Why Does the Outer Jacket of a Cryogenic Vessel Need Its Own Relief Device?

Because the vacuum space can become pressurised, and when it does there is nowhere for that pressure to go. The annulus between the inner vessel and the outer jacket is evacuated. If the inner vessel develops a leak, cryogenic liquid enters that space, warms against the outer shell and expands enormously as it turns to gas. The outer jacket was never designed to hold pressure, so without a relief path it becomes the weakest part of a system under load.

That is why BCGA Code of Practice 36 requires the outer jacket relief device to be set to open at not more than 0.5 bar. It is a low setting on purpose. The device is not there to regulate anything in normal operation, it is there to fail open early and vent the annulus before the jacket is stressed.

It is separate from the inner vessel protection, which under the same code of practice requires at least two independent safety devices on a pressurised tank, typically a pressure relief valve and a bursting disc. The three devices protect against different events and none of them substitutes for another.

Two practical points. The jacket relief device is usually a small plug or disc that is easy to overlook during a visual check, so include it in the inspection routine explicitly. And a jacket relief device that has operated is evidence of an inner vessel problem, not a nuisance to be replaced and forgotten. Take the vessel out of service and have it examined.