Dry Chemical Weighing Systems

Dry Chemical Weighing Systems continuously measure and control the feed rate of powdered chemicals like lime, alum, or polymer into water treatment processes. These gravimetric systems use loss-in-weight feeders or gain-in-weight hoppers with load cells to achieve feed rate accuracies of ±1-2% at flow rates from 5-5,000 pounds per hour. The system automatically adjusts chemical dosing based on plant flow conditions and residual monitoring feedback, eliminating manual batch weighing and improving process consistency. However, these systems require significant capital investment ($50,000-200,000) and need regular calibration maintenance, making them most cost-effective for plants using over 500 pounds per day of dry chemicals.

• Lime Feed for Water Softening: Dry weighing systems meter hydrated lime (Ca(OH)₂) for pH adjustment and alkalinity addition in 2-25 MGD plants. Connected upstream to lime storage silos and downstream to slurry makeup tanks, these systems provide ±2% accuracy versus ±10% for volumetric feeders. Selected for consistent dosing despite lime's poor flow characteristics and density variations.
• Polymer Feed for Dewatering: Accurately weighs dry polymer powder for belt filter press and centrifuge applications in 1-10 MGD facilities. Feeds directly to polymer makeup systems, eliminating liquid storage concerns. Critical for maintaining 0.5-3.0% solution concentrations needed for optimal flocculation.
• Alum Feed for Coagulation: Weighs dry aluminum sulfate in 5-50 MGD treatment plants where bulk delivery economics favor dry storage over liquid. Integrates with dissolving tanks and chemical feed pumps, providing precise coagulant dosing for turbidity removal and phosphorus precipitation.

Daily Operations: Operators monitor feed rates via HMI displays, checking actual versus setpoint values hourly. Hopper levels require visual inspection twice per shift, with refill initiated at 25% capacity. Feed rate adjustments respond to process demands - typically 10-50 lb/hr for lime systems and 1-15 lb/hr for polymers. Weight calibration verification performed weekly using certified test weights.

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Maintenance: Load cell calibration required monthly using span weights, performed by certified technicians. Screw conveyor bearings need lubrication every 500 operating hours. Dust collector bags replaced annually or when pressure drop exceeds 6" WC. Full PPE including respirators mandatory during chemical handling. Requires millwright-level mechanical skills for conveyor maintenance and instrumentation technician for calibration work.

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Troubleshooting: Erratic feed rates indicate worn screw flights or bridged material in hoppers - typically occurs after 2-3 years. Weight signal drift suggests load cell contamination or mechanical binding, requiring immediate attention. Material buildup causes dust collection alarms and reduced accuracy. Service life averages 10-15 years for mechanical components, 5-7 years for load cells in corrosive environments.

• Loss-in-Weight Feeder: Continuously weighs hopper contents while discharging material via variable-speed screw conveyor. Stainless steel construction with 50-2000 lb hopper capacity. Load cells provide 0.1% accuracy feedback for closed-loop control. Selection based on required feed rate (5-500 lb/hr typical) and material characteristics.
• Dust Collection System: Baghouse or cartridge filters capture airborne particles during material transfer and feeding. 99.9% collection efficiency required for operator safety and environmental compliance. Sized for 2-4 air changes per minute in feed areas.
• Material Handling Equipment: Pneumatic conveyors, bucket elevators, or screw conveyors transport chemicals from bulk storage to day hoppers. Carbon steel or stainless construction depending on chemical compatibility. Capacity typically 500-5000 lb/hr for municipal applications.
• Control System: PLC-based controllers integrate weight signals, feed rate setpoints, and alarm functions. Provides 4-20mA output to SCADA systems and maintains ±1% feed rate accuracy across 10:1 turndown ratios.

