Fiberglass Reinforced Plastic Pressure Tanks

Fiberglass Reinforced Plastic (FRP) pressure tanks store pressurized water or air in municipal treatment systems, providing surge protection, pressure maintenance, and flow equalization. These composite vessels utilize a fiberglass shell with internal bladder or diaphragm to separate water and compressed air phases. Typical municipal installations achieve 10-15 year service life with operating pressures from 100-300 psi. However, FRP tanks require careful sizing calculations and regular bladder replacement, making them more maintenance-intensive than steel alternatives while offering superior corrosion resistance in aggressive water chemistries.

• Hydropneumatic Storage Systems: FRP pressure tanks maintain system pressure (40-80 psi) in booster pump stations and distribution systems. Connected downstream of high-service pumps with pressure switches and check valves, they reduce pump cycling and provide emergency storage. Selected for corrosion resistance and 20+ year service life versus steel alternatives requiring frequent recoating.

• Chemical Feed Systems: Pressurized solution tanks for sodium hypochlorite, ferric chloride, and polymer feed systems operate at 10-25 psi. Upstream connections include chemical delivery ports and level sensors, downstream to metering pumps. FRP resists chemical attack better than steel, eliminating contamination risks and extending service intervals.

• Backwash Water Storage: Pre-pressurized tanks (15-30 psi) store filtered water for filter backwashing in 2-20 MGD plants. Connected to clearwell discharge with automated valving for rapid backwash initiation. FRP provides clean storage without interior coatings that could flake into process water.

Daily Operations: Operators monitor system pressure via gauges, checking cut-in/cut-out cycling (typically 40-60 psi range). Pressure switch operation verified during pump starts. Air pre-charge pressure checked weekly using tank-mounted gauge. Automated systems require minimal adjustment beyond pressure switch calibration and periodic bladder pressure verification.

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Maintenance: Quarterly inspection of fittings for leaks and pressure gauge calibration. Annual bladder pre-charge verification and air valve service. Tank exterior cleaned annually with mild detergent. No confined space entry required for FRP tanks. Basic mechanical skills adequate for routine maintenance. Safety glasses and gloves sufficient PPE for most tasks.

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Troubleshooting: Rapid pressure loss indicates bladder failure or air leakage. Waterlogged tanks show no pressure differential and constant pump cycling. Visible bulging suggests over-pressurization. Typical service life 15-20 years for bladder, 25+ years for tank shell. Warning signs include erratic pressure readings, unusual pump cycling patterns, and visible tank deformation.

• Vessel Shell: Filament-wound fiberglass construction with polyester or vinyl ester resin. Typical municipal sizes range 120-8,000 gallons with 150-300 psi ratings. Wall thickness varies 0.25-0.75 inches based on pressure rating and diameter. Selection depends on chemical compatibility and required pressure rating.

• Bladder/Diaphragm: Butyl rubber or EPDM membrane separates water from compressed air. Sized for 25-40% tank volume at operating pressure. Material selection based on water quality and chemical exposure. Pre-charge pressure typically 2 psi below cut-in pressure.

• Connection Fittings: Stainless steel or brass fittings include water inlet/outlet (2-8 inch), air valve (0.5-1 inch), and drain connections. NPT or flanged connections based on pipe size. Pressure gauges and level indicators mounted in accessible locations.

• Support Structure: Steel cradle or saddle supports distribute tank weight. Designed for full tank load plus seismic/wind loads per local codes. Anchor bolts sized for uplift resistance in pressurized applications.

• Operating Pressure Range: 50-300 psi working pressure, with 150 psi being most common for municipal distribution systems. Safety factor of 4:1 minimum per ASME RTP-1.

• Volume Capacity: 120-15,000 gallons typical for municipal applications. Standard sizes: 120, 220, 315, 525, 750, 1,060, 1,580, 2,640, 4,220, 6,320, 8,450, and 15,000 gallons.

