Valve Standard Reference
API 641
AmericanQualification Testing of Quarter-Turn Valves for Fugitive Emissions
API 641 is the primary qualification testing standard for fugitive emissions from quarter-turn valves — primarily ball valves and butterfly valves.
Overview — API 641
API 641 is the primary qualification testing standard for fugitive emissions from quarter-turn valves — primarily ball valves and butterfly valves. It establishes a type-testing protocol to qualify valve designs for low fugitive emission service, requiring valves to demonstrate stem-seal leak rates below defined limits across multiple thermal and mechanical cycles. API 641 compliance is increasingly required in petrochemical, refinery, and LNG specifications alongside ISO 15848-1.
Scope
Applies to quarter-turn valves (ball valves and butterfly valves) of NPS ½" to 12" intended for fugitive emission service. Valves are tested as representative of a 'design family' — once a valve size and design passes, smaller sizes of the same design are considered qualified.
Applicable Valve Types
Ball Valves — floating and trunnion-mounted, quarter-turn
Butterfly Valves — double-offset, triple-offset (API 609)
Applicable Pressure Classes
Key Requirements — API 641
310 mechanical cycles at ambient temperature (open-close-open = 1 cycle)
3 thermal cycles between −29°C and +200°C (or applicable service temperature range)
Leak rate measurement at each stage using methane or helium tracer gas per EPA Method 21
Acceptance criterion: leak rate ≤ 100 ppm (EPA Method 21) at stem and body-bonnet joint
Optional enhanced test to ≤ 500 ppm for less stringent fugitive emission applications
Valve must be representative of production design — no special or prototype construction
Test equipment calibration traceable to national measurement standards required
Testing Requirements — API 641
310 mechanical operating cycles at ambient temperature per API 641 procedure
Thermal cycling from −29°C to +200°C (3 cycles minimum)
Stem and body-bonnet leak measurement per EPA Method 21 at every stage
Leak rate result reported as ppm methane-equivalent
Final test pressure: 10% of rated pressure for leak measurement
Documentation Required for API 641 Compliance
API 641 type test report from accredited third-party test laboratory
Test report must include valve design drawings, packing material specification, and leak rate data at each test stage
Statement of qualified valve sizes and pressure classes covered by the test
Live-loaded packing specification if specified for qualification
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