Category
Safety & Relief
Subcategory
Relief Device
Applicable standards
8
Issuing bodies
4
Description
Spring-loaded or pilot-operated pressure relief valve — primary overpressure protection.
Applicable Standards (8)
API — 5 standards
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).
API 520 Part II
Sizing, Selection, and Installation of Pressure-relieving Devices — Installation
Installation guidance — inlet/outlet piping, reaction forces, discharge.
Key requirements (9)
- Installation of pressure-relieving devices in refineries — companion to API 520-1 (sizing).
- Inlet line pressure loss ≤ 3% of set pressure at rated flow (rule preventing chatter).
- Outlet (built-up) backpressure ≤ 10% set pressure for conventional valves; ≤ 50% for balanced bellows.
- Inlet line schedule: typically same as PRV nozzle, no reductions.
- Block valves under PRVs: car-sealed open (CSO) or locked open (LO) per OSHA 1910.119.
- PRV elevation: discharge piping should self-drain back to disposal header.
- Reactive force calculation required for open atmospheric discharges (especially relief to atmosphere).
- Tail pipe sizing: avoid two-phase flow regime transitions.
- Multiple PRVs: staggered set pressures to prevent simultaneous popping.
API 521
Pressure-relieving and Depressuring Systems
Causes of overpressure, relief load determination, depressuring, flare/disposal system design.
Key requirements (6)
- Sizing and design of pressure relief and depressuring systems.
- Relief scenarios: blocked outlet, fire, control valve failure, tube rupture, power failure, thermal expansion, runaway reaction, abnormal heat input.
- Fire heat input: Q = 21,000·F·A^0.82 BTU/hr (vapor space < 25 ft elevation; F = environment factor 0.3–1.0).
- Depressuring: typically blowdown to 50 % of design pressure or 100 psig (whichever lower) within 15 minutes for vessels exposed to fire.
- Flare radiation: max permissible 1.58 kW/m² (continuous), 4.73 kW/m² (limited exposure), 6.31 kW/m² (emergency).
- Disposal system sizing: must handle simultaneous events — 'global event' analysis.
API 526
Flanged Steel Pressure-Relief Valves
Standard orifice sizes (D through T) and dimensional/pressure ratings for flanged steel PRVs.
Key requirements (6)
- Flanged steel pressure relief valves — sets dimensions, materials, pressure-temp ratings.
- Orifice designators: D, E, F, G, H, J, K, L, M, N, P, Q, R, T (D = 0.110 in², T = 26 in²).
- Pressure classes: 150, 300, 600, 900, 1500, 2500.
- Body materials: CS, alloy steel, stainless options.
- Spring chamber materials matched to bonnet temperature.
- Companion to API 520 (sizing) and API 527 (seat tightness).
API 527
Seat Tightness of Pressure Relief Valves
Acceptance criteria for seat leakage of metal- and soft-seated PRVs.
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.
BIS — 1 standard
IS 4622
Spring-loaded Safety Valves for Process Fluids
Indian PRV / safety valve standard.
Key requirements (6)
- Specification for rising-stem pressure-relief valves.
- Indian counterpart of API 526 / API 520.
- Set pressure tolerance: ±3% (typical).
- Allowable accumulation: 10% (single), 16% (multiple), 21% (fire).
- Hydrotest and seat-tightness testing per IS 4733.
- Pressure ratings: PN 16 to PN 250.