Valve Leakage Classes: ANSI FCI 70-2 and API 598 Explained

Valve seat leakage — the flow through a valve when it is nominally closed — is a critical performance parameter for isolation and control valves. Zero leakage in a mechanical seal is physically impossible; all seat designs allow some leakage. The question is how much leakage is acceptable for a given application. Standards bodies ANSI (via FCI 70-2) and API (via API 598) define standardised leakage classes that allow engineers, valve manufacturers, and inspection bodies to communicate leakage performance requirements unambiguously.

ANSI FCI 70-2 Leakage Classes

ANSI/FCI 70-2 (Control Valve Seat Leakage) defines six leakage classes, primarily intended for control valves but widely referenced for all valve types:

Class VI: Bubble-Tight Shut-Off

Class VI is the highest leakage class and is described as 'bubble-tight.' The test method uses air or nitrogen at 3.5 bar (50 psi) differential with the outlet immersed in water. The maximum allowable leakage rate is expressed as a number of bubbles per minute, which varies from 1 bubble/min for a 1" valve port to 45 bubbles/min for a 4" port (per ANSI/FCI 70-2 Table I). Ball valves with PTFE seats inherently achieve Class VI. Metal-seated ball valves, gate valves, and globe valves typically achieve Class IV to Class V unless lapped seats are specifically specified and tested.

API 598: Valve Inspection and Testing

API 598 is the inspection and testing standard for gate, globe, check, ball, butterfly, and plug valves. It defines three types of tests: shell test (body hydrostatic), backseat test (stem seal under pressure), and closure test (seat leakage). API 598 closure tests are performed at both low pressure (5.5 bar with air or water) and high pressure (1.1× rated pressure with water). API 598 defines test acceptance criteria that are broadly equivalent to ANSI FCI 70-2 Class IV for metal-seated valves and Class VI for soft-seated valves.

Which Leakage Class to Specify

• Class VI (bubble-tight): Gas isolation valves, valves on toxic or hazardous fluid lines, valves where any leakage creates a safety hazard, ESD valves on gas pipelines — specify soft-seated ball valves or PTFE-seated butterfly valves
• Class V: High-pressure liquid hydrocarbon isolation, refinery process block valves, pump isolation where significant liquid leakage is unacceptable
• Class IV: Standard for most process gate, globe, and ball valves — API 600, API 602, API 6D valves tested to API 598 achieve Class IV as standard
• Class II / III: Only appropriate for non-critical general utility services — water, steam, compressed air where minor leakage is acceptable
• Note: Never over-specify leakage class — Class VI metal-seated valves require precision lapping and are 2–5× more expensive than standard. Over-specification is as costly as under-specification.

Leakage Class and Valve Type

Documentation Requirements

Valve test certificates (Material Test Reports, Dimensional Inspection Reports, and Hydrostatic / Leakage Test Certificates) should be requested for all valves in critical service. For Class V and VI testing, the test pressure, test medium, test duration, and observed leakage rate must all be recorded on the certificate. Third-party inspection (TPI) by Lloyd's Register, Bureau Veritas, SGS, or other approved body is required for PED Category III/IV valves and for projects with specific quality plans.