A bayonet lock closes the door by turning it so that lugs on the door slide behind matching lugs on the vessel, which is fast to operate but puts the whole pressure force through those lugs, so every lug must bear equally and the door must be physically unable to turn while pressure remains. A partly turned door, a worn lug face or an unlocked rotation is the usual route to a door being thrown off.
Part of Door and Closure Mechanism
A quick twist-lock closure for an autoclave that is opened often: how the lugs carry load and how a bayonet door fails.
Every Lug Carries Its Share Only If All Of Them Touch
The carrier describes the bayonet lock as an interrupted thread or lug system: insert the door, rotate it by 30 to 60 degrees, and the lugs engage behind matching lugs on the vessel, with a safety pin or interlock to stop the door turning under pressure. It is faster than a bolted ring (Bolted Flange), which is why it is chosen for equipment that is opened many times a day.
The load path is short and concentrated. All of the pressure force on the opening goes into the lugs. For the 400 mm opening at 1 bar gauge the force is 12,566 N (Door and Closure Mechanism). If the closure has N lugs bearing equally, each carries 12,566 ÷ N. The lug count and size belong to the drawing, not to this article.
The important point is the word equally. Machined lugs are never perfectly alike. If one lug touches first and the others do not quite reach, the first lug carries the whole load until it deforms enough for the next to take over. A badly fitted door can therefore put 12,566 N through a single lug, N times its intended share. Wear in service makes it worse, because the faces that meet most often wear fastest.
What You Need
- A lug pattern from a code or certified vessel, with lug thickness, bearing face and weld detail.
- Lugs welded to the vessel with full-penetration welds, because a lug is a stress concentration at the vessel wall and a crack there can run into the shell.
- A rotation stop so the door can only turn between the open and fully-closed positions.
- A lock against rotation under pressure, either a pin in the door or the interlock (Door Interlock).
- A gasket that tolerates being compressed by a turning motion (Gasket Types).
Method: Closing, Checking And Maintaining A Bayonet Door
Step 1: Insert the door square. Offer it up level so the door lugs pass between the vessel lugs without catching the gasket.
Step 2: Turn to the stop. The handle should reach the rotation stop and the lock indicator, and nothing short of that counts as closed. A door turned most of the way and left is the commonest unsafe condition, because it appears closed and holds at low pressure.
Step 3: Engage the locking pin or interlock before the heat goes on.
Step 4: Check lug contact when cold. With the door closed and cold, look at the lug faces: contact should be even all round. A bright polished mark on only one or two lugs means the others are not bearing. Correct the fit before use.
Step 5: Inspect lugs periodically, checking for hairline cracks at the root of each lug (dye penetrant), mushrooming of the bearing face, and cracks in the weld to the shell. Replace or repair, and re-prove with water, anything found cracked.
Step 6: Open only at zero pressure, turning with the gauge reading zero and the vent open, standing beside the door.
Common Mistakes
- Treating the “nearly turned” position as closed. Make the stop and the indicator unambiguous.
- Filing one lug to “make it go in”. That lug stops bearing and the load moves onto the rest.
- Skipping the locking pin on small loads. The pin is what prevents rotation when steam pushes on the door.
- Re-welding a cracked lug and returning the door to service untested. A repaired closure is a new closure.
- Lubricating the lugs with an oil that burns or breaks down in steam.
Safety
A bayonet door that turns under pressure opens instantly and completely
No bayonet door may be capable of rotating while the vessel holds pressure. The lock against rotation must engage on its own and must not depend on the operator remembering. Never defeat, remove or shorten the pin or interlock to speed up the cycle.
What’s Next
- Learn next: Door Interlock — the device that stops the turn under pressure
- Combine with: Failure Modes — how worn lugs and cracked roots end in a failure