Polyvinyl Chloride Tanks

Polyvinyl Chloride (PVC) tanks serve as chemical storage and process vessels in municipal water and wastewater treatment plants, primarily housing corrosive chemicals like sodium hypochlorite, ferric chloride, and acids. These thermoplastic tanks resist chemical attack through PVC's inherent corrosion resistance, eliminating the need for protective linings required with steel or concrete alternatives. PVC tanks typically handle concentrations up to 15% sodium hypochlorite with service lives exceeding 20 years when properly specified. The primary trade-off is temperature limitation - PVC tanks are restricted to approximately 140°F maximum operating temperature, making them unsuitable for heated chemical applications or high-temperature process streams.

• Chemical Storage for Sodium Hypochlorite: PVC tanks store 12.5% sodium hypochlorite solution at water treatment plants, typically ranging 500-5,000 gallons for 0.5-50 MGD facilities. Selected for excellent chlorine resistance and UV protection when properly formulated. Fed downstream to chlorine contact basins via chemical metering pumps with CPVC piping connections.

• Polymer Feed Systems: House diluted polymer solutions (0.25-0.5%) for coagulation and flocculation processes. Tank sizes range 100-1,000 gallons depending on plant capacity. PVC resists polymer fouling better than steel alternatives. Connected upstream from rapid mix chambers with PVC or HDPE piping and progressive cavity pumps.

• Acid/Base Storage: Store sulfuric acid (93%) or sodium hydroxide (25-50%) for pH adjustment in treatment processes. Typically 1,000-3,000 gallon capacity. Selected for corrosion resistance and lower cost than fiberglass. Requires secondary containment and connects to chemical feed systems via CPVC piping with leak detection.

Daily Operations: Operators monitor tank levels via sight gauges or ultrasonic sensors, checking for proper chemical feed rates and any visible leaks around fittings. Temperature monitoring critical as PVC softens above 140°F. Chemical concentration verified through grab samples and titration. Tank rotation practiced when multiple units installed to prevent stagnation.

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Maintenance: Monthly visual inspections for stress cracking, particularly around fittings and support points. Annual cleaning requires confined space entry procedures with proper ventilation and gas monitoring. PPE includes chemical-resistant suits, SCBA, and fall protection. Fitting gaskets replaced every 2-3 years. Requires millwright skills for fitting replacement and basic chemistry knowledge for cleaning procedures.

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Troubleshooting: Stress cracking appears as white lines, typically from over-tightening fittings or thermal cycling. Fitting leaks often indicate gasket failure or improper installation torque. UV degradation shows as surface chalking after 8-10 years without proper inhibitors. Expected service life 15-20 years with proper installation and maintenance, versus 25-30 years for fiberglass alternatives.

• Tank Wall Construction: Fabricated from virgin PVC resin with 0.25-0.75 inch wall thickness depending on capacity (100-5,000 gallons typical municipal range). Wall thickness increases with diameter and specific gravity of stored chemical. Selection based on chemical compatibility charts and structural requirements per ASTM D1998.

• Fitting Integration: Molded-in flanged connections for inlet/outlet (2-6 inch), drain (2-4 inch), and overflow (3-8 inch). Fittings use same PVC material as tank body to prevent galvanic issues. Sizing based on flow requirements and pump specifications.

• Support Structure: Requires concrete pad or steel framework due to PVC's thermal expansion (3-5 times steel). Pad design accounts for full tank weight plus seismic/wind loads per local codes.

• Secondary Containment: Required for hazardous chemicals - typically 110% tank volume using PVC-lined concrete or HDPE. Includes leak detection and emergency drain systems.

• Tank Capacity: 500-50,000 gallons for municipal applications, with standard sizes at 1,000, 2,500, 5,000, 10,000, and 15,000 gallons for chemical storage and process applications

• Operating Pressure: Atmospheric to 50 psig maximum for standard PVC construction. Full vacuum capability standard for most manufacturers like Poly Processing and Snyder Industries

• Temperature Range: 32°F to 140°F continuous operation, with brief excursions to 160°F acceptable. Critical for chlorine solution storage where temperatures above 100°F accelerate degradation

• Chemical Compatibility: pH range 2-12 for standard PVC formulations. Sodium hypochlorite concentrations up to 15% at ambient temperature, with reduced concentrations at elevated temperatures following manufacturer derating curves

• Wall Thickness: Minimum 0.25" for tanks under 5,000 gallons, 0.375" for larger vessels. Calculated using ASME RTP-1 or manufacturer proprietary standards

• Specific Gravity: Design basis typically 1.2-1.5 for chemical solutions, though PVC tanks can handle up to 2.0 SG with proper support

• Retention Time: 1-7 days typical for chemical storage, 2-24 hours for process mixing applications, affecting tank sizing relative to daily consumption rates

• What chemical concentration and temperature combination will the tank handle? Standard PVC handles 12.5% sodium hypochlorite at 70°F, but only 10% at 100°F. Wrong selection leads to premature cracking, leaks, and safety hazards. Requires detailed chemical compatibility charts and site temperature data.

• Should the tank be vertical or horizontal configuration? Vertical tanks minimize footprint but require 2:1 height-to-diameter ratio maximum for stability. Horizontal tanks offer better mixing but need more floor space. Wrong choice affects building design, mixing effectiveness, and structural requirements.

• What level of secondary containment is required? EPA SPCC rules mandate 110% containment for tanks over 660 gallons near waterways. Integral containment costs 40% more but eliminates separate concrete containment. Inadequate containment risks regulatory violations and environmental liability.

• Is the tank for storage or active mixing duty? Storage tanks use standard wall thickness, while mixing tanks need reinforced construction for agitator loads. Mixing tanks require 25% thicker walls and reinforced nozzles. Wrong specification leads to structural failure under dynamic loads.

• Division 40 - Process Integration

• Section 40 05 23 - Process Piping and Tanks

• Alternative: Section 33 56 39 - Potable Water Storage Tanks (for finished water applications)

• Material/Equipment Verification: UV stabilization certification for outdoor installations, NSF-61 compliance documentation for potable water contact, Wall thickness verification against hydrostatic pressure calculations

• Installation Requirements: Level concrete pad with 4-inch minimum thickness, Proper tie-down anchoring for wind loads, Secondary containment for chemical storage applications

• Field Challenges: Temperature expansion joints often overlooked, Access limitations for large tank delivery

• Coordination Issues: 8-12 week lead times for custom fabrications

• Pentair Aquatic Systems - X-Series vertical tanks up to 15,000 gallons for chemical storage applications

• Norwesco - N-Series horizontal tanks ranging 500-6,000 gallons, popular for sodium hypochlorite storage

• Poly Processing - IMFO series with full-drain capabilities for caustic soda applications

• Tank Depot - Custom fabricated rectangular tanks up to 10,000 gallons for contact chambers

• Fiberglass tanks cost 20-30% more but offer superior chemical resistance for aggressive applications like ferric chloride storage

• Stainless steel tanks run 3-4x higher cost but provide longest service life for critical applications

• HDPE tanks offer similar cost to PVC with better impact resistance, preferred for cold climate installations where thermal cycling is severe

Establish relationships with regional distributors rather than going direct to manufacturers - they stock common sizes and provide faster delivery. For chemical storage, specify tanks with 1.9 specific gravity rating even for lighter chemicals to allow operational flexibility. Consider modular installations using multiple smaller tanks instead of single large units to reduce crane requirements and improve redundancy during maintenance outages.