API 520

Covers — sizing procedures and selection criteria (Part I) and installation practices (Part II) for pressure-relief devices protecting unfired pressure equipment with MAWP ≥ 15 psig (103 kPag), for gas/vapor, steam, and liquid relief, including fire and operating contingencies.

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

• Determining relief loads / disposal (flare, blowdown) system design → API 521.
• Standard orifice designations and PRV dimensions → API 526.
• Seat-tightness acceptance → API 527; in-service inspection → API 576.

API 520 sizes and selects the device; API 521 determines the loads it must handle and the disposal system.

API 520 sets minimum requirements for relief-device sizing and installation:

• Relief Scenarios — identify governing overpressure cases (blocked outlet, fire, thermal expansion, control failure, etc.).
• Sizing — compute required orifice area for gas/vapor, steam, or liquid flow using the standard's equations and coefficients.
• Selection — choose device type (conventional, balanced-bellows, pilot-operated) for the back-pressure and service.
• Installation — inlet/outlet piping rules to avoid excessive pressure drop and chatter (Part II).

API 520 is the primary reference for sizing, selecting, and installing pressure-relief devices. Part I gives the equations and coefficients to compute the required relief orifice area for gas, vapor, steam, and liquid service under the governing overpressure scenario; Part II covers installation to avoid excessive inlet/outlet pressure drop and instability.

The computed area is matched to a standard letter orifice from API 526, while the relief loads themselves and the flare/disposal system come from API 521.

Engineering Note: Inlet line pressure loss above ~3% of set pressure invites valve chatter — check the installation rules in Part II, not just the orifice area.