The DCS spirit lives on everywhere

Classic distributed control systems typically remain only in high-availability refining and power applications, but their strategies and capabilities persist in descendants and today’s sophisticated networks.

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Erfurdt adds that Emerson’s personnel helped solve the OPC interface challenge by setting up a lab at their home facility to verify interoperability of the safety shutdown system, and identify issues that could affect system performance or operation. As a result, the refinery’s modernization program is allowing Shell Deer park to operate its catalytic cracker at 100% of planned uptime or greater with no surprises and no unscheduled stoppages. The cracker now processes about 70,000 barrels of feedstock per day, and is expected to show an increase in utilization of at least 1%.

“The availability of sound diagnostics from the field will enable our operators to have a better sense of what’s going on in their processes, and make it easier for them to avoid upset conditions or at least minimize the effects of upsets,” adds Erfurdt. “If we can prevent the loss of three or four days of production in a year, this control modernization program will achieve its objective. We expect to be successful.”

Mimics Go Mainstream
The long debate over whether combining other technologies can adequately recreate DCS capabilities is still going strong. Many developers and users claim that PLCs, HMI/SCADA software, a data historian, an OPC connection, and other components can be merged into systems that can do most if not all DCS jobs, and that traditional DCS functions are just a subset of these overall digital architectures. “We’ve installed more than 50 large-scale, high-end, batching systems with more than 500 I/O each over the past eight years. These were PLC and HMI based, and tied to relational databases to get directions,” says Mark Hoffman, of Automation & Control Concepts Inc., a system integrator in St. Louis, Mo. “None of these applications missed having DCSs at all.”    

However, other observers say these combined systems still aren’t integrated enough to provide true DCS availability. “The real acid test is every year when you need to update your operating system (OS), add new functions, or remove software bugs, can you migrate your DCS from one to another version without shutting it down? If you truly need high availability, how can you add software patches and fix viruses?” asks Sweet. “Only a DCS can add a new OS and patches, and fix bugs without shutting down. High-availability has to be architected in. DCSs are built to add upgrades without going off process. Combined PLC/HMI-based displays typically start and stop every day or shift. They’re still nibbling at DCS’ fringe, but it isn’t happening.”    

New Regions, Greener Fields
DCSs also stay relevant because there are so many “distributed” geographic regions, such as India and China, where users are building plants that need high-availability controls. “The big challenge there is how to deploy massive amounts of control in places where people are just learning the technology,” says Sweet. “We love working with engineering staffs in India because they’re inventive, aggressive, and they have the chance to make really big percentage gains in productivity, safety, and quality of life.”

Solig Gas Power

Solig Gas Power’s new DCS system performs balance-of-plant control and links to a 126-megawatt GE PG9171E gas turbine.
Source: GE Energy

For example, GE Energy’s optimization and control division recently installed its DCS solution at the first combined-cycle power plant at the Inner Mongolia Solig Gas Power Co., Ltd.’s facility in Dabuchake town and Wushengqi Eruduosi city in the nation’s autonomous region (See Figure 2). The Solig plant’s DCS system performs balance-of-plant control, and links to a 126-megawatt GE PG9171E gas turbine, which is controlled by a Mark VI control system. The DCS also controls the heat recovery steam generators, the electrical system and the circulating water controls for Solig’s two sets of combined-cycle units, which started operating in April and July 2006. “Standardizing our plant on one supplier for our plant will give us the best service and reduce maintenance and operating costs,” says Yue Jianhua, Solig’s chief director vice chief engineer for Inner Mongolia Power (Group) Co., Ltd.

Networking and Wireless
The next big journey for DCSs—like all other process-control devices and systems—is further into the Ethernet and wireless realms. This may be a familiar trip because DCSs and PLCs have long used fieldbuses to transmit I/O signals via twisted-pair wiring, and now use Ethernet to communicate to other devices and make graphics available via local area networks (LANs), virtual LANs (VLANS), and even wireless LANs (WLANs).

“Just eight years ago, you needed an $18,000 proprietary gateway to make a Delta V DCS communicate with an Allen-Bradley PLC, but now you can do it with a $10 Ethernet gateway,” says Superior Controls’ Pierro. “Communication between different DCS and PLC packages and third-party devices also is simplified by OPC, which makes it easier to read register values for temperature, pressure and pH. Previously, we had to write costly and time-consuming I/O drivers.” [See this month’s Industrial Networking cover story, "Multiple Choices," for Superior’s implementation of a wireless-enabled DCS in a bioreactor application.] 

Aramco's Shedgum Gas Plant

Operators inaugurate the then-new central control room at Saudi Aramco’s Shedgum Gas Plant in early 1998.

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