Industrial Vaporisers: Managing Dew Point, Pressure Collapse, and Heavy Ends in High-Draw LPG Systems

The Thermodynamics of Natural vaporisation Failure

Many legacy commercial operators rely solely on the ambient heat transfer of a bulk LPG tank's steel shell to boil the liquid into a usable vapor. In warm climates, or under minimal draw (e.g., a restaurant oven), this 'natural vaporisation' succeeds. However, LPG is not a pure substance; it is a blend of hydrocarbons, primarily Propane (C3) and Butane (C4). In the Southern Hemisphere (particularly the South Island), operators also battle trace amounts of Pentane (C5) and heavier, sluggish molecules.

When a large industrial burner ignites, it pulls massive volume from the top of the vapor space. Lighter hydrocarbons like propane boil off instantly at lower temperatures (boiling point -42°C). Simultaneously, heavier ends like butane require entirely different thermodynamic energy to boil (boiling point -0.5°C). The gas naturally 'fractionates' inside the pressure vessel. Over months of continual refilling on top of an un-emptied tank, this preferential vaporisation leaves behind a stagnant, non-boiling puddle of heavy 'dead' liquid.

The Crisis of Dew Point and Pressure Collapse

The primary symptom of a heavy-ends crisis is massive cold-weather pipeline condensation. Because the tank is largely pure butane remaining in late winter, the vapor exiting the tank requires warmer pipes to stay gaseous. As the piping runs through freezing ambient air or snow, the heavy vapor hits its dew point and rapidly re-liquefies inside the regulators and manifolds.

• Burner Flameout: Liquid droplets slamming into low-pressure burner orifices cause dangerous flare-ups followed immediately by soot, CO alarms, and full outages.
• Tank Freeze-Off: The tank's wetted surface area 'sweats' and freezes over, creating an insulating jacket of ice that completely halts any further natural heat transfer, dropping internal tank pressure to near 0 kPa.
• Calorific Value Drift: As the fuel composition shifts erratically from pure propane gas to pure butane vapor, the Wobbe Index wildly fluctuates, rendering finely-tuned air/fuel modulating valves largely useless.

Forced Liquid Withdrawal: The Engineering Solution

The ultimate engineering resolution for consistent fuel composition and guaranteed pressure delivery is eliminating natural vaporisation entirely. High-demand sites must pivot to forced liquid-withdrawal.

Instead of drawing vapor from the tank headspace, a liquid dip-tube extracts the bulk liquid straight from the bottom of the vessel. The liquid is piped directly into a dedicated electric, water-bath, or direct-fired vaporiser skid. Inside the heat exchanger, 100% of the liquid blend (propane, butane, pentane) is flash-boiled simultaneously.

Always ensure the downstream vapor piping from a forced vaporiser is adequately insulted and trace-heated if running extremely long distances in freezing weather, as a vaporiser does not permanently alter the gas's physical dew point - it merely guarantees steady homogenization.