Butterfly valve leakage class is a standardized rating that sets the maximum internal leakage allowed through the valve seat when fully closed. ANSI/FCI 70-2 and API 598 are the two most widely referenced standards in industrial valve procurement.
The leakage class rating measures the amount of fluid that passes through a closed valve seat under test conditions. Industrial standards assign a leakage class that sets the expectations of procurement teams and engineers before installation.
This article explains how leakage classes work, compares soft-seated and metal-seated valve performance, and outlines the factors that affect sealing in service.
Two Standards that Define Valve Leakage Class
There are two standards that govern most industrial butterfly valve applications. These standards define the maximum allowable leakage through a valve seat when it is fully closed.
- ANSI/FCI 70-2: Used widely for control valves. It defines Classes I through VI, each with a progressively tighter leakage limit.
- API 598: Applied to isolation valves. It separates metal-seated and resilient (soft)-seated valves with distinct test requirements.
European and international markets also reference EN 12266-1 and ISO 5208, which use similar logic with slightly different test parameters.
Leakage class is determined during a factory seat test. The test uses air, water, or nitrogen at a specified pressure with the valve in the closed position.
The result is the benchmark for that valve’s sealing performance throughout its service life.
What Are the Maximum Leakage Rates for Each Class?
The maximum leakage rates vary widely across classes. Selecting the wrong class can lead to product loss, safety incidents, or regulatory non-compliance.
The table below covers the classes most relevant to butterfly valve selection:
| Class | Standard | Max Leakage | Test Medium | Typical Valve Type |
|---|---|---|---|---|
| Class IV | ANSI/FCI 70-2 | 0.01% of rated flow capacity | Air or water | High-performance butterfly valve (HPBV), double-offset |
| Class V | ANSI/FCI 70-2 | 5 × 10⁻⁴ mL/min per psi per inch of port diameter | Water at max differential pressure | Triple-offset butterfly valve (TOV), precision metal seat |
| Class VI | ANSI/FCI 70-2 | Bubble-tight; allowable bubbles/min based on valve size | Air or nitrogen at 45–50 psi | Resilient-seated (soft-seated) butterfly valve |
| API 598 Metal | API 598 | Drops or bubbles per minute — quantity depends on valve size | Air or water | All new metal-seated isolation valves |
| API 598 Resilient | API 598 | Zero leakage during factory test | Air or water | All new soft-seated isolation valves |
Classes I through III are rarely specified for butterfly valves. Class IV is the practical minimum for metal-seated designs. Class VI is the standard for soft-seated butterfly valves.
Class IV permits leakage of up to 0.01% of the valve’s maximum rated flow capacity. Class VI (soft-seated) limits leakage to a defined number of bubbles per minute based on valve size. At 2 inches, that is 0.15 mL/min. At 8 inches, it rises to 1.70 mL/min.
Note: Always confirm the test pressure and medium on the valve datasheet. A Class VI rating tested at 50 psi is not equivalent to one tested at 80 psi.
Stem: 2Cr13, SS304, SS316, Monel
Comparing Soft-Seated vs. Metal-Seated Butterfly Valves
The seat material is the single biggest factor in determining the valve’s leakage performance. Both seat types serve distinct operating conditions.
Soft-Seated Butterfly Valves
Soft-seated valves use elastomeric or polymer seat materials. Common materials include EPDM, NBR, PTFE, Neoprene, and Viton.
When the disc closes, the soft seat deforms slightly against the disc edge. This creates a conforming, bubble-tight seal.
These valves perform best in water treatment, HVAC, food processing, and general utility services. Typical working pressure is up to 16 bar, and temperature stays below 200°C.
In high-temperature or abrasive media, zero leakage is not achievable with soft seats. Hence, a metal seat at Class IV is the practical alternative.
Achievable class: Class VI (ANSI/FCI 70-2) or zero leakage per API 598 Resilient*. Both qualify as a zero-leakage butterfly valve under standard factory test conditions.
Note: API 598 Resilient refers to the testing standard established by the American Petroleum Institute (API) for soft-seated valves.
