A pressure vessel is dangerous because it stores the energy of whatever it holds, and a vessel of hot water stores far more than a vessel of gas at the same pressure, because the water flashes into steam the moment the pressure falls. The wall is stretched in every direction by the pressure, so its thickness, its material, its welds and its relief device must come from a recognised pressure-vessel code or from an existing certified vessel, never from a rule of thumb or from appearance.
Part of Autoclave Construction
The physics of containing steam under pressure: what loads the wall, what the vessel stores, and why the design figures cannot be improvised.
A Pressure Vessel Is A Store Of Energy Waiting For A Crack
Fill a closed steel shell with steam and the steam pushes outward on every square centimetre of the inside surface with the same force. Nothing about that force is dramatic while the shell holds. The danger is what the shell is holding back. Compressed gas, once released, expands and does its damage as a pressure wave and flying metal. Hot water under pressure does the same and then something worse: water held above 100 °C only stays liquid because of the pressure on it. When the wall opens, the pressure drops and a fraction of that water boils off at once. The Boiler Construction article puts the expansion at roughly 1,700 times the volume of the liquid, and says a boiler at only 30 psi stores enough energy to do this in milliseconds. That is why a failed steam vessel wrecks a building where a failed vessel of cold compressed air mostly wrecks only itself.
An autoclave is a small pressure vessel with a very large consequence factor: it sits in a room, people stand next to it, and it is opened by hand. The rest of this tree is written on the assumption that the reader has accepted that, and wants to build one that fails safely or does not fail.
What The Pressure Does To The Wall
Pressure pulls a cylindrical shell apart in two directions at once. It tries to stretch the ring of the shell outward (hoop stress) and it tries to push the two end caps off, stretching the shell along its length (longitudinal stress). For a thin-walled cylinder the hoop stress is twice the longitudinal stress, which is why a cylinder bursts by splitting along its length. The full derivation, and the reasons the simple formula is only a floor and never a design, are in Hoop and Longitudinal Stress.
Three things the formula cannot see decide whether a real vessel lives:
- Weld quality and joint design. A seam is the weakest part of a rolled shell. A weld with a lack-of-fusion flaw behaves like a pre-cut crack.
- Openings and corners. Every hole for a gauge, valve or drain concentrates stress at its edge, and a sharp internal corner does the same.
- Time. Corrosion thins the wall and cycling fatigues it, so the vessel that was sound on the day it was built is not the vessel running five years later. See Failure Modes.
The Pressure Ladder Every Vessel Has
A properly specified vessel has a fixed order of pressures, and every safety decision hangs on keeping them in order:
- Operating pressure — what the process needs (for steam sterilisation, the pressure that gives the required temperature).
- Relief setting — the pressure at which the relief device opens. It must sit above operating pressure so it does not chatter, and at or below the maximum the vessel is rated for.
- Maximum allowable working pressure — the rating the vessel’s designer assigns, which comes from the wall, the welds and the material.
- Test pressure — what the vessel is proven to with water before first use.
- Burst pressure — where the wall actually gives way. Nothing in normal practice is allowed near it.
Pressure and temperature are tied together in saturated steam. The Pressure Gauge article tabulates 15 psi gauge as 121 °C and 30 psi gauge as 135 °C, which is why sterilisation sets the operating rung of the ladder and nothing else does. The other four rungs are the engineer’s job and are the reason this tree gives no wall thickness.
What You Need
- A code or a certified vessel to copy. The design pressure, wall, head shape, weld procedure and test pressure of a real vessel built to a recognised code are a tested package. A freehand design is not.
- A history for any vessel you did not make. What it held, for how long, at what pressure. A cylinder of unknown service is not a candidate.
- A water-filled test setup that never uses air or steam for the proof test (see Failure Modes).
- A relief device the operator cannot adjust or block (Pressure Relief Valve).
Common Mistakes
- Designing from a formula alone. The thin-wall formula assumes a seamless, unpierced, uncorroded cylinder. Real vessels are none of those.
- Trusting the gauge as the safety system. A gauge shows pressure; it stops nothing. The relief device is the safety system.
- Treating a scavenged cylinder as free material. Its wall may have been thinned inside where nobody can look.
- Believing “stronger” cannot hurt. Extra thickness is safe, but extra pressure rating on the same wall is not; the rating belongs to the vessel, not to the person who wants more.
Safety
Stored energy, not the pressure reading, is what kills
A small vessel of hot water at modest pressure can destroy a room if its wall opens, because the water boils off in an instant. Never build, buy or run a pressure vessel without a code-derived design or a certified existing vessel, a relief device that cannot be adjusted or blocked, and a water-only proof test. Nobody stands in front of the door or over the vessel during the first pressurisation.
What’s Next
- Learn next: Hoop and Longitudinal Stress — the arithmetic behind the wall, and why it is only a floor
- Combine with: Vessel Construction — how the shell, heads and door come together