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We have a not very usual CCPP with 6 CT, each one has an HRSG associated, and all the steam production is sent to a single steam header to feed only one Steam Turbine.
We had a problem in the FeedWater CV of one HRSG and its drum flooded. A lot of water flowed from the drum to the superheaters, then to the steam header where, mixed with the steam coming from the others HRSG, made the steam temperature suddenly decrease. This cold and wet steam fed the ST some minutes until it was manually tripped. We had to overhaul both, the ST and the HRSG to inspect. Fortunately, after a very costly inspection, some minor damage was found.
I’ve to say that we were lucky this time, but need to improve our DCS. We realized that a high water level in the drum of any of our HRSGs, will only produce an alarm signal to the Operator but, will not trip the HRSG neither the ST, neither the associated CT. The DCS will just advice the operator that there is high water level in a drum, and nothing else. No automatic protection actions.
Also, in the ST there is only an alarm signal if the steam temperature decreases 40°F from nominal temp. But if the steam temperature continues to decrease, or if it suddenly falls say 200°F, nothing else happens...no automatic protection actions.
We are trying to develop an automatic protection in our DCS for the HRSGs and the ST to prevent future similar faults. We believe that a high-high level in a drum should:
1st) trip the HRSG: close supplementary fire gas valve, close FW inlet valve and steam outlet valve. Not sure if it should also trip the CT.
2nd) trip the ST: if in addition to the high-high level in the drum, low steam temperature is detected upstream the main steam valves of the ST. Not sure which should be this low temperature trip value.
Any feedback is welcome. thank you for shearing your experience.
We had a problem in the FeedWater CV of one HRSG and its drum flooded. A lot of water flowed from the drum to the superheaters, then to the steam header where, mixed with the steam coming from the others HRSG, made the steam temperature suddenly decrease. This cold and wet steam fed the ST some minutes until it was manually tripped. We had to overhaul both, the ST and the HRSG to inspect. Fortunately, after a very costly inspection, some minor damage was found.
I’ve to say that we were lucky this time, but need to improve our DCS. We realized that a high water level in the drum of any of our HRSGs, will only produce an alarm signal to the Operator but, will not trip the HRSG neither the ST, neither the associated CT. The DCS will just advice the operator that there is high water level in a drum, and nothing else. No automatic protection actions.
Also, in the ST there is only an alarm signal if the steam temperature decreases 40°F from nominal temp. But if the steam temperature continues to decrease, or if it suddenly falls say 200°F, nothing else happens...no automatic protection actions.
We are trying to develop an automatic protection in our DCS for the HRSGs and the ST to prevent future similar faults. We believe that a high-high level in a drum should:
1st) trip the HRSG: close supplementary fire gas valve, close FW inlet valve and steam outlet valve. Not sure if it should also trip the CT.
2nd) trip the ST: if in addition to the high-high level in the drum, low steam temperature is detected upstream the main steam valves of the ST. Not sure which should be this low temperature trip value.
Any feedback is welcome. thank you for shearing your experience.