A pressure vessel (PV) is a container that has pressure different from that of the atmosphere. Pressure vessels come in many shapes. There are many types of pressure vessels and they come in all shapes and sizes depending on their operation. Generally they can be grouped into: thin walled, thick walled, strong tanks, transportable containers, propane bottles and gas cylinders. They can also be lined (cladded) with various metals, ceramics, or polymers to prevent leaking and protect the structure of the vessel from the contained product. The most common shape is a cylinder with domes at both ends; these can be installed horizontally and vertically.
The classifications of the PVs affect its 
design; this includes the shape, material type and thickness, the cladding, location of internal features, nozzles, and inspection regimes which are all important factors to consider for robotic inspection.
PV may have internals; these may affect access by robots as they divide the PV internal space into compartments. Internals such as baffles, distribution piping, trays, mesh- or strip-type packing grids, catalyst bed supports, cyclones, pipe coils, spray nozzles, demister pads, and quench lines can be found in many PVs. Large spheroids may have internal bracing and ties, and most vacuum PVs have either external or internal stiffening rings. Some PVs have heat exchangers or re-boilers located in the lower shell area. Other factors, such as the role and the position of manholes that allow access to the PVs internals or to different sections for cleaning inspection and internal repair, are equally important. In addition, the content of PVs must be considered as they may be filled completely with catalyst and packing; this requires removal to allow internal inspection. Additionally, there may also be access restrictions due to the external environment.
PV’s are found in upstream, midstream and downstream systems. The many conditions in which PVs operate result in a wide range on inspection requirements (in terms of timing and the actual inspection tasks to be carried out). Inspection is therefore handled on a case-by-case basis, either using a time-based or a risk-based inspection approach. Considerations such as the size, construction material and product are all contributing factors that must be considered during the inspection process. The design and requirements of robot inspection tools as well as the operation and deployment of these systems are all important factors.