3D modelling eliminates field rework during retrofit5 June 2002
By modelling a power plant and its proposed selective catalytic reduction system in 3D, it was possible to eliminate interference problems that would otherwise have led to extensive rework during installation.
This project was particularly susceptible to interferences because the new prefabricated steel had to fit in among a great deal of existing steel, ductwork, and piping. With 2D drawings, it would have been very difficult to find instances of interference because the steel, ductwork, and piping were represented on separate sets of drawings. By creating a 3D model of the facility and then routing the piping and cable trays for the SCR through and around it, engineering designer Neill and Gunter was able to design a system that fitted perfectly into the existing structure. "The SCR was installed during a plant shutdown, so every hour of downtime was critical," says Tom Cyr, project manager at Neill and Gunter. In the event the construction contractor was able to detect and correct interferences before the plant shutdown, thus saving a significant amount of time and rework.
Neill and Gunter specialises in upgrades to existing industrial facilities. This project involved the adding of SCR to an existing coal-fired power plant on the US east coast. The purpose of the retrofit was to reduce the level of NOx emissions in accordance with the Clean Air Act and involved adding an SCR system to each of the plant's three boilers. SCRs work by injecting ammonia (NH3) into the flue gas stream where, in the presence of a catalyst, the NH3 and NOx combine to form nitrogen and water.
The construction company subcontracted the balance-of-plant design to Neill and Gunter. This involved designing all the foundations, the piping to tie the SCR equipment to the existing plant, and the new electrical systems. The main challenge for the designers was the routing of cable trays and piping through a network of new SCR support steel that was being designed by the SCR manufacturer, and also through existing steel work, some that would be removed after the SCR installation and some that would remain in place. "The new steel work was by far the more challenging issue. It was a complex design involving a significant amount of steel," Cyr says. "We were given over 200 orthographic drawings showing where all this steel was going. Along with those, we had some older drawings of the existing plant where steel members were on one set of drawings and ductwork was on another. We realized that if we tried to use all those different drawings, it would be very difficult to route our components and not have interferences."
It was decided to use these drawings to create a 3D model of the existing plant and the new steelwork. The software they selected was AutoPLANT, an AutoCAD add-on from Californian company Rebis. As well as 3D modelling capabilities it has the capacity to store materials and plant information.
The first step involved creating a 3D model of the new and existing steel using a design module called 'structural modeller' which covers 3D grid placement, steel placement and database management, steel editing and display options, steel annotation, and access way placement. The model was created from structural drawings and old drawings of the power plant. However, because some of the old drawings were dimensionally inaccurate and didn't show all the existing ductwork, this was supplemented by photogrammetry, a co-ordinate measuring technique that uses photographs as the medium for metrology by creating lines of sight from several photographs. This extra step ensured a very high level of accuracy in the model and led to the discovery of some interferences between the existing facility and the new steel. The steel design firm was impressed enough to eventually buy AutoPLANT for themselves.
After creating an accurate model of the existing steel and ductwork and the new steel and ductwork, engineers were ready to route piping and cable trays through the model. Before the engineers started creating 3D objects, they entered the material specifications into the AutoPLANT database. This made it possible to pull in components from the database, automatically creating a database of the materials required for the project, so that the completed model includes data files for new steel, existing steel, and new and existing ductwork. It also provides a degree of visualisation that enables engineers to see the components they have to avoid during the route design process.
Neill and Gunter was able to create orthographic drawings, all balance-of-plant systems and a bill of materials (usually a time consuming process) for all piping and cable tray systems. The orthographics were created by selecting suitable views of the 3D model then giving the command to generate the drawing. Title blocks and dimensions etc were then added. An alternative Rebis module can automatically produce isometric drawings from the 3D plant model.
The company took the extra step of creating rendered images of the 3D model, also an automatic process, to provide 'snapshots' for the construction crew of complicated areas. A bonus was that they avoided interferences not just in their own work, but in others' drawings too.
This project was the first one Neill and Gunter had designed in 3D. As a tool for designing interference-free plant systems, and as a producer of time-saving benefits, the software was reckoned a complete success.