API 6D and API 594 are core standards for different valve types in the oil and gas industry. The main differences are as follows:
Different scope of application
API 6D
Specifies the design, manufacturing, testing and documentation requirements of ball valves, gate valves, plug valves and check valves in pipeline systems, and is suitable for high-pressure scenarios such as long-distance pipelines and refineries.
API 594
Specially for check valves, it covers the design, materials and testing standards of flanged, lug, wafer and butt-weld check valves, especially for conditions to prevent backflow of media.
Differences in core requirements
- Design and manufacturing
API 6D: Mandatory reference to API 6DX (valve actuator specification), new design verification appendix and new requirements for bolt materials, and emphasis on full life cycle quality control (QSL classification).
API 594: Focuses on the structural strength and sealing of check valves, and specifies details such as valve body thickness and valve disc movement mechanism.
- Test requirements
| Item | API 6D | API 594 |
|---|---|---|
| Shell test | NPS ≥ 20 The valve needs to be subjected to hydrostatic test for 30 minutes | Conventional hydrostatic test time is short (usually ≤5 minutes) |
| Sealing test | The default medium is fresh water, which can be replaced by light oil; low-pressure gas seal needs to be agreed separately | Allows high pressure liquid or low pressure gas sealing test |
| Leakage rate | More stringent requirements (allowed leakage rate lower than API 598) | Comply with common valve testing standards (such as API 598) |
Comparison of application scenarios
API 6D: Applicable to key shutoff valves for main pipelines (such as ball valves for gas transmission trunks and gate valves for stations), which must meet the requirements of high pressure, large diameter and long life.
API 594: For anti-backflow systems such as pump outlets and compressor pipelines, ensure that the check valve closes quickly and seals reliably.
Correlation description
Some check valves must meet both API 6D and API 594 (such as swing check valves for pipelines).
API 6D covers a wider range of valve types, while API 594 is a special standard in the field of check valves.
