Everyone knows the inner vessel has relief devices. The one people cannot find is the relief on the outer shell, and it exists for a specific failure.
What it protects against
If the inner vessel develops a leak, liquid nitrogen enters the annular space. That space is a vacuum jacket, not a pressure vessel, and it was never designed to hold pressure. Nitrogen boiling into it would pressurise the outer shell rapidly. The jacket relief device is what gives that gas somewhere to go.
The setting
BCGA Code of Practice 36 specifies that the outer jacket relief device should be set to open at a pressure of not more than 0.5 bar. That is a low figure precisely because the shell has very little pressure capability.
Why it gets overlooked
It never operates in normal service, so it attracts no attention and is easy to obstruct with lagging, trolley fittings or stored items. A relief device that cannot vent is not a relief device.
Locate it on every vessel you hold, confirm it is unobstructed, and include it in your routine visual checks alongside the inner vessel devices.
