Target Flow Meters

Target flow meters measure flow by detecting the force exerted by flowing fluid against a circular disc (target) suspended in the pipe centerline. The target connects to a force-sensing element outside the pipe that converts drag force into flow rate. These meters typically achieve ±1-2% accuracy across a 10:1 turndown ratio in municipal applications from 4-inch to 48-inch lines. While robust and suitable for dirty water applications, target meters create permanent pressure loss and require periodic recalibration due to target fouling in wastewater service.

• Raw Water Intake Monitoring: Target flow meters excel in large diameter raw water lines (24"-72") feeding treatment plants, measuring flows from 2-50 MGD. The robust target design handles debris and sediment that would damage other meter types. Installed between intake screens and low-lift pumps, these meters provide critical data for treatment process control and regulatory reporting.
• Effluent Discharge Measurement: Commonly installed in 18"-48" effluent lines for final discharge flow measurement and NPDES compliance reporting. The meter's accuracy across wide flow ranges (0.2-20 fps) suits variable discharge patterns. Positioned downstream of chlorine contact chambers and upstream of outfall structures, providing reliable data despite temperature variations and residual chlorine exposure.
• High-Solids Process Streams: Target meters handle activated sludge return lines (6"-24") and waste activated sludge flows where suspended solids reach 8,000-12,000 mg/L. The unobstructed flow path prevents clogging issues common with other technologies, making them ideal for measuring 0.1-5 MGD flows in solids-handling applications.

Daily Operations: Operators monitor flow readings through SCADA systems, comparing target meter data against pump curves and downstream process indicators. No routine adjustments required during normal operation. Typical monitoring includes verifying flow totalization matches expected values and checking for signal drift or erratic readings that indicate potential fouling or mechanical issues.

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Maintenance: Quarterly calibration verification using portable ultrasonic meters or pump performance curves. Semi-annual inspection requires confined space entry for target disc cleaning and force sensor inspection. Maintenance requires millwright skills for mechanical components, I&E technician for electronics. Standard PPE includes gas monitoring, fall protection, and electrical safety equipment for wet well work.

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Troubleshooting: Common failure modes include target disc fouling (causing low readings), force sensor drift from temperature cycling, and cable moisture intrusion. Warning signs include erratic signals, readings inconsistent with pump operation, or gradual signal degradation over weeks. Typical service life: 15-20 years for mechanical components, 8-12 years for electronics in municipal environments.

• Target Disc Assembly: Stainless steel disc (6"-36" diameter) positioned perpendicular to flow creates differential pressure proportional to flow velocity squared. Target sizing typically 0.6-0.8 times pipe diameter for municipal applications. Material selection includes 316SS for standard service, Hastelloy for aggressive environments.
• Force Transmission System: Strain gauge or capacitive sensor converts target deflection force to electrical signal. Sealed transmission prevents moisture ingress in wet well environments. Typical measurement range 0.1-100 lbf for municipal pipe sizes, with 0.25% accuracy across 10:1 flow turndown ratio.
• Flow Conditioning: Upstream straight pipe requirements: 5-10 diameters minimum, 20 diameters preferred after elbows or valves. Flow straightening vanes often installed to eliminate swirl effects. Critical for accuracy in space-constrained municipal installations where piping configurations aren't ideal.
• Transmitter Electronics: Microprocessor-based units provide 4-20mA output, pulse output, and digital communications (Modbus, HART). Temperature compensation standard for municipal applications. NEMA 4X enclosures required for wet environments, intrinsically safe versions available for methane-producing processes.

• Flow Velocity Range: 3-30 ft/s (optimal 8-20 ft/s) to ensure adequate force on target while avoiding excessive pressure loss
• Pipe Size Range: 4-48 inches for municipal applications, with 6-24 inches most common for 0.5-50 MGD plants
• Accuracy: ±0.5% of reading for flows >10% of full scale; ±2-5% at lower flows typical in municipal service
• Pressure Loss: 0.5-5 psi at design flow, varies with target design and flow velocity
• Operating Pressure: Up to 250 psig standard, 600 psig available for high-pressure applications
• Temperature Range: 32-180°F for standard units, extended ranges available
• Turndown Ratio: 10:1 typical, up to 20:1 with advanced electronics
• Minimum Straight Pipe: 10D upstream, 5D downstream for ±0.5% accuracy; reducible to 5D/3D with flow conditioning
• Reynolds Number: >10,000 for reliable operation in turbulent flow regime
• Target Force: 2-200 lbf depending on pipe size and flow rate, with force beam sizing critical for accuracy

• 1. What is the expected flow range and required accuracy? Municipal flows vary 3:1 to 10:1 daily. If peak flow exceeds 20 ft/s or minimum drops below 3 ft/s, consider multiple meters or alternative technology. Inaccurate sizing results in poor low-flow performance or excessive pressure loss.
• 2. Is adequate straight pipe available upstream and downstream? Standard installation requires 10D upstream, 5D downstream. Constrained spaces may require flow straighteners (adding $3,000-8,000) or alternative meter locations. Insufficient straight pipe causes ±5-10% accuracy degradation.
• 3. What are the fluid characteristics and fouling potential? High solids (>500 mg/L TSS) or fibrous materials can interfere with target movement. Wastewater applications may require upstream screening. Heavy fouling requires self-cleaning designs or frequent maintenance affecting lifecycle costs.
• 4. What output signals and communication protocols are required? Standard 4-20mA output versus digital protocols (Modbus, HART) affects control system integration. Wrong selection requires additional signal conversion equipment ($1,000-3,000) or limits future SCADA expansion capabilities.

• Primary: 40 05 23 - Flow Measurement and Control
• Secondary: 40 05 13 - Control and Instrumentation for Process Water and Wastewater Treatment
• Target flow meters fall under process instrumentation for water/wastewater treatment systems.

• Material/Equipment Verification: Verify 316SS construction for potable water service, Confirm AWWA C700 compliance for buried applications, Check transmitter NEMA 4X rating for outdoor installations
• Installation Requirements: Minimum 10D upstream, 5D downstream straight pipe, Concrete thrust blocks for larger sizes (24"+), Electrical classification verification (typically non-hazardous)
• Field Challenges: Target damage during hydrostatic testing, Calibration drift in high-solids applications, Access limitations for maintenance in confined spaces
• Coordination Issues: 12-16 week lead times typical, Factory calibration scheduling coordination required

• Emerson (Micro Motion) - 8700 Series for municipal applications up to 48" diameter
• Yokogawa - digitalYEWFLO AXF Series with field-proven reliability in water plants
• FCI (Fluid Components International) - ST Series target meters popular in smaller municipal facilities
• Eldridge Products - Model TM target meters commonly specified for raw water applications in 12-36" sizes

• Magnetic flow meters - Preferred for clean water applications, 15-20% higher cost but superior accuracy and no pressure drop
• Ultrasonic meters - Better for large diameter (36"+) applications, similar cost to target meters
• Venturi meters - Lower cost option for constant flow applications, but require more straight pipe and permanent pressure loss makes them less attractive for pumped systems

Establish relationships with manufacturer field service early - calibration verification is critical and requires specialized equipment. Consider purchasing spare transmitter electronics for critical applications; the sensing element rarely fails but electronics are vulnerable to lightning. Negotiate factory witness testing for meters >24" diameter. Bundle multiple meters for better pricing and consistent delivery schedules.