Process Safety, API 520 Pressure-Relief Sizing & Mechanical Integrity
API 520 Liquid Relief Valve Orifice Area & API 526 Formula
Calculate API 520 Part I Liquid Relieving Differential Pressure (psi), Required Effective Orifice Area (in²), Rupture-Disk Combination Area (in²), and Selected API 526 Standard Orifice.
Size ASME liquid relief valves at 10% overpressure (1.10 * P_set - P_back)—and round UP to an API 526 letter!
Under API Standard 520 Part I for ASME Section VIII non-fire liquid relief (e.g., blocked-outlet positive-displacement or centrifugal pump overpressure), full valve lift occurs at 10% overpressure above set pressure: Relieving Differential Pressure is dP_rel = 1.10 * P_set_psig - P_back_psig, and Required Effective Discharge Area (in²) is A_req = (Q_GPM * SQRT(SG)) / (38 * K_d * K_w * K_c * K_v * SQRT(dP_rel))! Never order a custom-drilled bore: engineers select the next LARGER standard API 526 lettered orifice (e.g., J = 1.287 in², K = 1.838 in², L = 2.853 in²)!
Verify on a Blocked-Discharge Hydrocarbon Pump PRV—Relieving_Flow Q = 400.0 GPM (B2), Specific_Gravity SG = 0.81 (SQRT = 0.90, B3), Certified_Liquid_Kd = 0.65 (B4), Viscosity_Factor K_v = 0.98 (B5), Set_Pressure = 100.0 psig (B6), and Constant_Backpressure = 10.0 psig (K_w = 1.00, B7): Relieving Differential Pressure equals 1.10 * 100 - 10 = 100.00 psi (E2), Standalone Required Area (K_c = 1.0) is (400 * 0.90) / (38 * 0.65 * 0.98 * 10) = 1.4872 in² (E3), Area With Upstream Rupture Disk (K_c = 0.90) is 1.6525 in² (E4), and Selected API 526 Orifice is "K (1.838 in2)" (E5)!
| API 520 Liquid PRV Sizing KPI (400 GPM, SG=0.81, Kd=0.65, Kv=0.98, 100/10 psig) | Formula | Verified Output |
|---|---|---|
| Relieving Diff Pressure (psi) | 1.10 * Set_psig - Backpressure_psig | 100.00 psi |
| Required Orifice Area Kc=1.0 (in²) | Q * SQRT(SG) / (38 * Kd * Kv * SQRT(dP)) | 1.4872 in² (1.8590 @ 500 GPM) |
| With Rupture Disk Kc=0.9 (in²) | Required_Area_Kc1 / 0.90 | 1.6525 in² |
| Selected API 526 Standard Orifice | Next Standard Letter >= Required_Area | K (1.838 in2) (L @ 500 GPM) |
API 520 Liquid Effective Discharge Area Formula
Validate Flow_GPM > 0, SG > 0, K_d in (0.40, 0.90], K_v in (0.30, 1.0], Set_psig >= 15, and 0 <= Backpressure < 0.80 * Set_psig!
=LET(q_gp, B2, sg_l, B3, kd_c, B4, kv_f, B5, ps_g, B6, pb_g, B7, IF(OR(q_gp <= 0, sg_l <= 0, kd_c <= 0.4, kd_c > 0.9, kv_f <= 0.3, kv_f > 1, ps_g < 15, pb_g < 0, pb_g >= 0.8 * ps_g), "Invalid API 520 Input", LET(dp_r, 1.1 * ps_g - pb_g, ROUND((q_gp * SQRT(sg_l)) / (38 * kd_c * kv_f * SQRT(dp_r)), 4))))Pump Impeller Upgrade Letter Step-Up Check (Set Relieving Flow = 500 GPM)
Increase Relieving_Flow B2 from 400 GPM to 500 GPM (a larger pump impeller): Required Orifice Area E3 rises to 1.8590 in², exceeding K-orifice capacity (1.838 in²) and automatically stepping E5 up to "L (2.853 in2)"!
=IF(ISTEXT(E3), E3, IF(E3 <= 1.287, "J (1.287 in2)", IF(E3 <= 1.838, "K (1.838 in2)", "L (2.853 in2)")))Try the example
Use the original values or change them to test your understanding.