I. Application Areas
API 6A
API 6A is mainly for oil and gas wellhead equipment (such as oil extraction trees, blowout preventers) and well control equipment, and is suitable for high temperature, high pressure and corrosive media oil and gas extraction scenarios.
API 6D
Dedicated to oil and gas pipeline systems (such as long-distance pipelines, LNG storage and transportation, refining units), focusing on fluid control, pressure resistance and corrosion resistance.
Valve type and structure
API 6A
API 6A covers wellhead gate valves, throttling valves, safety valves, etc. The structure needs to be designed to adapt to the high pressure at the wellhead (e.g., blowout risk) and frequent operation.
Common materials are high-grade alloy steel (such as ASTM A182 F22), which need to meet the extreme working conditions of wellhead equipment.
API 6D
API 6D valves are mainly ball valves and flat gate valves, with full bore design to reduce pressure drop, suitable for large diameter (NPS 4~60) pipelines.
Valve body materials include stainless steel (ASTM A351 CF8M) and sulphur-resistant alloy steel, which meets the requirements of NACE MR0175 for resistance to hydrogen sulphide corrosion.
Performance and Testing Requirements
API 6A
Emphasises high-pressure tightness and blowout protection, and is subject to wellhead unit integrity testing (e.g., gas-tightness test).
Pressure ratings are typically higher than those of API 6D to accommodate extreme pressure fluctuations at the wellhead.
API 6D
Requires case strength test (1.5 times nominal pressure) and seal test (1.1 times nominal pressure).
An API 607/6FA fire test is required to ensure a tight seal in a fire environment.
Design and Certification Differences
Structural Design
API 6A valves tend to be compact to accommodate wellhead space constraints; API 6D valves are more long-life and low-maintenance for long-term pipeline operation.
API 6D allows for multi-piece gate configurations (e.g., resilient gates), whereas other standards, such as API 600, may be limited to a single gate.
Scope of Certification
API 6A certification covers the performance of the wellhead assembly as a whole, including blowout preventers and valve linkage control; API 6D certification focuses on the performance of the valve as a unit and pipeline compatibility.
