Core positioning differences
| Standard | Applications | Valve Type | Typical calibre range |
|---|---|---|---|
| API 6D | Oil and gas long-distance pipelines (off-station transmission) (e.g. ASME B31.4/B31.8 pipelines) |
Pipeline valves (ball valves, gate valves, etc.) | NPS 4 to 60 (large calibres predominate) |
| API 608 | Station process piping (e.g. refineries, chemical plants) | General Metal Ball Valves | NPS ≤ 24 (small and medium calibre) |
Essential differences:
API 6D is for high pressure, long distance pipelines and focuses on safety and media integrity;
API 608 is for low and medium pressure piping within plants, with a focus on economy and versatility.
Comparison of key technical requirements
1. Channel design
API 6D: mandatory full bore design (channel aperture = line diameter), reducing flow resistance and preventing media deposition;
API 608: allows reduced design (channel aperture < pipe diameter), reduce costs but increase pressure drop.
2. Pressure Rating and Sealing
| Terms | API 6D | API 608 |
|---|---|---|
| Pressure Coverage | Class 150~Class 1500 | Class 150/300 only |
| Sealing Requirements | Zero leakage in both directions (mandatory for high pressure conditions) | Unidirectional seal (usually downstream seal) |
| Low Pressure Hermeticity | Mandatory (0.6 MPa gas tightness test) | not required |
3. Materials and structures
API 6D:
The valve body is required to resist sulfur corrosion (NACE MR0175 certification is required for sulfur-containing service);
Valve seats are welded with hard alloys (e.g. Stellite).
API 608:
Allows soft (PTFE/PEEK) or metal seals;
Material versatility (carbon steel/stainless steel).
4. Drive and fire protection
API 6D:
Compatible with pneumatic-hydraulic linkage, electric remote control;
Mandatory fire resistance certification (API 6FA/607).
API 608:
Manual/worm gear/electric based;
Fire resistance not mandatory.
Differences in testing standards
| Test Type | API 6D | API 608 |
|---|---|---|
| Shell test | 1.5×PN(water pressure) | 1.5×PN(water pressure) |
| High pressure sealing test | 1.1×PN(water pressure) | 1.1×PN(water pressure) |
| Span | ≥10minutes(Extension beyond Class 600) | ≥3minutes |
Special tests: API 6D requires a stem seal performance test (no leakage at 1.1 times pressure).
Engineering selection guide
Select API 6D:
Oil and gas long-distance pipeline (≥NPS 4);
Sulfur-containing media, high pressure (Class 300 or more) or fire safety certification is required.
Select API 608:
Internal plant process piping (≤ NPS 24);
Clean media, low to medium pressure (Class 150/300) and cost sensitive scenarios.
Synergistic relationship: API 608 ball valves are still required to meet ASME B16.34 pressure-temperature ratings and are not a substitute.
Summary: API 6D is a special safety standard for high-pressure full bore ball valves for oil and gas pipelines, while API 608 is an economical solution for small and medium bore general purpose ball valves. The core of the difference is reflected in the diameter design, pressure coverage, sealing grade and risk resistance.
