LPG Sinks, and That Changes Where It Is Dangerous

Most people's intuition about gas leaks comes from natural gas, which is lighter than air, rises, and disperses through a high vent. LPG does the opposite. Propane and butane vapour is roughly one and a half to two times as dense as air. A leak does not rise and thin out — it flows along the floor like an invisible liquid, finds the lowest point available, and stays there.

Where it goes

Released LPG follows gravity. It runs into floor drains, cable trenches, inspection pits, basements, lift shafts and the gap under a kitchen counter. On still days it can travel a surprising distance before finding an ignition source that the person who caused the leak never considered: a pilot light in the next room, a pump motor in a basement, a vehicle parked beside a drain.

The concentration that matters is small. LPG is flammable at around two percent in air, so a pool of vapour that nobody can smell from standing height can still be well inside the flammable range at ankle height.

What this means for design

• Ventilation at low level. Openings for LPG areas belong near the floor, not the ceiling. A high vent that works perfectly for natural gas does almost nothing here.
• Detectors near the floor. An LPG detector mounted at head height is measuring the wrong layer of air. It should sit low, typically within a short distance of the floor, near the likely leak points.
• No storage above openings to below-ground spaces. Cylinders and tanks must be kept away from drains, pits and basement openings, so a leak cannot run straight into an enclosed space.
• Outdoor cylinders stay outdoors. Moving a cylinder indoors to protect it from sun or theft converts a manageable outdoor leak into a trapped indoor one.

The kitchen pattern

The most common arrangement we find in restaurants, hotels and staff canteens is a cylinder bank moved closer to the cooking line over time for convenience, flexible hoses lengthened and routed along the floor, and a detector — if there is one — fixed on the wall at eye level. Each change looked reasonable when it was made. Together they produce exactly the conditions in which a small leak becomes an explosion.

Checking your own site

Stand in your LPG area and ask one question: if a leak started now, where would the gas flow? Follow the floor to the lowest point. If that point is a drain, a pit, a doorway to a room below, or anywhere near an ignition source, the installation needs attention regardless of what the last certificate says.