Why a fuel can breathes, and what heat and altitude do to a sealed container.
Temperature and altitude change the pressure inside a sealed can of fuel, and a fuel that gives off vapour needs somewhere for that pressure to go.
Quick answer: Why does a fuel can build up pressure?
Heat and altitude. Fuel gives off vapour as it warms and the air above it expands, so a can filled cool swells in the sun; outside pressure falls with height while the inside does not. Petrol drives it harder than diesel, whose vapour pressure is under a kilopascal. Open a warm can slowly, pointed away from you.
Fuel expands as it warms and gives off vapour, and the air above it expands too. A can filled in the cool of the morning and left in the sun builds internal pressure. Carried up in altitude, the lower outside pressure has the same effect. A rigid sealed can can bulge or, when opened, spray. The swing between a cool fill and a hot afternoon is enough to feel when you crack the cap. Petrol gives off vapour readily, so it drives this harder than diesel, and a diesel can sits quieter in the same heat.
Two separate effects add up and it helps to keep them apart. The first is the trapped air, which follows the gas law. Warm a sealed headspace from 10 °C to 50 °C and the absolute temperature rises from about 283 to 323 kelvin, so the pressure rises by around 14 per cent before any fuel has evaporated. On its own that is roughly 14 kPa of gauge pressure, which you would feel at the cap and which no can would fail on.
The second effect is vapour pressure and it dwarfs the first. Petrol's Reid vapour pressure is regulated seasonally under EN 228, with a summer maximum around 60 kPa and winter grades allowed considerably higher because a cold engine needs a more volatile fuel to start. Diesel's vapour pressure at ambient is under a kilopascal, which is the whole reason a diesel can sits quiet in the same sun.
That seasonal split produces a counter-intuitive result. Winter-grade petrol is formulated to evaporate more readily, so a can filled in February and left sealed into a warm spring afternoon builds more pressure than the same can filled with summer fuel would. The dangerous combination is not high summer; it is winter fuel meeting the first warm day.
A non-vented can runs the same physics in reverse. Heat it in the sun, then let it cool overnight, and the vapour inside contracts. With no way to draw air back in, the can pulls a vacuum and its walls dent inward. A can that bulged by afternoon and caved by morning has been fighting a seal that will not let it breathe either way.
The damage is cumulative. Each cycle works the metal at the same fold, and work-hardened steel eventually cracks there. A can that has panelled and recovered a hundred times is closer to failure than one that has never done it, even though both look serviceable. On a plastic can the same cycling shows as a permanent distortion that stops the can seating flat or stacking square, which is the point at which it starts causing problems for everything stored with it.
A vented cap or a separate vent lets the can equalise: it releases built-up pressure and lets air in as liquid pours out for a smooth pour. It trades a perfect seal for controlled breathing, which suits volatile fuels and hot climates.
The mechanism separates two jobs that a plain hole does badly. A pressure-relief function opens above a set differential and reseals below it, so the can vents the surplus and stops. A vacuum relief function does the same in reverse, admitting air as the contents cool so the walls never panel in. A cap that does both breathes on demand and seals the rest of the time, which is the behaviour you actually want and is not what an open vent provides.
The pouring vent is a third thing again and is often confused with the other two. Liquid leaving a container has to be replaced by air, and a single opening makes air and liquid take turns through the same hole, which is the glugging that throws fuel off target. A second opening, or an internal air path in the spout, lets air in while liquid leaves and produces a smooth stream. Emissions rules removed the separate hole, so on a compliant can that air path lives inside the spout and works only while pouring.
Before a full pour, burp a warm can. Loosen the cap a little, let the hiss settle, then open it the rest of the way. You release the pressure on your terms instead of at the spout, where a sudden push throws fuel further than you aimed. On a hot day, keep the habit every time you open one.
