Several years ago, author worked in a combined cycle power plant, which is 3×1 combined cycle power plant, due to design error, cause HRSG#2 HP drum evaporator overheating and seriously damage the evaporator tube cause over million-dollar repair and plant out of the operation over three months
1, the plant Situation
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.
2, the Accident happen
One days at around 8:00am, after CT1 start for STG synch then CT2 and CT3 start, , after CTG1 go to 50% load, operator experience the bigger than normal drum make up water, and in the meaning time to find the HSRG2 drum level could not keep to normal level either the feed water speed reach maximum , then call field operator to check found the sharp noise from the HP drum internal enclosures, there is a leaking at inside of HP boiler, CTG#2 stop and for inspection.
Open the top hatch of the HP evaporator find the there is water leakage from the HP drum evaporator panel as figure 1 ,
Figure 1 HP drum evaporator leakage
also found some tube was bowing, open the HSRG access door entire inside of the penal, and found the north panel most tube was over heating and bowing, figure 2, one tube was found broken as figure 3 .
Figure 2, tube bowing due to overheat
Figure 3 1 tube is broken due to overheating
3 Root cause
HP drum has two evaporator, which is north side and south side, each evaporator includes two 28 inch downcomer to connect to bottom of the evaporator header then absorb the gas turbine exhaust heating evaporating steam through cyclone steam water separator, steam will move to superheater.
After three days cooling down, inspector access the HP drum and found the northside of the panel of the evaporator downcomer (28inch) downcomer vortex breaker was sucked down figure 4, vortex barker covered the entire of the downcomer, then the north panel which includes 4 headers, all tubes overheated, each header has three rows, and each row has 23 tubes, so total the tube 3x23x4 (276) tubes was over heated
Figure 4 28-inch vortex breaker sucked down
As figure 4, the vortex breaker was tag welding inside of the three supports, due to the flow cause the vibration, the support tag welding is cracked and final suck it down and block the water way
Figure 5 each vortex breaker has one tag welding
Plant immoderately to stop another two units HP, IP and LP drum, and found HRSG#1, 3 HP drum have one of the vortex breaker welding has crack, one of the vortex brakers is half way to down to the HP down comer nozzle.
4 Repair
Due to the tubes was over heated, all tubes must to be changed out, figure 6 showing the entire damaged tube was cut out, the repaired job was huge,552 welds plus inspection and heat treatment, requested welder two shift working the total replacement take around three month, and due to the working place was so narrow, each time only have space for 4 welder inside, almost to mobilized the entire local area qualify welder, several good retired welder was recall to do the welding qualified test then to put in the shift to support this job, the welding job is in the summer time, so the working environmental is confined space and high temperature inside of the boiler, every 2 hours welding time then need come out for half hour rest.
Each weld must to do at least MT and RT test to make sure the welder provided the qualify welding
Figure 6 all damage tube cut out
Figure 7 Start welding top welds
Figure 8 welder working on lower header side
5 Reinforce the vortex breaker supports
Due to the manufacture design fault, plant install extra support bar at HP and IP LP downcomer vortex barkeeper support bar welding attached with the original bar but is also welding to top plate, which is not welding to the downcomer nozzle to avoid the crack and heating treatment of the nozzle, the measurement will prevent future inside tag welding crack and sucked down to seal the downcomer
6 Results
The repair is very successful, after three months, the tubes repaired and hydrostatic water test no leaking, CT2 successfully start up one time, no leaking, after 8 years operation, no any downcomer has experience crack and vortex breaker never crash down again , but the design defects cause over million dollar repair cost and three months downtime and no generation which is over ten million dollar revenue lost.
This accident was absorbed by the new downcomer design now for the manufacture, no top plate on the downcomer which is use the mesh screen insisted of the plate, then there is no chance to block the water pass way.
