1 Accident process
6 years ago, author was working on a combined cycle power plants which located at Ontario, Canada, the plant Fueled by natural gas, the power generating block consists of three Siemens Westinghouse 501 F combustion gas turbine generators, three Deltak 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.
Plant installs 5 start up silencer, STG install two silencers one for the start up and another for the cold reheating ventilation (Figure 1) , each HRSG install one silencer, the purpose of start up silencer is to decrease the noise level during unit start up, when the unit start up, the HRSG and STG drain valve need to keep open to drain the all water in the system to prevent the water hammer to damage the pipe system and steam turbine. The city by law requests the plant noise level must control to 40 dB at local residuals, which means the noise level must control to 93dB which at outlet of the silencer,
Figure 1 STG start up silencer (right) and STG cold reheater sky vent silencer
After the plant commissioning and operation 4 years, the STG silencer experienced seriously damage and the entire plant was shutdown, lost million dollars, this paper will describe the root cause and how to fix it.
At one days in summer time, the floor operator found the STG blowdown tank has large amount of the water come out to the flow, the amount of water very fast to occupy the entire STG floor and go to condensate pump pit and in the short time the condensate pump motor was be submerged to the water, the STG trip due to condensate pump trip and plant immediately shutdown gas turbine operation, which plant operated at based load full ducts over 1000MW. Also Stop the both circulation water pump then the leakage stopped.
After the plant circulation water pump stopped, it is clearly found the water from circulation water pump outlet to the STG blowdown quench water pipe under ground line is broken. The HPDE under ground pipe connector is broken due to over heating. Figure 2
Figure 2 the broken HPDE pipe connector
Figure 3 STG start up silencer damaged
Figure 4 contractor disconnect the connector
Figure 5 blowdown tank inside
2 Root cause analysis
Inspection the silencer and found the start up silencer acoustic fills protection line is corrupted and the 2 third of the acoustic fill is left, and also inspected the silencer inlet diffusion pipe and found the diffusion holes was plugged by calcium carbonate scale, which cause the blowdown tank over pressure and push the hot water and steam back to inlet ground line which use the HDPE pipe, the blowdown tank only have a local pressure gauge. the highest temperature of HDPE is only can hold maximum 130 degree C, when the steam and with hot water push back to touch with the HDPE pipe it will overheat to broke up immediately. the blowdown tank uses the cooling tower water as quench water which includes high level calcium hardness, the calcium was evaporated with steam and react with CO2 bring it to form calcium carbonate and attached to the silencer inlet diffusion pipe which was equipped 0.2 mm small holes to evenly distribution the flow to the silencer , the calcium carbonate scale attached to the small hoes gradually plugged off the diffusion pipe. Plant was design for two sift operation due to the nuclear capacity at Ontario at night time, so every day need to be start up and shutdown, over 300 starts per year, the plant steam turbine blowdown tank used every day, after three years operation the diffusion pipe completed plug to damage the underground pipe cause entire plant shutdown.
The calcium carbonate Reaction process:
CaO + H2O → Ca(OH)2
Ca(OH)2 + CO2 → CaCO3↓ + H2O
3 the measure anti future
After three days plant outage, Plant to cut the plugged diffusion pipe header installed a new diffuser to release pressure and install a remote pressure sensor connect to control room to monitory the pressure build up on the blowdown tank. the online and local pressure gauge was showing the pressure is build is fast almost every three month the tank pressure will increase to damage level request to stop to cleaning the diffuser.
In the meaning time, plant discuss with the OEM (Fluid Kinetics ) to design a new silencer which use stainless steel material instead of carbon steel,
After discuss with the manufacture new silencer will use stainless material to instead of the carbon steel to prevent the acoustic protection layer corrosion , in the meaning time, due to the diffuser is installed at inlet of the silencer which is not reachable for the normal design, so plant request to install a manway at silencer wall which just beside of the silencer and it can use to access diffuser for the cleaning and also design flange cover ( figure 7) at top of the inlet pipe diffuser for access the inside of the diffuser to use mechanical and chemical way to cleaning plugged hole and scale on the inner surface , plant requested to increase the hole size form 0.2mm to 0.5 mm (figure 6) which can extend the time to cleaning and due to reinforced acoustic fill thickness so the silence outlet noise level not increase.
Figure 6 calcium carbonate build on the new silencer diffuser
Figure 7 new silencer installed a end cap to support cleaning
4 HRSG silencer operation situation:
Plant all inspected the HRSG silencer, compare with STG start up silencer which collect the HP, LP, HRH and CRH 3×1 long distance pipe drain water and also includes STG cylinder and stop valve drain, the amount of the drain water is smaller and also the period of each HRSG drain line open is shorter, HRSG start up silencer has good shape, the acoustic fill protection layer was not damage but the crack of silencer welds can be found anywhere, so plant decided to change of them to stainless silencer .
Figure 8 HSRG silencer cracked
