“Man-Years” of Work in ST2 Conversion Become Reality
When you talk about the amount of engineering and design work that went into the fuel conversion of Steam Unit 2 (ST2) from coal to pipeline natural gas, you can’t appreciate it if you talk about “man hours.”
“It was literally multiple man-years,” said Nathen Hatch, AzGT generation engineering manager, and turbine overhaul project manager. “This was an undertaking that probably had up to 20 engineers working on it, putting in thousands of man-hours each, for two years or more,” said Hatch.
Add the amount of work involved in equipment procurement, fabrication, testing and more, and only then can one begin to appreciate what a major undertaking this was.
“It actually takes years to do. For this outage we did a lot of work both on the turbine and on the boiler. On the boiler, a complete conversion, a fuel swap from coal to natural gas, and on the turbine, major work was involved because of issues we discovered after we got into it,” Hatch said.
Chuck Reece, AzGT principal engineer, pointed out that the engineering work that was needed to lay the foundation for the conversion began more than two years ago.
“We started on this project about two and a half years ago, writing the specifications to change the boiler, and we contracted with Riley Power to do the design and fabrication of the parts, so that part’s been ongoing for about a year and a half,” Reece said. “The lead time on the equipment was about a year. We got the large flue gas recirculation fan in over the summer and got it installed in time for startup, so we were ready there,” Reece said.
“The most difficult part of changing the boiler was making sure we could ft the new flue gas ductwork from the new recirculation fan into the boiler, but we got that done as well. I and some other people with Riley came out and we measured the equipment and the existing structures and the existing open spaces to make sure the new ductwork would fit in, so we didn’t have to demolish or take out anything other than the existing fan and existing ductwork,” Reece said. The other major aspect of the boiler conversion involved the new natural gas burner system that had to be designed and engineered – from scratch – then fabricated, and installed.
“We’ve completely redesigned the burner front and installed twice the number of gas guns and new gas shut-oﬀ valves. We installed a large-capacity flue-gas recirculation fan so we can decrease the oxygen at the burners. That lowers the flame temperature, which yields lower nitrous oxide emissions. We’ve installed a low-NOx system and permanently converted ST2 to natural gas; it’s going to result in lowering NOx emissions by two-thirds,” said Reece. Then there was the turbine and the engineering, design, and fabrication that had to be done to it – much of which wasn’t anticipated.
“The turbine is what drove the duration of the overhaul,what we call the ‘controlling job,’” said Hatch.
“We originally planned to have it all done by the end of October, with the turbine taking only six weeks, so it was going to done by mid-October, but when we exposed the turbine we found a lot of issues. Specifically the first and second stage buckets (blades) were deteriorated, they had a lot of foreign object damage. We had to remove the buckets onsite, and we found that the dovetail, the part of the turbine that holds the buckets on the shaft, had extensive cracks. We had to bring GE (General Electric) out here as the OEM (original equipment manufacturer) and we did extensive non-destructive examination of the cracks to determine the cause, and GE said we had to fix it or they would not guarantee it for safe operation,” said Hatch.
“We had to ship the rotor back to Virginia where they machined the dovetails oﬀ, welded the material back out, stress relieved it, and then cut a new dovetail back on to match the original,” Hatch said.
“We weren’t anticipating any of the damage we found. To fnd out what caused it, we tracked it back to the last time these buckets were installed in 2005 and the dovetail cracks were right at the edge of each individual bucket. There’s square geometry where they mate together and that’s where the cracks were, so it looks like they had been rotating and vibrating,” Hatch said.
Not only were there thousands of man hours involved, there were thousands of moving parts, and individual tasks, all of which had to be coordinated – and that’s a huge challenge as well.
“You’ve got dozens of project managers overseeing diﬀerent aspects, and you have to get everyone together periodically and go over the diﬀerent tasks that everyone is assigned to. It’s a huge undertaking – the biggest challenge is getting everyone on the same page, because even though you’re looking at the same drawing, not everyone is visualizing it the same way. There may have been a change in the field over 30 years of operation that is not reflected in the drawings, so the basis for what you’re working with is wrong to start with,” Hatch said.
“It has to be precise,” Hatch said.
Finally, the week of January 8, it all came together, and the moment approached when the boiler would be lit and the turbine would start spinning. Hatch and Reece said they slept well the night before startup.
“I’m pretty confident it’s all going to come together,” Hatch said.
“I’m confident,” said Reece.
“Because of the diligence we spent during the design, engineering, fabrication, and the checking of the engineering drawings from the vendors and manufacturers; how we oversaw the construction – we worked hard to make sure everything went together properly,” Reece said.
The conversion work began in with a major overhaul in fall of 2017.
Scores of AzGT plant employees and contract workers were swarming over the Apache Generating Station and Steam Unit 2 (ST2) as the eight-week major overhaul that includes work to convert the unit to natural gas was underway.
“We’ve started to disassemble the steam turbine and the generator from top to bottom, the burner fronts and burner deck area, all the necessary repairs and refurbishing done,” Mike Nelson, executive director of power production said.
Safety is the first priority, Nelson said.
“It’s safety above all, so we’ve got four safety officers on site making sure we do all of this with the safety of our workers in mind,” Nelson said.
Contractors are always involved in a major overhaul, but because the conversion involves work that is new to this site, contractors were being hired to work with AzGT plant workers to keep the project on schedule.
“A major general contractor is doing the work on the back side of our scrubbers and precipitators, we’re using a new boiler contractor, TEIC, a subsidiary of Riley Power, that’s doing the boiler retrofit, and we’ve got two new cleaning contractors and a new turbine contractor, and two different scaffolding contractors, so we have a lot of individuals here who are new to our site,” Nelson said.
“It’s a challenge to make sure everything is coordinated,” Nelson said.
The conversion also means the machinery that has been used to process coal to the boilers will no longer be needed.
“All of the coal-handling and processing machinery, bunkers, feeders, classifiers, is being cleaned and mothballed, never to be used again. There will be no fly ash residue, all the ductwork is being vacuum cleaned, the scrubber modules, all of it has to be exceptionally cleaned, and this is a lot of additional work that would not normally be associated with a major overhaul,” Nelson said. However, should the coal-related machinery be needed in the future, it will be on hand.
Conversion of ST2 to burn pipeline natural gas instead of coal as its primary fuel is part of the EPA’s Regional Haze final rule as it applies to operations at the Apache Generating Station. The agency’s first rule, called the proposed rule, would have required the use of untested technologies and major emissions control upgrades that would have cost more than $200 million. Arizona G&T Cooperatives was able to show that overall emissions would be lower than what the EPA was seeking by upgrading emissions controls on Steam Unit 3 while still allowing it to burn coal, and converting ST2 to natural gas. This alternative solution is being accomplished at a cost of approximately $32 million, saving the cooperative almost $170 million.
More images from the first week of the ST2 overhaul, September 4 – 8, photos courtesy of Chad Kew, instrument technician:
Images from the second week of the ST2 overhaul, September 11-14, photos courtesy of Chad Kew, instrument technician:
Images from the third week of the ST2 overhaul, September 18-21, photos courtesy of Chad Kew, instrument technician:
Images from the fourth week of the ST2 overhaul, September 25-28, photos courtesy of Chad Kew, instrument technician:
Images from the fifth week of ST2 overhaul, October 2-6, photos courtesy of Chad Kew, instrument technician:
Images from the seventh week of the ST2 overhaul, October 19th, photos courtesy of Chad Kew, instrument technician::
Images from twelfth week of the ST2 overhaul, November 20 – 26, photos courtesy of Chad Kew, instrument technician: