CAD is the process of drawing and modelling of diagrams and products in either two and three dimensions on a computer package, such as AutoCAD. 

This serves many purposes:  in the first instance CAD is used to produce manufacturing drawings of components, which the manufacturer uses to produce components; secondly, these component drawings can be assembled together within the CAD package, in particular in a 3D view, enabling visualisation of assemblies before construction, and ensuring that one can assemble the components in the desired manner. Furthermore, this aids in fault finding in assemblies before manufacture and construction. 

CAD is also used to design plant layout. Given the details of a plant room, for example, CAD makes it possible to optimise the layout of plant items within the allocated space. Where there is limited space for the installation of valve modules, then alternative arrangements of components can be tried out inexpensively using CAD, before settling on the final design.