BY
P. Gergerich - Director of Operations, A.C.S.

APPLIED CONTROL SYSTEMS

426 South Main Street

Pittsburgh, Pennsylvania 15220

This paper covers all aspects of the Distributed Control System (DCS) at USX Fairfield Boiler House, including the control philosophy developed and implemented for this application. The DCS consists of distributed control processing units, fiber optic data highway, workstations and Modbus Communications to the Blast Furnace Turbo Blower. The DCS controls two 500 KPPH boilers firing multiple fuels of Natural Gas (NG) and Blast Furnace Gas that includes burner flame safety and combustions control. The control philosophy is based on burning all the available BFG first only supplementing with NG when necessary. The DCS also controls multiple steam header pressures of 900 PSIG and 325 PSIG. The 900 PSIG steam header is controlled through pressure reducing stations or by one turbine / generator set. The 325 PSIG steam header is controlled also though pressure reducing stations or three turbines / generator sets. This paper covers the cost savings recognized by USX since the new control system and philosophy have been installed.

The USX Utility Boilers and Generators for USX Powerhouse in Fairfiled Alabama, consist of the following major components:

1. Two, multiple fuel fired 900# - 500 KPPH boilers manufactured by                  Combustion Engineering.
2. Three, 20 MW, 325 # condensing steam turbo-generators manufactured  by General Electric
3. One, 21 MW, 900 # non-condesning steam turbo-generator manufactured by Dresser-Rand Company.
4. Three, 325 # - 300 KPPH boilers maufactured by Babcox & Wilcox
5. One distributed Control System (DCS) manufactured by Max Control Systems, Inc. 

This paper covers all the basic aspects of the Distributed Control System at the Powerhouse, including the control philosophy developed and implemented for the USX application. Also, a brief description of the DCS architecture is presented.

The Power House Distributed Control System (DCS) is based on a commercially available Distributed Control System manufactured by Max Control Systems, Inc.

DCS ARCHITECTURE

The control system concept for the Powerhouse is as shown in Figure 1. The figure illustrates the interface between the plants' components and the control system. In order to realize this concept, the DCS at the Powerhouse consists of three basic components:

1. Fiber Optics Data Highway
2. Distributed Processing Units
3. Workstations

Fiber Optics Data Highway

The data highway consits of a token passing bus in a physical loop configuration. This bus is made of a fully redundant pair of 200 micron fiber optic cables that make communication possible between workstations at the control room and the Distributed Processing Units at the DCS cabinets.

Distributed Processing Units

The DPU ia a data highway resident, self-contained control unit that plugs into an input/output rack. The DPU has an integral high speed input/output processor and a dedicated data highway processor. The DPU scans and process's information for use in reports, logs, calculation and graphics by other DCS devices. Each DPU can support one to one backup to provide maximum reliablility.

Workstations

The workstation is the operations human-system interface. Each workstation consists of a variety of PC processors (Graphic, Application, and Real Time Processors), each processor having its own duty, but working together to provide the operator/engineer with valuable information. These processors are connected together on each workstation via an industrial standard SCSI interface.

          Graphic Processor

This is the operator's interface with the plant process. This processor runs with Microsoft Windows Operating System and a graphic screen builder. The Graphic Processor's functions include:

• Monitor and display process information from the DPU's.
• Manipulation of process.
• Display trending that has been collected.
• Run pre-defined reports.

Applications Processor

This processor runs the UNIX Multi-user operating sytem and a commercial relational database manager. Its functions are:

• Build control blocks, data blocks and EXCEL (Extended Control Engineering Language) code for customized control.
• Install control configuration into RTP's and DPU's.
• Access and query all system data.
• Generate and print operational reports on demand or automatically.
• Implement historical database configurations.
• Access to and from Write Once Read Many (WORM) 800 MB optical disk.

Real Time Processor

This is a highway resident processor which represents the interface between the DPU's and the owrkstations. It collects and stores current analog or discrete trend points of informations fro mvarious DPU's via the data highway and buffers the information on a hard disk for access by the Application and Graphics Processors.

