A door interlock is a mechanical lock that keeps the autoclave door shut for as long as the chamber holds pressure, and it must fail to the locked state, never to the open one, so that a broken spring, a leaking piston or a blocked sensing port cannot unlock a pressurised door. Even a small residual pressure pushes hard on a large door, so the best designs make the first movement of the handle open a vent and only the second release the lock.
Part of Safety Devices
The device that prevents the door being opened while the vessel is pressurised: how the pressure-piston design works, how it fails, and how to build it so the operator cannot defeat it.
An Interlock Must Fail Locked, Not Open
The carrier’s design is a mechanical latch that locks the door’s rotation. A small piston connected to the internal pressure holds the latch in place. Pressure above 0.1 bar keeps the latch from releasing, and only when pressure falls to near zero can the door be unlocked. This is the right idea: the pressure itself holds the lock, so no operator decision is involved.
The dangerous failure of such a design is a false zero. If the sensing passage to the piston clogs with scale or condensate, the piston sees no pressure while the chamber is full, and the latch lets go. The same happens if the piston seal is stuck or a spring breaks the wrong way. A good interlock is therefore designed so that every foreseeable failure leaves the door locked, and so that the sensing line is short, wide, and on the steam space and not at the drain.
A Residual Pressure Still Pushes Hard
The carrier’s release threshold, 0.1 bar, sounds small. Its effect on the door is not. For the 400 mm opening used in Door and Closure Mechanism:
- Area: π × (0.2 m)² = 0.1257 m².
- Pressure: 0.1 bar = 10,000 Pa.
- Force: 10,000 × 0.1257 = about 1,257 N.
- Weight equivalent: 1,257 ÷ 9.81 = about 128 kg, pressing the door outward.
A door released at that threshold will be pushed open by the equivalent of a heavy person’s weight, with steam behind it. That is why the release should be staged:
- First movement of the handle opens a vent, letting remaining steam out of the chamber and away from the operator through a pipe.
- Only after the vent is open and the gauge reads zero does the latch release.
- The door is then cracked open a little, with the operator beside it and not in front.
This vent-before-unlock arrangement is a recalled design principle used on commercial sterilisers; it means the interlock’s final safety does not rest on a single sensor.
Interlocks For The Other Closures
- Bayonet door. The lock must stop rotation. A pressure-held pin or a bar driven into place by internal pressure is the usual form (Bayonet Lock).
- Bolted flange. The flange cannot be released by a single motion, so the interlock is a guard plate over the nut ring, held by a lock whose key stays trapped in the vent valve while the valve is closed. The key can only be removed once the vent is fully open, and the key alone releases the guard. This trapped-key principle is a recalled industrial practice (Bolted Flange).
- Plug door. The pressure itself holds it, but a latch should still keep it from being pulled open at zero pressure with residual hot water.
What You Need
- A pressure-sensing passage on the steam space, wide enough not to block and drained of condensate.
- A latch and piston from a code drawing or a certified vessel, with a spring that holds the latch locked when the piston is not under pressure.
- A vent that the first handle movement opens or a separate vent valve linked by the trapped-key method.
- A clear gauge next to the handle, reading in the operator’s line of sight.
- A record of the daily interlock check in the vessel log.
Method: Checking The Interlock
Step 1: With the vessel cold and empty, try the handle with the lock engaged. It must not move to open.
Step 2: Run the vessel to a low water-only pressure. The latch must stay locked and the piston must move freely. Release the pressure and confirm the latch frees only when the gauge reads zero.
Step 3: Look at the sensing passage. Blow it clear or rod it out. A passage you cannot see through is a passage that may be blocked.
Step 4: Check the spring and the piston seal for corrosion and wear.
Step 5: Write the result in the log. A skipped check is a missing record.
Common Mistakes
- A latch that releases at a pressure above zero. The 1,257 N figure above is why.
- A sensing port at the bottom. Water and scale gather there and block it.
- Tying or wedging the latch to speed up cycles. That removes the device.
- Forcing a stuck interlock. The correct response is to leave the door shut, let the vessel cool completely with the vent open, and find out why it stuck.
- A spare key left hanging near the vessel. A key that anyone can fetch is a bypass.
Safety
An interlock that can be overridden will be overridden
No keyed override, no jumper, no wedge. If the interlock fails, the vessel is withdrawn from service, vented and cooled, and repaired. The Failure Modes article explains what a door opened under pressure does.
What’s Next
- Learn next: Heat Source — where the pressure that the interlock holds back comes from
- Combine with: Pressure Relief Valve — the other acting device, protecting the vessel itself