Where you do it matters as much as how. Crack the cap outdoors, downwind, away from anything with a flame or a spark, and with the opening pointed away from your face and from anyone else. Vapour released at the closure is heavier than air and will fall rather than disperse, so it goes to the ground around your feet. Do this away from a doorway, a pit or a drain, not beside one.
Emissions rules pull the other way. EPA and CARB portable fuel containers use an automatic-closing spout with no open vent, to limit vapour loss. A modern compliant fuel can manages pressure through its spout design rather than an open hole. The auto-shutoff spout carries an internal vent path, so the can breathes only while you pour and seals shut when you stop. See EPA and CARB compliance, and how a pouring spout does both jobs at once.
Which is why cans sold into different markets behave differently in the hand for reasons that have nothing to do with build quality. A vented cap that a user in one market takes for granted is an uncontrolled emissions path in another, so the same manufacturer ships two closure systems. If a buyer complains that a compliant can pours worse than the one it replaced, that is the trade the regulation made, and it is worth explaining rather than defending.
The frequent one is brimming a can to the top in the cool morning, then leaving it sealed in full sun. With no expansion room and no way to vent, the pressure has nowhere to go. A second mistake follows the first: forcing a stuck cap on a can that is already under pressure. If a cap fights you, the can is not ready to open flat out. Ease it, let it breathe, then finish the turn.
A third follows both and is the one that damages the can rather than the operator. Forcing a cap that is stuck under vacuum tears the gasket against the neck, and a torn gasket does not announce itself: the can reseals, looks fine, and weeps on the next warm day. If a cap will not move, warm the can rather than lever it, and replace the gasket if it has been forced.
Pressure gets the attention because a swollen can is visible. The reverse case does the quiet damage: a can filled warm, sealed, then cooled (driven down from altitude, moved into shade or left overnight) develops a partial vacuum as the vapour condenses and the remaining gas contracts.
A thin-walled can panels inward under that vacuum, and repeated cycles work-harden the creases until the metal splits or the seam opens. It is also why a cap that seals perfectly against internal pressure can be very hard to remove after cooling, and why forcing it is how gaskets get torn.
Ambient pressure falls with height while the pressure inside a sealed can does not, so the differential grows on the way up regardless of temperature. A can sealed at sea level and carried over a mountain pass is doing the same thing as one left in the sun, and the two compound. Aviation is stricter than road transport about carrying fuel containers for exactly this reason.
Standard atmospheric pressure at sea level is about 101 kPa; at 2,500 metres it is nearer 75. A can sealed at the coast and driven over an alpine pass therefore carries about 26 kPa of differential from altitude alone, roughly twice what a 40-degree temperature swing produces on the trapped air. Add a warm afternoon on top and the two work in the same direction.
The aircraft case is stricter again because a cargo hold is pressurised to an altitude equivalent of roughly 2,400 metres, and because a leak in a confined, ventilated-but-enclosed space behaves differently from a leak on a road. That is why the air rules on carrying fuel containers are the tightest of the three modes, and why an empty-but-unpurged can is treated as carrying its previous contents rather than as an empty container.
The practical rule for both directions is the same: equalise deliberately rather than letting the container do it. Crack the closure in shade, away from ignition sources, opening pointed away, and let it settle before removing the cap fully. On a long descent, do it before the can has fully cooled rather than after.
Leave expansion room when filling, store fuel cool and out of direct sun, and open a warm can slowly to release pressure before it sprays. Good caps and gaskets seal when you want them to and give way when you ease them, so check yours before a trip. Read storing fuel safely.
Check them warm rather than cold, because a gasket that has taken a compression set seals well enough at rest and fails once the can is pressurised. The simplest check is to fill a can, leave it in the sun for an afternoon, then ease the cap and look for a weep at the neck before the hiss arrives. A seal that passes that will hold on a hot tailgate.
