Relief Valve Sizing: J to N
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) |
|---|---|---|---|---|---|---|---|---|---|
| J | 0.85 | 1 | 0.975 | 1 | 1.287 | 830 | 3 | J orifice typical 3x4 connection | 4 |
| K | 0.84 | 1 | 0.975 | 1 | 1.838 | 1,186 | 3 | K orifice typical 3x4 connection | 4 |
| L | 0.83 | 1 | 0.975 | 1 | 2.853 | 1,841 | 4 | L orifice typical 4x6 connection | 6 |
| M | 0.82 | 1 | 0.975 | 1 | 3.6 | 2,323 | 4 | M orifice typical 4x6 connection | 6 |
| N | 0.82 | 1 | 0.975 | 1 | 4.34 | 2,800 | 6 | N orifice typical 6x8 connection | 8 |
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