PC-based control system improves boiler operation, efficiency
A new boiler control package installed as part of a PC-based control system solved operating problems for the Roanoke, VA, Locomotive Shops of Norfolk Southern Corp.
A new boiler control package installed as part of a PC-based control system solved operating problems for the Roanoke, VA, Locomotive Shops of Norfolk Southern Corp. Combustion efficiency of the company's four 1950s vintage, stoker-fed, coal-fired boilers improved within the first few hours of the system's operation. In addition, the need for manual intervention in the control loops was virtually eliminated, safety was improved, and oxygen levels in the stacks were reduced.
Improvements were achieved using a moderately priced, PC-based control system with a preconfigured boiler control rather than costly distributed controls and custom-engineered strategies. The new package, a DeltaV control system from Fisher-Rosemount, Inc., replaced a DCS that had been used for the last 15 yr. The steam from the four boilers, each rated at 70K pph at 160 psig, is used to power three air compressors, heat water for parts washers, and heat the facility.
Although state of the art when it was installed, the old control system had become outdated. Limitations made efficient boiler operation difficult and changes hard to make. For example, when two units operated at the same time, only one could be managed automatically. The other had to be controlled manually with loads fluctuating between the two. A substantial number of control loops had to be manually controlled at all times, requiring close operator attention.
Fan control also was a serious problem. The induced draft fan ran erratically and the control system occasionally shut it off without stopping the forced draft and overfire fans. The furnace would become pressurized, forcing the furnace doors open and creating the potential for dumping burning coal onto the floor. To ensure safety, operators had to control the fan manually to stabilize boiler pressure and prevent smoke emissions.
The new control system combines the look and feel of a Microsoft Windows-based PC operating system with the security, interactive displays, and information accessibility of a DCS. Combining PC-based hardware with preconfigured software designed for specific types of industrial applications has brought control to many types of operations once considered too small for a fully-automated control system.
The new system automatically controls about 250 I/O points, significantly more than the old DCS could handle. Equipment controlled includes the four boilers, economizers, precipitators, ID fans, water decarbonization units, deaerators, condensate storage, and feedwater pumping system. Sufficient capacity remains to expand control to the water treatment system and air compressor at some time in the future.
The installation was completed in minimal time by Control Dynamics Co., Richmond, VA. A rental boiler was used to carry the load during the changeover. Existing hardware was removed; then the hundreds of wires were reconnected with I/O interfaces and controllers installed in existing control system cabinets. Cables were run to two desktop workstations facing the boilers and a third in the boilerhouse superintendent's office.
Within eight days, the first boiler was successfully refired and placed under automatic control. The installation might have been completed even quicker if not for some equipment problems discovered during startup. Inappropriate responses from several control valves revealed positioner problems. Several were replaced and all were recalibrated, ensuring efficient operation once the valves were under automatic control. At startup, each boiler was fired in the manual mode, then switched to automatic one loop at a time. Although some minor problems were encountered, all were quickly corrected and within an hour all units were operating in the automatic mode.
Applying the strategies
Because a good deal of auxiliary equipment is involved, control of steam production is quite sophisticated. Four interdependent loops -- water, air, fuel, and steam pressure -- are involved, and typically strategies had to be custom-designed for the control hardware. In this case, however, a preconfigured strategy was developed for use with the new control system. The software simplifies system engineering, installation, configuration, use, and maintenance, while providing tight control of the medium-sized boilers. Cost was a fraction of that generally required for a custom-engineered strategy.
The control system was adapted from handling gas or oil to coal with a minimum of effort. The air and fuel parallel positioning strategy was replaced with one that is cross-limited to ensure a proper fuel/air ratio and full combustion. Because coal is sometimes wet and slow to burn, the strategy adds air as long as heat is generated, resulting in greater fuel efficiency.
During startup, each boiler was load-tested to be certain the fuel/air mixture was proportioned properly at all load levels. As each boiler was run from minimum to maximum load and back down, a fuel/air curve was derived and the information fed into the control package. Oxygen in the stack was measured and recorded and the fuel/air ratios adjusted in the program to maintain minimum excess oxygen in the stacks without increasing emissions.
The new control package now operates the boilers automatically, even when more than one boiler is online. The system also monitors all the inputs from field sensors continuously. If preset operating limits are exceeded, an alarm sounds and control of the loops involved reverts to manual to allow the operators to correct the problem.
During normal operation, one or both monitors in the control room are turned on and switched to a screen showing all operating parameters. More information about a particular area (feedwater pumps, for example) is available by using the mouse to click on the appropriate point. A new display appears with information about the area selected.
The 25 displays that provide the information the plant needs were designed by the plant superintendent and boiler operators, then implemented by the installer using graphical elements contained in the basic control system package. Bright colors and high resolution help generate eye-catching, easy-to-read graphics.
Acceptance of the system by the operators has been excellent. Most were surprised that the PC-based system could handle the entire operation automatically. Essentially every aspect of the boiler operation has been improved. Trend reports, available for every 24 hr of operation, let the superintendent analyze any given time period to determine if any changes need to be made. Totalizers in the system track fuel use and steam production, providing information for financial performance reports.
Capacity of the scalable system is such that additional areas can be controlled in the future. The architecture of the system gives the company the ability to integrate intelligent field devices, asset management solutions, and business applications to provide the information, control, and management capabilities needed to increase performance levels and control costs even further.
Information for this article was provided by Mearis Martin, Jr., powerhouse superintendent for Norfolk Southern Corp., Roanoke, VA; and Randall Hayes, PE, energy consultant, Fisher-Rosemount Systems, Inc., Austin, TX. Fisher-Rosemount Systems, Inc., may be reached at 8301 Cameron Rd., Austin, TX 78754-3895; 512-834-7154; or check out its web site at www.frco.com.