Category
Safety & Relief
Subcategory
Relief Device
Applicable standards
3
Issuing bodies
3
Description
Non-reclosing pressure relief device for primary or backup overpressure protection.
Applicable Standards (3)
API — 1 standard
API 520 Part I
Sizing, Selection, and Installation of Pressure-relieving Devices — Sizing and Selection
Sizing methodology and device selection for pressure-relieving devices.
Key requirements (7)
- PRV sizing for gas, vapor, liquid, two-phase relief.
- Effective discharge coefficient Kd: 0.975 (gas, certified), 0.65 (liquid certified).
- Backpressure correction Kb (conventional vs balanced bellows).
- Two-phase via Omega method (HEM) or DIERS.
- Set pressure: ≤ MAWP. Allowable accumulation: 10% (single PRV), 16% (multiple), 21% (fire case).
- Inlet line: max 3% loss at rated flow (rule of thumb to prevent chatter).
- Outlet line: backpressure ≤ 10% set (conventional) or ≤ 50% (balanced bellows).
ASME — 1 standard
ASME BPVC Sec VIII Div 1
Rules for Construction of Pressure Vessels
Mandatory rules for design, fabrication, inspection, testing and certification of pressure vessels operating above 15 psig (~1 barg).
Key requirements (12)
- Applies to vessels with internal pressure > 15 psig (1.03 barg) up to ~3000 psi (Div 1 sweet spot).
- Joint efficiency E: 1.0 (full radiography), 0.85 (spot RT), 0.70 (no RT) for Type 1 butt joints.
- Shell thickness: t = P·R/(S·E – 0.6·P) for circumferential stress (UG-27).
- Head thickness: t = P·D/(2·S·E – 0.2·P) for 2:1 ellipsoidal heads (UG-32).
- Hydrostatic test pressure: 1.3 × MAWP × (S_test/S_design) — UG-99.
- Pneumatic test (when hydro impractical): 1.1 × MAWP — UG-100.
- Corrosion allowance is added to calculated thickness (not included in S formula).
- MDMT and impact testing per UCS-66 / Figure UCS-66; below the curve, Charpy V-notch impacts are required.
- PWHT thresholds per UCS-56 — generally required above 19 mm thickness for P-No. 1 carbon steels.
- Nozzle reinforcement: area replacement method (UG-37) within reinforcement zone.
- Maximum allowable stress S: from ASME II Part D, Table 1A/1B/3 — temperature dependent.
- Flanges per ASME VIII-1 Appendix 2 (or B16.5 for standard sizes).
Note: Most widely used pressure vessel code worldwide.
ISO — 1 standard
ISO 4126 (series)
Safety devices for protection against excessive pressure
Multi-part standard on safety valves, bursting discs, pilot-operated relief, and combinations.
Key requirements (6)
- Safety devices for protection against excessive pressure (international counterpart of API 520/526).
- Parts: 1 (safety valves), 2 (rupture discs), 3 (CSPRS), 4 (pilot-operated), 5 (CSPRS), 6 (rupture disc selection), 7 (general data).
- Set pressure: ≤ PS (PED design pressure).
- Pop tolerance: ±3% set (typical) for spring valves.
- Certified discharge coefficient by manufacturer testing.
- Used as harmonized PED standard in Europe.