Valve leakage classification defines how much internal leakage (seat leakage) is allowed when a valve is fully closed. These classifications are standardized under ANSI/FCI 70-2 (also known as IEC 60534-4 internationally).
This guide explains all six leakage classes, how they are tested, and how to select the correct class for your application.
Why Leakage Classification Matters
Choosing the right valve leakage class ensures:
Proper system shutoff
Safety and regulatory compliance
Correct actuator sizing
Long-term sealing performance
Compatibility with the process fluid
Incorrect leakage class selection can lead to energy loss, contamination, overheating, or dangerous pressure buildup.
Leakage Classifications Overview (Class I–VI)
ANSI/FCI 70-2 defines six leakage classes:
| Leakage Class | Description | Typical Valve Type |
|---|---|---|
| Class I | No test required (basic seat leakage) | General service valves |
| Class II | ≤ 0.5% of rated capacity | Metal seated control valves |
| Class III | ≤ 0.1% of rated capacity | Better metal-seated valves |
| Class IV | ≤ 0.01% of rated capacity | High-quality metal seats |
| Class V | Very low leakage: ≤ 0.0005 ml/min per inch of port | Precision metal seats |
| Class VI | “Bubble tight” (soft-seat valves only) | Ball valves, butterfly valves |
Leakage Class Definitions (Full Explanation)
Class I — No Leakage Test Required
Basic construction valves
No specific shutoff performance guaranteed
Usually cheaper valves
Used in: low-severity service, non-critical applications.
Class II — 0.5% Rated Valve Capacity
Moderate leakage allowed
Metal seats
Actuator must supply specific seating force
Used in: low-pressure steam, water, and gas service.
Class III — 0.1% Rated Valve Capacity
Tighter shutoff than Class II
Stronger seating force required
Used in: industrial process lines with moderate shutoff requirements.
Class IV — 0.01% Rated Valve Capacity
Called “metal-to-metal tight shutoff”
Most common in control valves
Requires well-machined seat and plug
Used in: oil & gas, chemical plants, refinery processes.
Class V — Very Tight, High-Performance Metal Seat
Leakage limit:
≤ 0.0005 ml/min per inch of port diameter per psi differential
Example:
2-inch valve at 100 psi →
0.0005 × 2 × 100 = 0.1 ml/min allowed
Used in:
High-pressure services
Hazardous fluids
High-temperature applications
Class V is the highest for metal-seated valves.
Class VI — Soft Seat “Bubble-Tight” Shutoff
Lowest leakage class
Uses elastomeric or PTFE soft seats
Zero visible bubbles allowed in test
Common in ball and butterfly valves
Used in:
Air, gas, and vacuum systems
Shutoff isolation
Safety systems
Class VI = the tightest leakage class available.
Leakage Class Comparison Table
| Class | Leakage Allowed | Seat Type | Typical Valve |
|---|---|---|---|
| I | Not tested | Metal | Basic isolation valves |
| II | 0.5% of rated flow | Metal | General service control valves |
| III | 0.1% of rated flow | Metal | Industrial control valves |
| IV | 0.01% of rated flow | Metal | Refinery, chemical valves |
| V | 0.0005 ml/min/in/psi | Metal | High-pressure critical valves |
| VI | Bubble-tight | Soft seat | Ball/butterfly valves |
How Leakage Classification Is Tested
Testing follows ANSI/FCI 70-2:
For Class I–V
Valve closed
Apply test pressure
Measure leakage through the seat
For Class VI (soft seat)
Valve submerged in water
Air applied to seating
No bubbles allowed
How to Choose the Right Leakage Class
For control valves
Class III or IV is typical
Class V for high pressure or hazardous fluids
For on/off isolation
Class VI (bubble-tight)
For metal-seated high-temperature service
Class IV or V
For inexpensive, non-critical valves
Class I or II
Common Mistakes
❌ Choosing Class VI for high-temperature service (soft seats may fail)
❌ Using Class II/III in hazardous gas lines
❌ Forgetting that higher leakage class requires a larger actuator
❌ Assuming all ball valves are Class VI (not always true!)
Conclusion
Valve leakage classifications (Class I–VI) define how much seat leakage is allowed when a valve is fully closed.
Choosing the correct class ensures safe, efficient, and reliable system operation.
In summary:
Class I: No test
Class II–III: Basic metal seat shutoff
Class IV: Standard industrial tight shutoff
Class V: High-pressure precision shutoff
Class VI: Bubble-tight soft-seat shutoff
