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Posted by Rick on December 03, 2003 at 18:45:33:
Last night we were base loading a 2x1 CC unit for the first time . We had 2 GT's at base load and the steam turbine at 180mw. We fired duct burners to 10 then 20% load . At the same time that we fired the ducts to 20% our HP steam temp from the #2 unit began nosedive from 1030* and HP drum pressure began to rapidly increase . Before we could get the steam turbine tripped unit 2's HP superheater outlet temp had fallen to 538* , our mainsteam temp at the steamer had fallen to 750* , eccentricity went to 23.5 , axial position went way negative and vibrations had spiked . When we E-stopped the STG one of the MSV's initially failed to close and as a result, since both MSV's were not closed , the breaker did not open and therefore the before and afterseat drains could not open. HP drum level on unit 2 fell below setpoint of 0" to -4" during the episode , but the decrease in drum level seems to correspond with the increase in HP drum pressure. HP feedwater flow to the unit increased from 350K to nearly 900K , and drum level came back to setpoint and stabilized. There was no attemporation from the HP desuperheater as indicated by the position of the block and control valve , no spray waterflow and no delta T across the desuperheater , yet the HP drum inlet temp , and the hp pimary SH outlet tempsand the HP sec. SH outlet temps were all very close to the same. The MSV that did not initially close did after a few minutes
go closed and the STG breaker was manually tripped from the aux sub though the generator was motored for approx. 3 min . Fortunantly when the sun came up here this morning we still had a steam tturbine sitting on the turbine deck .
What appears to have happened to me is that for whatever reason , we obviously carried water, and a lot of it, over from the HP drum into the SH's .That is the only explanation I can find for the rapid decrease in temps and the increase in drum pressure due to the back pressure in the SH. The MSV that did not close initially was probably due to the stem being quenched and then as temps stabilized it had enough memory to straighten back out and go closed. We still have a turbine there because there was enough SH in the mainsteam header from unit 1 to keep the steam from being completly saturated . We didn't burn the generator up because there was still excitation and we are damm lucky . We are investigating for damage to the HP drum and SH's though it was still too hot today to get inside . What puzzels me and my question is how did the water get from the drum and into the SH's without the drum level transmitters ever seeing it. On a long shot I looked at the raw count form the LT's and they never spiked . Any ideas out there????