In this brief article, the use of engineering models in process plant design, planning and operations is explored.
USE OF ENGINEERING MODELS AND SOME GENERAL NOTES
By Paul Bowers, Bir Khangura and Kevin Noakes
In the long history of technical drawings, it has been necessary to depict designed objects in multiple views, simply because three dimensional things cannot be effectively described in 2D space (such as on paper, backs of envelopes or cave walls). At minimum, usually two views of the object(s) are required; think of architectural house drawings which typically include floor plans as well as side views.
Using models, the general arrangement of the facility to be built can be easily conveyed without relying on multiple drawings and documents. Whether virtual (i.e., digital, using computer assisted design/drafting - CAD) or physical, one major advantage of engineering models over engineering drawings is ease of visualization.
Prior to the now ubiquitous use of 3D modelling software to generate engineering drawings for process plants, scale models were often constructed to assist for visualization of the designed facility. These models were generally built to 1/4”=12” scale and some completed plant models at this scale could easily fill a standard size ISO shipping container .
The components used to create these models were custom made by the engineering firm's model shop from balsa wood and acrylic, and ABS, as well as purchased as pre-manufactured butyrate moulded components (pipe fittings, valves, structural steel shapes, etc.) such as those available from a company called Plastruct. Many Plastruct components are also used by model railroad hobbyists to construct complex railway dioramas.
All equipment, piping and structures in the plastic process plant were color-coded according to process fluid service, mainly grey, red, yellow, green, white, black and blue and were built to plastic design model specifications and procedures provided by engineering companies in order to standardize and control the workflow and presentation of the plastic model. Hand made sketches, fully dimensioned and annotated were made from the model and used for fabrication of pipe spools. After the model was completed in the office, it was shipped to the construction site where it was used as a reference to put fabrication spools together and to assist construction and installation planning.
The technicians building these scale models were often a subset of piping designers and draftsmen, because one needed to beable to read technical drawings and know what these things were, who were willing to get their hands dirty with plastic, solvents, balsa wood dust and paints; oh my. There are anecdotal stories of fingers being glued together, inhalation of noxious fumes (from plastic solvents, adhesives and cements), lacerations and scrapes from cutting tools and dissolved fingerprints. As a comparison, today's process plant 3D CAD modellers might suffer from carpal tunnel syndrome, email overload or blue
screens of death.
While today's 3D CAD models are physically intangible and require powerful computer hardware and software to create and use, they offer many more useful features than their physical predecessors, such as automatic documentation generation. Creating the initial 3D model can be as time consuming as manual drafting methods but the time and work-saving occurs downstream in the workflow.
As the digital model is being 'built', each component is either created from scratch or selected from a digital catalog (of geometric shapes) and then inserted into the model as well as automatically added to the associated model database (information file cabinet).
The database allows for automatic generation/extraction of orthographic drawings of plant areas, bills of materials, line designation tables, equipment lists, work packages, characteristics and geometry for pipe and structural stress analysis, piping isometrics, plot plans, etc; deliverables that had to be produced by hand either before or after the plastic model was built. The 3D model-database as well minimizes the possibilities of error while transposing information from the model to deliverable documents.
SOME LIMITATIONS AND CAVEATS REGARDING DIGITAL MODELS
While 3D CAD models offer the ability to easily re-use previously-designed equipment, components and common assemblies, the concept of GIGO (Garbage-In, Garbage-Out) still applies.
This is especially true for vendor-sourced equipment models created from 2D supplier drawings by the engineering firm, example equipment being pumps, compressors, vertical and horizontal vessels, heat exchangers, storage tanks, etc. As opposed to pipe fittings, valves and other inline piping components, which are standardized in dimensions, major equipment is most often custom-designed for each process facility. If these components are inaccurately modelled , the interconnecting piping between equipment will not fit, resulting in expensive field fixes, schedule delays and the use of profanity on-site during construction.
The use of component catalogs by the modellers (pick item from menu, insert in model) also tends to eliminate the need of the modellers to know such details as standard dimensions and material makeup of the inserted components. To some, this may not be a concern, but it tends to minimize the 'nuts-and-bolts'-level detailed knowledge of the designer.
Another phenomenon that can occur is overconfidence in the digital model, simply because they tend to project precision  and there is a 'wow factor' associated with the visualization technology.
The compatibility of the format of the information files can also be an issue if, for example, a project was designed utilizing one software application package and a subsequent upgrade to the project needs to be performed using a different software application package . There are data translators and convertors, but typically the conversion process will not function 100% correctly, leaving unwelcome (and possibly hidden) surprises to be discovered at the most inopportune moment.
Similar to file format incompatibility is the issue of hiring trained and experienced modellers who are familiar with the software used on a new project. As design software tends to be complex, it may be necessary to train designers in the program's operation to use a specific required software.
One advantage of 'the good old days' was that designers could get up to speed and draw just as fast and accurately with Staedler as with Pentel, Berol or Rotring mechanical pencils – no training needed or learning curves to overcome. At one time you instinctively knew how to use your hand to draw and now you have to learn how to tell a machine to do it for you!
 A possible solution to this could be for vendors to provide 3D CAD models of their own equipment.
 The difference between 'precision' and 'accuracy' must be noted.
 Common plant design software packages are AVEVA PDMS, Intergraph SmartPlant 3D, CodeCAD CADWorx (also Intergraph, as is PDS), Bentley AutoPLANT and AutoCAD Plant 3D.