There are only three honest routes to an autoclave chamber: copy a design from a recognised pressure-vessel code, reuse an existing certified vessel whose whole history is known, or have the work done and inspected by a qualified fabricator. A found cylinder of unknown service is not a route, because corrosion and past overpressure cannot be seen from outside, and every new or repaired vessel must pass a water-only proof test before it ever sees steam.
Part of Autoclave Construction
The chamber of an autoclave: shell, heads, closure and openings, and the sequence from drawing to first proven cycle.
The Vessel’s History Matters More Than Its Appearance
A clean, painted steel cylinder tells you nothing about what it held, at what pressure, for how long, or what was left inside it. Corrosion thins a wall from the inside, near the bottom, under lagging and around drains. Overpressure in the past can leave a vessel stretched or cracked while still looking round. A vessel that held chemicals, fuel or compressed gas of unknown kind is not a candidate at all.
Choose in this order:
- A certified vessel with a record. A purpose-built sterilising vessel or pressure canner with a known design pressure, nameplate and service history is the best starting point.
- A vessel built to a recognised code by a qualified shop, with the weld procedure, inspection and test records handed over.
- A vessel made locally to a code-derived drawing, by welders who have been tested on the actual procedure and with every weld inspected.
A cylinder of unknown service goes in none of those groups. Use it, if at all, for something that holds no pressure.
What You Need
- A drawing taken from a code or from an existing certified vessel, giving the dimensions, wall, head shape, openings and weld details. This tree does not supply those figures. See Pressure Vessel Theory for the reasons.
- Steel of known identity. Mild steel is the carrier’s material, with stainless steel as the better choice for medical work but not required, since the steam does the sterilising and not the metal; see Stainless Steel Production. Cast iron must never be used for any part holding pressure, because it shatters instead of bending; the Boiler Construction article states this plainly.
- Qualified welders and a written procedure. The carrier asks for a full-penetration longitudinal seam, ground flush, and inspection by X-ray or dye penetrant where possible.
- A hand pump, a calibrated gauge and water for the proof test.
- A vessel log (see below).
Method: From Drawing To First Proven Cycle
Step 1: Fix the size from the load, not the other way round. The carrier’s typical chamber is an inner diameter of 300-400 mm and a length of 500-700 mm. Check what that means in volume: a cylinder of 300 mm diameter and 500 mm long is π × 0.15² × 0.5 = about 35 litres, and one of 400 mm diameter and 700 mm long is π × 0.2² × 0.7 = about 88 litres. Those match the carrier’s 35-90 litre range. A bigger chamber holds more water and more stored energy, so do not make it larger than the work needs.
Step 2: Roll and weld the shell to the drawing. The longitudinal seam is the highest-stressed joint. It must be a full-penetration weld with no lack of fusion or inclusions, because a flaw there behaves like a cut.
Step 3: Fit the heads and the nozzles. One end is permanent and welded. The other carries the door. Every opening (gauge, relief device, thermometer, steam inlet, drain) is a stress concentration and must be reinforced as the code drawing shows. See End Caps (Heads) and Cylindrical Body.
Step 4: Inspect every weld before closing anything up. Dye penetrant shows surface cracks; radiography, where it exists, shows internal flaws. A weld that cannot be inspected is a weld that cannot be trusted.
Step 5: Fit the door and seal (Door and Closure Mechanism, Sealing Systems).
Step 6: Hydrostatic test. Fill the vessel completely with water, bleed all air from the highest point, and raise the pressure slowly with a hand pump to the test pressure that the governing code specifies for this vessel. The Boiler Construction article describes the corpus’s own boiler test as 1.5 times working pressure held for 30 minutes; use the code figure for the vessel in front of you. Look for weeping at every weld, nozzle and gasket. Mark leaks, release the pressure fully, repair, and test again.
Step 7: Fit and test the safety devices, then run a water-only trial with the heat source and relief device in place and everyone clear of the door. Only then run the first sterilising cycle.
Worked Example: The Vessel Log
Every vessel gets a bound book that never leaves it. The first page records who built it, what steel, what drawing, which welders, and the weld inspection and hydrostatic test results. After that, each use adds the date, the maximum pressure reached, whether the relief device lifted, and any leak or repair. A vessel that is later handed to someone new is only as trustworthy as this book. The record-keeping habits in Record Keeping apply directly.
Common Mistakes
- Using a “free” cylinder. Its history is its risk, and the saving is not worth it.
- Welding a repair and returning it to service untested. A repaired vessel is a new vessel until it passes the proof test again.
- Making it bigger than needed. Every extra litre of water is extra stored energy.
- Skipping the log. Without it the next owner cannot tell a sound vessel from a ruined one.
Safety
Weld flaws and unknown history are the two ways a built vessel fails first
Never weld on a vessel that has held fuel or an unknown substance. Never pressurise a new or repaired vessel with anything but water for the first test. Never stand in line with the door or the weld seams during a proof test.
What’s Next
- Learn next: Cylindrical Body — rolling, welding and inspecting the shell
- Combine with: Safety Devices — the fittings that every nozzle exists to carry