Safety, Health, and Environment
Scope
Relief system design, flammability, hazard evaluation, and consequence analysis.
Concepts to own
- Relief scenarios: blocked outlet, external fire, runaway reaction, thermal expansion, control valve failure. Size on the credible worst case, not normal flow.
- Vapor relief: critical versus subcritical nozzle flow; two-phase and flashing service require homogeneous or API methods; account for set pressure, allowable accumulation (usually 10% for fired and unfired vessels per code), and back pressure limits.
- Flammability: flash point, LFL/UFL, autoignition temperature, minimum ignition energy, and inerting below the limiting oxygen concentration.
- Purging: after n pressure cycles between P_low and P_high, y = y0 (P_low/P_high)^n.
- Dust explosions: Kst, Pmax, grounding, and housekeeping.
- Hazard analysis: HAZOP guide words, LOPA, SIL assignment, PHA, management of change, incident investigation.
- Toxic exposure: IDLH, TLV-TWA, ERPG tiers, and dispersion modeling.
Traps
- Sizing a relief device for normal flow rather than the fire or blocked-outlet case.
- Forgetting that positive displacement pumps and compressors need a relief device regardless of controls.
- Using gauge pressure where absolute is required in a critical-flow ratio.
- Applying ambient LFL values to a hot process stream.
Worked example
A relief valve set at 150 psig with 10% accumulation relieves at 165 psig, or 165 + 14.7 = 179.7 psia. For k = 1.4, critical flow requires downstream/upstream pressure below (2/2.4)^3.5 = 0.528.
Rule of thumb: never quietly drop a scenario from the list - document the basis and have the PHA team confirm it.