What is a Check Valve? Types, Working Principle & Selection Guide
A check valve (also called a non-return valve or NRV) is a one-way valve that allows fluid to flow in one direction and automatically prevents reverse flow. It is one of the most critical safety devices in any piping system — protecting pumps from backflow, preventing contamination, and avoiding dangerous water hammer events.
Unlike gate, globe, or ball valves, check valves have no handle, wheel, or actuator. They operate entirely by the pressure differential of the fluid flowing through them.
Table of Contents
1. How a Check Valve Works
A check valve works on a simple principle: pressure differential.
- Forward flow (valve opens): When upstream pressure (P1) exceeds downstream pressure (P2) by more than the "cracking pressure" of the valve, the internal disc, plate, or ball is pushed open by the fluid force, allowing flow.
- No flow / Reverse flow (valve closes): When flow stops or reverses (P2 > P1), gravity and/or a spring force pushes the disc back onto the seat, creating a seal that blocks reverse flow.
Key Terms
- Cracking Pressure: Minimum upstream pressure required to open the valve (typically 0.5–5 PSI)
- Full Open Pressure: Pressure at which the disc reaches maximum travel (typically 3–5x cracking pressure)
- Backpressure: Downstream pressure the closed valve must withstand without leaking
- Slam: Impact of the disc on the seat during rapid closure — causes water hammer
2. Types of Check Valves
2.1 Swing Check Valve
The most common type of check valve. A single disc is hinged at the top of the body. Forward flow pushes the disc open in a swinging arc. When flow stops, gravity swings the disc back onto the seat.
Best for: Clean fluids, large pipelines, low velocity applications, general isolation. Commonly used in water supply, oil & gas, and power plants.
Limitation: Slow closing can cause water hammer in pump discharge applications with frequent starts/stops. Not suitable for vertical upflow in large sizes.
2.2 Dual Plate Check Valve (Wafer Check Valve)
Uses two semicircular plates (half-moon discs) mounted on a central hinge pin with torsion springs. The plates fold open with forward flow and snap shut when flow reverses. Extremely compact — fits between flanges in a wafer configuration.
Best for: Pump discharge, compressor systems, systems prone to water hammer. The spring-loaded plates close before flow reversal completes, dramatically reducing slam.
Why it's popular: 70-80% lighter than swing check valves. 50-60% shorter face-to-face. Can be installed horizontally or vertically. Most cost-effective for pump protection.
2.3 Piston / Lift Check Valve
The disc (piston) lifts vertically off the seat when forward flow lifts it. Similar internal geometry to a globe valve — the flow makes a 90° turn, passes through the seat, lifts the piston, then turns 90° again to exit. Available in horizontal (lift) and vertical (piston) configurations.
Best for: Steam service, high-pressure applications, small sizes, applications where a globe valve pattern body is already in the piping layout. Commonly used in boiler systems.
2.4 Tilting Disc Check Valve
The disc is mounted on a pivot point offset from the center. This design allows the disc to open with a short tilting motion (15-20°) rather than the full swing arc of a swing check. Faster closing and reduced water hammer compared to swing check valves.
Best for: Large diameter pipelines where low pressure drop is needed but faster closing than a swing check is required. Used in pump stations, power plants, and water transmission mains.
2.5 Non-Slam Check Valve (Silent Check Valve)
A spring-loaded check valve specifically designed to prevent water hammer. The spring pre-loads the disc to close before flow reversal occurs. The disc begins closing as soon as flow decelerates (not waiting for reversal), achieving "non-slam" or "silent" operation.
Best for: Pump stations with frequent starts/stops, systems with rapid flow changes, applications where water hammer could damage piping or equipment. Critical for centrifugal pump protection.
2.6 Ball Check Valve
Uses a spherical ball as the closure element. The ball sits on a conical seat. Forward flow lifts the ball; gravity returns it to the seat when flow stops. Simple, no moving parts to wear or corrode. Used primarily in small sizes and for viscous or particulate-laden fluids.
3. Check Valve Comparison Table
| Feature | Swing | Dual Plate | Piston/Lift | Tilting Disc | Non-Slam |
|---|---|---|---|---|---|
| Closing Speed | Slow | Fast | Fast | Fast | Very Fast |
| Water Hammer Risk | High | Low | Low | Low | None |
| Pressure Drop | Low | Moderate | High | Low-Mod | Moderate |
| Weight | Heavy | Very Light | Medium | Heavy | Medium |
| Face-to-Face | Long | Very Short | Long | Long | Short-Medium |
| Vertical Install | No* | Yes | Yes (up only) | No | Yes |
| Best Size Range | 2"–48" | 2"–48" | ½"–12" | 6"–48" | 2"–24" |
| Cost | Medium | Low | Medium | High | Medium-High |
| Pump Discharge | Fair | Excellent | Good | Good | Excellent |
*Swing check valves can be installed in vertical pipelines with flow upward in small sizes with counterweight, but this is not common practice.