Metal-Seated Butterfly Valves
Metal-seated valves use stainless steel, Stellite hardfacing, Inconel, or similar alloys for the seat surface.
The sealing mechanism is metal-to-metal contact. The disc closes against a precisely machined seat ring. Small leaks are normal because of surface variations.
Metal seats handle high-temperature steam, cryogenic service, fire-safe applications, and corrosive media. They also hold up better under high-cycle operation.
The trade-off is higher leakage versus soft-seated designs. For most critical isolation duties, Class IV is acceptable.
Achievable class: typically Class IV under ANSI/FCI 70-2. Triple-offset butterfly valves (TOV) with precision-lapped seats can reach Class V.
| Feature | Soft-Seated | Metal-Seated |
|---|---|---|
| Leakage Class | Class VI / API 598 Resilient (zero leakage) | Class IV (standard), Class V (precision TOV) |
| Temperature Range | Up to ~200°C | -196°C to 600°C+ depending on alloy |
| Media Suitability | Water, HVAC fluids, food-grade liquids, mild chemicals | Steam, cryogens, hydrocarbons, corrosive process media |
| Seat Life | Moderate; seat wear increases over high-cycle operation | Long; resists wear under high pressure and temperature |
| Best Application | Water treatment, HVAC, chemical dosing, general isolation | Refinery, power generation, LNG, fire-safe service |
Key Factors That Affect Valve Leakage in Service
Valve seat leakage does not stay constant over a valve’s service life. Several factors influence how well a valve maintains its rated class in real operating conditions.
- Seat material and hardness. Softer elastomers conform better at low pressures. Harder metals last longer but tolerate less surface imperfection.
- Disc geometry. Double and triple offset disc designs reduce seat contact friction. This lowers seat wear and improves sealing over many operating cycles.
- Differential pressure. Leakage increases when the pressure difference across the closed valve rises. Verify that the specified class applies at your actual operating differential.
- Operating temperature. Heat causes elastomers to harden or creep over time. This alters the contact geometry and reduces sealing integrity.
- Media compatibility. Corrosive or abrasive fluids attack seat surfaces. Surface roughness increases, and leakage follows.
- Actuator torque. An under-torqued actuator prevents the disc from fully seating. An over-torqued one accelerates seat wear. Both outcomes increase butterfly valve leakage over time.
- Maintenance history. Worn seats that have not been replaced will not hold their original leakage class. Schedule periodic seat inspections in high-cycle or corrosive service.
How Do You Choose the Right Leakage Class for Your Application?
Choosing the right leakage class starts with understanding your process requirements.
- Specify Class VI or API 598 Resilient for potable water, chemical dosing, and any service where visible leakage is not acceptable. A soft-seated design meets this standard in most conditions.
- Specify Class IV (API 598 Metal) for high-temperature steam, fire-safe service, and refinery processes where elastomers cannot survive.
- Consider Class V only when process criticality justifies the added cost. LNG terminals, power plant steam isolation, and some petrochemical duties fall into this category.
- Confirm test conditions. Check that the class stated on the datasheet matches the test pressure and medium used during factory inspection.
- Request a factory test certificate with every order. The certificate confirms the supplied valve meets the specified class before it reaches your site.
Conclusion
Matching leakage class to your application protects process integrity and reduces total cost of ownership over the valve’s service life.
Butterfly valves can achieve zero leakage with soft-seated butterfly valves. These soft-seated valves with EPDM, PTFE, or NBR seats meet Class VI under ANSI/FCI 70-2 and zero-leakage per API 598 Resilient. These valves qualify as a zero-leakage butterfly valve under standard factory test conditions.
TG Valve guarantees the full range of leakage class requirements. Our soft-seated products achieve extremely low valve seat leakage under most working conditions. They are built for water treatment, HVAC, chemical isolation, and general industrial service.
Our metal-seated and high-performance butterfly valves meet Class IV and above for demanding thermal, corrosive, and fire-safe duties.
All TG Valve products undergo factory seat leakage testing before delivery. Test certificates are available upon request. Our manufacturing meets ISO 9001 standards, while our valves comply with API 609, API 607, ISO 10497, and European CE marking requirements.
