A jacket is a second pressure container applied to the outside of a pressure vessel shell, to either apply cooling or heating to the vessel contents.
 

Big differences between jacket and shell temperatures can create huge thermal stresses, which can lead to thermal fatigue. 

The internal pressure in the jacket will normally act as an external pressure on the inner shell; this can be further compounded if the inner shell pressure can drop to full vacuum.

The inner shell design should be capable of withstanding at least 80% of the jacket test pressure at ambient temperature; otherwise there is a risk that the inner shell will buckle when the jacket is tested.

Openings and penetration into jackets should be treated as nozzles, and normal compensation rules applied.  Square openings should be avoided and large openings should be given special consideration.

Jacket Permutations
There are many permutations of jackets; the ASME code lists 5. However, as far as calculations are concerned there are two basic types: -
 

Type 1	Where the diameter of the inner shell under the jackets does not reduce, and the closure ring is only subjected to a pressure load.
Type 2	Where the diameter of the inner shell under the jacket reduces or completely covers the end; thus creating an end thrust on the sealing ring.

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Page Last Modified  10 March 2002