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How does an LPG cylinder work?

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Quick answer

An LPG cylinder holds propane and butane squeezed into liquid form. Above the liquid sits a layer of gas under pressure. When you open the regulator, that gas flows to the stove at a reduced, steady pressure, and more liquid boils to replace it. This continues until the liquid runs out.

An LPG cylinder works by storing propane and butane as a liquid under pressure, with a pocket of gas above it that feeds the stove. As gas is drawn off through the regulator, some liquid boils to replace it, keeping supply steady until the cylinder is nearly empty.

Step by step

  1. At the bottling plant, liquefied propane and butane are pumped into a steel cylinder until it is about 80 percent full.
  2. Some liquid evaporates into the space above it until the gas pressure balances the liquid's vapour pressure.
  3. Opening the valve lets this pressurised gas flow into the regulator, which lowers it to a safe, steady pressure.
  4. Gas travels through the hose to the burner, mixes with air and burns as a hot blue flame.
  5. Liquid inside boils continuously to replace the gas drawn off, drawing heat from the cylinder walls.

Key things to understand

  1. Liquefied petroleum gas is mainly propane and butane, which turn to liquid under modest pressure at room temperature.
  2. The steel cylinder is filled only to about 80 to 85 percent, leaving vapour space for the liquid to expand.
  3. Gas collects above the liquid at a pressure set by temperature and blend, several times higher than atmospheric pressure.
  4. A regulator on the valve drops that pressure to a low, steady level that the stove burners are designed for.
  5. As gas leaves, the liquid boils to replace it, absorbing heat, so a heavily used cylinder can feel cold.

Labelled diagram

  1. Liquid LPG

    Propane and butane stored as liquid filling most of the lower cylinder.

  2. Vapour space

    Pressurised gas above the liquid that actually flows out to the stove.

  3. Valve

    Opens and seals the cylinder outlet, with a built-in safety relief device.

  4. Regulator

    Cuts high cylinder pressure to a low, steady supply pressure for burners.

  5. Burner

    Gas mixes with air and burns as a hot blue flame.

Inside an LPG cylinder feeding a stove

Real-world examples

  • Indian households commonly use 14.2 kg domestic LPG cylinders delivered by the oil marketing companies.
  • Restaurants and dhabas often use larger 19 kg commercial cylinders for heavy cooking loads.
  • Campers use small butane canisters that work the same way on portable stoves.
  • A patio gas heater in a British garden typically runs on a propane cylinder fitted with its own regulator.

Common mistake

✗ Myth: An LPG cylinder is packed with highly compressed gas, like an oxygen tank.

✓ Fact: Most of the cylinder's contents are liquid at modest pressure. Only the space above the liquid holds gas, which is why pressure stays steady until the cylinder is nearly empty.

People also ask

Is lpg stored as liquid or gas?
LPG is stored as a liquid. Propane and butane liquefy under moderate pressure at normal temperatures, so a cylinder holds mostly liquid with a pocket of vapour above it. The vapour is drawn off for cooking, and more liquid evaporates to replace it, which is why LPG is called liquefied petroleum gas.
What does a gas regulator do on an lpg cylinder?
A gas regulator reduces the cylinder's pressure, which changes with temperature and fill level, to a low, constant pressure suited to the appliance. Inside, a spring-loaded diaphragm opens or closes a small valve to keep outlet pressure steady. Most domestic regulators also include a knob or lever that shuts off supply.
Why does an lpg cylinder get cold?
Turning liquid into gas requires energy called latent heat. As the stove draws gas, the liquid LPG boils and takes that heat from itself and the steel walls, so the cylinder cools. Under heavy, continuous use, moisture from the air can even condense or frost on the outside.
Why does LPG smell if the gas is odourless?
Pure propane and butane have almost no smell, so a sulphur compound such as ethyl mercaptan is added during bottling. Its strong rotten-cabbage odour lets people notice a leak quickly, even in very small concentrations.
Why does leaked LPG collect near the floor?
LPG vapour is roughly one and a half to two times denser than air, so it sinks and pools in low spots such as floors, drains and basements. This is why ventilation near floor level matters in kitchens.
Why is a cylinder never filled completely?
Liquid LPG expands noticeably as it warms. Leaving a vapour space gives that liquid room to swell on a hot day, so the pressure does not climb dangerously. A relief valve provides a further safeguard.
How can you tell how much gas is left?
Pressure stays roughly constant until almost all the liquid has boiled away, so a pressure gauge is a poor guide. Weighing the cylinder and subtracting its stamped empty weight, called the tare, shows how much gas remains.

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