PLANT CONTROL OBJECTIVES

OBJECTIVES

The control system has the following objectives:

The primary objective is to operate the plant within safe operating limits and applicable regulations.

The second objective is to control the boilers to supply steam reliably and in sufficient quantity to the 900 # steam header and the 325 # steam header.

The third objective is to burn the maximum amount of blast furnace gas. A purchased fuel, natural gas or oil, is used to supplement the steam generation requirement and to stabilize the blast furnace gas flame.

The fourth objective, should the availability of blast furnace gas exceed the required steam demand and maximum generation, the boiler will automatically back down the fire rate demand and exhaust the excess blast furnace gas through the plant flare stack.

PLANT ARRANGEMENT

Figure 2 shows a single line plant arrangement diagram of the facility.

The steam demands on the 900 # steam header , 325 # steam header and the availablility of blast furnace gas and BTU value will vary widely and rapidly. The control system will regulate the boilers and generators to balance the various conditions.

Boilers "9" and "10" will discharge into the 900# steam plant header. The boilers can be independently fired with any combination of blast furnace gas, natural gas or oil. Blast furnace gas is the preferred fuel and will be fired to its capacity. Natural gas or oil will be fired, as necessary, to supplement blast furnace gas. For simplicity, the following descriptions will discuss the firing of blast furnace gas and natural gas only. It is understood that oil can be substituted for natural gas.

 The 900# steam header supplies steam, as required, to the "F" turbo-blower, #4 topping turbine generator, #1 & #2 boiler feed-water turbine pumps, #1 & #2 cooling water turbine pumps, air compressor turbine, 600kw critical load turbine-generator. these devices, except for the "F" blower are non-condensing turbines that exhaust steam into the 325 # steam header.

The 325 # steam header, in turn, supplies steam to other users within the steel mill,  #1, #2 and #s steam condensing turbine generators, standby "E" turbo-blower, boiler #5, #7, and #8 F.D. fan turbines, boiler #5, #7, and #8 I.D. fan turbines, low pressure feed-water pump, blower "E" circulating water turbine pump, auxiliary and standby turbines, air ejectors, boiler #9 and #10 F.D. fan turbines, boiler #9 and #10 I.D. fan turbines, treated water turbine pumps, #3 boiler feed-water turbine pump, booster turbine pump, feed-water heater and finally "F" blower condensate turbine pump. These devices are non-condensing turbines that exhaust steam into the #5 or #10 steam headers.

PLANT CONTROL STRATEGY

The control objectives are met when the following controlled vairable are satisfied:

1. Blast furnace gas header perssure is maintained at a minimum pressure that assures maximum utilization consistent with stable firing.
2. The 900# and 325# steam header pressures are stable, indicating that the steam demand is satisfied.

PLANT OPERATING MODES

Depending upon the availability of blast furnace gas and the steam demand requirements, the plant will operate in one of the following three modes:

1. MODE 1 - BLAST FURNACE GAS SUPPLY ADEQUATE

   The supply of blast furnace gas is adequate to meet the steel plant steam demand and minimum generation. There is surplus blast furnace gas which can be fired to produce additional steam if required. Mode 1 of operation will maintian the 900# steam header pressure by increasing or decreasing the firing rate demand on the boilers. The minimum requirement steam generation in this mode is the steam demand of the 900# steam header pressure and the minimum generation requirement of the "F" turbo-blower and #4 topping turbine. Any steam generated in excess of this minimum requirement is exhausted or dumped to the 325# steam header which is then converted to electricity by #1, #2, and #3 steam turbine-generators. 

    

2. MODE 2 - BLAST FURNACE GAS EXCEEDS REQUIREMENTS

   The supply of blast furnace gas is greater than that required to meet the steel plant steam demand and the spinning capacity of the generators. Under this condition, the excess blast furnace gas must be flared-off. Mode 2 of operation will control the following should the blast furnace gas exceed the combined steam and generation capability. The on-line generators #1, #2 and #3 will reach their maximum generation capabilities and #9 and #10 boilers will regulate the blast furnace gas firing rate in a conventional boiler follow combustion control system. Any surplus blast furnace gas mus be vented.

