- Plant situation
The plant Fueled by natural gas, the power generating block consists of three 501 F combustion gas turbine generators, three heat recovery steam generators, a single 517mw Toshiba steam turbine generator, and a wet mechanical draft cooling tower. The facility, with a nameplate capacity of 1,005 MW, which includes Selective Catalytic Reduction (SCR) systems for controlling nitrogen oxide exhaust emissions
Plant is on two shift operation, plant will start in the morning time 7:00 am and shutdown on at 11:00 pm , the three Deltak heat recovery steam generators includes HP, IP, LP three pressure steam loop, the HP boiler maximum allow working pressure 2700 psig , boiler heating surface 312038 square feet, steam capacity is 847000lbs/hr. three siemens 501F air cooling gas turbine and air cooling gas turbine provide the fast start up and stop function to support plant shift operation.
2 gas turbine cooling
Gas turbine gas path has very temperature which requests cooling for the rotation parts, the combustor outlet temperature at F class machine reach 1400 degree C, for the G machine at outlet of the combustor can reach 1500 degree C, the highest combustor outlet temperature can reach 1700 degree C, the higher temperature the thermal cycle can get higher efficiency, use steam cooling gas turbine can get higher efficiency than air cooling method which is due to the cooling air will exit to the hot gas after cooling then it will decrease the hot gas temperature hence to decrease the efficiency. Steam cooling is separated with gas path which not impact with hot gas temperature. But air-cooling gas turbine is more flexibility than steam cooling, it is also not requested complicated cooling steam system and cooling steam warm sequence, two shift operation gas turbine want fast start then most then request using air cooling gas turbine.
Cooling technology (Figure 1) , as applied to gas turbine components is composed of five main elements, (1) internal convective cooling, (2) external surface film cooling, (3) materials selection, (4) thermal-mechanical design, and (5) selection and/or conditioning of the coolant fluid. Cooled air will return back to hot gas path, figure 2 is showing Schematic of a blade cooling circuit. Pursuit high efficiency way to cooling the gas path components is the goal of the all the manufacture and if there is mistake, then the catastrophic damage will happen immediately
Figure 2 : Schematic of a blade cooling circuit.
Due to the hot gas path entire parts operated in the hot temperature environmental, manufacture need to carefully design the hot gas path cooling to prevent any of the parts due to overheat to damaged the hot gas path
3 hot gas path damage process
At 24/02/2011, at around 5:35 am, the CT1 operation at based load 182MW, CTG#1 operation on based load 182MW, turbine exhaust vibration x and Y direction step up from 47 μ m, 56 μ m to 68 μ m, 88.18 μ m, the operator experiences the CT1 suddenly output decrease from 182MW to 151MW, after the damage, it also can see rotor motor position move to exhaust side 6 μ m based on the extra force push the rotor. blade path temperature #15 decrease 9 degree C. compressor shell pressure is stable, IGV cut back a little, and gas fuel flow is no any change, and in the meaning time, the gas turbine high vibration alarm comes, around 1 minutes later, gas turbine tripped due to high vibration.
After the CT1 stopped, Plant did borescope of the CT1 gas turbine hot gas path and found gas turbine was damaged seriously from first stage to end of third stage, plant apply the forced outage and open the gas turbine cylinder to do inspection.
4 Gas turbine hot path damage situation
The hot gas path was found completed damaged due to rubbing with lost parts and nozzle and blade also damaged at stage 1, 2,3, the rotor desk also found rubbing the entire rotor which can’t be used. Below are more detail damage pictures :
PowerPoint-Präsentation
- Found impact damage at the Row 1 turbine blades and vanes with reducing impact severity in subsequent rows.
- Transition exit mouth seal pins found sheared in 5 locations -seals shifted circumferentially. No indications of other upstream distress
- TE of Row 1 vanes had indications of impact/metal splatter. No LE vane impact distress observed
- Disassembly of the Row 1 blade ring found 4 of the upper half row 1 static seal segments liberated (positions 12-15)
- Varying amounts of impact was observed on every Row 1 turbine disc fir tree
- One small piece of debris was found in the exhaust that is suspected to be a piece of a liberated static seal segment assembly.
- First stage static nozzle damaged: First stage nozzle damaged, the protection coating was overheated and some nozzle had damaged like small hole. The inlet was in good shape of the nozzle but the outside was damaged
Figure 3 first stage static nozzle inlet side
Figure 4 First stage nozzle damaged, exit side was showing damaged by loss bolt
- First stage blade damaged
First stage all blades was seriously damage, due to the lost bolts to cut the corner of the blades and also cause the overheat the blades to cause the protection coats on the surface of the blade was damaged
Figure 5 first stage blade damage, coating lost, blade tips broken
Figure 6 First stage blade was rubbing with damaged material and broken tips
Figure 7 first stage blade was rubbing with damaged material and broken tips
Figure 8 first stage blade was rubbing with damaged material and broken tips
Figure 9 First stage blade was rubbing with damaged material and broken tips
Figure 10 outlet side of the second stage blade
Figure 11 second stage nozzle damaged
Figure 12 Second stage blade was rubbing with damaged material and broken tips
- Third stage blade inlet side also damaged by the debris of the upstream of the hot gas path
Figure 13 third stage blade was rubbing with damaged material and broken tips
Figure 14 Rotor desk rubbing with the damaged pieces
4 Root cause
After the incidents the OEM did root cause and analysis and found the static seal ring upper half 4 segments missing due to lack of the cooling air, and overheating , the tight bolts lost, and seal ring was found crack 2 years before, was short of the cooling air and the blots was over heated and expanded and lost the tightness and finally the bolts was lost and to damage the hot gas path, after this incidents and OEM redesigned the cooling path and provide more air to cooling first stage nozzle retaining bolts and to prevent the overheating
Figure 15 B root cause analysis
5 Repair
Due to seriously damage, and the hot gas path all the components couldn’t be reused and must removed, the rotor desk damaged due to seriously rubbing and couldn’t be recover on the site, OEM provide a completed new gas turbine rotor and the damaged one was shipped to OEM for repair.
Figure 16, all the hot gas patch component was removed then ship to OEM.
6 Conclusion
Hot gas patch damaged due to shortage of the cooling is nightmare for the manufacture and also the power plant, the hot gas path cooling problem cause a gas turbine hot path completed destroyed with very short time and it must to replace all to new components, for this incidents plant lost three-month generation and over ten million dollars lost for make a new rotor.