Double Disc Check Valves

Double disc check valves prevent backflow in municipal water and wastewater systems by using two hinged semicircular discs that open under forward flow and close under reverse pressure. The split-disc design reduces water hammer compared to swing check valves while maintaining low headloss, typically 0.5-2.0 psi at normal operating velocities. These valves are commonly installed downstream of pumps, at pipeline high points, and in gravity flow applications where backflow prevention is critical. The primary trade-off is higher initial cost compared to swing checks, though this is often justified by reduced maintenance requirements and superior slam-shut characteristics that protect upstream equipment.

• High Service Pump Discharge Lines (2-36 inch): Installed immediately downstream of centrifugal pumps feeding distribution systems. Prevents reverse flow during pump shutdown, protecting against water hammer and backflow contamination. Connects between pump discharge flange and distribution header with flanged or grooved connections.
• Raw Water Intake Lines (12-48 inch): Positioned after intake pumps to prevent siphoning during power outages. Critical for plants drawing from elevated sources where gravity backflow could drain wet wells or damage equipment. Typically flanged installation with upstream strainer.
• Chemical Feed System Protection (1-6 inch): Protects chemical metering pumps from backpressure in polymer, coagulant, and disinfection systems. Prevents cross-contamination between chemical lines and maintains dosing accuracy. Threaded or small flange connections common.
• Backwash Water Supply (6-24 inch): Prevents contaminated backwash water from entering clean water systems during filter backwash cycles. Essential for maintaining water quality separation between process streams.

Daily Operations: Visual inspection of valve position indicators where provided. Monitor upstream and downstream pressure gauges for proper differential pressure operation. Check for external leakage at flanged connections and bonnet seals. No routine adjustments required during normal operation, as valves operate automatically based on flow direction.

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Maintenance: Annual inspection recommended with 3-5 year overhaul intervals depending on service conditions. Requires confined space entry procedures for large diameter installations. Basic mechanical skills sufficient for seat replacement and spring adjustment. Standard PPE includes hard hat, safety glasses, and work gloves. Valve removal requires system isolation and proper rigging for sizes above 12 inches.

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Troubleshooting: Disc flutter indicates insufficient closing pressure or worn springs - typically occurs after 8-12 years in clean water service. External leakage suggests gasket failure or bolt loosening. Chattering noise during flow indicates debris on seats or damaged disc hinges. Complete valve replacement often more economical than field rebuilding for sizes below 8 inches.

• Dual Disc Assembly: Two spring-loaded semicircular discs that pivot independently on hinge pins. Typically 316 stainless steel or bronze construction for municipal service. Discs sized for 2-48 inch nominal pipe diameters with face-to-face dimensions per ANSI B16.10.
• Body and Bonnet: Cast iron (ASTM A126) or ductile iron (ASTM A536) construction for 125-250 PSI municipal applications. Flanged ends per ANSI B16.1 with optional wafer-style bodies for space constraints. Internal surfaces often epoxy-coated for corrosion protection.
• Spring Mechanism: Torsion springs provide disc closing force and maintain seal under low differential pressure. 316 stainless steel springs sized for specific cracking pressures (typically 0.5-2.0 PSI). Spring tension adjustable on some models.
• Seat and Seal System: Replaceable elastomer seats (EPDM, Buna-N, or Viton) provide bubble-tight shutoff. Seat design accommodates thermal expansion and wear. Metal-to-metal seats available for high-temperature applications.

• Flow Velocity: 3-12 fps typical municipal range. Size for 6-8 fps optimal performance to minimize pressure loss while ensuring reliable closure. Velocities below 3 fps may cause flutter; above 12 fps increase headloss significantly.
• Pressure Rating: ANSI Class 125-250 standard for municipal applications. Class 125 adequate for most distribution systems (up to 200 psi working pressure). High-service pumping may require Class 250 (up to 400 psi).
• Valve Sizing: Available 3"-48" diameter. Municipal applications typically 6"-36". Size to match pipeline diameter - no upsizing/downsizing recommended as it affects closure dynamics.
• Pressure Loss: 0.5-2.0 psi at design flow rates. Quality valves (Mueller, APCO, Cla-Val) achieve <1.0 psi loss. Critical for high-service pumping where every psi matters for energy costs.
• Closure Time: 0.5-2.0 seconds typical. Faster closure reduces water hammer but increases slam forces. Adjustable with optional dashpots on larger valves (12"+).
• Temperature Range: 33°F-180°F standard. Adequate for all municipal water applications including heated buildings return lines.

• What is the maximum allowable pressure loss at design flow? Municipal systems typically budget 1-3 psi for check valve losses. Exceeding this affects pump sizing and energy costs. Need: system hydraulic analysis, pump curves, and energy cost projections.
• What reverse flow velocity will the valve experience during pump shutdown? Critical for sizing closure mechanism. Velocities >4 fps reverse require heavy-duty springs or dashpots to prevent slam. Need: transient analysis, pump rundown curves, and downstream storage volumes.
• Is the installation location accessible for maintenance? Double disc valves require periodic inspection of hinge pins and disc condition. Buried installations need valve boxes; elevated locations need platforms. Wrong decision means deferred maintenance and eventual failure. Need: site layout, maintenance access requirements.
• What are the system's water hammer characteristics? Fast-closing valves can amplify pressure surges in systems with long pipelines or minimal surge protection. May require slow-closing or cushioned designs. Need: surge analysis, pipeline profiles, and existing surge protection inventory.

• Division 40 - Process Integration
• Section 40 05 00 - Common Work Results for Process Integration - Primary specification location for municipal treatment plant check valves
• Also applicable: Section 33 30 00 (Sanitary Sewerage Utilities) for wastewater applications and Section 33 10 00 (Water Utilities) for water distribution check valves

• Material/Equipment Verification: Verify NSF-61 certification for potable water service, Confirm pressure ratings match system conditions, Check spring materials for chlorine resistance
• Installation Requirements: Minimum 5D upstream, 2D downstream straight pipe, Horizontal installation preferred; vertical acceptable with manufacturer approval, Support piping independently - valve not designed for pipe loads
• Field Challenges: Access for spring adjustment often overlooked in design, Differential settlement can bind disc mechanism
• Coordination Issues: 12-16 week lead times for larger sizes (>24"), Coordinate with electrical for position switches if specified

• Cla-Val: Model 39 Series - widely specified for municipal pump stations, available 2"-48"
• Val-Matic: Surgebuster Series 201 - popular for high-head applications, proven in 500+ ft TDH installations
• APCO: Model 1400 Series - cost-effective option for standard duty, common in smaller municipalities
• Mueller: Pratt Industrial double-disc models - established track record in water distribution systems

• Swing Check Valves: 30-40% less expensive, better for low-velocity applications but require more headroom and create higher pressure losses.
• Ball Check Valves: Preferred for slurry service and raw water with debris; 20-30% higher cost but superior solids handling.
• Tilting Disc Check Valves: Lower pressure drop than double-disc, ideal for gravity flow applications; similar cost but limited to horizontal installation only.

Establish relationships with manufacturer reps early - they provide invaluable field support during commissioning and can expedite warranty issues. Specify spare spring sets for critical applications; springs are the most common wear item. Consider upgrading to stainless steel internals for raw water service - the 15-20% cost premium pays back through reduced maintenance. Always verify actual system pressures before final sizing; many engineers oversize based on conservative design assumptions.