    

3. MODE 3 - BLAST FURNACE GAS LESS THAN REQUIREMENTS

   The supply of blast furnace gas cannot meet the steel plant steam demand and minimum generation, it will be necessary to fire natural gas. Mode 3 of operation will control the following should the blast furnace gas supply be inadequate to meet the combined requirements of steam demand and minimum generation. The on-line generators #1, #2, #3 and #4 will reach their minimum generation set-points. #9 and #10 boilers can only then maintian the required steam demand  by supplementing natural gas firing through the conventional boiler follow combustion control system.

BOILER CONTROL STRATEGY

The boiler control strategy determines the total plant steam requirements, allocates that requirement between both boilers and precisely delivers the correct amount of energy to each boiler to meet the required steam demand. The plant steam requirement is based upon the total steam flow, corrected by the ratio of the boilers energy demand set-point versus the actual energy delivered into the boilers.

BOILER ALLOCATION

The total energy requirement is then allocated between the boilers by a unique participation function block. Participation includes the following characteristics, all of which contribute to improved plant performace:

1. Manual bias provides a convenient means of changing the proportional loading between the boilers. The bias also provides a smooth and stable means of bringing a boiler online. The bias works on all boiler demands, thus reducing disturbances.
   
2. Any boiler may be base loaded should the operating conditions require. The participation function will recognize the contribution made by the base loaded boiler or boilers and directs the remaining plant requirement to the other boiler.
   
3. Should one of the boilers become restricted (e.g. from firing or equipment problems) the participation function will automatically determine the magnitude of the restriction and automaticalyy re-allocate the steam requirement, without the need for operator intervention.

FUEL CONTROL

The boiler allocation assures that each boiler is loaded in accordance to a desired schedule, subject to the plant availability. The established boiler demands for each boiler are matched to their respective firing rates.

Its is recognized that the heat value of the fuels (particularly blast furnace gas) is not constant and that the fuels can be burned in any combination. The control system utilizes a heat release calculation to determine the actual energy delivered by the fuels. The heat release calculation is independent of the fuel delivery system and any undedected and uncontrolled variations in those systems.

The unique combination of the calculated boiler demand, matched to the boiler heat release, assures excellent steam generation response to a wide range of plant requirements and operating conditions.

BMS CONTROL 

The Burner Management System (BMS) was implemented into the DCS. The BMS was implemented to the current NFPA standards for this type of application. The DCS monitored and controlled all of the points that were associated with the 4 corner burner system. The DCS was responsible for the shutdown and start-up of each boilers fuel introduction including but not limited to the following functions:

1. Boiler Purging
2. Igniter Start / Stop
3. Booster Start / Stop
4. Automatic / Manual Natural Gas Nozzle Firing
5. BFG Nozzle Firing
6. Master Fuel Trip
7. Process Fuel Trips

MISCELLANEOUS BOILER CONTROLS

The DCS was also responsible for the control of the following loops:

1. 900 # steam header temperature control at 900 F.
2. 325 # steam header temperature control at 600 F.
3. Boiler draft pressure control
4. Boiler forced draft air flow control
5. Single / three element drum level control
6. Boiler blow-down control
7. Boiler oxygen control 

MISCELLANEOUS COMMON CONTROLS

The DCS was also responsible for the control of the following common loops:

1. Boiler feed-water pressure control.
2. DA level control
3. 900 # steam pressure header control through Pressure Reducing Stations (PRV) and #4 topping turbine generator.
4. 325 # steam pressure header control through Pressure Reducing Stations (PRV) and #1, #2 and #3 turbien generators.
5. Blast furnace gas pressure control
6. Minimum MW generation.

FINANCIAL ESTIMATES

In the proposal phases USX estimated their Return On Investment (ROI) to be as follows:

Amount:        Capital                  $                 1,200.000
                     Cost                     $                   300.000

These figures are based on a study that USX did prior to implementing the system.