PSV-cal

Pressure Safety Valve (PSV) Sizing Calculator

API RP 520 (Sizing & Selection) & API RP 521 (Overpressure Protection / Fire Case) — Metric Units (Kg/cm²g, °C, Kg/hr, cP)

Page size A4 A3

Orientation Vertical Horizontal

Formulas: API RP 520 Part I (9th/10th Ed. metric), API RP 521 (fire case)

Engineering note: This tool performs preliminary PSV orifice sizing per the standard API RP 520 metric equations. Final valve selection, Kb/Kw/Kv correction factors, rated capacity certification, inlet/outlet piping (3% / 10% pressure-drop checks per API 520 Part II), and relief-header backpressure analysis must be verified by the responsible process/safety engineer and cross-checked against vendor capacity certification (NB/ASME) data before issue for construction / MOC closure.

1. General / Common Data

Tag No.

Equipment / Service

Fluid

Set Pressure, kg/cm²g

Allowable Overpressure, %10% single valve; 16% fire case; 21% supplemental

Built-up Backpressure, kg/cm²g

Atmospheric Pressure, kg/cm²a

Molecular Weight, kg/kmol

k = Cp/Cv

Compressibility Factor, Z

Relieving Temperature, °C

Liquid Specific Gravity, G (rel. water)

Liquid Viscosity, cP

Kd — Discharge Coeff. (vapor) i

✕Kd — Effective Coefficient of Discharge (vapor/steam). Standard value 0.975 for a certified PRV. Not read from a curve — it is the ASME/NB-certified value stamped on the valve nameplate.
Ref: API RP 520 Part I, §5.6.

Kd — Discharge Coeff. (liquid) i

✕Kd — Effective Coefficient of Discharge (liquid). Standard value 0.65 for a certified PRV in liquid service; 0.62 when sizing a rupture disc alone (no PRV).
Ref: API RP 520 Part I, §5.6.

Kb — Backpressure Corr. (vapor) i1.0 conventional; from Fig.30/31 for bellows

✕Kb — Capacity Correction Factor for Backpressure, vapor/gas. Kb = 1.0 for conventional (non-bellows) valves within the allowable backpressure limit. For balanced-bellows valves, read from the backpressure-ratio curve.
Ref (7th Ed.): Fig.30 (bellows, vapor/gas); Fig.35 (conventional, when backpressure exceeds normal limit). Figure numbers shift between editions — confirm against your copy of API RP 520 Part I.

Kw — Backpressure Corr. (liquid) i

✕Kw — Capacity Correction Factor for Backpressure, liquid, balanced-bellows valves. Kw = 1.0 for conventional valves; for bellows valves, read from the backpressure-ratio curve.
Ref (7th Ed.): Fig.31. Numbering varies by edition — confirm against your copy.

Kc — Rupture Disc Combination i1.0 = no RD upstream; 0.9 = with RD

✕Kc — Combination Correction Factor, rupture disc upstream of PRV. Kc = 1.0 with no rupture disc; 0.9 when a disc + PRV combination has not been NB-certified together (use the certified Combination Capacity Factor if available). Formula/table-based, not read from a figure.
Ref: API RP 520 Part I, §5.6.5 / §5.7.

Kv — Viscosity Correction (liquid) i1.0 if Re>~100,000 (light HC); else from API 520 curve

✕Kv — Viscosity Correction Factor, liquid service. Function of Reynolds number at the trial orifice area (iterative). Kv = 1.0 for turbulent flow (Re typically > ~16,000–100,000, most light hydrocarbons).
Ref: API RP 520 Part I, viscosity-correction curve (numbered Fig.33/34 depending on edition — confirm against your copy).

2. Equipment Fire-Exposed Area Module (API RP 521 §4.4 / Annex D)

Add every item of equipment within the common fire-exposed zone (vessel, drum, exchanger shell, compressor casing, etc.) protected by this PSV / relief header. Each item computes its own wetted (liquid-containing) or total exposed (gas-filled) surface area from its geometry, or a direct area entry — the areas are then summed to a single Total Fire-Exposed Area which feeds the Fire Case tab. One common Environmental Factor F and drainage credit is applied to the summed area (standard API 521 practice for a shared fire zone).

