Valve Leakage Class Cross-Reference
API 598, ISO 5208 and BS 6755 all grade seat leakage on a lettered scale — but the same letter does not mean the same numeric rate. The cross-reference so you cite the right class on the right standard.
A datasheet that just says "Class D" or "Rate D" without naming the standard is ambiguous. API 598 Class D is defined per NPS (inch); BS 6755 Rate D is defined per mm DN (metric). They land at different positions in their own standard's scale for different reasons and are not interchangeable — always cite the standard alongside the class.
| Class / Rate | Definition | Numeric Limit | Basis | Typical Use |
|---|---|---|---|---|
| Class A | Zero leakage (bubble-tight) | zero (no rate) | — | Metal seats, safety valves |
| Class D | Soft-seat maximum leakage | 0.18 mL/min per NPS (for NPS ≤ 2" liquid) | per NPS (inch, nominal pipe size) | General soft-seated valves |
API 598 also defines intermediate Classes B and C between A and D; their specific numeric leak rates are not recorded in this reference — consult API 598 directly.
| Class / Rate | Definition | Numeric Limit | Basis | Typical Use |
|---|---|---|---|---|
| Rate A | Zero leakage — no measurable leakage | zero (no rate) | — | Gas and toxic fluid service |
| Rate D | Standard rate for most industrial valves | zero (no rate) | — | General industrial valves |
ISO 5208 defines Rates A through F (F = most relaxed); only Rates A and D have numeric definitions recorded here — consult ISO 5208 directly for B, C, E and F.
| Class / Rate | Definition | Numeric Limit | Basis | Typical Use |
|---|---|---|---|---|
| Rate A | Zero leakage (bubble-tight) at test pressure | zero (no rate) | — | Toxic, hazardous or zero-leakage applications |
| Rate B | Maximum leakage per unit nominal bore | 0.006 cm³/min per mm DN | per mm DN (metric, nominal diameter) | Gas and vapour service |
| Rate C | Maximum leakage per unit nominal bore | 0.18 cm³/min per mm DN | per mm DN (metric, nominal diameter) | General liquid service |
| Rate D | Maximum leakage per unit nominal bore | 1.8 cm³/min per mm DN | per mm DN (metric, nominal diameter) | Lower-criticality liquid isolation duties |
FAQ
Does 'Class D' in API 598 mean the same leak rate as 'Rate D' in BS 6755?
No. API 598 Class D is defined per NPS (nominal pipe size in inches); BS 6755 Rate D is defined per mm of DN (nominal diameter in millimetres). They are different measurement bases, not the same numeric rate, even though both sit in the 'D' position of their own standard's lettered scale.
Is a 'zero leakage' or 'bubble-tight' class the same across API 598, ISO 5208 and BS 6755?
Qualitatively yes — API 598 Class A, ISO 5208 Rate A and BS 6755 Rate A all define zero measurable leakage (bubble-tight) at test pressure. This is the one class where the three standards align, since 'zero' has no unit to differ by.
Which standard should I cite on a datasheet — API 598 or ISO 5208 or BS 6755?
Match the project's governing standard framework: API 598 for API/ASME-based (US/Middle East oil & gas) specifications, ISO 5208 for ISO-based international projects, and BS 6755 for UK-influenced or older British-specification projects. Citing a leakage class without naming which standard it's from is itself a common specification error this reference exists to prevent.
Connected Engineering
Specifying a leakage class on your datasheet?
Tell us the service and the governing standard — we supply valves tested and certified to the exact leakage class your specification calls for.