How to design a vessel under External Pressure
External pressure can be caused in pressure vessels by a variety of conditions and circumstances. The design pressure may be less than atmospheric due to condensing gas or steam. Often vessels are design for some amount of external pressure, to allow for steam cleaning and the effects of the condensing steam. Other vessels are in vacuum service by nature of different devices, connection to a vacuum pump for example, vent of a vessel during draining, or from improperly sized vents. External pressure can also be created when vessels are jacketed or when components are within multi-chambered vessels.
The failure mechanism of external pressure is different from the failure induced due to internal pressure; therefore, different methods are required to design vessels to handle these two separate conditions safely. Internal pressure failure can be understood as a vessel failing after stresses in a part or a large portion, exceeding the materials strength. In contrast, during external pressure failure, the vessel can no longer support its shape and suddenly, irreversibly takes on a new lower volume.
A stable system is one that is stronger than required. When the vessel is pushed on, it pushes back and returns to its original shape. As external pressure is added to the system, the vessel has less reserve strength left to push back. Eventually the vessel reaches a point where it has very little reserve strength. The wall of the vessel is again pushed on and it cannot push back. At this point the vessel will change its shape to a smaller volume configuration. The change is sudden and irreversible, and if you watch some videos, very scary.
Unlike vessels which are designed for internal pressure alone, there is no single formula, or unique design, which fits the external pressure condition. Instead, there is a range of options available to the designer which can meet the requirements. The thickness of the shell is only one part of the design. Other factors which affect the design are the length between supports, the use, size, and spacing of stiffening rings.
Designing vessels for external pressure is an iterative procedure. First, a design is selected with all variables included, and then the design is checked to verify if it is adequate. If inadequate, the procedure is repeated until an acceptable design is reached. The geometry of the equipment is the critical factor rather than material strength. Failures can occur suddenly, by collapse of a component (normally the shell).
If you want to know more:
ASME VIII | Design of Pressure Vessels
TEMA | Design of Shell & Tube Heat Exchangers
