Relief Valve Sizing: P to T
API 526/520 standard pressure relief valve orifice designations — effective orifice areas, rated discharge coefficients (Kd), back-pressure correction factors (Kb), and inlet/outlet connection sizes for orifices D through T
| orifice | Kb balanced bellows 50pct (dimensionless) | Kb conventional (dimensionless) | Kd (dimensionless) | Ksh sat (dimensionless) | area in2 (in2) | area mm2 (mm2) | inlet size in (in) | notes | outlet size in (in) |
|---|---|---|---|---|---|---|---|---|---|
| P | 0.81 | 1 | 0.975 | 1 | 6.38 | 4,116 | 6 | P orifice typical 6x10 connection | 10 |
| Q | 0.8 | 1 | 0.975 | 1 | 11.05 | 7,129 | 8 | Q orifice typical 8x10 connection | 10 |
| R | 0.8 | 1 | 0.975 | 1 | 16 | 10,323 | 10 | R orifice typical 10x16 connection | 16 |
| T | 0.8 | 1 | 0.975 | 1 | 26 | 16,774 | 12 | Largest standard orifice; T orifice typical 12x16 connection | 16 |
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