TOPIC SERIES
External-pressure check, wall thickness, seal-face stiffness, and what actually separates 304L from 316L.
2 articles in this series
The structural question for a vacuum chamber is not simply whether the wall is thick enough. Three things decide the outcome: stability under external pressure, stiffness of the sealing faces, and whether thermal and cleanliness constraints can be met at the same time. Material selection trades corrosion resistance against weldability and low outgassing.
2026-11-01
The order matters: how the load enters, where the heat goes, how thick the shell must be, how large the pumps are, how the interlocks are wired. Get the order wrong and the door, the hot zone and the wall thickness all get redone.
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2026-10-27
Most chambers are adequately served by 304L. 316L is selected where the process atmosphere is corrosive or the drawing specifies SUS316L. Where the choice is unclear, state the operating conditions in the RFQ and the recommendation will be based on them.
Read article →Once evacuated, atmospheric pressure becomes the dominant load and the governing failure mode is elastic buckling rather than strength. Buckling occurs well below yield, so an external-pressure stability check is required rather than an internal-pressure strength formula with a margin.
Use 304L for ordinary vacuum service without chlorides where cost matters; use 316L (2–3% molybdenum) when process gas contains halogens, higher corrosion resistance is needed, or frequent cleaning and sanitisation is expected. Both are low-carbon grades with better intergranular corrosion resistance after welding than the non-L grades. The process medium and customer specification govern.
