- Flammability/explosion: the vapour is highly flammable; in the flammable range with air it can be ignited by any ignition source, causing fire or explosion.
- Cryogenic (extreme cold) hazard: LPG is carried near -50 deg C; spillage causes severe cold burns, embrittlement and fracture of steel, and condensation of moisture.
- Asphyxiation/toxicity: heavier-than-air vapour displaces oxygen causing oxygen deficiency and asphyxiation in enclosed and low spaces; some gases are toxic.
- Pressure hazard: high vapour pressure with temperature rise can over-pressure tanks and piping; uncontrolled release can cause BLEVE (boiling liquid expanding vapour explosion).
- Heavy vapour cloud dispersion: the vapour is denser than air, flows along deck/water, travels to distant ignition sources and accumulates in enclosed spaces, bilges and accommodation.
- Equipment/materials failure and corrosion: leakage from piping, valves and flanges due to thermal contraction, corrosion and incompatible materials, plus liquid carry-over into machinery spaces causing damage.
These require gas-tight equipment, gas detection, venting, exclusion of ignition sources and careful temperature/pressure control.
A free-standing prismatic tank is a self-supporting rectangular tank built of cryogenic nickel-steel or aluminium, carried in a hold and supported on the hull. Sketch: a rectangular tank with flat or corrugated plates, a top dome carrying the cargo piping, safety and pressure/vacuum valves, access hatch and level gauging; the tank is supported on load-bearing seatings (wood/mild-steel chocks) at the bottom, with side hoppers and a roof, and is surrounded by insulation (polyurethane foam) to keep the cargo cold and reduce boil-off.
Features:
- Self-supporting (type A or B) prismatic tank, adequately insulated externally; a secondary barrier (drip tray/gas-tight) is fitted so any leakage is contained.
- The tank bottom transfers loads through seatings to the hull; the tank top dome carries the cargo pipes, relief valve, pressure-vacuum valve, gas detection and remote-operated valves.
- The tank is designed to withstand liquid sloshing and cryogenic temperature, with corrugated internal stiffening and notch-tough welded joints.
- Venting systems, dry-pipe vent risers and flame arresters prevent vapour accumulation; high-level and emergency shut-down (ESD) systems are fitted.
- The cargo is carried at atmospheric or slightly above pressure; a relief valve protects against over-pressure.
The sketch should show the tank seatings, insulation, dome, and secondary barrier.