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Article Summary
  • NCEMC faced issues with back-flowing liquid fuel, causing coking and the inability to switch back to gas operation.
  • Liburdi Turbine Services assessed the failed fuel nozzle and determined thermal shock during fuel gas start-up as the root cause.
  • LTS developed a repair solution and collaborated with Liburdi Dimetrics to build a simulator for testing.
  • The repaired fuel nozzles were successfully tested and installed by NCEMC, and now operate normally, providing significant cost savings compared to replacing them.
The Challenge:
North Carolina Electric Member Corporation (NCEMC) Anson and Hamlet Generation Facilities have 24 P&W FT8-3 gas turbines. The sites operate primarily on natural gas fuel with distillate oil backup. Both distillate oil and gas operations use water injection for NOx control.
Liquid fuel was back-flowing into the gas supply system, causing coked fuel nozzles, coked gas supply hoses & headers (See Figure 1), and the inability to return to gas operation after liquid fuel operation.
Figure 1: Coking in Fuel Nozzle, Gas Supply Header & Hoses
Liburdi's Assessment:
Liburdi determined the root cause to be thermal shock experienced during the fuel gas start-up cycle when initiating water injection. A review of nozzle history demonstrates that nozzles tend to fail at roughly 1100 start-up cycles (See Figure 2). The internal stress cracking creates a cross-flow between the nozzle's liquid and gas sections.
Figure 2: Fuel Nozzle History Sample Data - Note Several Fuel Nozzles Scrapped with Approximately 1100 Cycles
Note that the dual-fuel nozzle design is identical for single and dual-fuel units. Water and fuel oil are premixed before the nozzle shares the same piping.
Issues discovered after liquid fuel operation:
  • Coked fuel nozzles (gas orifices & supply port), gas supply hoses, and gas supply manifold does not affect liquid fuel operation or supply piping
  • Unable to transfer back to gas fuel rendering forced outages
Liburdi's Solution
LTS developed and executed the repair of one fuel nozzle. LTS worked with their "sister" company Liburdi Dimetrics to ensure the repair's viability. Together, LTS and Liburdi Dimetrics designed, engineered and built a simulator replicating the start-up conditions of an operating fuel nozzle (see Figure 4). For the test, LTS utilized three fuel nozzles, one new, one service ran, and one that Liburdi repaired.
Although not experienced frequently at the NCEMC plants due to PEMS (predictive emissions monitoring based upon 5-year testing), Liburdi corrected the high CO2 issues measured during the scheduled testing by replacing the affected nozzles. It's, therefore, reasonable to assess that prematurely mixing water and gas within the nozzle before combustion could result in more frequent emissions issues when measured via CEMs (continuous emissions monitoring). Liburdi included this information in their complete evaluation and plan for their client to maintain optimally running turbines while saving their clients money in replacement parts and downtime.
Figure 3: Fuel Nozzle Cutaway View
Figure 4: Fuel Nozzle Test Cell
The Results:
Liburdi tested all three fuel nozzles simultaneously, replicating the start-up cycle by heating and thermally shocking with induced water injection for 2000 cycles. Liburdi examined each fuel nozzle after 250 processes using a pressure test and Fluorescent Penetrant Inspection (FPI).
Upon completion, NCEMC was confident in the integrity of the repair and installed LTS-repaired fuel nozzles in their units. The repaired fuel nozzles currently operate normally. The OEM would have scrapped these same fuel nozzles using current test criteria.
Thanks to Liburdi's advanced technologies and skilled technicians, FT8-3 Fuel Nozzles can now be repaired for a fraction of the cost of new parts, significantly reducing maintenance costs. To see how Liburdi can help you, contact Jeff Chapin at jchapin@liburditurbine.com.