Liquid Chemical Weighing Systems

Liquid Chemical Weighing Systems provide precise gravimetric dosing of treatment chemicals by continuously measuring chemical weight loss from supply tanks during feed operations. These systems use load cells mounted beneath chemical storage tanks to monitor real-time weight changes, automatically calculating actual chemical feed rates independent of pump performance variations. Typical municipal installations achieve ±2% dosing accuracy across flow ranges from 0.1 to 500 gallons per day, significantly outperforming volumetric feed systems that can drift ±10-15% due to pump wear, viscosity changes, and calibration issues. The primary limitation is higher capital cost compared to basic metering pumps, typically adding $15,000-40,000 per chemical feed point depending on tank size and instrumentation complexity.

• Polymer Feed for Dewatering Operations: Liquid weighing systems precisely meter concentrated polymer solutions (0.25-2.0% active) to belt filter presses, centrifuges, and gravity thickeners. Systems typically handle 5-50 gph flows with ±0.5% accuracy, connecting upstream from polymer makeup units and downstream to injection points
• Sodium Hypochlorite Disinfection: Municipal plants use weighing systems for primary and secondary disinfection, handling 12.5% sodium hypochlorite at flows of 2-25 gph for 1-20 MGD facilities. Systems connect downstream from bulk storage and upstream of contact chambers
• Phosphoric Acid pH Control: Weighing systems deliver 75-85% phosphoric acid for biological nutrient removal processes, typically 1-15 gph flows. Connected upstream from rapid mix chambers, these systems provide precise pH adjustment for enhanced biological phosphorus removal

Daily Operations: Operators monitor tank levels via SCADA displays, checking for consistent weight loss rates that match programmed feed rates. Daily tasks include verifying pump operation, recording chemical usage, and adjusting feed rates based on process demands. Systems require minimal operator intervention once properly calibrated, with most adjustments made remotely through the plant control system.

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Maintenance: Monthly calibration verification using certified test weights, quarterly pump maintenance including diaphragm inspection and valve cleaning. Operators require basic electrical safety training and chemical handling certification. Load cell calibration requires specialized technician every 6-12 months. PPE includes chemical-resistant gloves, safety glasses, and appropriate respiratory protection based on chemical hazards.

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Troubleshooting: Common failures include load cell drift (indicated by inconsistent readings), pump cavitation from low suction pressure, and control system communication errors. Warning signs include erratic feed rates, pump cycling, or weight readings that don't correlate with visual tank levels. Typical service life: load cells 10-15 years, pumps 3-5 years, tanks 15-20 years with proper chemical compatibility.

• Load Cell Assembly: Strain gauge load cells rated 50-500 lbs capacity with 0.02% accuracy, constructed with stainless steel or aluminum alloy
• Chemical Storage Tank: HDPE, fiberglass, or stainless steel tanks ranging 50-500 gallons, mounted on load cell platform
• Metering Pump: Progressive cavity or diaphragm pumps rated 0.1-50 gph with variable speed drives
• Control System: PLC-based controllers with 4-20mA outputs, featuring weight-loss algorithms and flow totalizers

• Flow Rate Capacity: 1-500 gpm per feed line, with typical municipal systems handling 5-150 gpm based on plant size and chemical type
• Weighing Accuracy: ±0.1% to ±0.25% of full scale for custody transfer applications; ±0.5% acceptable for process control
• Tank Capacity: 500-15,000 gallons typical, sized for 7-30 days chemical storage depending on delivery frequency and consumption rates
• Operating Pressure: Atmospheric to 150 psig discharge capability. Suction lift capacity 10-25 feet with positive displacement pumps
• Temperature Range: -20°F to 120°F ambient operation with heated enclosures for cold climates
• Loading Rate: 50-300 gpm truck unloading capability with 4-6 inch connections
• Control Integration: 4-20mA analog outputs, Modbus/Ethernet communication protocols for SCADA integration

• Batch vs. Continuous Weighing Configuration: Batch systems (100-5,000 gallon batches) provide higher accuracy (±0.1%) but require storage buffering. Continuous systems offer simpler operation but lower accuracy (±0.5%)
• Single vs. Multiple Load Cell Design: Single load cell systems cost $15,000-25,000 less but create single point of failure. Multiple load cell configurations (3-4 cells typical) provide redundancy and better accuracy
• Integral Mixing vs. External Dilution: Integral mixing tanks require 500-2,000 gallon minimum volumes with 1-5 HP mixers. External dilution systems need accurate flow measurement (±2%) and ratio control
• Truck Loading Interface Requirements: Dedicated truck scales ($75,000-150,000) versus portable systems ($15,000-35,000) depends on delivery frequency

• Primary: Division 40 33 43 - Chemical Storage and Handling Equipment
• Secondary: Division 40 33 13 - Chemical Feed Equipment (when integrated with metering pumps)
• Division 40 33 23 - Chemical Storage Tanks (for tank-only procurement)

• Material/Equipment Verification: Verify load cell accuracy class (typically Class III for chemical feed), confirm chemical compatibility with wetted materials, check electrical classification ratings for installation area
• Installation Requirements: Level concrete pad with vibration isolation, dedicated electrical service with proper grounding, secondary containment sizing per local codes
• Field Challenges: Scale calibration requires certified weights on-site, piping thermal expansion affects accuracy, electrical interference from VFDs
• Coordination Issues: SCADA integration points with plant DCS, chemical delivery truck access and unloading connections, lead times: 12-16 weeks for integrated systems

• Neptune Chemical Pump Company - NPT Series metering pumps with integral weighing systems, strong municipal presence in chlorination applications
• Grundfos - DDA and DDC series digital dosing pumps with Smart Digital technology, popular for smaller plants
• Hach (Xylem) - ChemTainer systems with integrated weighing and controls, common in larger municipal facilities
• ProMinent - Dulco Flex peristaltic pumps with gravimetric control, growing market share

• Rotameters with manual adjustment - $500-2,000 per point, suitable for stable flow applications where precision isn't critical
• Magnetic flow meters with PID control - $3,000-8,000 per loop, preferred when chemical costs are low and flow varies significantly
• Peristaltic pumps with stroke counting - $2,000-5,000 per pump, good for corrosive chemicals but limited accuracy compared to gravimetric systems

Establish relationships with local scale service companies early - they're essential for annual calibrations and troubleshooting. Many plants save 15-20% by purchasing chemical storage tanks separately and integrating with manufacturer's weighing/pump packages. Always specify redundant level indication beyond the weighing system. Consider remote monitoring capabilities for after-hours chemical deliveries and inventory management.