Combination Air Valves

Combination air valves automatically release trapped air during pipeline filling and admit air during draining to prevent vacuum conditions in municipal water distribution and transmission mains. These dual-function valves incorporate both large and small orifices - the large orifice (typically 2-4 inches) rapidly vents air at low pressures during initial filling, while the small orifice (1/16 to 1/4 inch) continuously releases accumulated air pockets during normal operation. Municipal installations typically achieve air release rates of 200-800 CFM at 2 psi for large orifices and 5-15 CFM for small orifices. The primary trade-off is increased complexity and maintenance requirements compared to single-function air valves, with potential for small orifice clogging in systems with poor water quality.

• Transmission Mains (High Points): Installed at pipeline summits and direction changes on 8"-48" transmission lines. Connected via 2"-6" branch taps with isolation valves. Selected for automatic air release during filling and vacuum breaking during draining, preventing water hammer and maintaining hydraulic efficiency.

• Raw Water Intake Lines: Mounted on 12"-36" intake headers before treatment plants. Upstream from raw water pumps, downstream from lake/river intakes. Critical for removing entrained air that reduces pump efficiency and causes cavitation. Combination design handles both small air pockets during operation and large air volumes during startup.

• Clearwell Discharge Lines: Located on 8"-24" finished water lines exiting clearwells and elevated storage tanks. Positioned before booster pumps to prevent air entrainment. Prevents negative pressures during high-demand periods that could compromise water quality through backflow.

• Force Main Summits: Essential on 6"-18" wastewater force mains at high points and pump station discharge headers. Releases accumulated gases (H2S, methane) while preventing siphon breaks during pump cycling.

Daily Operations: Operators perform visual inspections during routine rounds, checking for water discharge indicating float problems or excessive air venting. No adjustments required during normal operation. Monitor system pressures upstream and downstream - pressure drops may indicate air accumulation requiring valve inspection.

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Maintenance: Quarterly inspection of external condition and mounting hardware. Annual internal inspection requires 2-person confined space procedures with gas monitoring equipment. Float chamber cleaning and seal replacement typically needed every 3-5 years. Basic mechanical skills sufficient - no specialized training required beyond standard safety protocols.

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Troubleshooting: Continuous water discharge indicates damaged float or worn seals - typically occurs after 8-12 years in municipal service. Inadequate air release causes pump cavitation and reduced flow capacity. Vacuum breaker failure creates negative pressures during demand surges. Warning signs include unusual pump cycling, pressure fluctuations, and visible air in downstream piping.

• Air Release Orifice (1/32"-1/16"): Small precision orifice in float-controlled chamber releases entrained air during normal operation. Stainless steel construction resists corrosion. Sizing based on pipeline diameter and flow velocity - typically 1/32" for 8"-12" mains, 1/16" for larger lines.

• Vacuum Breaker Assembly (1"-8" throat): Large orifice with spring-loaded disc opens under vacuum conditions. Cast iron or ductile iron body with EPDM sealing. Throat diameter typically 1/4 to 1/2 of pipeline diameter for adequate air inflow capacity.

• Float Mechanism: Stainless steel or composite float operates both orifices. Lever arm ratio determines operating pressures - typically 2-5 psi closing pressure. Corrosion-resistant materials essential for long service life in municipal applications.

• Body Construction: Ductile iron or cast iron bodies with epoxy coating. Flanged connections 2"-8" for municipal installations. Internal components accessible through top-mounted cover for maintenance without pipeline shutdown.

• Orifice Sizing: Air intake/exhaust orifice diameter typically 1" to 12" for municipal pipelines 6" to 48". Size based on pipeline volume and filling/draining rates - generally 1 CFM per gallon of pipeline volume for filling applications.

• Operating Pressure: Standard municipal range 25-250 PSI working pressure. High-pressure applications up to 400 PSI available. Pressure rating must exceed maximum system pressure plus surge allowance.

• Air Flow Capacity: Intake capacity 50-15,000 SCFM depending on orifice size. Exhaust capacity typically 10-20% of intake due to smaller exhaust orifice. Critical for pump station surge protection and pipeline filling operations.

• Float Specifications: Stainless steel or composite floats sized for specific gravity 0.85-0.95. Float displacement volume determines closing pressure differential, typically 0.5-2.0 PSI.

• Body Materials: Ductile iron with epoxy coating standard for buried service. Stainless steel bodies for exposed installations or corrosive environments. Working temperature range -20°F to 180°F.

• Connection Size: Flanged connections 2" to 12" matching pipeline requirements. Thread connections limited to 2" and smaller applications.

• What is the required air flow capacity for system filling and draining operations? Calculate pipeline volume and specify filling rate (typically 2-5 feet per second). Undersized valves cause vacuum conditions and potential pipe collapse. Need: pipeline diameter, length, elevation profile, and desired filling time.

• What operating pressure and surge conditions must the valve withstand? System pressure plus water hammer surge (typically 1.5x operating pressure minimum). Insufficient pressure rating leads to valve failure. Need: maximum operating pressure, pump characteristics, and surge analysis results.

• Should valve include vacuum relief capability for pump shutdown protection? Critical for pump discharge lines and long force mains. Without vacuum relief, negative pressures can collapse pipes or damage pumps. Need: pump curves, system head loss calculations, and downstream restrictions.

• What level of corrosion protection is required for the installation environment? Buried applications need enhanced coatings; exposed coastal installations require stainless construction. Inadequate protection leads to premature failure and costly replacement. Need: soil conditions, atmospheric exposure, and maintenance accessibility.

• Division 40-05-23 - Combination Air Valves (Water Distribution)

• Alternative: Division 46-05-23 for wastewater force main applications. Use Division 40 for potable water systems, Division 46 for non-potable water and wastewater conveyance systems.

• Material/Equipment Verification: Verify stainless steel internals and corrosion-resistant coatings, Check pressure ratings match system design pressure plus surge allowance, Confirm AWWA C512 compliance and NSF-61 certification

• Installation Requirements: Coordinate vault sizing - minimum 4' x 4' for 6" valves, Specify proper approach piping with isolation valves, Plan for 12-18 week lead times for larger combination valves

• Field Challenges: Vault flooding during construction requires temporary dewatering, Access road coordination for maintenance vehicles

• Coordination Issues: Early coordination with electrical for vault lighting and ventilation

• Val-Matic - Series 201 combination valves, strong municipal presence with 30+ year track record

• APCO - Model 142 series, widely specified for water transmission mains

• Crispin Valve - Model 16CA combination valves, popular in Western US municipalities

• A.R.I. (Applied Research International) - D-040 series, growing North American market share with proven European municipal heritage

• Separate Air Release and Vacuum Valves - Lower initial cost ($800 vs $1,200 for 2" size) but requires two vault penetrations and more complex piping. Preferred for critical applications requiring redundancy.

• Surge Anticipating Valves - 3-4x cost premium but essential for pump station discharge lines with severe surge potential.

• Manual Air Vents with Operator Intervention - Lowest cost option ($200-400) suitable only for non-critical applications with regular operator rounds.

Establish direct relationships with manufacturer technical support - they provide invaluable sizing verification and troubleshooting assistance. Consider standardizing on one manufacturer across your system for parts commonality and operator familiarity. Specify removable bonnets on larger valves (4" and up) to enable field servicing without full valve replacement. Group purchase multiple valves to negotiate better pricing - 15-20% savings possible on orders over $50k.