API 521

Covers — causes of overpressure, determination of individual and total relief loads (including the fire case), and the design of pressure-relieving and vapor-depressuring systems and their disposal (flare, blowdown, effluent handling) for refineries, petrochemicals, gas plants, and LNG.

Does NOT cover (exclusions) — the following are governed elsewhere:

• Sizing/selecting the individual relief device → API 520.
• Standard orifice designations/dimensions → API 526; seat tightness → API 527.
• In-service inspection of devices → API 576.

API 521 sets the loads and disposal system; API 520 sizes the device that handles each load.

API 521 sets minimum requirements for relief-system design:

• Overpressure Causes — systematic identification of scenarios (blocked outlet, fire, loss of cooling/reflux, control failure, tube rupture, thermal expansion).
• Load Determination — quantify relief rates per scenario, including fire-case heat input and the governing (largest credible) load.
• Disposal Design — flare/knockout/blowdown system sizing, back-pressure management, and radiation/dispersion considerations.

API 521 is the system-level relief and depressuring standard. It first identifies every credible overpressure cause, then quantifies the relief load for each — including the often-governing external fire case — and finally guides the design of the disposal system: flare headers, knockout drums, and blowdown.

It works hand-in-hand with API 520 (which sizes each individual device) and feeds the flare-network hydraulics. An error in the fire-case load propagates through the entire relief and flare system.

Engineering Note: The largest single relief load rarely equals the total flare load — confirm which scenarios are simultaneous before sizing the flare header.