What This Means
The relief load is the mass the system must move when the chosen scenario fires. The required flare load adds a contingency margin on top — the flare and header are sized for that bigger number, not the bare relief rate.
Why It Matters
Undersize this and every downstream component — header, KO drum, stack — is undersized with it. The whole flare system is only as good as the load you feed it.
Common Design Mistakes
- Using normal operating flow instead of the relief-scenario flow.
- Ignoring simultaneous relief loads — fire can lift several valves at once.
- Forgetting the fire case, which usually governs the total flare load.
Field Notes
Build a load summation table per scenario before trusting any single number. The governing case is rarely one valve — it is the worst credible combination relieving together, and that is what the header has to swallow.
Standards & Methods
No. It is a preliminary, single-source sizing aid. A full design needs a flare network backpressure model, transient depressuring with heat transfer, dispersion, and vendor tip data — confirm with API 521/520/2000 and a qualified engineer.
External fire can relieve several vessels at once, so the summed load is larger than any single blocked-outlet or control-failure case. That is why API 521 fire relief so often sets the flare system size.