• Feed Rate Capacity: 5-2,000 lbs/hr for typical municipal applications. Polymer systems: 10-500 lbs/hr. Lime systems: 100-2,000 lbs/hr. Size based on maximum day demand plus 25% safety factor.
• Accuracy Requirements: ±0.5% of full scale for gravimetric feeders, ±2% for volumetric systems. Critical for chemical cost control and process optimization.
• Hopper Capacity: 8-72 hour storage at average feed rate. Minimum 500 lbs for polymer, 2,000 lbs for lime to prevent frequent refilling. Consider delivery truck capacity (typically 40,000 lbs bulk).
• Conveying Distance: Maximum 300 feet horizontal, 50 feet vertical for pneumatic systems. Pressure requirements: 8-15 psig for dense phase, 2-5 psig for dilute phase.
• Environmental Conditions: -20°F to 120°F operating range. IP65 enclosures minimum for outdoor installations. Humidity considerations for hygroscopic chemicals like lime.
• Power Requirements: 480V/3-phase standard. 5-25 HP for blowers, 1-5 HP for feeders depending on capacity.
• Control Integration: 4-20mA signals, Modbus/Ethernet communication. Flow pacing capability from 0.1-20 MGD plant flows.

• 1. Gravimetric vs. Volumetric Feeding? Gravimetric required for feed rates >100 lbs/hr or accuracy <±2%. Volumetric acceptable for smaller polymer systems with consistent bulk density. Wrong choice results in 5-10% chemical waste or process upsets. Need: chemical properties, accuracy requirements, budget constraints.
• 2. Pneumatic vs. Mechanical Conveying? Pneumatic for distances >50 feet or multiple feed points. Mechanical for short distances <25 feet with single destination. Pneumatic costs 2-3x more but offers flexibility. Wrong choice creates maintenance nightmares or limits future expansion. Need: plant layout, chemical abrasiveness, maintenance capabilities.
• 3. Bulk Storage vs. Bag Systems? Bulk storage economical above 200 lbs/day usage (typical 5+ MGD plants). Requires truck access, dust collection, and higher capital cost. Bag systems cost 20-40% more for chemicals but lower installation cost. Need: chemical usage rates, site constraints, labor availability.
• 4. Dust Collection Requirements? EPA fugitive emissions standards require collection for lime systems >50 lbs/hr. Baghouse typically required, adds $15,000-50,000 to project cost. Need: chemical type, feed rates, local regulations.

• Primary: Division 46 - Water and Wastewater Equipment, Section 46 21 00 - Water and Wastewater Pumps
• Secondary: Division 40 - Process Integration, Section 40 91 00 - Process Control and Instrumentation (for control systems and integration components)

• Material/Equipment Verification: Verify load cell accuracy ratings (±0.5% typical municipal requirement), Confirm explosion-proof ratings for enclosed chemical storage areas, Check belt material compatibility with specific chemicals
• Installation Requirements: Level concrete pads within ±0.125" for accurate weighing, Dedicated 120V circuits for controls, avoid shared neutrals, Minimum 6-inch clearance around load cells for calibration access
• Field Challenges: Dust control systems often undersized in specifications, Vibration isolation critical near pumps/blowers, Temperature compensation required for outdoor installations
• Coordination Issues: 16-20 week lead times for custom municipal configurations, Early coordination with SCADA integrator for communication protocols

• Acrison - Model 105 Weigh Belt Feeder (1-4,000 lb/hr range, popular for lime/soda ash)
• Schenck Process - MultiBelt WeighFeeder (municipal polymer applications, 0.5-500 lb/hr)
• K-Tron - T35 Twin Screw Feeder with weigh scale (precise polymer dosing, 2-200 lb/hr)
• Hapman - Weigh Belt Conveyors (cost-effective for larger municipal plants, lime/carbon applications)

• Volumetric Dry Feeders - 30-40% lower cost, acceptable for non-critical applications like lime softening where ±5% accuracy sufficient. Loss-in-weight systems offer better accuracy but 50% higher cost.
• Liquid Chemical Systems - Preferred for polymers and smaller plants (<5 MGD). Higher chemical costs offset by simpler equipment and reduced maintenance.
• Pneumatic Conveying with Weigh Hoppers - Better for dusty chemicals like lime, 25% higher capital cost but reduced labor for bag handling.

Establish relationships with local manufacturer representatives early - they provide valuable troubleshooting support and spare parts inventory. Consider standardizing on one manufacturer across multiple systems for operator familiarity and parts commonality. Specify redundant load cells where possible; single-point failures shut down entire chemical feed systems. Request factory acceptance testing for complex installations. Many plants save 15-20% by purchasing directly through manufacturer representatives rather than general contractors.