Reference Tables
API 526 Orifice Letter Table
Standard PSV orifice designators (D through T) with effective area in in² and inlet/outlet flange size combinations per API Std 526.
README
API 526 Orifice Letter Table — Reference
| Purpose | Standard orifice letter sizes per API 526 for flanged, spring-loaded PSVs |
|---|---|
| Source | API Standard 526, 7th Edition (2017), Table 1 |
| Source | API Standard 520 Part I (2020) for sizing equations |
| Usage | Match calculated orifice area to NEXT LARGER standard letter |
| Note | Effective areas shown here are the API 526 'catalog' values (not manufacturer certified) |
| Note | For ASME-certified sizing, use manufacturer-specific Kd and actual orifice |
Orifice Table
API 526 Standard Orifice Designations
| Letter | Effective Area (in²) | Effective Area (mm²) | Typical Inlet (NPS) | Typical Outlet (NPS) | Max Set Pressure (psig) | Notes |
|---|---|---|---|---|---|---|
| D | 0.11 | 71 | 1 | 2 | 6000 | Smallest standard; bonnet vent service |
| E | 0.196 | 126 | 1 | 2 | 6000 | |
| F | 0.307 | 198 | 1-1/2 | 2 | 6000 | Most common small PSV |
| G | 0.503 | 324 | 1-1/2 | 3 | 6000 | |
| H | 0.785 | 506 | 1-1/2 | 3 | 6000 | |
| J | 1.287 | 830 | 2 | 3 | 6000 | |
| K | 1.838 | 1186 | 3 | 4 | 6000 | Watch for inlet/outlet flange sizing |
| L | 2.853 | 1841 | 3 | 4 | 6000 | |
| M | 3.6 | 2323 | 4 | 6 | 3300 | |
| N | 4.34 | 2800 | 4 | 6 | 3300 | |
| P | 6.38 | 4116 | 4 | 6 | 1500 | |
| Q | 11.05 | 7129 | 6 | 8 | 1500 | |
| R | 16 | 10320 | 6 | 8 | 900 | Popular low-pressure flare valve |
| T | 26 | 16770 | 8 | 10 | 600 | Largest standard letter |
P T Limits
Typical P-T Ranges per API 526
| Class 150 | 285 | 140 | 1" through 10" |
|---|---|---|---|
| Class 300 | 740 | 550 | 1" through 10" |
| Class 600 | 1480 | 1095 | 1" through 10" |
| Class 900 | 2220 | 1640 | 1" through 8" |
| Class 1500 | 3705 | 2735 | 1" through 6" |
| Class 2500 | 6170 | 4560 | 1" through 4" |
Sizing Formulas
API 520 Part I — Quick Sizing Reminders
| Liquid (non-flashing) | A = Q/(38*Kd*Kw*Kc*Kv) * sqrt(G/(1.25*Pset - Pback)) | A in in², Q in gpm, G = SG |
|---|---|---|
| Vapor (critical) | A = W/(C*Kd*P1*Kb*Kc) * sqrt(T*Z/M) | API 520 Eq 3.6; W lb/hr |
| Vapor (subcritical) | A = W/(735*F2*Kd*Kc) * sqrt(T*Z/(M*P1*(P1-P2))) | Sub-critical flow |
| Steam | A = W/(51.5*P1*Kd*Kb*Kc*Kn*Ksh) | Napier equation |
| Two-phase | Per API 520 Part I, Annex C (omega method) | Use manufacturer Kd |
Source: API526_Orifice_Letter_Table_Reference_v1.xlsx
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