Factory Acceptance Testing
Unlike the traditional straightforward 4-20 mA interfacing between the control system and monitoring equipment and instruments, digital bus projects must deal with the communication interfaces necessary for an open control system design. Proponents have suggested that one of each like-kind of instruments or devices be sent to the DCS manufacturer’s shop for functional testing and to ensure that the communications interface is verified. For like-kind devices, we believe it is critical that the exact software and appropriate revision used for factory acceptance testing is the same as what will be deployed on the project. We believe this approach was well worth the additional coordination expense incurred.
Construction and Commissioning
One of the greatest benefits of digital bus architecture is the reduced need for expensive analog system cables. Fluor evaluated the cable savings potential at project initiation versus a similar-size reference plant that had traditional control system architecture with the majority of the I/O hardwired. Beyond the direct cable savings, benefits were realized in fewer cable trays, smaller cabling corridors/rooms, less physical congestion, and lower labor density in work areas. All of these benefits contributed to greater construction productivity. For TSPP, the projected cable savings was approximately 30% of the total cable footage.
In the event that an instrument is located too far from the assigned segment protector, construction can add an additional segment protector daisy-chained to take full advantage of the Foundation Fieldbus available trunk line length limits rather than change the way the instruments are assigned in the DCS (Figure 5).
5. Plan ahead. A typical level transmitter is shown with excess prefabricated cable coil. The excess cable is due to using precut lengths with shop-installed connectors to ease installation. Courtesy: Fluor
The typical commissioning process includes a point-to-point check or "loop check" of each control and signal wire from the control room DCS to the field device. Loop checks using traditional 4-20 mA hardwire communications serving as the primary means of data transfer always seem to eventually become the critical path tasks to complete construction and to begin the commissioning tasks.
The digital bus architecture used at TSPP streamlined much of the traditional loop checking, thereby significantly accelerating the wire-checking process. The traditional loop check metrics are often based on the number of loops completed per day in a given shift. Though the rate of loops completed per shift can be improved on a digital bus project with more field device technicians, control room density for the other end of the loop usually limits the number of checks possible at any particular time (Figure 6).
6. Digital domain. The control room operators oversee the network of device communications at Newmont’s TSPP. Courtesy: Fluor
Another advantage of the streamlining that the DCS enabled concerned labor. The remote, high-desert area around TSPP made it difficult to attract and retain experienced electricians. Any design planning or advanced technology that could help reduce the need for onsite labor was especially beneficial at TSPP given the plant’s remote location.
Future Digital Networks
Newmont’s TS Power Plant project provided Fluor the unique opportunity to broaden its experience using digital bus communications in the power industry. Since entering commercial operation in June 2008, TSPP has operated with an annual availability greater than 95% and an availability of 100% thus far for 2009 going into its first planned annual outage in May, demonstrating that the digital bus architecture is here to stay.
We have built on the lessons learned at TSPP and further broadened the use of digital communications on a more recent power project, a 2 x 800-MW supercritical coal-fired facility that is currently being constructed and commissioned. As with any new technology advancement, experiences gained will benefit future project executions and the way engineers approach implementation.
The authors wish to acknowledge the technical expertise and support of Patrick Wilhelm, an electrical design engineer at Fluor. His contributions to this article and to the successfully implementation of the digital bus network at TSPP are greatly appreciated.
—Ali Abdallah, PE is electrical and ocntrols supervisor and James H. Brown, PE, PMP (james.brown@fluor.com) is director of design engineering—solid fueled projects for Fluor Corp.
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