Flow Metering Weirs

Flow metering weirs are engineered overflow structures that measure flow rates by creating a controlled hydraulic relationship between water level and discharge. Water flows over a precisely shaped notch (typically V-notch, rectangular, or trapezoidal), with upstream head measurement converted to flow rate using established hydraulic equations. V-notch weirs achieve ±2-5% accuracy across flow ranges from 0.1 to 50 MGD in municipal applications. The primary trade-off is significant head loss (typically 6-18 inches) which can limit gravity flow systems and increase pumping requirements in constrained elevation profiles.

• Primary Clarifier Effluent: V-notch weirs measure clarified water flow (0.1-15 MGD) entering secondary treatment. Selected for accuracy at variable flows and self-cleaning properties. Upstream connection from clarifier launders; downstream feeds aeration basins or trickling filters.

• Plant Effluent Monitoring: Rectangular weirs measure final discharge (0.5-50 MGD) for regulatory compliance and NPDES reporting. Chosen for wide flow range capability and ease of calibration. Located in effluent channel before outfall, often with upstream flow straightening section.

• Return Activated Sludge (RAS): Sharp-crested weirs measure RAS flow (0.2-10 MGD) in gravity return lines. Selected for low head loss and visual flow verification. Positioned in RAS channels between secondary clarifiers and aeration basins.

• Scum Removal Systems: Small V-notch weirs (0.01-0.5 MGD) measure scum box overflow rates. Used for process control and equipment cycling verification in primary and secondary clarifiers.

Daily Operations: Operators visually inspect weir crest for debris accumulation and verify level sensor readings against observed flow conditions. Flow totalizer readings are recorded for daily reports and compared to plant influent for mass balance verification. No routine adjustments required during normal operation.

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Maintenance: Monthly cleaning of weir crest and approach channel to remove accumulated debris and biofilm. Level sensors require quarterly calibration verification and annual replacement of wearing components. Safety requires confined space procedures for below-grade installations. Basic mechanical skills sufficient for routine maintenance tasks.

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Troubleshooting: Erratic readings typically indicate debris on weir crest or sensor fouling. Consistently low readings suggest approach channel sediment buildup or downstream submergence. Sensor drift develops gradually over 12-18 months. Service life exceeds 20 years for stainless steel weir plates; sensors require replacement every 5-7 years in typical municipal environments.

• Weir Plate: Stainless steel 316L construction with machined crest edges. Thickness 0.25-0.5 inches for structural integrity. V-notch angles typically 90° or 60°; rectangular widths 1-20 feet. Selection based on expected flow range and accuracy requirements.

• Approach Channel: Concrete or fiberglass structure providing 10:1 length-to-width ratio upstream of weir. Includes flow straightening baffles and sediment collection zone. Sized for approach velocities under 1 fps to ensure accurate measurement.

• Level Measurement System: Ultrasonic or bubbler sensors measuring head above weir crest. Mounted 6-12 inches upstream of weir plate. Range selection based on maximum expected head (typically 0.5-4 feet for municipal applications).

• Downstream Collection: Concrete channel or flume directing measured flow to next process. Designed with adequate freeboard and proper hydraulic profile to prevent submergence of weir crest.

• Flow Range: 0.1-50 MGD (0.15-77 cfs) for municipal applications. V-notch weirs handle 0.05-5 MGD; rectangular weirs 1-50 MGD; Parshall flumes 0.1-100 MGD.

• Head Loss: V-notch weirs require 6-24 inches; rectangular weirs 3-12 inches; Parshall flumes 2-6 inches. Critical for gravity systems with limited elevation.

• Accuracy: ±2-5% typical for properly installed weirs; ±1-3% for Parshall flumes with upstream flow conditioning. Accuracy degrades with approach velocity >1 fps or downstream submergence >70%.

• Approach Channel: Minimum 10× weir width upstream; velocity <1.5 fps; depth >2× maximum head over weir. Requires 20-50 feet straight approach for accurate measurement.

• Materials: 316SS standard for potable water; concrete/FRP acceptable for wastewater. Thickness: 0.25-0.5 inches for steel plates; 6-12 inches for concrete structures.

• Installation Requirements: Level tolerance ±0.01 feet; perpendicular to flow ±1 degree. Ventilation required for suppressed weirs. Downstream clearance prevents submergence effects.

• What flow turndown ratio is required? V-notch weirs achieve 10:1 turndown; rectangular weirs 3-4:1; Parshall flumes 5-8:1. Insufficient turndown causes measurement errors during low flows, critical for compliance monitoring and process control accuracy.

• Can the system tolerate head loss? Weirs require 6-24 inches head loss versus 2-6 inches for Parshall flumes. Excessive head loss in gravity systems can cause upstream flooding or require costly pump station modifications. Need accurate hydraulic grade line analysis.

• What accuracy is needed for the application? Compliance monitoring requires ±5% accuracy; billing applications need ±2%; process control can accept ±10%. Higher accuracy demands better approach conditions, precise installation, and potentially more expensive Parshall flumes over simple weirs.

• Is downstream submergence possible? Submergence >70% destroys weir accuracy; Parshall flumes tolerate 95% submergence with correction factors. Requires downstream hydraulic analysis including storm conditions and potential backwater effects from receiving streams or downstream restrictions.

• MasterFormat 40 05 23 - Flow Measurement and Control: Primary section covering weirs, flumes, and associated flow measurement devices for water/wastewater treatment facilities. May also reference 33 30 00 for site utilities integration.

• Material/Equipment Verification: Verify weir plate thickness (typically 0.25" minimum stainless steel), Confirm approach channel dimensions meet 4:1 length requirements, Check nappe ventilation provisions

• Installation Requirements: Level installation critical - ±0.01" tolerance typical, Upstream flow conditioning length: 10x channel width minimum, Downstream submergence limits: <70% for accurate measurement

• Field Challenges: Debris accumulation at weir crest requires regular cleaning access, Freeze protection in northern climates, Accurate level measurement in turbulent conditions

• Coordination Issues: SCADA integration for level transmitters (4-20mA standard), Electrical rough-in for lighting/instrumentation, Lead Times: 8-12 weeks for custom stainless fabrication

• Openchannelflow: Fiberglass weir boxes, Model WB-series for 0.1-10 MGD applications

• Thel-Mar Engineering: Stainless steel Palmer Bowlus flumes, PB-series for 0.5-50 MGD

• Badger Meter: ModMAG electromagnetic systems with weir integration

• Plasti-Fab: HDPE weir structures, custom fabrication for 1-25 MGD municipal plants

• Electromagnetic Flowmeters: Higher accuracy (±0.5% vs ±2-5% for weirs), no head loss, but 3x cost. Preferred for critical billing applications.

• Ultrasonic Flow Measurement: Non-contact operation, good for existing channels, ±1-3% accuracy, moderate cost.

• Parshall Flumes: Better debris handling than weirs, self-cleaning action, but require more space and higher construction costs. Weirs typically 20-30% less expensive than flume alternatives.

Installation Insight: Always specify removable weir plates - permanent installations become maintenance nightmares when calibration drift occurs. Thel-Mar's bolt-in plates save thousands in future modifications.

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Manufacturer Relations: Establish relationships with local fabricators for quick repairs; national manufacturers often have 6+ week lead times for replacement parts.

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Cost-Saving: Consider HDPE construction for non-critical applications - 40% cost reduction versus stainless steel with similar accuracy.