Valve Leakage Class: API 598 vs FCI 70-2

Quick Answer

Valve leakage class defines the maximum allowable seat leakage when the valve is closed. Two standards dominate: FCI 70-2 / ANSI/ISA 75.02 (for control valves, Classes I through VI) and API 598 (for isolation valves). Class V (0.0005 mL/min per inch of port diameter per psi differential) and Class VI (bubble-tight) are the most commonly specified for oil and gas applications.

FCI 70-2 Leakage Classes

Class	Leakage Rate	Test Medium	Test Pressure	Typical Application
I	No test required	-	-	Non-critical, manual isolation
II	0.5% of rated Cv	Air or water	45-60 psi	General purpose control
III	0.1% of rated Cv	Air or water	45-60 psi	Better-than-standard control
IV	0.01% of rated Cv	Air or water	45-60 psi	Tight shut-off, metal seat
V	0.0005 mL/min per inch of port OD per psi differential	Water	Max operating differential	High-performance metal seat
VI	Bubble-tight (see table below)	Air or nitrogen	50 psi (3.4 bar)	Soft seat, critical isolation

Class VI Bubble Rates

Class VI leakage is measured as the number of air bubbles per minute through an inverted test tube submerged in water:

Valve Port Diameter	Maximum Bubbles per Minute
1” (25 mm)	1
1.5” (38 mm)	2
2” (50 mm)	3
3” (76 mm)	6
4” (102 mm)	11
6” (152 mm)	27
8” (203 mm)	45

FCI 70-2 vs API 598

Feature	FCI 70-2 / ANSI/ISA 75.02	API 598
Primary scope	Control valves	Isolation valves
Classes/Rates	Class I through VI	Rate A and Rate B
Test condition	Operating or specified differential	1.1x CWP (high pressure) and 0.6 bar (low pressure)
Zero leakage	Class VI approaches zero	Soft seat = zero visible leakage
Metal seat	Class IV or V (measurable leakage allowed)	Rate A (limited drops per minute)
Used by	Control valve specifications, ISA datasheets	Valve purchase orders, manufacturing QC
Standard body	Fluid Controls Institute / ISA	American Petroleum Institute

FCI 70-2 vs ISO 5208

FCI 70-2 Class	Approximate ISO 5208 Equivalent
Class VI (bubble-tight)	Rate A (zero visible leakage)
Class V	Rate AA to B
Class IV	Rate C to D
Class III	Rate D to E
Class II	Rate E to F

These are approximate equivalences. The test conditions differ between standards, so exact cross-referencing requires comparing actual leakage values at the same test conditions.

Selecting the Right Leakage Class

Application	Recommended Class	Reason
ESD / emergency isolation	Class V or VI	Safety-critical; minimal leakage required
Control valve (general)	Class IV	Standard metal-seated control valve
Control valve (tight shut-off)	Class V or VI	Process requires near-zero leakage when closed
Ball valve (soft seat)	Class VI / API 598 zero	Soft seat provides bubble-tight
Gate valve (metal seat)	Class IV / API 598 Rate A	Metal seat allows controlled leakage
Butterfly valve (triple offset, metal seat)	Class V	High-performance metal-to-metal
Globe control valve	Class IV or V	Depends on process requirements
Cryogenic valve	Class V or VI	Prevent product loss and ice formation

Practical Considerations

Specifying Class VI (bubble-tight) for all valves is unnecessary and expensive. Metal-seated gate valves and globe valves cannot reliably achieve Class VI because the metal-to-metal contact inherently allows some leakage. Requiring Class VI for these valves forces manufacturers to use soft inserts (PTFE, elastomer), which may not survive the service conditions (high temperature, fire, chemical attack).

Class V is the practical upper limit for metal-seated valves and is achievable with lapped metal seats. Class VI requires soft seats.

Common Mistake

Specifying FCI 70-2 leakage classes on isolation valve purchase orders. FCI 70-2 applies to control valves tested under operating-type conditions. For isolation valves, specify leakage acceptance per API 598 or ISO 5208 instead. Mixing standards creates confusion at the factory and during inspection.

Read the full guide to valve types