PSVCalc Pro — API 521 / 520 / 526 Process Pressure Relief & Sizing Workbench

Select API 521 Overpressure Scenario

API 521 §4.4

1A. External Fire — Wetted Vessel Inputs

API 521 §4.4.13.2.4

Base Scenario Calculation Trace & Geometry Visualizer

Multi-Scenario Overpressure Summary & Governing Case Matrix

Compares all 6 API 521 scenarios side-by-side and identifies the Governing Orifice Area

API 521 Scenario	Overpressure	Relieving P₁	Relief Load (W)	Est. Req. Area	API 526 Orifice	Status / Action

Fluid Phase & HYSYS Property Inputs

API 520 Part I

1. Relieving Fluid Phase

2. Fluid Property Library (or Manual HYSYS Override)

Quick-Load Fluid Property Preset HYSYS DB

Molecular WeightM

kg/kmol

Specific Heat Ratio (Cp/Cv)k

Cp/Cv

Compressibility FactorZ

dim.

Liquid / Inlet Densityρ

kg/m³

Dynamic Viscosityμ

Steam Superheat (ΔT_sh)ΔT_sh

°C

Leung Omega Parameterω

API 520 Ann.C

3. Relieving Process Conditions

Required Relief RateW

kg/h

Relieving TemperatureT₁

°C

Normal Operating PressureP_op

barg

PSV Set Pressure (≤ MAWP)P_set

barg

Allowed OverpressureOP%

Atmospheric PressureP_atm

bara

Superimposed BackpressureP_bs

barg

Built-Up BackpressureP_bu

barg

4. Valve Construction & Correction Factors

Upstream Rupture Disc?K_c

Backpressure Factor ModeK_b / K_w

API 520 Part I Sizing Result & API 526 Orifice Selection

CRITICAL (CHOKED) FLOW

Selected API 526 Orifice

Inlet × Outlet: 4" x 6"

Utilization: 65.4% (Margin: 34.6%)

Required Effective Area (A_req) 1203.8 mm² 1.8659 in²

Selected API 526 Area (A_sel) 1840.6 mm² 2.8530 in² (Orifice L)

Relieving Pressure (P₁) 25.76 bara 373.7 psia (P_cf = 13.71 bara)

Total Backpressure (P₂) 1.95 barg 8.7% of P_set (P₂/P₁ = 0.115)

Correction Factors Kd=0.975 · Kb=1.00 Kc=1.00 · Kv=1.000

Valve Rated Capacity (W_rated) 43,501 kg/h 95,903 lb/hr (for Line Sizing)

API 526 Standard Orifice Scale (D to T)

API 526 7th Ed.

Pressure Relationship & Choking Ladder

API 520 Pt I

PSV Mechanical Cutaway & Nozzle Flow

Balanced Bellows

Backpressure Correction Curve (K_b / K_w)

API 520 Fig. 30 / 32

Step-by-Step API 520 Equation Breakdown (Whitebox Calculation Trace)

API 520 §5.6 Gas/Vapor

Interactive Inlet — PSV — Outlet Tailpipe Hydraulic Schematic & Axial Pressure Profile

Evaluation Flow Basis:

Inlet Line Sizing & 3% Non-Chatter Criterion

ΔP_in = 1.58% ≤ 3.0% PASS

Inlet Nominal Pipe SizeNPS_in

Pipe ScheduleSch

Equivalent Straight LengthL_in

Sum of Inlet Fittings KΣK_in

dim.

Multi-NPS Inlet Pressure Drop Screening Table

NPS	ID (mm)	Velocity (m/s)	ΔP_inlet	% of P_set	API 520 Pt II (≤3%)

Outlet Line Sizing, Backpressure & Reaction Force

P_bt = 13.9% PASS

Outlet Nominal Pipe SizeNPS_out

Outlet Pipe ScheduleSch

Tailpipe Length to HeaderL_out

Sum of Outlet Fittings KΣK_out

dim.

