Valve Leakage Classes & Sealing Standards: API 598, FCI 70-2, ISO 5208 Explained
Why Leakage Class Matters
"Zero leakage" is not a single number — it is a class. Specifying the wrong seat-leakage class either wastes budget on bubble-tight seats you do not need, or lets a supposedly shut valve weep in a critical line.
API 598 (Shell and Seat Testing)
API 598 defines acceptance tests for gate, globe, ball, check, and plug valves. Seat leakage is reported as allowable drops or bubbles per minute depending on size and type.
FCI 70-2 (ANSI/FCI 70-2) Seat Leakage Classes
FCI 70-2 is the most cited seat-leakage standard for control valves.
| Class | Allowable Leakage (air/water) |
|---|---|
| Class I | No requirement (assembled-only) |
| Class II | 0.5% of rated Cv per psi |
| Class III | 0.1% of rated Cv per psi |
| Class IV | 0.01% of rated Cv per psi |
| Class V | 5×10⁻⁴ ml/sec per inch of port dia per psi |
| Class VI | Bubble-tight (max bubbles/min by port size) |
ISO 5208 Leakage Rates
ISO 5208 uses letter grades A, B, C, D, E and AA through EE for tight shutoff, increasingly stringent. Grade A is the least tight; EE approaches bubble-tight.
Choosing the Right Class
- On/off isolation: Class VI (FCI) or Grade E/EE (ISO) for bubble-tight.
- Throttling control: Class IV is the common default; Class V for cleaner fluids.
- Safety/relief tie-in: match the upstream classification of your system.
The seat material (soft vs metal) decides the achievable class — see our material compatibility guide for trim choices.
Test Medium and Direction
Leakage is always specified for a given test medium (air or water) and flow direction. A valve that passes air bubble-tight may weep with viscous fluid. Cross-check against valve standards before purchase.
Per-product leakage class and test data are catalogued across valve brands in the ValveSpecs Pro database.
Frequently Asked Questions
What is the tightest valve leakage class?
FCI 70-2 Class VI is bubble-tight (soft seat); ISO 5208 Grade EE is the tightest letter grade. Metal-seated valves rarely achieve bubble-tight.
Is Class IV good enough for control valves?
Class IV (0.01% of Cv per psi) is the common default for throttling control valves and is suitable for most process services.
Does API 598 cover seat leakage?
Yes. API 598 specifies both shell and seat leakage acceptance tests for the main valve types, reported as allowable leakage rates.
Can a metal-seated valve be bubble-tight?
Generally no. Bubble-tight (Class VI) requires a soft seat. Metal-seated valves are used where temperature or abrasion rules out soft seats, accepting some leakage.
Where do I find a product's leakage class?
Leakage class is on the manufacturer test certificate. The ValveSpecs Pro database records seat type and test class for products across 90-plus brands.