The stoker coal boilers on the powerhouse at FMC's soda ash mining plant in Granger, Wyoming, had become a serious problem. "The boilers were not running well, and when they did run, they would not run at full load," said Barbara Hamilton, senior industrial energy consultant, Emerson Process Management, in her presentation Wednesday morning at Emerson Global User Exchange 2014 in Orlando, Florida.
There were also safety concerns, Hamilton said. "They were running on manual, and whenever they opened the door to pull ash, they had positive pressure issues." FMC and Emerson brought in Jansen Combustion and Boiler Technologies, and they all went to work. "Jansen works on air flow, and they do a lot to improve combustion," Hamilton said. "Meanwhile, I went over every valve, pressure transmitter, flow transmitter and the controls."
The first problem was air flow measurement. "The Venturi design was excellent, and they had good instruments, but they had not configured them," said Hamilton. "They had no square root extraction, so automatic control didn't work. The operators were using differential pressure as flow—they had learned what to do by trying things and seeing what happened. So we got air flow measured and into the control system."
Then there was fuel. "They had a gate that opened and closed to let more or less coal onto the belt that threw it in," Hamilton said. The final elements had position feedback, so the original control engineer had put PID on them. "This just added dynamics and delays," she said.
Finally, the flue gas control was being done using the damper and fan speed, but the induction draft damper was set at 60% with no gap action. "Essentially, there was no turndown on this boiler; it was not responsive at all," Hamilton said. "The air was not right, the fuel was not right and the flue gas control was not right. Other than that, it ran great."
The safety systems had their own issues. "We did a complete NFPA 85 safety audit," said Andrew Verdouw, also a senior industrial energy consultant with Emerson. "A lot of the hard-wired sequences were not in play, so when they took a safety trip, it was a hard trip."
The boiler fires up on oil. "I was trying to find the serial numbers on the injectors, and I couldn't—they had made and installed the oil guns themselves," Verdouw said. The whole oil system was homemade. "It was a very challenging system. It was only used 12 hours a year, but it had to meet NFPA 85," he said.
"This was not a DeltaV site," said Hamilton. Emerson did a complete Scientific Apparatus Makers Association (SAMA) documentation, functional specification, design and management of change (MOC) review. "They had to learn how to read SAMAs; they had never seen one before," she said. Planned changes included new pre-trip and conditional alarming programmed in the DCS, as well as corrections to the fuel and air flow controls. The upgrades were made without shutting down, which got the plant's full attention. "We developed a very detailed cutover plan," she said. "We put it on a whiteboard with the existing control system in one color and the additions in another."
The migration was planned to take four days and was essentially completed in two, Hamilton said. "We left the boiler in manual overnight, and it was still there the next morning." Throughout the project, Emerson worked with Jessica Pauley, process supervisor, FMC, who was not available to co-present as she left FMC a week ago for a new job with SpaceX in California.
"Powerhouse availability rose from 94% to 99%, and productivity increased 10%," said Hamilton. "There's a multiplier effect because productivity suffers more from a trip as everything goes down and has to come back up."
FMC's powerhouse superintendent is very pleased. He reported, "We have eliminated a serious safety concern, and our boiler room is now an advantage, rather than a hindrance, to our production."