1 SGEC plant operation
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. the three by one confirguration requred long common steam pipe system to connect each HRSG to single steam turbine.
Figure 1, 3×1 combine cycle power plant reques longer steam system
2, Accident happen
On the middle of December, the operator start up the CT1 as first start up gas turbine, the plant was stopped at last nights 11 pm, this start is hot start, like usual after 30 minutes, CTG1 synchrized to the power grid, the CT1 ramp up to increase the HP steam temperater, when HP steam temperature reach certain degree the HP to auxiliary steam valve open to laydown steam to auxiliary steam system to provide STG gland steam to prepare STG build the gland steam pressure for pulling vacum at condenser, after vacuum build up, the STG will start up as hot start to reach full spreed no load then completed synchrized to the power grid. Then start up the CTG2, CTG3 at same time, when the both HRSG$2,3 HP , IP and LP reach the requirement then blending steam with HRSG#1, approxiliaterly 2 hours, all three HRSG will online, then start up process completed. During CT2 and CT3 start time, floor operator heard the bumping noise from hot reheating pipe system and also found the pipe is moveing back forth, the pipe system hanger all moving with the pipe moving, after synchrized, plant engineer inspected the hot reheating pipe system all hanger was defromed and system experience the steam hammer damage
Figure 2 HRH pipe system hanger deformed
Figure 3 HRH pipe system hanger deformed
Figure 4 HRH pipe system hanger deformed
Figure 5 HRH pipe system hanger deformed
Figure 6 HRH pipe system hanger deformed
Figure 7 HRH pipe system hanger deformed
4 root cause
4.1 steam hammer
This is happed at HRH steam systems known as steam hammer. In a steam system, a water hammer most often occurs when some of the steam condenses into water in a horizontal section of the piping. The rest of the steam picks up the water, forming a “slug“, and throws down this at high velocity into a pipe fitting, pipe hanger and support system, creating a loud hammering noise and greatly stressing the pipe. This condition is usually caused by a poor condensate drainage strategy: having more condensate in the pipe makes the slug easier to form. Vacuum caused by condensation from thermal shock can also cause a steam hammer.
Steam hammer can be avoided by using sloped pipes and installing steam traps. Where air-filled traps are used, these eventually become depleted of their trapped air over a long period through absorption into the water. This can be cured by shutting off the supply, opening taps at the highest and lowest locations to drain the system (thereby restoring air to the traps), and then closing the taps and re-opening the supply.
4.2 Root cause
HRH hanger was damaged on the middle of the December, the root cause was found is during STG speed up to sync speed, if something happen the synch time postpone, the cold reheater ventilation valve will open to prvent STG exhaust hood overheated, which bring down the HRH line pressure, if this pressure below the standby HRSG HRH pressure, the balance valve will passing, but the drain valve HS1405,HS2405,HS3405 was not open to drain water which accumulated on the pipe due to logic set up, until the CTG start up, so drain water in the HRSG HRH header pass though balance valve to cause steam hammer to damage the HRH hanger.
Figure 8 drain valve not open accuse the water accumulated on HRSG#2, 3 HRH pipe cause lower temperature
Figure 9 drain valve not open accuse the water accumulated on HRSG#2, 3 HRH pipe cause lower temperature
5 Measure
To change HS1405,HS2405,HS3405 drain saturation tempearture <20 degree C AND gate with CTG start up to open to OR gate to open drain valve, previously logical want to keep the heat in the HRH system until the gas turbine start up, nornally the locial thinking operation unit would had higher pressure than standby unit to keep the balance valve in close condition. But when unit stop and specially in the winter time, HRH header temperature drop very fast then water accumulated in the drain port, the balance valve would open any time if the header pressure is lower than than standby unit, so the drain water will pass through then catch by steam to cause steam hammer
In the meaning time changed the HP superheater drain line logical to keep HP S.H drain valves in open condition till pressure reach 500 kpa or saturation temperaturemore than 50C. Also keep Reheater and IP supper heater drain valves in open condition till pressure over 250 kpa or steam staturation temperautre is over 50C.
6 hanger and support inspection
Inspect the steam hanger system is very important during outage time, as figure10 is showing some hander the travel pin was not remove, which should be removed when the hydrostatic test completed, the travel pin not remove cause the hanger still on the cold poisition when the plant operation, this would cause the extra stress for the pipe system to potential cause pipe steam damage
Figure 11 is showing HP steam to Auxiliary steam horizonal line hander damaged due to the start up time drain valve is not open ontime to drain the water cause the steam hammer
Figure 10 some hanger the travel pin not remove
Figure 11 HP pipe system hanger deformed
Inspect and log the hot position and cold position also important to demonstrate the pipe system moving condition to prevent the pipe stress build up. Also check the travel pin should be removed when hanger at normal operation, check the hanger root connection no crack indication. Finally keep hanger and support health is important for the high energy system