an open channel flow measurement system is referred to as the secondary device.

Types of Primary Devices • Flumes Parshall Palmer Bowlus Trapezoidal Leopold Lagco H-flumes • Weirs Rectangular (contracted and noncontracted) V-notch

Parshall Flume • Advantages - Self cleaning. - Provides good resolution throughout the entire flow range - Available in a wide range of sizes 1” thru 12’ - Low head loss. • Disadvantages - Relatively expensive. - Not recommended for widely varying flows or low flows - Small throat flumes can trap debris.

Parshall Flume Installations • Good installation • Bad installation

Palmer Bowlus Flume • Advantages - Ease of Installation; especially in an existing pipe invert. - Self cleaning - Minimal head loss. • Disadvantages - Poor turndown. - Not easily installed in a flat-bottom channel.

Trapezoidal flume • Advantages - High turndown (flows down to 1. 0 gpm). - Can pass large debris. - Can handle extremely low flows. Disadvantages - Poor resolution at higher flows. - Limited sizes- 6200 GPM max in largest size.

Leopold Lagco Flume • Obsolete flume, but still a few out there. • Similar in design to Palmer Bowlus

H-Flumes Used primarily for agricultural runoff applications

Rectangular weir (non-contracted) • Advantages - Simple installation (flat plate at end of channel). Disadvantages - High head loss. - Debris buildup behind weir.

Rectangular Weir, Contracted • Advantages - Contraction can increase resolution. • Disadvantages - High head loss. - Requires flow to be backed up (true of all weirs). - Debris buildup behind weir. Formula: Q = 3. 33 ( L - 0. 2 H) H 1. 5 Where: Q=flow of water in cu. ft. per sec. L=width of weir opening in ft. (4 to 8 times H) H=head of weir in ft. (measured at least 6 ft. back of weir opening) a=at least 3 H

Difference between Contracted and Uncontracted Rectangular Weirs 4 ft Rectangular Weirs Flow rate (GPM) level Rectangular weir w/o Rectangular weir with end (ft) end contractions % error 0. 1 189 188 0. 5 0. 2 535 530 1. 0 0. 3 982 968 1. 5 0. 4 1512 1483 2. 0 0. 5 2114 2061 2. 5 0. 6 2778 2696 3. 1 0. 7 3501 3380 3. 6 0. 8 4278 4108 4. 1 0. 9 5104 4876 4. 7 1 5978 5681 5. 2 1. 1 6897 6520 5. 8 1. 2 7858 7389 6. 3 1. 3 8861 8288 6. 9 1. 4 9903 9212 7. 5 10982 10162 8. 1 1. 6 12099 11134 8. 7 13250 12128 9. 3 1. 8 14437 13142 9. 9 15656 14174 10. 5 2 16908 15223 11. 1

Rectangular Weir Design

V-notch weir • Advantages - Good low flow resolution. - High turn-down. • Disadvantages - Susceptible to clogging at notch.

Cipoletti Weir Similar to contracted rectangular weir

Compound Weir Example: V-notch weir for low flow Rectangular weir for high flow Total flow = V-notch flow + Rectangular flow Advantage: • ability to measure high and low flows. Disadvantage: • Need to manually develop level to flow relationship in flowmeter. • Questionable accuracy, especially at transition flow

Other weirs Plan view

Packaged Metering Manholes

Stilling wells

I don’t have a primary device, and it’s gonna cost a lotta money to install one

Water flowing over 1 ft thick concrete wall

Concrete wall = Broad Crested Weir Flow = ? x(width)x. H 1. 5 ^

Effluent from Detritor

30’ rectangular weir without contractions

Flow Meter Functions • Measure level • Transmit level via 4 -20 ma to SCADA, recorder, or chlorinator • Local indication of flow rate • Local indication of total • Pace sampler or remote totalizer via contact output

Types of Open Channel Flowmeters Device to measure level, convert to flow rate • • • Ultrasonic Radar Bubbler Submerged pressure RF capacitance Float (showing my age)

Flow Meter

Level Sensor Placement Parshall Flume

Level Sensor Placement D D/2

Palmer Bowlus flume accessories

Level Sensor Placement Weirs

Flume Accuracy Open Channel Flow accuracy statement from flume manufacturer • • • There are three basic criteria for any open channel primary device (such as a flume) to function properly: 1) The proper type and size of flume must be selected according to the flow range and installation conditions. This flume must be installed level both directions and must maintain dimensional integrity. 2) The upstream conditions for flumes require a low (subcritical) velocity, non-turbulent flow entering the flume. Commonly high velocities will show up in the form of turbulence and surface ripples or waves. High velocity can also result in false low readings as it pushes through the throat faster then normal. 3) The downstream conditions must be adequate and capable of taking the water away from the flume such that the flow does not create backwater submergence beyond the tolerance of that flume. If these three conditions are met, the flume can operate in an accuracy range of 2% to 5%. For the meter, we believe one must also add another 1% to 5% potential additional error to the overall performance of the metering system. We are aware that there are laboratory studies showing that a meter can be achieve accuracy at 1% to 2%. However, in a field installation, even if the meter is working flawlessly, human calibration and other factors can create larger errors. As such, a good flume installation may possibly to be out as much as 5% to 10 % from reality.

Sources of error with Open Channel Measurement • • • Dirt buildup behind weir or flume. Excessive algae Clogging of throat. Submerged flow. Foam on surface. Inadequate straight run. Excessive slope ahead of flume. Flume deformed during installation Improperly sized primary device

Calibration Error Example Flow error from 0. 25” error in reading level Actual level = 0. 10’ (1. 2”) Actual flow = 15. 8 gpm Measured level= 0. 12’ (1. 45”) Measured flow = 20. 99 gpm 33% error in flow from reading level incorrectly by 0. 25”

Other examples of inaccuracy due to ¼” measurement error 3 foot Rectangular weir Head (ft) Flow (GPM) 0. 50 1, 585 0. 52 1, 681 Error: 5. 7% 12" Parshall Flume Head (ft) Flow (GPM) 0. 50 625 0. 52 664 Error: 5. 8%

Measuring level incorrectly On flumes with elevated throats, measure from zero flow level, not bottom of flume

Staff Gauges A staff gauge properly installed will make for a much more accurate calibration than trying to read from a tape measure inserted in the flow stream. Note: many staff gauges read in tenths of foot, not inches

Submerged flow Restriction of flow downstream of primary element can cause submerged flow condition

What does Submerged Flow Look Like?

Open Channel Flow Utility Found at www. gilsoneng. com

Other technologies • Manning formula • Area-velocity meters

Questions? Email any questions to: jb@gilsoneng. com If you need a PDH certificate, please send chat request, along with your PE#