Double-Check Valve Assembly: Installation and Initial Inspection

Double-Check Valve Assembly: Installation and Initial Inspection

What Is a Double-Check Valve Assembly?

A double-check valve assembly (DCVA) is a backflow prevention device designed to protect potable water supplies from contamination under low-hazard, non-health-threatening conditions. Examples of low-hazard applications include lawn irrigation systems, commercial ice makers, and fire sprinkler systems supplied by a potable source where no toxic or chemical additives are present.

The DCVA contains two independently operating spring-loaded check valves arranged in series inside a single body. Each check valve opens when forward water pressure overcomes the spring, allowing water to flow in the intended direction. If downstream pressure rises above upstream pressure — a backpressure condition — or if upstream pressure drops below downstream pressure — a backsiphonage condition — both check valves seat tightly and block reverse flow. Requiring two check valves to fail simultaneously before contamination reaches the supply line is the key safety advantage of the DCVA over a simple single-check valve.

Component Overview

Understanding each part before installation prevents errors and speeds field troubleshooting:

• Body: The cast or ductile-iron housing that contains both check valve modules.
• Check valve No. 1 (upstream): The first line of defense; closes against backpressure.
• Check valve No. 2 (downstream): The redundant backup; closes independently of No. 1.
• Resilient discs: Rubber or elastomeric seating surfaces on each check valve that must be free of nicks, debris, or deformation to seat properly.
• Strainer: Installed upstream of check valve No. 1 to keep debris from holding a disc off its seat. Always inspect and clean the strainer during maintenance.
• Test cocks (No. 1 through No. 4): Small ball valves located at specific test ports along the assembly. They are used only during official field testing; they remain closed at all other times.
• Isolation valves (shutoff valves): Gate or ball valves located immediately upstream and downstream of the DCVA body. They isolate the device for testing and repair without shutting down the whole system.
• Union joints: Allow the body to be removed for maintenance without cutting pipe. Torque must be even to prevent cross-threading and leaks.

Flow Direction and Orientation

Every DCVA body is marked with a flow-direction arrow cast or stamped into the body. Installing a DCVA backwards — against the arrow — means the check valves will be held open by forward flow rather than closing against it, rendering the device completely ineffective. Before threading a single union nut, confirm the arrow on the body points in the direction of intended water flow from supply to service.

DCVAs may be installed horizontally or vertically depending on the manufacturer's specifications and local code requirements. In horizontal installation, which is the most common, keep the test cocks positioned so a field tester can attach differential pressure gauge hoses without awkward angles. Consult the manufacturer's installation sheet and your Authority Having Jurisdiction (AHJ) for acceptable orientations.

Safety Before You Begin

• Verify the system is de-pressurized and isolated before handling any pipe component. Use the upstream isolation valve and confirm zero pressure with a gauge or by slowly cracking a test cock — never assume a system is off.
• Wear safety glasses whenever working near pressurized water lines or using wrenches. Sudden pressure release can spray water or debris.
• Inspect threads and unions before assembly. Cross-threaded unions can fracture under pressure, becoming a projectile hazard.
• Never over-torque unions. Follow manufacturer torque specifications. Excessive force cracks plastic seat components and distorts seals.
• Keep test cocks closed except during an authorized field test. An open test cock bleeds treated water and can introduce contamination if a cap is missing.

Installation Procedure

1. Confirm flow direction on the rig piping by tracing the supply line. Mark an arrow on a piece of tape on the bench as a reference.

2. Position the DCVA body so its cast arrow aligns with the flow direction you identified.

3. Hand-thread both union joints — upstream first, then downstream — to confirm alignment before applying wrench force.

4. Torque unions evenly with a pipe wrench and backup wrench, following the torque value in the manufacturer's instructions or your instructor's direction for the training rig.

5. Confirm isolation valves on both sides are closed before slowly pressurizing for leak check.

6. Slowly open the upstream isolation valve a quarter-turn to introduce pressure gradually; listen and watch for leaks at unions.

7. Open the upstream isolation valve fully, then open the downstream isolation valve fully. The device is now in service.

8. Verify all test cocks are in the closed position (handle perpendicular to the body of the test cock).

Visual Initial Inspection

Before any differential pressure field test can be run, ASSE field test procedures require a visual initial inspection to confirm the device is properly installed and safe to test. On the ASSE form, you must document:

• Device manufacturer, model, serial number, and size
• Location and application (what it is protecting)
• Flow direction correct (yes/no)
• Strainer present and accessible
• Test cocks numbered, operable, and closed
• Isolation valves operable (open fully for service, closeable for test)
• No visible leaks at unions or body
• Device is appropriately supported and labeled per local requirements

All fields must be complete before you proceed. An incomplete initial inspection form means the field test result is not valid for submittal to the AHJ. This is not a suggestion — it is an ASSE procedural requirement and a key competency for the ASSE Backflow certification exam.

Why This Matters for Your Career

Backflow incidents have caused documented illness outbreaks when unprotected cross-connections allowed fertilizers, industrial chemicals, or sewage to enter drinking water distribution. A correctly installed and inspected DCVA is the physical barrier that prevents this. As a licensed journeyman and ASSE-certified backflow tester, your signed field test report is a legal document. Getting installation and inspection right every time protects the public, protects your license, and is the foundation of ASSE Backflow certification.