Drainage & Fire-fighting (applies to group total) Adequate drainage & firefighting (C₁=43,200) Inadequate drainage / no firefighting (C₁=70,900)

Environmental Factor, F (applies to group total) Bare vessel (F=1.0) Insulated — typical 1" (F=0.3) Insulated — typical 2" (F=0.15) Insulated — typical 4" (F=0.075) Water-application facility (F=1.0, credit via C₁ instead) Earth-covered storage (F=0.03) Below-grade / underground (F=0.0)

F — custom value used in calc

Equipment	Geometry	Computed Area, m²

Total Fire-Exposed Area (sum of all equipment), A
— m²

Manually override Total Fire-Exposed Area (skip the geometry calculation above)

Manual Total Area, m²

3. Relief Load Cases

Step 1 — Select PSV Duty / Fluid State

Gas/Vapor PSV selected — applicable relief scenarios shown below: External Fire, Blocked Outlet/CV Fail, Tube Rupture. Switch above to change fluid state; the summary in Section 4 will only compare cases relevant to the selected duty.

Step 2 — Applicable Relief Scenarios

Case 1: External Fire

Case 2: Blocked Outlet / CV Fail

Case 3: Liquid Thermal Expansion

Case 4: Exchanger Tube Rupture

Case 5: Blocked Outlet (Liquid)

Case 1 — External Pool Fire (API RP 521 §4.4)

Fluid Generated Gas / Vapor (hydrocarbon)Steam

Latent Heat of Vaporization, kJ/kg

Flow Type Critical Flow (to flare header)Sub-critical Flow

Ksh — Superheat Correction i

✕Ksh — Steam Superheat Correction Factor. Ksh = 1.0 for saturated steam. For superheated steam, read from the superheat-correction table (interpolated by pressure and temperature).
Ref: API RP 520 Part I, Superheat Steam Correction Factor table (commonly Table 8/9 — confirm against your edition).

Kn — Napier Correction (steam) i

✕Kn — Napier Steam Correction Factor. Kn = 1.0 for set pressures below ~10,340 kPa (1500 psig). Above this, apply the Napier correction curve/table up to ~20,680 kPa (3000 psig); beyond that, use the critical-flow steam model directly.
Ref: API RP 520 Part I, Napier Correction Factor table (commonly Table 8 — confirm against your edition).

Heat Input, Q
— kW

Vapor Relief Load, W
— kg/hr

Required Orifice Area
— mm²

Case 2 — Blocked Discharge / Control Valve Fail-Open / Gas Blowby (Vapor)

Relief Load, W, kg/hr

Fluid Type Gas / Vapor (hydrocarbon)Steam

Flow Type Critical FlowSub-critical Flow

Ksh — Superheat Correction i

✕Ksh — Steam Superheat Correction Factor. Ksh = 1.0 for saturated steam; for superheated steam, read from the superheat-correction table (interpolated by pressure and temperature).
Ref: API RP 520 Part I, Superheat Steam Correction Factor table (commonly Table 8/9 — confirm against your edition).

Kn — Napier Correction (steam) i

✕Kn — Napier Steam Correction Factor. Kn = 1.0 for set pressures below ~10,340 kPa (1500 psig); above this apply the Napier correction curve/table up to ~20,680 kPa (3000 psig), beyond which the critical-flow steam model is used directly.
Ref: API RP 520 Part I, Napier Correction Factor table (commonly Table 8 — confirm against your edition).

Required Orifice Area
— mm²

Case 3 — Liquid Thermal Expansion (blocked-in liquid-full line/exchanger, API 521 §4.14)

Q_relief (m³/hr) = (β × H × 3600) / (ρ × Cp)  —  β = cubical expansion coeff. (1/°C), H = heat input (kW), ρ = density (kg/m³), Cp = specific heat (kJ/kg·°C). This case typically governs the smallest orifice (often D or E) and is rarely limiting.

Cubical Expansion Coefficient, β — Guideline (API RP 521, Table 2)

Quick-pick β by liquid gravity i

✕API RP 521, Table 2 — Typical Values of Cubic Expansion Coefficient for Hydrocarbon Liquids and Water (at 60°F/15.6°C basis). These are generic hydrocarbon-liquid correlations by gravity band; for a specific fluid/mixture, use actual process/simulation data (e.g. HYSYS/Aspen) where available — the API table is a fallback when fluid-specific data isn't at hand. Water does not follow the hydrocarbon correlation (its real β is much lower and non-linear with temperature) — use the dedicated Water row.

