Check valves of the Metal Work Line-on-Line series

Check Valves: Function, Types, and Industrial Application Criteria

Check valves prevent the backflow of media in pipelines and systems by allowing flow in only one direction. In the manufacturing industry, they serve to protect pumps, compressors, control and measurement systems, and to prevent process disruptions caused by pressure surges and cavitation. When selecting and installing, material compatibility, connection type, sealing materials, pressure/temperature range, and installation position are crucial.

Technical Principles and Operating Mechanism

The basic principle is based on a movable closing element (flap, ball, diaphragm, or piston) that opens under forward pressure and is pressed against a seat surface under back pressure. Typical operating principles include spring-assisted closures for fast closing times, weight-guided flaps for simple applications, and pivoting flap seats for larger nominal diameters. Important parameters include response pressure, pressure loss (Kv value), maximum switching frequency, and permissible differential pressure. For design purposes, the valve's characteristic curve must be checked, especially the flow coefficient Kv at the intended Reynolds number and viscosity of the medium.

Materials and Corrosion Resistance

Material selection depends on the medium (water, oil, gas, steam, aggressive chemicals), temperature, and ambient conditions. Common materials include stainless steel (1.4404/316L) for corrosion resistance and hygienic applications, brass and bronze for water installations and oil, and carbon steel-coated variants for high mechanical stress. For aggressive media, special materials such as Hastelloy, duplex, or Monel alloys are used. At high temperatures, ceramic or PTFE-coated seat surfaces are also possible.

Seals and Seating Systems

The choice of seal influences tightness, friction, service life, and approval for media contact. Elastomer seals made of NBR (nitrile) cover many hydraulic applications, FKM (Viton) is suitable for high temperatures and fuels, EPDM is resistant to hot water and steam, PTFE offers chemical resistance and low friction under low stress. Metallic seals or metal-backed PTFE seats are used where elastic materials are not permissible or high abrasion resistance is required. The seal design also determines the dead volume and thus the risk of deposits and contamination in sensitive processes.

Designs and Connection Types

Check valves are available as inline elements for pipelines, screw-in valves for fitting connections, inline modules, or as compact block housings. Designs differ in flow path (straight, angled), mechanism (ball, disc, spring-loaded piston), and mounting (flange, weld, clamp, or threaded connection). For pneumatic applications, line-on-line modules with compact sizes are common. For industrial piping networks, DIN and ANSI flange versions are relevant. Threaded connections follow common standards such as ISO228/1, BSP, NPT, or metric threads; the choice influences assembly effort and tightness.

Pressure and Temperature Ranges

Check valves for low-pressure applications in water and pneumatics typically operate up to 16 bar, while hydraulic and steam-powered systems require versions up to 400 bar or more. The temperature range and material coordination are critical: elastomer seals limit the maximum temperature, while metallic seats allow higher temperatures but may have a higher leakage rate in the micro-range. Manufacturer specifications for permissible operating pressure, test pressures, and burst pressure are binding for selection.

Leakage Classes, Test Regulations, and Standards

For industrial applications, leakage guidelines according to VDI, DIN EN, or API are relevant. Valves for hygienic processes must also meet food-grade or FDA-compliant materials and surface roughness requirements. Test regulations such as DIN EN 12266 (Testing of Industrial Valves) provide criteria for tightness and function. For safety-relevant installations, regular test intervals, test pressure, and test protocols must be documented.

Design Aspects and Installation Instructions

Correct installation prevents malfunctions: valves must be installed marked in the direction of flow; for spring-loaded types, the response pressure must be matched to the system pressure. The installation position influences closing behavior and abrasion: horizontal position is standard; for vertical installation, attention must be paid to the flow direction (upward flow preferred for gravity-affected media). Media containing particles require protective filters or strainers upstream of the valve to prevent seat damage. For pulsating pressure conditions, damping measures such as accumulators or check dampers should be considered.

Practical Application Examples

Example 1 – Pump protection in a cooling water circulation system: In a cold water circulation, a spring-loaded ball check valve prevents backflow after the pump is shut down. The valve is made of stainless steel 316L, with a PTFE seat, flanged connection according to DIN, nominal size DN50. The combination of low leakage rate and corrosion-resistant material prevents backflow of the cooling fluid and relieves the return network. A 200 μm fine filter is located upstream of the valve to protect the seat surfaces. Periodic inspection of the seal and cleaning of the filter are documented in the maintenance interval.

Example 2 – Pressure stabilization in a compressed air network: In an industrial compressed air distribution network, compact line-on-line check valves with low response volume are installed to prevent back pressure after compressed air accumulators. The valves are equipped with NBR seals for temperatures up to +80 °C and are mounted in narrow busbars. In the event of pressure loss in the supply line, the valves prevent escape from connected consumers and protect sensitive measurement and control technology.

Example 3 – Protection of measuring instruments in chemical plants: For corrosive media, a check valve with a Hastelloy housing and a PTFE-backed seal was used. The valve is designed as a pivoting flap with minimal dead volume to prevent resinification. Through the material- and seal-specific selection, chemical attacks are prevented, and measurement deviations due to contaminated samples are avoided.

Selection Criteria and Test Parameters

The following selection criteria serve for quick orientation:

• Medium and chemical resistance, temperature and viscosity; permissible pressure and nominal size; connection type and installation position; leakage rate and reaction time; maintenance accessibility and conformity to standards.

Installation, Maintenance, and Failure Modes

Typical maintenance work includes visual inspection for corrosion, leak testing under operating pressure, and replacement of elastomer seals. Failure modes include continuous leakage (worn seat or damaged seal), delayed closing (stuck ball/piston), and noise generation due to cavitation. Causes include foreign bodies, unsuitable installation position, excessive flow velocity, or material fatigue. Measures include cleaning, replacement of sealing inserts, installation of a flow restrictor, or switching to a more robust design.

Standards, Certifications, and Documentation

Essential standards include DIN EN 12266 (Valve Testing), DIN EN ISO 4126 for safety-relevant components, and specific ASTM/ASME standards for high pressures and temperatures. For use in food or medical technology applications, FDA and EC-1935/2004 compliant material certificates and surface roughness declarations are required. Technical data sheets, material certificates (EN 10204 3.1), and test protocols must be documented and accessible.

Further Information and Application Examples

For technical details, materials, and specific application cases, please refer to the overview page Technology and the collection of specific practical cases Application Examples. There you will find further tables on materials, sealing options, and installation diagrams.

FAQ

1. How do I choose the right check valve for my system?

Choose based on the medium, temperatures, pressure ranges, nominal size and connection type, as well as the required leakage rate. Check material compatibility and sealing materials. For abrasive or particle-laden media, protective filters should be planned. For safety-critical applications, normative test requirements apply. Document the manufacturer's data sheet and test protocol.

2. Which sealing materials are suitable for high temperatures and aggressive media?

For high temperatures and aggressive media, PTFE-backed seats or metallic seats with PTFE filling are advantageous. Hastelloy or duplex housings offer corrosion resistance. Elastomers such as FKM (Viton) cover moderate to high temperatures, EPDM is suitable for hot water/steam, NBR for many oil applications. The final selection is made based on a chemical resistance test against the operating medium.

3. What causes frequent failures of check valves?

Frequent causes include foreign body impact on seat surfaces, incorrect installation direction, unsuitable installation position, exceeded pressure/temperature limits, insufficient filtration, and unsuitable sealing materials. Measures include consistent filtration, appropriate material selection, regular maintenance, and, if necessary, switching to a more robust design with a metallic seat.