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When Does a Diesel Tank's Emergency Relief Valve Activate?
1. Primary Trigger: Excessive Internal Pressure
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Activates when pressure exceeds MAWP (Maximum Allowable Working Pressure)
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Thermal Expansion: Fuel expands ~0.07%/°C.
10,000-gal tank at 20°C → 60°C may reach 10–15 psi (MAWP: 5–8 psi).
- Blocked Primary Vents: Clogs can cause pressure to rise 1.5–2x MAWP before ERV triggers.
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Thermal Expansion: Fuel expands ~0.07%/°C.
2. Secondary Triggers: Fire & System Failures
- Fire Exposure: Radiant heat can push vapor pressure to 30 psi (ERV set point: 10–20 psi).
- Overfilling: Pump failures can generate hydrostatic pressure exceeding ERV limits in ASTs.
3. Design Specifications & Activation Thresholds
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Typical ERV Set Points:
- Small tanks: 5–10 psi (0.34–0.69 bar)
- Industrial tanks: 10–20 psi (0.69–1.38 bar)
- Marine tanks: 8–15 psi (0.55–1.03 bar) (ISO 10816)
- Difference from P/V Valves: Primary vents handle 1–3 psi; ERV activates at >10 psi.
4. Consequences & Post-Action Protocols
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Risks:
- Flammable vapor release
- Environmental contamination from liquid discharge
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Immediate Steps:
- Shut off fuel supply and heating systems
- Inspect for vent blockages or pump defects
- Recalibrate or replace ERV to meet API 2000 standards
5. Regulatory Standards
- API 2000: Requires ERVs to handle worst-case scenarios (e.g., fire).
- NFPA 30: ERV must activate below 90% of MAWP (e.g., 13.5 psi for 15 psi MAWP).