Hydraulic Coupling: Sync calculated Built-Up Backpressure (1.73 barg) into Step 2 Orifice Sizing

API 520 Part II Hydraulic & Reaction Force Equations

Centrifugal Pump Loop Hydraulics, NPSHa, Control Valve Cv & Line Sizing ("HYTOS" Module)

Per API 610 (12th Ed.), API RP 14E Erosional Velocity, and IEC 60534-2-1 Control Valve Sizing

EPC Pump Loop Preset:

1. Service, Liquid Properties & Flow Basis

API 610

Pump Equipment TagTag

Pump Service DescriptionService

Normal Volumetric FlowQ_norm

m³/h

Rated Over-Design MarginMargin %

Pumping Liquid Densityρ_L

kg/m³

Liquid Viscosityμ

Vapor Pressure at T_pumpP_vap

bara

Pumping TemperatureT_pump

°C

2. Suction Circuit & NPSH Basis

Suction Vessel PressureP_s

barg

Suction Static Elevation HeadZ_s

Suction Pipe NPS (ASME B36.10)NPS_s

Suction Pipe LengthL_s

Suction Fittings ΣK + StrainerΣK_s

dim.

Vendor Required NPSHr (3%)NPSHr

3. Discharge Circuit, Control Valve & Pump Efficiency

Destination Vessel PressureP_d

barg

Discharge Static Elevation HeadZ_d

Discharge Pipe NPSNPS_d

Discharge Pipe LengthL_d

Discharge Fittings ΣKΣK_d

dim.

Exchanger / Filter / Meter ΔPΔP_eq

bar

Control Valve Min ΔP (or 30% Dyn)ΔP_CV

bar

Selected Control Valve Rated CvCv_100%

Pump Hydraulic Efficiencyη_pump

Shutoff Head Rise (% of Rated)Rise %

API 610 Pump Duty, NPSHa Margin & Motor Sizing Summary

NPSHa PASS

Pump Head H(Q), System Resistance & NPSHa vs. NPSHr Curves

Control Valve Sizing Envelope (IEC 60534 Min / Norm / Rated)

Pump Differential Pressure & Energy Breakdown Waterfall (Suction → Static Lift → Friction → Equip → CV → Dest)

Suction & Discharge Line Sizing Screening (Click Row to Apply NPS)

Pipe NPS	ID (mm)	Suction Vel (m/s)	Suction ΔP/100m	Discharge Vel (m/s)	Discharge ΔP/100m	Apply NPS

API 610 / API 14E / IEC 60534 Calculation Trace

Instrument & Equipment Process Datasheet (IPDS)

Formatted for interdisciplinary review (Instrumentation, Mechanical, Rotating Equipment, Piping & Vendor Print Check)

Project PSV Tag Register & Relief Load Summary

Persisted automatically in browser LocalStorage. Load, compare, or export all PSV tags in the project.

Import JSON

Tag No.	Service / Equipment	Phase	Governing Scenario	P_set	Relief Rate (W)	A_req (mm²)	API 526 Orifice	Inlet Line (ΔP%)	Outlet Line (P_bt%)	Actions

Zero-Backend 1-Click Deployment & Offline Distribution

100% Static HTML/CSS/JS

Download Standalone Package

PSVCalc Pro is engineered as a self-contained, zero-dependency static web application (index.html, styles.css, engine.js, app.js). It requires no database server or runtime licenses and runs offline in any modern desktop or tablet browser.

Download Standalone Web App (.ZIP)

Deploy Free in 30 Seconds

Option A: Netlify Drop / Cloudflare Pages (Drag & Drop)

Drag and drop the unzipped folder onto app.netlify.com/drop or pages.cloudflare.com for an instant HTTPS link to share with process & instrument teams.

Option B: GitHub Pages / Vercel

Push the 4 static files to any GitHub repository and enable GitHub Pages (Root branch), or run npx vercel --prod.

Option C: Offline Plant / Site Laptop Execution

Unzip anywhere on Windows/macOS and double-click index.html — works completely offline during site walkdowns or HAZOP sessions.