We have two GTs and one ST. It was cold-start up and at the time when the plant was tripped due to high high trip for the water level in IP drum, one GT produced 8 MW after synchro. The evaporator in IP drum is naturally circulated. There is a valve located on a pipe which delivers superheater from IP drum to ST turbine and the valve is programmed to be opened step by step when IP drum reaches 10 bar. When IP drum really reached 10 bar, the valve was opened, about 4%. Then, all of sudden, the pressure in IP drum abruptly dropped to 5 bar and the water was swelled to high--high level and the plant was tripped. Is there anyone who can help me understand what could cause this supposed that all equipment in the plant is operated well?

 

You don't provide enough information. The problem is whatever caused the IP drum pressure to drop. The drop in pressure is what caused the swell and high-high level trip.

Does the IP superheated steam go directly to the ST or does it go to cold reheat (assuming there is a reheater)? Do you have pegging from the HP drum to the IP drum? What type of valve are you stepping open - gate, globe, ball; MOV, air operated?

Was the IP steam actually going to the ST or was it being bypassed to the condenser? Assuming this has worked correctly in the past, what has changed? Maintenance?

Who is the manufacturer of the plant - GE, Seimens, MHI, Alstom, other?? What is the configuration? (You only say 2 GT's and 1 ST. Do the GT's each have their own HRSG? Do they only exhaust through the HRSG(s) or do you have dampers and bypass stacks?)

 

I feel I need to explain more about what happened.

I doubt on cold start-up since this problem does not occur in warm and hot start-up. After we had this problem, IP drum level high-high trip, we made warm start-up in a few hours and it simply worked out. Therefore, I think this problem may have something to do with cold start up.

The water in IP drum is evaporated through natural circulation system. There is a PCVpressure control valve between IP drum and the first IP steam superheater in the stack and this PCV is programmed to be opened step by step when IP drum pressure reaches 10 bar. Until this PCV is fully opened, IP drum pressure should sustain 10 bar. Between this PCV and IP drum, there is no way that the steam can go out except for very small dimension of sampling line.

At the time when the plant was tripped due to high-high IP drum level, PCV valve was opened, 4% and the LCVlevel control valve located between feed water pump and IP drum. The sampling line was closed, so there is no where IP steam can go out except for 4% opened PCV. Blow-down tank line was opened so that the water in IP drum could go out to the blow-down tank to control IP drum level.

My question is what could cause IP drum pressure drop to 5 bar all of sudden during cold start-up though the PCV was opened, only 4% and there was no other way that the IP steam could go out. So I doubt that this problem may have something to do with cold start-up, for example, something was wrong in natural circulation system to evaporate the water, since there has been no problem in warm and hot start-up after one failure in cold start-up.

I wish I can find some clue for this problem and I would appreciate your help.

Thank you.

Phillip

 

Thank you for your concern and answer. Please see below information.

The water and steam flow is like this :

Feed Water pump ---> Level control valve ----> IP drum -----> Evaporator -----> IP superheater ----> PCV ----> Cold reheater -----> Hot reheater -----> IPSV ----> IP turbine

And Level control valve was closed and PCV was opened, 4% when the plant was high-high tripped for IP drum level. Before IP turbine, there is IPSV (IP stop valve) and it was closed and as PCV was opened, IP steam was intended to go to the condenser through by-pass line until the steam pressure and temperature became high enough to be supplied to IP turbine. There is a drain valve before IP superheater inlet, but it was closed when the IP drum pressure was dropped to 5 bar from 10 bar all of sudden.

PCV valve we opened is globe valve operated by air.

The plant manufacturer is MHI and each GT has its own HRSG and flue gas is only exhausted to HRSG.

If there is any other information you need to give some comments, please let me know.

 

the indicated water level is driven by the density and how your transmitters are installed.

Cold start will show higher indicated level than indicated by electrode measurement or sight glass, so you might be getting a false trip

your work around is a redundant level measurement one of which goes by actual level, or set up a cold start bypass based on temperature - equivalent to a "blow-down switch". The latter is a safety issue so be careful.

 

It is still not clear where the bypass connection for the IP steam is. The arrangement I am familiar with on a multishaft combined cycle plant is:

HP bypass dumps to cold reheat;
IP superheated steam also dumps to cold reheat;
IP bypass is from hot reheat to condenser.
Hot reheat connects to IP (RH) turbine stop valves and on to intercept valves.
Steam turbine is rolled off through the IP / hot reheat steam.

IP drum and superheater pressurizes with drains open; drain valve closest to the PCV is kept open until the PCV is at least 25% open.
It is necessary to bring on the IP gradually, and a step to 4% sounds like it should work, as long as there was good drain flow established and the line was hot prior to opening that PCV. How do you know the valve was only at 4%? Is this based on position feedback? Or command output? If position feedback, how reliable is the transmitter? (Potentiometer devices are notoriously inaccurate.) If command output, have you checked the positioner and verified that it strokes linearly with command signal? Have you checked the valve seat? Is there a non-return valve in the line that may be sticking and then opening suddenly?

I don't know if it matters, but was this GT/HRSG the first unit up, or was the steam turbine already running and this one was being added on?

 

The instrument was ok. We made several perfect warm starts after the trip. When there was trip, ST was not working and it was like first GT/HRSG unit up.

Like you mentioned, I think we should open drain valve closest to PCV. There might be full of air trapped from IP drum to PCV and hot steam generated by IP drum was not able to be flowed because of full of air. When PCV was opened, trapped air exited though PCV and hot steam was flowed, but it was condensed because pipeline from IP drum to superheater inlet and from superheater outlet to PCV might be cold condition due to trapped air. I think it could make sudden pressure drop in IP drum because steam was condensed. This is what I can think of by now though I am not sure I understand the situation correctly. I must check temperature history of the pipeline to see if it was colder than saturation temperature. I think I can check it tomorrow. Thank you for valuable comments.