Liquefied Petroleum Gas (LPG)
Core concepts
LPG is a mixture of propane and butane (with small amounts of propylene/butylene) stored as a liquid under pressure. Candidates must know that LPG liquid expands roughly 0.2–0.3% per °C and that the vapour space above the liquid provides room for expansion - filling ratios exist precisely to prevent hydraulic lock. Understand vapour pressure versus temperature (propane is far more volatile than butane, so a commercial propane/butane mix behaves differently in winter), relative density of vapour (about 1.5–2.0, so it pools in low points and drains), and the flammable range (roughly 1.8–10% by volume in air). Know cylinder and bulk-tank fittings: POL/left-hand threads on cylinders, the vapour off-take on a cylinder versus liquid off-take for vapourisers, and the role of the excess-flow valve, pressure relief valve and the regulator (first stage, second stage). Combustion requires correct primary aeration; incomplete combustion produces CO, and the classic symptom is a lazy yellow flame with sooting. Know the gas installation pipework sizing principle (pressure drop budget) and tightness-testing with air or inert gas, never with a flame.
Traps
Questions love the left-hand thread on cylinder connections; confusing vapour and liquid off-take; assuming LPG is lighter than air; and mixing up the roles of the relief valve (thermal protection) versus the regulator (pressure reduction). Another trap: forgetting that a leak test on LPG pipework uses a pressure gauge and a stable test period, not a soap bubble only.
Worked rule of thumb
For a cylinder vapour off-take, usable vapour rate drops steeply as the cylinder cools from vaporisation; as a rule of thumb, don't exceed about 1 kg/h per 9 kg cylinder for continuous draw - use multiple cylinders or a vapouriser for higher loads.