Vortex-Shedding Flow Meters

Vortex-shedding flow meters measure volumetric flow by detecting pressure oscillations created when fluid flows around a bluff body obstruction in the pipe. As water passes the shedder bar, alternating vortices form downstream at a frequency directly proportional to flow velocity. These meters typically achieve ±1% accuracy across a 10:1 turndown ratio in municipal applications from 2-inch to 24-inch lines. The primary limitation is sensitivity to flow disturbances and vibration, requiring adequate upstream/downstream straight pipe runs and stable mounting conditions that can be challenging in retrofit installations.

• Raw Water Intake Lines (12-48" pipe): Installed downstream of intake screens with 10-20 pipe diameters upstream straight run. Selected for accuracy in variable flow conditions and minimal pressure loss. Connects between intake pumps and chemical feed systems for dosing control.
• Secondary Effluent Monitoring (6-36" pipe): Positioned after clarifiers before chlorine contact basins. Chosen for reliability with clean water and no moving parts requiring maintenance. Provides flow data for chlorine residual control and NPDES reporting with ±1% accuracy.
• Plant Water/Service Water (2-12" pipe): Measures backwash water, building supply, and washdown systems. Selected for wide turndown ratio (10:1 typical) handling variable demands. Minimal straight pipe requirements make installation feasible in congested plant areas.
• Recirculation Flows (8-24" pipe): RAS/WAS lines and internal recycle streams. Preferred over magnetic meters in applications with occasional air entrainment. Handles mixed liquor without fouling concerns that affect other meter technologies.

Daily Operations: Monitor flow readings on SCADA for process control verification. Check totalizer values against plant water balance calculations. Observe diagnostic alarms for sensor fouling or electronics faults. No routine adjustments required - meters maintain calibration for years without intervention.

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Maintenance: Annual verification using portable ultrasonic meter (±2% comparison acceptable). Quarterly inspection of sensor connections and enclosure seals. Requires confined space entry for large pipe installations. Standard electrical PPE sufficient. Basic instrumentation skills needed for calibration verification and output testing.

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Troubleshooting: Erratic readings indicate air entrainment or upstream flow disturbances. Signal loss suggests sensor failure or loose wiring connections. Low flow cutoff prevents false readings below 1 ft/sec velocity. Typical service life 15-20 years with sensor replacement around year 10. Temperature drift indicates electronics aging requiring recalibration.

• Bluff Body/Shedder Bar: Triangular or trapezoidal obstruction spanning pipe diameter. 316SS construction standard for municipal applications. Sizing based on pipe Reynolds number - typically 0.2-0.3 times pipe diameter. Creates consistent vortex formation across 4-200" pipe sizes.
• Vortex Sensor: Piezoelectric or capacitive sensor detecting pressure oscillations. Dual sensors common for redundancy in critical applications. Temperature-compensated electronics handle -40°F to 180°F municipal ranges. Frequency output proportional to flow velocity.
• Signal Conditioning Electronics: Converts sensor frequency to 4-20mA output with pulse/digital communications. NEMA 4X enclosures standard. Battery backup maintains calibration during power outages. Built-in diagnostics monitor sensor health and signal quality.
• Flow Straightening Elements: Internal or external straightening vanes when adequate straight run unavailable. Reduces installation requirements from 20D to 5D upstream. Particularly valuable in retrofit applications with space constraints.

• Flow Range: 0.5-30 ft/s velocity (Reynolds number >20,000 minimum). For 6-inch lines: 200-4,500 GPM; 12-inch: 800-18,000 GPM; 24-inch: 3,200-72,000 GPM typical municipal ranges.
• Pressure Rating: 150-300 PSIG standard for municipal water systems. Wastewater applications typically 150 PSIG maximum due to lower system pressures.
• Accuracy: ±0.75% to ±1.0% of reading over 10:1 turndown ratio. Best performance between 3-25 ft/s velocity range.
• Temperature Range: -40°F to 400°F standard, though municipal applications rarely exceed 100°F.
• Pipe Requirements: Minimum 10D upstream, 5D downstream straight pipe runs. Critical for accuracy - insufficient straight runs cause 2-5% measurement errors.
• Materials: 316L stainless steel wetted parts standard. Hastelloy C-276 for aggressive wastewater applications.
• Signal Output: 4-20mA with HART, Modbus, or Foundation Fieldbus communication protocols.
• Turndown Ratio: 10:1 to 30:1 depending on manufacturer. Rosemount 8800D series offers 30:1; Yokogawa digitalYEWFLO 25:1 typical.

• What is the minimum and maximum flow velocity range? Must maintain 3-30 ft/s for reliable vortex generation. Below 3 ft/s, weak signal causes measurement drift. Above 30 ft/s, excessive pressure drop and potential cavitation. Need 6-month flow data to verify turndown requirements.
• Are there upstream/downstream flow disturbances within 15 pipe diameters? Elbows, reducers, valves, or pumps create turbulence affecting accuracy by 3-8%. Insufficient straight pipe runs require flow conditioners, adding $3,000-8,000 to project cost. Survey existing piping layout during design phase.
• What is the minimum process pressure at operating temperature? Must maintain pressure 2x vapor pressure plus vortex-induced pressure drop to prevent cavitation. Typically requires 25-50 PSIG minimum depending on temperature. Inadequate pressure causes measurement instability and potential meter damage.
• Is the fluid single-phase with <5% entrained air/solids? Vortex meters fail with two-phase flow or high solids content. Wastewater applications require upstream screening and degritting. Consider electromagnetic meters for high-solids applications instead.

• Primary: 40 05 19 - Water Meters and Gages
• Secondary: 46 05 19 - Water Treatment Equipment Instrumentation
• Vortex-shedding meters fall under water system instrumentation for municipal applications, covering both potable water distribution and process water measurement within treatment facilities

• Material/Equipment Verification: Verify wetted materials match process chemistry (316SS standard, Hastelloy for aggressive conditions). Confirm temperature/pressure ratings exceed design conditions by 25%
• Installation Requirements: Enforce 15D upstream, 5D downstream straight-run minimums. Specify vibration isolation pads for large diameter installations. Require bypass provisions for maintenance access
• Field Challenges: Coordinate electrical rough-in before concrete pours. Plan lifting equipment access for meters >24" diameter
• Coordination Issues: 12-16 week lead times typical for custom configurations

• Yokogawa - digitalYEWFLO vortex meters, strong municipal presence with 4-48" sizes
• Endress+Hauser - Proline Prowirl series, popular for wastewater applications
• Emerson/Rosemount - 8800D series, widely specified by consultants
• ABB - ProcessMaster FEV600, competitive pricing for municipal budgets
• All offer NEMA 4X enclosures and Hart communication standard for municipal SCADA integration

• Magnetic flowmeters - Preferred for dirty water applications, no moving parts, 20-30% higher cost but superior reliability in solids-laden flows
• Ultrasonic clamp-on meters - Ideal for retrofit applications, 40% lower installed cost, accuracy limitations in turbulent flow
• Differential pressure (orifice plates) - Lowest first cost option, requires more maintenance, pressure loss concerns in low-head applications

Establish relationships with manufacturer field service reps early - they provide invaluable commissioning support and can expedite warranty claims. Specify factory calibration certificates rather than field calibration to avoid 2-week delays during startup. Consider purchasing spare electronics modules for critical applications; they're 15-20% of total meter cost but eliminate extended downtime. Always verify actual pipe ID matches meter bore sizing - schedule 40 vs. schedule 80 discrepancies cause significant accuracy issues.