Upgrading on the fly
Producing more than 33% of an electric power system’s generating capacity can be a daunting task. Located on the Tombigbee River near Leroy, AL, the Charles R. Lowman Power Plant is Alabama Electric Cooperative’s (AEC) primary generating source. With its three coal-fired generating units, Plant Lowman’s generating capacity equals 556 MW of electric power.
Coal serves as the primary fuel, with No. 2 fuel oil used for startup and flame stabilization. Each year, Plant Lowman burns around 1.5 million tons of coal to produce the steam needed to make electricity. When coal is burned, most of the ash is carried out of the furnace in the flue gas. The plant’s three operating units are equipped with electrostatic precipitators, which prevent particulate matter from entering the atmosphere. The ash is collected in hoppers arranged beneath the precipitators.
Plant Lowman’s Unit 1 (84 MWs) was built between 1965 and 1969, and commercial operation started in 1969. Commercial operation of Units 2 and 3 (236 MW each) began in 1979 and 1980, respectively. Units 2 and 3 were equipped with a wet/dry flyash removal system as part of the original equipment. The original manufacturer of this equipment went out business shortly thereafter, leaving plant personnel to service this equipment themselves.
Over the years, the plant initiated upgrades to the ash system valves, removed the ash cooler and replaced the pneumatic controls with programmable logic controllers (PLCs). In 2004, the plant decided the dry ash system performance and capacity had deteriorated to the point where the system needed to be replaced.
The Babcock and Wilcox Co. (B&W) headed a project team to execute on this turnkey project. B&W’s Allen-Sherman-Hoff (ASH) subsidiary supplied the ash system; Industrial Services of Mobile (ISM) was used for erection services; and Fossil Power Systems (FPS) modified the ash system controls. B&W provided overall project management services.
The project consisted of two replacement ash systems, both capable of 100% plant capacity. The existing Rockwell Automation Allen-Bradley (A-B) PLC-5 ash control systems were modified, and A-B PanelViews were installed to serve as the operator interface. Since the existing ash system had wet/dry capabilities, it was decided that the new collectors could be installed and commissioned with the boilers on line, utilizing wet operation.
The design selected included ASH’s vacuum/pressure transfer arrangement. Ash from the precipitator hoppers (Units 1, 2 and 3) is conveyed under vacuum to either of two collectors (Units 2 and 3) by means of vacuum-conveying air supplied by a vacuum pump. Each collector functions as a temporary storage bin to filter and store the ash collected by the vacuum portion of the system before it is conveyed through the pressure section of the same system.
A three-part equalizing valve functions to equalize the pressure between two unequal pressure zones. At the bottom of the collector, airlock valves cycle continuously on preset time intervals to transfer ash from one pressure zone to another without interrupting the conveying system operation. As the ash discharges from the airlock valves, a transport blower conveys the ash to either a market silo or waste silo. The stored ash is periodically unloaded to transport vehicles.
Dividing the work
ASH supplied duplicate vacuum/pressure transfer stations with top bag removal walk-in plenums, abrasion-resistant piping and fittings to adapt to the existing vacuum piping. It also used structural steel and foundation engineering, valves and instrumentation and system capacity engineering to maintain adequate conveying rates.
ISM performed demolition services on the existing collectors and erected the new vacuum/pressure transfer station in three weeks for each unit. They also were responsible for erecting instrument air lines, instrument mounting and stairwell modifications to access the new equipment from existing platforms.
The existing Allen-Bradley PLC-5 fly ash controls on Units 2 and 3 were reprogrammed to remove the obsolete functions of the existing collectors and insert the new vacuum/pressure transfer station logic and instrumentation. Programming was written to incorporate control and transfer of dry ash from Unit 1.
“The controls retrofit portion of the project was a huge undertaking,” says Andre Phillips, the B&W project manager. “FPS and AEC instrumentation and control personnel did an outstanding job of working closely to accomplish a successful interface.”
Replacing the controls
When the plant was upgraded from the ash system pneumatic mimic board control to PLC-based control, the mimic board was used as a marshalling cabinet between the field devices and the PLC. The original documentation for the mimic board wiring was scarce, so every cable and wire had to be verified and either reused or abandoned. Because the mimic board is located on the ground floor closer to the system equipment, the A-B PanelView operator interface for each unit was installed at this location for local control, alarms and indication.
AEC instrumentation and control personnel were responsible for all instrument and valve field terminations, PLC I/O module terminations, cable and conduit installation, junction box mounting and wiring, and electrical modifications. FPS provided logic diagrams, process graphics, PLC programming and hardware addition specifications. FPS also provided installation engineering in the form of cable block diagrams, wiring diagrams and a cable schedule.
The A-B PanelView process graphics included vacuum pump and pressure blower status; ash hopper level and discharge valve status for Units 1, 2 and for intermittent or continuous filter collection operation; wet/dry transfer capabilities; and waste or market silo selection. Instrument indications, alarm history and vacuum-side performance trending also were included.
In addition to providing the mechanical equipment, ASH also provided the controls narrative and piping and instrumentation drawings (P&IDs). During commissioning, some of the field instrumentation installed on the collectors was at or near its temperature limits. The plant verified this with thermal imaging mapping. The instrumentation identified was upgraded to either a heavy-duty or high-temperature model.
“Although the mechanical portion of the project was challenging with demolition and erection taking place while the units were in operation, this was confined to the dry system only,” Phillips says. “With the controls, we had to be conscientious and maintain the integrity of the wet system throughout the project.”
Realizing the benefits
The upgraded ash system has afforded the plant tremendous flexibility. At any time, one collector can be taken out of service for a boiler outage or preventive maintenance without affecting system capacity. Ash from Unit 1 can be initiated and controlled from either Unit 2 or Unit 3. Ash collection sequencing is fully automated with hopper levels, valve status, alarms status and alarm history readily displayed.
“Before we installed the replacement ash system, the existing system did not have the capacity for full load operation ash production on each unit,” says Bryan Pansing, AEC plant superintendent. “Since the new vacuum/pressure transfer collectors have been in operation, one system can maintain full load ash production capacity on not just one unit, but all three units.”
More project details
The mechanical scope of supply consists of two new vacuum/pressure transfer stations to replace those installed on Units 2 and 3. The existing twin collector arrangement on Unit 2 and Unit 3 was replaced with a single collector per unit. The single collectors are sized for 100% plant capacity, allowing ash to be conveyed from any unit or all three units to either of the vacuum/pressure transfer stations. The system was designed based on ash production rates of 11 tons per hour (TPH) each from Units 2 and 3 and 3 TPH from Unit 1. The average handling rate of the vacuum/pressure ash system is 25 TPH. The foundations upon which the existing twin collectors were sitting was reused.
Approximately 563 ft of 7 in. and 8 in. pipe, which ran from the Unit 1 precipitator to the Unit 2 vacuum/pressure transfer station, was installed for the Unit 1 tie-in. Crossover piping and isolation valves were installed between Unit 2 and Unit 3, allowing ash to be conveyed from Unit 1 to either Unit 2 or Unit 3. Replacement of 386 ft of carbon steel piping and isolation valves was required to accommodate the new ash flow rates to the market and waste ash silos.
The existing vacuum pump used to transfer ash from the precipitator hoppers and the pressure blower used to transfer ash to the market and waste silos were reused with only slight modifications. Isolation and control valves at the market and waste silos were replaced.
|Vance VanDoren is a consulting editor with Control Engineering. He can be reached at Vance@control.com .|