4. Materials of Construction
| Component | Carbon Steel | Stainless Steel | Duplex | Special Alloys |
|---|---|---|---|---|
| Body | ASTM A216 WCB | ASTM A351 CF8M | ASTM A890 CD3MN | NAB C95800, Monel, Inconel |
| Disc | A216 WCB + Stellite | A351 CF8M + Stellite | CD3MN + Stellite | Alloy-specific |
| Seat Ring | SS410 + Stellite | SS316 + Stellite | Duplex + Stellite | Alloy-specific |
| Hinge Pin | SS420 / SS304 | SS316 | Super Duplex | Inconel 625 |
| Spring | Inconel 718 | Inconel 718 | Inconel 718 | Inconel 718 |
| Temperature | -29 to 425°C | -196 to 538°C | -46 to 315°C | Varies by alloy |
| Service | General, oil & gas | Corrosive, cryogenic | Seawater, offshore | Severe corrosion |
5. Water Hammer & How to Prevent It
Water hammer (hydraulic shock) is a pressure surge caused by sudden flow stoppage or reversal. In check valves, it occurs when the disc slams shut after reverse flow has already begun accelerating. The kinetic energy of the reversing fluid converts to a pressure wave that can exceed 10x normal operating pressure.
What causes water hammer in check valves?
- Slow-closing disc: Swing check valves with heavy discs take too long to close
- Oversized valve: Disc doesn't fully open, causing flutter and delayed response
- Rapid pump shutdown: Quick flow deceleration without proper check valve response
- Long pipeline: More fluid mass = more kinetic energy to absorb
How to prevent water hammer
Prevention Methods
- Use dual plate or non-slam check valves — spring-assisted closing responds before full reversal
- Size correctly — valve should be fully open at normal flow velocity (recommended: 1.5–3 m/s for liquids)
- Don't oversize — an oversized check valve has a partially open disc that flutters and slams
- Consider a dashpot — hydraulic dampener on the disc for large swing checks
- Use slow-closing pump controls — VFD ramping or soft starter to decelerate flow gradually
6. Sizing & Selection Guide
Proper check valve sizing is critical. The valve must be sized so that the disc is fully open and stable at normal operating flow. An undersized valve causes excessive pressure drop; an oversized valve causes disc flutter, premature wear, and water hammer.
Selection decision logic:
- Application: Pump discharge? → Dual plate or non-slam. Pipeline? → Swing or tilting disc. Steam? → Piston/lift.
- Size: Small (½"–2") → Piston/lift. Medium (2"–12") → Dual plate or swing. Large (12"+) → Dual plate, tilting disc, or swing with dashpot.
- Water hammer risk: High risk → Non-slam or dual plate. Low risk → Swing check is fine.
- Space: Limited → Dual plate (wafer). Space available → Swing or tilting disc.
- Orientation: Vertical pipe → Dual plate, piston, or non-slam. Horizontal → Any type.
- Budget: Economy → Dual plate wafer. Premium → Non-slam or tilting disc.
7. Applications by Industry
| Industry | Application | Recommended Type | Material |
|---|---|---|---|
| Oil & Gas | Pipeline isolation, pump discharge | Dual plate, Swing (API 6D) | WCB, Duplex |
| Power Plant | Boiler feed, cooling water, steam | Swing, Piston, Non-slam | WCB, WC6, CF8M |
| Water Treatment | Pump discharge, distribution | Dual plate, Swing | WCB, CI, DI |
| Chemical Plant | Process lines, acid/alkali | Dual plate, Piston | CF8M, Alloy 20, Hastelloy |
| Marine/Offshore | Seawater, ballast, process | Dual plate, Swing | NAB, Super Duplex, 254 SMO |
| HVAC | Chiller, cooling tower | Dual plate wafer | CI, DI |
| Fire Protection | Pump discharge, riser | Swing, Non-slam | CI, DI, WCB |
| Mining | Slurry pumping, tailings | Swing (rubber lined) | Rubber-lined CI/CS |
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