ST ran up out of sequence

Hi,

We have just come back from an outage where a lot has been done. The concern I have is that the ST ran up out of sequence. The control system we use is the MKV1. The units are CCGT, GE GT 9fa and ST A15.

The auto sequence operates as follows:
Seq: 20 "Master Start" looks at and controls
Seq: 21, 22, 23A + 23B.
Each Seq is made up of a number of steps.

Seq: 21 rolls GT to Synch.
Seq: 22 start temperature matching.
Seq: 23A rolls ST to Synch and completes temp matching.
Seq: 23B place ST in to IPC and completes the Auto start.

This is what happened.

The Auto Start command was give to the MKV1 to start the "Auto Start up" sequence.

The sequence carried through as normal until it reached Sequence 23A (step b).

Sequence 23A (step b). =
HP bypass open >3min & HP bypass flow

The sequence should hold at this step (23A step b) until all permissives are met, then move on through the rest of the sequence. i.e.
(step c / HP pressure >?)
(step d / HP Steam >?)
(step e / Temp match / GT exh temp ok)
(step f / ST ready)
(step g / ST start)

On this occasion the DCS (HMI) screen showed that the auto sequence was at Sequence 23A (step b). When the ST then ran up and synchronised with the grid with out any indication from the DCS (HMI)

The sequence step that permits the ST to run up to synch is Sequence 23A (step g).

There was no indication on the DCS that any of theses sages of the sequence (c, d, e, f or g) were or had been active.

The sequence on the DCS (HMI) then moved on from Sequence 23A (step b) until it reached Sequence 23A (step e) = ST ready state. Here it held fast waiting for the ST ready state permissive to be met before it would release the ST to roll.

Due to the fact that the ST had already ran up to 3000rpm and was on load these permissive (Reheat pressure < ? and all drain valves open and healthy) could not be met.

What was noticed is that when the MSV valve opened to heat the CV valve the IV valves opened up at the same time and it was this that ran the ST up.
As the ST ran an alarm (ST exh hood temp high) came in.

Due to not knowing what was going on and why the unit ran up we tripped the ST until some sense could be made of it.

Some for the work carried out were down/up loads and reboots of the MKV1 files.

My questions are:

1. Has any one experienced any concerns with the auto start sequence before?

2. Has any one experienced any concerns with ST running up with out a prompt?

3. If the ST was run up before all permissives were met, i.e. (steam temps & pressures, drains being blown down) would this give a ST exh hood temp high alarm and would that mean there was water in the unit?

Will update you with our findings.

Are you sure that after the download/uploads and reboots you made on the system, you still have the same sequence programmed in your ST MarkVI unit? Are you sure that the sequence is being called sequentially as required and as it should?

Hi jojo,

The answers to your questions are simply
No and No!

I afraid to say that we are not sure about anything yet. We are awaiting advice from the supplier and or a visit from them to do some investigation in to the concerns.

I was wondering if any one had come across this before and could advise the best way forward as the last thing I need is two 2shift the plant and find I have major concern. What I will do is run the units up in manual, this way I can control the sequence and prevent water being dragged in to the ST.

Q: does any one have any first hand experience of water getting in to ST, if so what happened and how was it rectified?

Well sorry to tell you but your no and no does not augur well.

When the system was originally set up was a backup of all the 'as installed' software made and more important kept in a safe place to be used as a reference only (i.e. no modifications done to this copy)?

I hope that the answer to this question is yes. Go back to it, compare it to the version you have copied inside your MkVI unit and see what went wrong.

If not, then somebody needs to study all the sequences programmed, and practically reprogram the whole system again.

With reference to your question of water reaching your steam turbine. If a steam turbine is hot (at its working temperature) and all of a sudden water gets in the turbine:

1. The water impinging on the rotating blades will damage the blades (in the extreme case even break them) given water has a far higher density than steam
2. Given the water has a much lower temperature than the steam, when the water gets in touch with all the internal hot components it will suddenly cool the parts, causing cracks in the component materials (the surface of the component cools whereas the inner material is still hot causing differential contraction across the material - similar, even though in opposite direction, when you pour hot water in a cold glass tumbler, it breaks). The only rectification to such eventuality is open up the steam turbine and inspect it completely.

The protection system of the steam turbine and/or the boiler should include protection for such eventuality (i.e. steam turbine tripped and not allowed to run if steam temperature is below a certain limit). If not make sure that such protection is added, before the unit is started up again. I don't think your steam turbine will resist another carry over (water entering the steam turbine).

I would recommend that you contact the steam turbine manufacturer and explain to him giving all details, even what you consider the most trivial, of what happened. He is in a far better position to assess the damage that was done to the steam turbine.