Pressure lives in the closure.
| Your use case | Recommended can | Why this one |
|---|---|---|
| A fuel can with a vented cap |  20L NATO Steel Fuel Can |
Vented screw closure manages pressure with temperature. |
| Vented and non-vented caps |  Caps, Closures & Gaskets |
The right cap for transport or for a smooth pour. |
| A pouring or auto-shutoff spout |  Pouring Spouts & Nozzles |
Directs the pour and manages vapour on a compliant can. |
| A plastic fuel can |  Fluorinated HDPE Jerrycan |
Fluorinated HDPE, with a closure to suit the fuel. |
Petrol vapourises, and the vapour pressure rises sharply with temperature. A can filled on a cool morning and left in the sun can swell noticeably by afternoon. Altitude adds to it, because the outside pressure drops while the inside does not.
Modern US-market containers stay sealed between pours, and any separate vent must close by itself (40 CFR 59.611), because an open vent releases the vapour the emissions rules exist to control. Pressure is managed by the container design and by relieving it deliberately before pouring, rather than by letting it escape continuously.
Open the closure slowly, in shade, away from ignition sources, with the opening pointed away from you, and let it equalise before removing the cap fully. A swollen can should be moved to shade and allowed to cool first rather than opened hot.
Heat can split a sealed fuel can, and the vapour that escapes can explode if it meets a spark or flame. Petrol gives off vapour readily, so a can filled cool and left in the sun builds pressure, bulges, weeps at the cap and sprays when opened, and repeated cycles can split a seam. Diesel sits quieter in the same heat. Leave expansion room when filling, store cans in shade and open a warm can slowly, pointed away from you.
Temperature swings in a sealed can cause both. Warm fuel gives off vapour and the air above it expands, so a can filled cool and left in the sun swells. Overnight the vapour condenses and the gas contracts, and with no way to draw air back in, the can pulls a vacuum and its walls cave in. Repeated cycles leave a plastic can permanently distorted. Steel cans bulge and panel too, until the metal cracks at the fold. A two-way vent prevents both.
They let air in while you pour and stay sealed the rest of the time. On an emissions-compliant can, the automatic-closing spout carries an internal air path, so the can breathes only during the pour and seals shut when you stop. The US rule allows a vent the user can open only if it closes again by itself (40 CFR 59.611). A two-way vented cap does a related job: it releases pressure above a set level and admits air as the can cools.
A sound can with a good seal releases little, but vapour escapes three ways. A vented cap breathes by design as the fuel warms. A worn, torn or wrong gasket weeps at the neck. And untreated HDPE lets petrol permeate through the wall, which is why plastic fuel cans are fluorinated; steel does not permeate. US rules cap portable fuel containers at 0.3 grams of hydrocarbons per gallon per day, counting venting and permeation together.
Use the vent the can was built with. A vented cap breathes on its own, letting air in as fuel pours out and relieving pressure as the can warms. A plain screw cap is vented by loosening it a little before a full pour and letting the hiss settle. A US-compliant can has no separate vent: its spout carries an internal air path that opens only while you pour. Do not drill a vent hole, which opens an uncontrolled emissions path.
The spout narrows the pour into a stream you can aim at a filler neck, and it lets air in as fuel flows out. Liquid leaving a can has to be replaced by air; with a single opening, air and fuel take turns and the pour glugs. A vented spout carries an internal air path so the stream runs smooth. On an EPA or CARB compliant can the spout also closes itself, so the can breathes only while you pour and seals when you stop.
Yes. A diesel tank needs a vent so air can replace the fuel as it leaves and expansion has a way out. For storage tanks, OSHA requires atmospheric tanks for flammable liquids to be vented against the vacuum and pressure that filling, emptying and temperature changes cause (29 CFR 1910.106(b)(2)(iv)). A diesel can follows the same physics. Its vapour pressure is under a kilopascal at ambient, so pressure builds far less than in a petrol can.
The standards and regulations this page relies on, at their issuing body. Where a market rule is named here, check the current revision before you specify against it.
Last reviewed by Clarence Cheang, Co-Founder.
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