Process Safety Engineering

Flare System Design & Analysis

API 521 / 537 / 14.10-compliant disposal network — Aspen Flarenet / PIPENET hydraulics, Hajek-Ludwig radiation, EPA 40 CFR 60.18 visibility

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Technical overview

Flare System Design
& Analysis

Flare system design integrates the disposal network from PRDs through collection headers, knockout drums, liquid seals, stack, and flare tip, ensuring safe routing of emergency and continuous relief loads without violating backpressure limits, thermal radiation thresholds, noise constraints, or environmental emissions standards. The discipline operates under API 521 / ISO 23251 (8th Ed., 2024) for sizing, API 537 (3rd Ed., 2017) for flare-tip selection and design, API STD 14.10 for offshore flare-and-vent, EPA 40 CFR 60.18 / 63.11 for environmental performance (98% destruction efficiency, exit velocity, visible emission limits), and the 2020 EPA Refinery MACT amendments tightening flare performance monitoring. Modern execution uses Aspen Flarenet, PIPENET, and Aspen HYSYS for hydraulic and thermodynamic modelling, FLACS / FRED for radiation and dispersion, and increasingly LIDAR-based flare-tip performance monitoring per the post-Texas City / Buncefield era. Common audit findings: undocumented simultaneous-relief assumptions, hot-case radiation exceedance at maintenance walkways, water-seal sizing not accounting for low-flow stagnation, and missing flare-gas-recovery integration for routine-flaring zero-tolerance commitments.

Flare System Design & Analysis — Overview
Engineering process

Flare System Design & Analysis workflow

Simultaneous Relief Load Study

Identify and justify all simultaneous relief load cases per API 521 Section 5; develop case matrix with fire credit, blocked-outlet simultaneity, and utility-failure scenarios.

Knockout Drum & Liquid Seal Sizing

Size knockout drum for 500 µm droplet separation per API 521; size water seal for purge-gas management and oxygen-ingress prevention across all relief scenarios.

Header Hydraulic Modelling

Model disposal network in Aspen Flarenet / PIPENET; verify Mach number (<0.7 sustained), pressure profile, erosional-velocity limits, and simultaneous flow distribution.

Flare Tip Selection & Stack Determination

Select flare tip type per API 537 (utility, steam-assist, air-assist, enclosed, sonic); determine stack height from radiation, dispersion, structural, and wind-loading requirements.

Radiation & Environmental Compliance

Calculate thermal radiation iso-contours per API 521 Annex D (Hajek-Ludwig / Brzustowski-Sommer); verify EPA 40 CFR 60.18 destruction efficiency and exit-velocity compliance.

FGR Sizing & Deliverable Package

Size flare gas recovery compressor for routine-flaring zero commitment; issue design basis, hydraulic model files, radiation contour plots, and environmental compliance package.

Flare System Design & Analysis — Scope
Scope of work

Every deliverable — from basis to handover

Complete Flare System Design & Analysis scope — every calculation, drawing, specification, and construction support activity.

Simultaneous relief load case development per API 521 Sec.5 with credit logic and case justification
Disposal-network hydraulic modelling in Aspen Flarenet / PIPENET with steady-state and transient
Header sizing with Mach-number and erosional-velocity check (typically Mach <0.7 sustained, <1.0 short)
Knockout drum sizing per API 521 — typically 500 µm liquid droplet separation
Water seal sizing for purge-gas / oxygen-ingress prevention
Flare tip selection per API 537 — utility / steam-assist / air-assist / enclosed ground / sonic
Thermal radiation per API 521 Annex D — Hajek-Ludwig, Brzustowski-Sommer, single-point
Stack-height determination per radiation, dispersion, and structural / wind loading
Flare gas recovery (FGR) compressor sizing for routine-flaring zero-commitment
EPA 40 CFR 60.18 compliance — destruction efficiency, exit velocity, visible emissions
Engineering outcomes

Outcomes of Flare System Design & Analysis

Thermal Radiation & Backpressure Control
  • Safe disposal of all simultaneous relief loads without backpressure exceedance
  • Thermal radiation exclusion zones protect personnel and occupied buildings
  • Prevents flare liquid carryover and potential ground-level explosion
  • Ensures reliable flare ignition in all weather conditions
API 521 / 537 / IPSC Flare Defence
  • API 521 backpressure compliance for all PRDs
  • EPA CAA continuous flare compliance monitoring
  • OSHA 1910.119 flare system mechanical integrity
  • Environmental permit noise and radiation limits
Flare Reliability & Smokeless Operation
  • Correct knockout drum sizing prevents liquid carryover to tip
  • Minimum purge rate calculation prevents flashback
  • Integration with flare gas recovery reduces routine flaring volume
  • Supports zero-routine-flaring commitments
Flare Sizing & Capacity Investment Value
  • Prevents costly flare system redesign post-startup
  • Optimises flare tip size to avoid unnecessary capital cost
  • Reduces environmental penalty exposure from excess flaring
  • Flare gas recovery generates return on capital
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