NO TANGLE formula

Designing the complex tangles of pipework that are the vital arteries of any process plant is laborious work. It can require the mental agility of a chess grand-master to keep tracks on exactly which pipes go in which directions, where they cross over and join, how the pumps fit in, and where the weld-points are. The advent of computer-aided design has, as in so many other areas, eased the problem, but it still remains one of the most complex tasks facing chemical engineers.

In the past, Sharp Design, a New Jersey-based multidisciplinary engineering design firm, tended to use the traditional two-dimensional computer aided design approach for most projects. This meant that plan drawings, section drawings and isometrics had to be drawn manually one view at a time. This approach has a major drawback, however: each drawing must be completed from scratch, even though it shows the same area as numerous other drawings. The designer has to maintain consistency between the different drawings. Inconsistencies frequently cause construction errors that require expensive rework. Moreover, it is very difficult to visualise equipment and piping layout in two dimensions, which often results in clashes or other design problems that need to be resolved during construction.

In an effort to overcome these problems, Sharp decided to adopt 3D design methodology. The key difference in the 3D approach is that the designers begin by developing an all-inclusive 3D model of the project, rather than producing the different individual drawings.

Once this model is completed, it can be used to produce a desired plan drawing, section drawing or isometric without additional design work. Consistency of the drawings is ensured, because each comes from the same source. The designers therefore have a 'birds-eye view' of the entire project during the design process, so they can almost always avoid clashes and other problems.

About a year ago, Sharp Design began looking for a 3D computer-aided plant engineering design software system. Sharp focused on packages that support the Intergraph PDS computer-aided plant engineering system because many of its customers use it. But a workstation-based system cost over $50 000 per seat including both hardware and software. So the company evaluated plant engineering systems that ran on personal computers.

The company selected the OMNI-SERIES computer-aided plant engineering system from Cheshire-based firm Rebis. OMNI-SERIES offers the key advantages of workstation-based systems full 3D modelling, automatic ISO generation, automatic generation of bills of materials and interference checks at about a fifth of the price of a UNIX-based system.

Instrumental break

With the new 3D software, the piping engineer starts with the piping and instrumentation diagram, which defines the number, material, dimensional specifications and contents of each pipe. The engineer begins working in the plan view and locates each piece of equipment. Then he or she locates the ends of each piping segment either by typing in coordinates or clicking with a mouse. Rather than simply entering elevation data on the 2D drawing, the elevation becomes an integral part of the 3D model. Using this third dimension allows the model to be viewed from any view or rotation angle.

An automated feature of the software places a full 3D pipe in the position that the engineer has defined. The centreline of the pipe is dressed in a cylinder with proper dimensions taken from the specifications database.

The system came into its own with a project to design an off-gas treatment unit for a major refinery. The unit was designed to reduce emissions by blowing off-gas through a caustic solution that reacts with hydrogen sulphide to reduce sulphur emissions. Sharp's design included two packed towers and a venturi scrubber. Each tower is 24 inches in diameter and five feet long, and filled with Monel packing to ensure a good interface between gas and solution.

Sharp Design developed a rough model of their proposed design based on a P&ID provided by the customer. This model was used to generate graphics that helped the customer visualise the proposed design. Using the specification database in developing the rough model made it possible for them to quickly generate a bill of materials that included overall dimensions and even made it possible to determine the number of welds required to assemble the unit.

Viewing the design in 3D made it possible for Sharp Design to optimise the placement of nozzles and evaluate different piping routes at the very beginning of the design process. This made it possible to accommodate the extremely tight footprint restrictions easily. Within the first few weeks, it was possible to begin feeding partial information to the contractors.

During one of the design review meetings, customer representatives pointed out some process issues that had not been clearly defined in the original proposal. These made it necessary to move an entire section of the unit to the other side of the layout. If the designers had been working in 2D, it would have been necessary to tear up nearly everything they had produced. With the 3D model, all that had to be done was select the section of the model and pull it over to the proper position.

The model was completed at the two-month point, basically finishing the design process. This was about half the time that would have been required to reach the same stage if the project had been done in 2D. Then all that was required was defining the plan drawings, section drawings and ISOs that the contractors needed. Since the model already incorporated the third dimension, extracting orthographic drawings, sections and detail views required only the simple process of 'clipping' the desired area from the 3D model and generating a hidden-line-removed file. To create the isometric drawing, all that was required was opening a view port to provide a 2D paper view. This view could be rotated in any direction and saved to a file that was referenced back to the original layout model so that any changes made to the original model were automatically reflected in the isometrics.

All orthographic and isometric drawings were generated from the 3D layout model during the last few days of the design phase. The time to produce a finished isometric drawing was one to two hours - as compared to about ten hours typically required for 2D drawings.

One of the major advantages of the 3D approach was an improvement in accuracy. Linking orthographics and isometrics to the 3D model ensured that all project drawings were consistent with each other.

The client was very happy with this project. The availability of realistic 3D models in the early phases of the project made it possible for the client to have more input into the design process than normal. Moreover, it was completed more cheaply and quickly than they expected. For Sharp Design, the project provided valuable experience in using 3D methodology to transform the design process. Based on this success, the company has decided to use this technology for future design work.