5.4 LPG Storage, Refrigeration & BLEVE Prevention

LPG Storage Modes & Thermodynamics

Liquefied Petroleum Gas (LPG) consists predominantly of commercial propane ($C_3H_8$), butane ($C_4H_{10}$), or mixtures thereof. LPG is gaseous at standard ambient temperature and pressure but readily liquefies under moderate pressure or reduced temperature. Storing LPG as a liquid increases its volumetric energy density by a factor of approximately 250:1.

                   LPG Bulk Storage Classification
                                  |
         +------------------------+------------------------+
         |                                                 |
         v                                                 v
  [ Pressurised Storage ]                       [ Refrigerated Storage ]
  - Ambient Temperature                         - Low Temperature (-42°C Propane)
  - High Pressure (8-10 bar Propane)            - Atmospheric Pressure (~1 bar)
  - Spheres & Horizontal Bullets                - Double-Walled Cylindrical Tanks
  - Subject to BLEVE Risk                       - Requires Boil-Off Gas (BOG) Units

1. Pressurised Ambient Storage

• Operating Conditions: Stored at ambient temperatures under equilibrium vapour pressure. At 20°C, pure propane exerts a vapour pressure of approx. 8.5 bar g, while butane exerts approx. 2.0 bar g.
• Vessel Types: Spherical vessels ("Horton Spheres") or horizontal cylindrical pressure vessels ("bullets"), designed per pressure vessel codes (ASME Section VIII or BS EN 13445).

2. Refrigerated Atmospheric Storage

• Operating Conditions: Stored at or near atmospheric pressure by chilling the liquid below its boiling point (-42°C for propane, -0.5°C for butane).
• Vessel Types: Large flat-bottomed, double-walled cylindrical tanks with outer steel/concrete secondary containment.
• Boil-Off Gas (BOG) Management: Require continuous refrigeration compressors to capture, re-compress, and condense boil-off gas produced by ambient heat leak. Loss of refrigeration leads to gradual tank overpressurisation.

3. Mounded Bullet Storage (Inherent Safety Benchmark)

Mounded bullets are horizontal steel pressure vessels installed at ground level and completely encased in a thick mound of compacted sand, gravel, or earth.

• Fire Protection: The earth mound provides absolute passive thermal insulation, preventing external pool or jet fires from impinging on the vessel steel.
• BLEVE Elimination: By eliminating direct fire impaction and projectile hazards, mounding represents the inherently safer design benchmark for bulk LPG storage worldwide.

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Anatomy of a BLEVE (Boiling Liquid Expanding Vapour Explosion)

A BLEVE is a physical explosion that occurs when a vessel containing a liquid held under pressure at a temperature significantly above its atmospheric boiling point suffers a sudden, catastrophic breach of integrity.

                  Sequence of a BLEVE Incident
                  
  [ Jet Fire Impingement ] ---> [ Vapour Space Overheating ]
                                       |
                                       v
  [ Structural Metal Failure ] <-- [ Yield Strength Drops ]
               |
               v
  [ Catastrophic Rupture ] ---> [ Depressurisation below T_sl ]
                                       |
                                       v
  [ Instantaneous Liquid Flash ] -> [ BLEVE Blast & Giant Fireball ]

Step-by-Step Mechanism of a BLEVE

1. Fire Impingement: An external flame (jet fire or pool fire) impinges on the vessel shell.
2. Vapour Space Overheating: Liquid in the lower portion absorbs heat via boiling ($h_{cool} \approx 1000\text{ W/m}^2\text{K}$), keeping the lower metal cool. However, heat transfer in the upper vapour space is poor ($h_{vap} \approx 50\text{ W/m}^2\text{K}$). The dry metal wall in the vapour space rapidly overheats, reaching temperatures above 500°C to 650°C.
3. Loss of Tensile Strength: Steel yield strength drops sharply above 400°C. Meanwhile, internal vessel pressure continues rising due to boiling liquid.
4. Catastrophic Rupture: The weakened steel shell tears open longitudinally.
5. Flash Boiling & Superheat Limit ($T_{sl}$): Depressurisation to atmospheric pressure drops vessel pressure instantaneously below the Superheat Limit Temperature ($T_{sl}$) of the liquid (approx. 53°C for propane). The subcooled liquid undergoes explosive homogeneous nucleated boiling, expanding over 200-fold in volume in milliseconds.
6. Fireball Ignition: If the liquid is flammable (LPG), the expanding vapour cloud ignites instantly from the initiating fire, creating a massive rising fireball.

