CONTROL SYSTEM OF USX FARIFIELD'S
UTILITY BOILERS & GENERATORS
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:
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.
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:
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.
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.
PLANT CONTROL 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.
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:
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:
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.
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.
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:
MISCELLANEOUS BOILER CONTROLS
The DCS was also responsible for the control of the following loops:
MISCELLANEOUS COMMON CONTROLS
The DCS was also responsible for the control of the following common loops:
In the proposal phases USX estimated their Return On Investment (ROI) to be as follows:
Total $ 1,500.000
These figures are based on a study that USX did prior to implementing the system.