— select to auto-fill β — °API 3–34.9 (SG 1.052–0.850) — β = 0.00072 /°C (0.0004 /°F) °API 35–50.9 (SG 0.850–0.775) — β = 0.0009 /°C (0.0005 /°F) °API 51–63.9 (SG 0.775–0.724) — β = 0.00108 /°C (0.0006 /°F) °API 64–78.9 (SG 0.724–0.672) — β = 0.00126 /°C (0.0007 /°F) °API 79–88.9 (SG 0.672–0.642) — β = 0.00144 /°C (0.0008 /°F) °API 89–93.9 (SG 0.642–0.628) — β = 0.00153 /°C (0.00085 /°F) °API 94–100+ / lighter (SG 0.628–0.611) — β = 0.00162 /°C (0.0009 /°F) Water (SG 1.000) — β = 0.00018 /°C (0.0001 /°F)

Cubical Expansion Coeff. β, 1/°Cheavier liquid (higher SG) → lower β; lighter liquid → higher β

Liquid Density, kg/m³

Liquid Specific Heat, Cp, kJ/kg·°C

Heat Input, H — Guideline & Helper

Two recognized bases are commonly used for the blocked-in thermal relief heat input, per API RP 521 and widely-published industry practice (not a single fixed API equation — engineering judgment applies):

• Solar radiation on the exposed surface of the blocked-in, liquid-full line/equipment: typical design flux 0.79–1.04 kW/m² (≈250–330 Btu/hr·ft²), applied to the exposed (or projected) surface area. Use the lower end for temperate climates / part-insulated lines, the upper end for full tropical sun exposure on bare pipe.
• Heat exchanger duty, when the blocked-in liquid is on one side of an exchanger and the other side continues to circulate a hot medium — use the actual exchanger duty from the datasheet/simulation (this is normally far larger than the solar case and will govern if applicable).

In practice many operating companies simply fit the smallest standard PSV (¾"×1", orifice D/E) on blocked-in liquid-full segments without a rigorous heat balance, since the computed relief rate is almost always negligible — reserve the full calculation for larger-bore / long-length blocked segments or where an exchanger duty applies.

H Estimation Method Manual / Direct Value (kW) Solar Radiation on Exposed Area Heat Exchanger Duty (from datasheet)

Exposed Surface Area, m²use the Equipment Area module (Section 2) result if applicable, or pipe OD×length

Solar Flux, kW/m² Temperate climate / typical (0.79 kW/m² ≈ 250 Btu/hr·ft²) Moderate-high sun (0.94 kW/m² ≈ 300 Btu/hr·ft²) Full tropical sun, bare pipe (1.04 kW/m² ≈ 330 Btu/hr·ft²)

Flux — custom value used in calc

Heat Input, H, kWsolar / fire exposure / adjacent hot stream

Volumetric Relief Rate, Q
— m³/hr

Required Orifice Area
— mm²

Case 4 — Heat Exchanger Tube Rupture (API 521 §4.9, single-tube rupture rule)

Flow through a single ruptured tube is taken as twice the flow area of one tube (conservative rule-of-thumb, full-bore rupture), based on the high-pressure side conditions flashing/flowing into the low-pressure side. Flow computed via nozzle equation at tube-rupture ΔP.

Tube Fluid Phase Gas / VaporLiquid

Tube ID, mm

High-Pressure Side Pressure, kg/cm²g

Low-Pressure Side (relief set) Pressure, kg/cm²g

HP-side Liquid Density, kg/m³

Tube-Rupture Relief Load, W
— kg/hr

Required Orifice Area
— mm²

Case 5 — Blocked Outlet / Pump Deadhead / Overfilling (Liquid)

Relief Load, Q, m³/hr

Required Orifice Area
— mm²

4. Governing Case Summary & PSV Selection

Relief Case	Basis	Relief Load	Required Area, mm²	Selected Std. Orifice	Orifice Area, mm²
Click "Calculate All Cases" to populate summary

Governing Case (Maximum Required Area)
—

API Standard Orifice Reference — API 526 (Horizontal Table)

Multiple flange-rating combinations exist per API 526 for a given orifice letter (e.g., 3J4, 3"×4" 150#/300#/600# etc.). The table above lists the most commonly applied inlet×outlet flange size; confirm final selection with valve vendor GA drawing, inlet piping 3% pressure-drop check and outlet piping built-up backpressure check (API 520 Part II) before MOC/PO issue.

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