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BLEVE Fireball Dynamics & Thermal Radiation Scaling

The physical dimensions, duration, and thermal radiation hazard of an LPG fireball are calculated using empirical scaling laws (TNO Yellow Book / CCPS standards):

BLEVE Fireball Thermal Radiation Model
         
            / ~ ~ ~ ~ ~ \
           /             \   <-- Diameter D = 5.88 * m^(1/3)
          |   FIREBALL    |  <-- Duration t = 0.45 * m^(1/3)
           \             /
            \ _ _ _ _ _ /
                 ^
                 |  Height H = 0.75 * D
                 |
         [ Storage Vessel ]

Empirical Scaling Equations

• Fireball Diameter ($D$, meters): $D = 5.88 \cdot m^{0.333}$
• Fireball Duration ($t$, seconds): $t = 0.45 \cdot m^{0.333}$
• Target Height ($H$, meters): $H = 0.75 \cdot D$

Where $m$ is the total mass of LPG involved in the explosion ($kg$).

Worked Example: 50-Tonne LPG Bullet BLEVE

For a vessel containing $m = 50,000\text{ kg}$ of propane:

1. Diameter: $D = 5.88 \times (50,000)^{0.333} = 5.88 \times 36.84 \approx \mathbf{216.6\text{ meters}}$
2. Duration: $t = 0.45 \times (50,000)^{0.333} = 0.45 \times 36.84 \approx \mathbf{16.6\text{ seconds}}$

Thermal Radiation Effects

Thermal radiation flux ($q$, $kW/m^2$) received by surrounding personnel or structural equipment causes immediate severe consequences:

• $4.7\text{ kW/m}^2$: Pain threshold reached within 15 seconds; maximum allowable exposure for emergency escape without protective clothing.
• $12.5\text{ kW/m}^2$: Second-degree burn threshold; structural wood ignition; piloted ignition of plastics.
• $37.5\text{ kW/m}^2$: 100% fatality threshold for exposed personnel within 10 seconds; immediate structural damage and failure of uninsulated process pipework.

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Active & Passive Fire Protection Systems for LPG

Preventing BLEVE accidents requires a combination of robust active cooling, passive thermal barriers, and rapid pressure reduction systems.

       Active Water Deluge Cooling (API 2510 / NFPA 15)
       
             Water Deluge Ring Header
                 /   |   |   \
               v     v   v     v  (High-Velocity Spray Nozzles)
            +-------------------+
            |  Pressurised LPG  | <-- Continuous 10.2 L/min·m² 
            |  Storage Sphere   |     Water Film Prevents Metal 
            +-------------------+     Overheating

1. Active Water Deluge Spray Systems (NFPA 15 / API 2510)

Fixed water spray systems are the primary active defense against vessel metal failure during external fire exposure:

• Application Density: Must deliver a uniform minimum application rate of $10.2\text{ L/min}\cdot m^2$ ($0.25\text{ gpm/ft}^2$) over the entire vessel shell surface area.
• High-Risk Zones: Spray nozzles must specifically target the upper vapour space and top fittings where metal overheating occurs fastest.
• Response Speed: Automated actuation via optical flame detectors or pneumatic heat-detecting quartz bulbs must initiate deluge flow within 30 seconds of fire detection.

2. Passive Fire Protection (PFP)

Coatings applied directly to vessel steel to insulate the metal from external flame temperatures:

• Materials: Intumescent coatings, lightweight subliming compounds, or dense concrete/cementitious sprays.
• Performance Standard: Must maintain vessel steel temperature below 400°C for a minimum of 2 hours under standard hydrocarbon jet-fire conditions (ISO 22899 test standards).

3. Emergency Depressurisation Systems (EDPVs)

Automated blowdown valves designed to depressurise the LPG vessel to atmospheric pressure within 15 minutes (per API RP 521 guidelines). Reducing internal vessel pressure lowers stress in the steel shell, preventing catastrophic rupture even if the shell metal suffers thermal weakening.