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What Is a Check Valve? Types, How It Works and Selection
A check valve (also called a non-return valve or one-way valve) is a mechanical valve that lets a liquid or gas flow in one direction only and closes automatically when the flow tries to reverse. It needs no actuator or operator: the pressure of the medium opens it, and a spring, gravity or the reverse flow itself closes it. Check valves protect pumps, compressors, boilers and meters from backflow and keep pipelines full.
How Does a Check Valve Work?
When the inlet pressure is high enough, it lifts the disc, ball or flap off its seat and the medium passes through. When the flow stops or reverses, the closing element returns to the seat and blocks the line. Depending on the design, closing is assisted by a spring or by gravity.
What Is Cracking Pressure?
The minimum inlet pressure at which the valve starts to open is called the cracking pressure. It depends on the design, the spring and the size of the valve. When selecting a check valve, make sure your system can always produce this pressure; otherwise the valve will not open fully.
How Does a Check Valve Close?
The valve closes when the inlet pressure falls below the cracking pressure or when pressure builds up on the outlet side.
Installation Direction
Every check valve body carries an arrow showing the flow direction. Check it before installation: a check valve installed backwards blocks the line completely.
Where Are Check Valves Used?
- Pump discharge lines: prevent water from flowing back and the pump from spinning in reverse when it stops. In parallel pump systems, each pump outlet gets its own check valve.
- Boiler feed lines: stop pressurised boiler water from pushing back into the feed pump.
- Steam and condensate lines: prevent backflow in condensate return lines and after steam traps. A stainless steel disco check valve is preferred for high temperatures.
- Compressor outlets: stop compressed air from returning to the compressor.
- Tank and well suction: keep the suction line full, so the pump does not have to re-prime at every start.
- Heating, cooling (HVAC) and water treatment: keep the flow direction in circuits and prevent dirty water from mixing with clean water.
- Fire protection and process industries: sprinkler systems, chemical, petrochemical and food plants.
Types of Check Valves
Spring-Loaded Inline and Y-Type Check Valves
A spring holds the disc against the seat. To open the valve, the inlet pressure must overcome both the cracking pressure and the spring force. Because the spring closes the valve, it can be installed horizontally or vertically.
Spring-Loaded Ball Check Valves
These spring-loaded check valves use a ball as the closing element.
Lift Check Valves
The disc is guided by the bonnet and moves up and down: it rises to open the orifice and drops to close it. The inlet pressure must overcome gravity and/or the spring force. The disc must stay exactly on the axis of its guide.
Swing Check Valves
Swing check valves, also called tilting disc check valves, open and close with a disc that swings on a hinge. If there is no spring to help closing, the installation direction must suit gravity.
- Body material: AISI 316, AISI 304, brass; cast iron for large flanged sizes
- Size: 1/2″ – 2″ (threaded)
- Working pressure: 16 bar
- Working temperature: -10 to +180 °C
- Applications: water and condensate lines, HVAC, mining, textile, chemical, food and marine industries
Wafer Swing Check Valves
Installed between two flanges, the flap opens up to 90° with the flow and closes by itself when pressure drops; back pressure presses it onto the seat. Sealing can be metal-to-metal or soft (PTFE, EPDM etc.); soft seals give quiet operation. Compact design, low weight, low pressure drop, easy installation and low cost are the main advantages.
- Connection: wafer, DN25 – DN1000
- Working temperature: max. 100 °C (working pressure 10 bar, max. 16 bar)
- Applications: water lines, HVAC, pump lines, mining, textile, chemical, food and marine industries
Disco Check Valves
Disco check valves are light, easy to install between flanges and suitable for high temperatures, which makes them one of the most versatile check valve types.
- Connection: wafer
- Body material: AISI 304, AISI 316, brass
- Pressure rating: PN40
- Size: DN15 – DN100
- Working temperature: max. 300 °C
- Applications: water and steam lines, HVAC, iron and steel, mining, textile, chemical, food and marine industries
Dual Plate Check Valves
Two spring-loaded plates close the valve quickly and quietly. Low pressure loss, light weight, short face-to-face length and tight shut-off make them the standard choice for large diameters.
- Body material: AISI 304, AISI 316, GG-25 cast iron, GGG-40.3 ductile iron
- Connection: wafer, DN50 – DN600
- Pressure rating: 16 bar
- Working temperature: max. 110 °C
- Applications: water and steam lines, HVAC, pump lines, iron and steel and process industries
Unox Check Valve Models
| Model | Type | Key Feature |
|---|---|---|
| Stainless Steel Disco Check Valve | Spring-loaded disc, wafer | High temperature (max. 300 °C), any mounting position |
| Brass Disco Check Valve | Spring-loaded disc, wafer | Economical choice for water and general piping |
| Ductile Iron Disco Check Valve | Spring-loaded disc, wafer | Ductile iron body |
| Swing Check Valve | Swing | Low pressure loss |
| Stainless Steel Swing Check Valve | Swing | Stainless body for corrosive media |
| Wafer Type Swing Check Valve | Swing, wafer | Compact, fits between flanges |
| Dual Plate Check Valve | Dual plate, wafer | Light and quiet in large sizes |
| Flanged Lift Check Valve | Lift | Flanged, tight shut-off |
See all models on our check valves page.
How to Size a Check Valve
Size a check valve by the flow rate and velocity in the line, not only by the pipe size. In an oversized check valve the disc cannot open fully; it flutters, and the seat and spring wear out early. An undersized valve increases pressure loss. For a detailed calculation, read how to determine check valve sizes.
What to Consider When Selecting a Check Valve
- Compatibility of the body and seal materials with the medium
- Maximum pressure and the correct cracking pressure
- Line size and flow rate
- Installation position (horizontal or vertical)
- Temperature
- Access for maintenance and repair
Common Check Valve Problems and Solutions
The most common problems are noise, water hammer, vibration, backflow, wear and leakage. Most of them come from wrong selection or installation. To prevent backflow and water hammer, choose a fast-closing (spring-loaded) check valve: fast closing prevents pressure surges.
Frequently Asked Questions
Can a check valve fail?
Yes. The most common causes are dirt caught on the seat, a worn seal and a fatigued spring. Installing a Y-strainer upstream of the check valve extends its life. For symptoms and solutions, read can a check valve fail?
What happens if a check valve is installed backwards?
It blocks the flow completely and the line does not work. The arrow on the body shows the flow direction; always check it before installation.
Can a spring-loaded check valve be installed vertically?
Yes. Spring-loaded disco and wafer check valves close independently of gravity, so they work in horizontal and vertical lines. With swing check valves without a spring, pay attention to the installation direction.
Does a check valve prevent odors?
In drain lines, a check valve stops backflow and can reduce odors coming back. For details, read does a check valve prevent odors?