• Drawdown Volume: 25-40% of total tank volume, depending on pressure differential. Pre-charge pressure typically set at 2 psi below cut-in pressure.

• Flow Rate Capacity: Sized for 10-30 minute drawdown time at peak demand. For 0.5 MGD systems: 350-500 gallon tanks. For 5 MGD systems: 2,000-4,000 gallon tanks.

• Pressure Differential: 20-40 psi between cut-in and cut-out pressures. Narrower differentials require larger tanks.

• Temperature Range: 35-120°F operating range for municipal water service.

• Diaphragm Material: EPDM standard for potable water, meeting NSF-61 requirements. Butyl available for specific chemical compatibility.

• Connection Sizes: 2-8 inch flanged connections typical, with 4-6 inch most common for municipal applications.

• What is the required drawdown volume for your peak demand scenario? Calculate based on pump cycling frequency (minimum 6 starts/hour for VFD systems, 4 starts/hour for constant speed). Undersized tanks cause excessive pump cycling, reducing equipment life. Need: peak flow rate, acceptable cycling frequency, and pressure differential.

• What working pressure rating matches your system requirements? Standard ratings: 100, 125, 150, 200, 250, 300 psi. Over-specifying increases costs 15-25% unnecessarily. Under-specifying risks catastrophic failure. Need: system operating pressure plus 25% safety margin minimum.

• Should you specify vertical or horizontal orientation? Vertical tanks require 30% less floor space but need 12-16 feet ceiling height for maintenance access. Horizontal tanks need larger mechanical rooms but easier maintenance. Need: available space dimensions and maintenance access requirements.

• What diaphragm pre-charge pressure is optimal? Set 2-5 psi below pump cut-in pressure. Higher pre-charge reduces effective volume; lower pre-charge allows water hammer. Incorrect setting reduces tank life 40-60%. Need: exact pump controller cut-in pressure and system static head.

• Division 40-48: 40 31 33 - Hydropneumatic Water Storage Tanks

• Primary specification section covering fiberglass reinforced plastic pressure tanks for potable water systems. Includes pressure vessel requirements, diaphragm specifications, and NSF-61 compliance for municipal applications.

• Material/Equipment Verification: NSF 61 certification for potable water contact, ASME pressure vessel code compliance documentation, Resin compatibility with local water chemistry

• Installation Requirements: Concrete pad design for full tank weight plus seismic loads, Minimum 3-foot clearance for bladder replacement access, Proper pipe anchoring to prevent tank movement stress

• Field Challenges: Bladder pre-charging requires specialized nitrogen equipment, Tank delivery coordination due to size constraints on 500+ gallon units

• Coordination Issues: Electrical coordination for level switches and pressure transducers, SCADA integration for remote monitoring capabilities

• Lead times typically 8-12 weeks for standard sizes, 16-20 weeks for custom fabrication.

• Pentair Water Solutions - AquaMatic series, widely used in 1-25 MGD facilities for distribution system pressure maintenance

• Wessels Company - WTA series bladder tanks, popular in smaller municipal systems (0.5-5 MGD)

• Well-X-Trol - WX series, common in booster stations and small treatment plants

• Flexcon Industries - FRP series, frequently specified for larger municipal applications requiring custom sizing

• Steel Pressure Tanks - 15-20% lower initial cost but require internal coating maintenance every 10-15 years. Preferred for temporary installations or budget-constrained projects.

• Hydropneumatic Systems with Multiple Smaller Tanks - Better redundancy for critical applications, easier maintenance access.

• Variable Frequency Drive Pumping - Eliminates pressure tank entirely, reduces energy costs by 20-30% but requires more sophisticated controls and backup power considerations.

Establish relationships with local bladder replacement contractors before installation - bladder life averages 8-12 years in municipal service. Specify stainless steel system connections over carbon steel to prevent galvanic corrosion issues common in FRP installations. Consider oversizing by 25% during design phase as retrofit expansion is expensive. Negotiate spare bladder inclusion in original purchase - replacement costs often exceed 40% of original tank price.