By Rich Merritt, Senior Technical Editor
IN OUR JULY 2005 issue, we noted that the servers are coming (“Remote Control,” CONTROL, July 2005). Soon, you will be able to control remote plants, pipelines and offshore platforms from afar, and you’ll be dealing with IT, condition monitoring, loop tuning, and process modeling software that runs in remote servers. In new systems, none of the enterprise-level software has to be in your plant. It can all reside on a server at your central engineering center, a vendor’s support center, or at a specialty supplier.
And, for truly remote systems, you won’t even need a local control room or an HMI/SCADA system. A portable laptop will suffice as an HMI when you or a tech has to visit the site.
All this requires segregating the real-time, field-based part of a process control system from the enterprise side, and making the real-time portion of the control system into a fortress that’s immune from attack by Internet creatures.
FIGURE 1: CONTINUING CONTROL
A Fisher control valve with Fieldvue is installed on a petroleum feedstock flow at a Texas oil refinery. It can control PID loops all by itself, thanks to fieldbus. If the fieldbus link to the host is lost, loop control continues. Source: Emerson Process Management
It also requires that you set up a control system that is capable of standalone operation (See Figure 1);
that is, if the links to higher-level enterprise servers are cut or interrupted, the real-time portion of the control system can soldier on, keeping the plant loops running until communications resume.
This is not a far-out concept. Some plants have been doing it for years. "Why not rely on field devices for control?” asks John Rezabek, control engineer at ISP Lima BDO Manufacturing in Lima, Ohio. “We're relying on them anyhow. Our site has logged five years with 80% of PID being solved in valve positioners. H1 fieldbus is much more robust than we anticipated."
In this article, we’ll look at some of the distributed intelligence that makes it all possible.A Mighty Powerful System
Mighty River Power’s hydroelectric operation in New Zealand is a perfect example of a distributed control system, a single server in a central location, and unmanned, remote controls. At Mighty River, nine hydroelectric stations are each monitored and controlled by individual Honeywell
Experion PlantScape PKS servers with the Distributed System Architecture option. The systems provide data via microwave and a leased fiber optic link to twin, redundant SCADA systems.
A single Experion central server in Hamilton, NZ, accesses whatever data it needs from the SCADA database, and runs enterprise software for the plants. This includes plant optimization, water optimization, inflow prediction, outage management, electronic dispatch and performance management enterprise software.
According to Mark Harvey, controls engineer at Mighty River, the Automation and Remote Control (A/RC) project was originally justified by estimated savings in labor costs and the increased efficiency of water usage. “The benefits considered at the time have proven to be wildly underestimated,” says Harvey. “Prior to the AR/C project, each of our nine power stations had 20 to 30 staff, with the larger stations having considerably more.” They also had seven people in Hamilton and 24 people in a centralized maintenance station. “Today, all maintenance is outsourced, and most stations have no staff on site for days at a time.”
What’s more, in case of a system failure, each power station is able to fully control any other power station via its local Experion server, the distributed network, and Honeywell’s Downtime Option. As a final advantage, by using a central server, Mighty Power has to pay only one license fee for the Downtime Option and the Network Server. Doing it the “old way” would require separate license fees for those functions in all 10 locations.
They plan to introduce tools such as an alarm/event manager and equipment health monitoring, both made possible by the distributed architecture.
Going to server-based, remote control, unmanned systems like Mighty River’s is the wave of the future. But to ride the wave, you’ll need distributed intelligence to run the actual plant. One way to accomplish this is to distribute the controller intelligence as far down into the process and as far out into the field as possible, making various nodes, components and systems as autonomous and self-sufficient as possible, and immune to network and communications interruptions.How Low Can We Go?
Steve Garbrecht, product manager at Wonderware
points out that segregating control systems is nothing new. “If we look back to the invention of the DCS by companies like Honeywell, the original architects designed two basic layers to the system: a regulatory control layer and a supervisory layer,” he points out. “It would have been easier to design a single layer so I can only assume that they had a good reason for it.”