Steam turbine shutdown

C

Thread Starter

C. Reihl

Started working at a new 2x1 combined cycle plant and they take the Gas Turbines offline prior to taking the Steam turbine offline. This is totally opposite on how I’ve been trained in shutting down a combined cycle unit. Is there a “right” way off taking a block offline?

[email protected]
 
I'm an combined cycle operator and We shutdown the steam turbine before the Gas turbine. the reason is to maintain
the good parameters of Steam to avoid the low quality.
 
S
this makes no sense to me! How do they maintain enough steam to operate the turbine? Do they close in the governor first?

Is there a bypass/ reducer valve for the turbine?

I'm just very curious...
 
Hey!

At our 2x1 plant we have 2 GE MS7001FA's exhausting to HRSGs that provide steam to an Alstom Steam Turbine. We operate the way your current plant does. If we receive the order to come offline and we are not already running in a 1x1 configuration then the first thing we do is give one of the CTs a stop command. Now, to bring the block offline we will give the second CT a stop command, the steamer will follow suit.

The Alstom turbine has a control program (Inlet Pressure Control/IPC) that is used while it is running that aims to keep pressure in the Main Steam line leading to the HP section at 750 psi. When the CT is given the stop command it will reduce load and un-sync itself from the grid. Its exhaust temp will drop accordingly (from ~1150F to about 900F initially then it will drop even further after fires go out). This obviously will affect steam creation, pressure in the drums and flow rates will drop. Since that program is still running, as long as the pressure in the MS header is above 750 psi the HP steam control valves leading into the HP section will be all the way open (it is trying to reduce the pressure in the line to the 750 setpoint). Once CT exhaust temp decreases and pressure in the HP drum decreases below 750 psi, the HP steam control valve will start to pinch off (trying to maintain that pressure in the header). As the HP control valve is pinching itself off, the steam bypass lines to the condenser are being opened. By the time the HP control valve is closed, the bypass lines are 100% open, dumping any residual steam in the lines or any steam being created by heat in the HRSG to the condenser.

One of the main reasons we take CT off first is to preserve the integrity of our condenser. If we were to take the steamer off first, the CT would still be exhausting at high temp and the HRSG would be producing massive quantities of steam which you could either vent to atmosphere or dump to the condenser. Venting them is not a viable option for us so that would leave us dumping it straight to the condenser for a short period of time. It is very undesirable to admit 1000F+ steam at pressures over 1000 psi to a condenser. We had many problems doing this when our plant was initially opened. In regards to admitting low quality steam to our Turbine as another poster mentioned, I haven't been told this but I am postulating that the IPC control mitigates that problem by denying steam entry to the turbine if it is under 750 psi.

Hope this helped!
 
caleb,

If you don't mind, I would like to question you about your comments. First, I have never seen a combine plant arrangement but have worked on utility steam turbines controls so I would like to learn about combined plant operation.

I was watching this thread because I was thinking similar as you that the if possible the GTs should come off first so ST could provide a gradual blow down of the HRSG and avoid severe BPV dumping to condenser.

what I do not understand is the following;
> "...As the HP control valve is pinching itself off, the steam bypass lines to the condenser are being opened. By the time the HP control valve is closed, the bypass lines are 100% open,..."

IF the BPV open as the CVs began to closed, the net effect would not be maintaining the 750 psi. Is it possible the CVs beginning closing at 750 to maintain 750, but when they are(nearly) closed, the generator trips on reverse power and the BPVs then open to continue the minimum amount of steam blow down to the condenser following turbine trip?

Following CV tripping, the BPVs should only be needed for a very little if they are trying to maintain the 750 header.

If the initial pressure is set for 750 psig, do you know the regulation, i.e., the amount of decay to fully close the CVs?
 
JFB,

Last night's explanation was a more high leveled one, but you are correct in your assertion. The BPV does not open until after then unit is un-synced from the grid. A more detailed explanation is as follows:

-The CT is given a stop command, it will ramp itself down slowly and eventually take itself offline (on reverse power).

-At about the 40MW range, the exhaust temperature has decreased sufficiently so that less steam is produced in the HRSG. As a result, pressure starts to drop below the 750 psi setpoint.

-The steamer sees this and responds by pinching off the HP control valve.

-It is important to remember, that this whole while (since the stop was issued to the CT) the steamer's electrical output has slowly been decaying due to decreased steam flow.

-The CT un-syncs itself from the grid. However, the CT is still at full speed no load, fires are still in and exhaust temp is anywhere from 700-900F still. It begins the process of lowering its speed.

-About a minute later (after CT un-synching), the MS pressure is still dropping and the HPCV is only 25% open.

-The pressure drop due to reduced steam production is so much that closing the valve no longer makes much difference. Yet the valve doesn't know this! It keeps pinching down until it closes all the way.

-Slightly before it is completely closed (maybe 13% open) the torque produced by the turbine is no longer sufficient to sustain a loaded generator. MWs drop and the turbine trips on reverse power.

-Now around this time, the CT has spun itself down to about 20% speed. At this point, the gas valves close and fires go out. Your steam production will decay very rapidly from here on out.

So right now what we have is a CT and a ST that are both disconnected from the grid. The CT is no longer generating any heat. The HRSG will start to cool down and steam production at this point is minimal (relatively speaking). Let us continue:

-Remember that at a HPCV value of ~13% the steamer was disconnected from the grid. The HPCV kept throttling itself down to 0% in an effort to maintain pressure in the line. This causes a dip in the speed which the turbine tries to correct by reopening the HPCV slightly. For reasons unknown to me, the steamer is still trying to maintain 3600 rpm for a few minutes, that is why it responds this way. (if anyone has insight about this, please share; I would think that after the HPCV closes the first time it would start slowing itself down, why wait another couple mins to being that process?)

-Approx 2 mins after the HPVC is fully closed (for the first time and then subsequently opened) the control system decides it is time slow the turbine down. It does this by closing the HPCV completely for a second time. At this point speed starts to decrease.

-When that valve is closed a second time, the line pressure sees a slight increase. The increase is enough that is surpasses the setpoint for the bypass valves. At this point the BPV opens, turbine and line drains are opened at this time too.

-Our plant makes it so our BPV and drains close pretty shortly after they open. We do this to maintain as much heat and pressure in the HRSG as possible. This makes the next startup much quicker.

I have a trend that I created that shows most of what is discussed above, if you would like a copy let me know your email and i'll send it to you.

Cheers
 
Hey dude,

I didn't even address your comments! I will do that here.

> IF the BPV open as the CVs began to closed, the net effect would not be maintaining the 750 psi. Is it possible
> the CVs beginning closing at 750 to maintain 750, but when they are(nearly) closed, the generator trips on reverse
> power and the BPVs then open to continue the minimum amount of steam blow down to the condenser following turbine trip?

This first one I think was covered by the previous post.

> Following CV tripping, the BPVs should only be needed for a very little if they are trying to maintain the 750 header.

Don't forget that the CV is trying to maintain the header pressure and it only does that while the steam turbine is on line. At the point when the CV is tripped, the ST generator has already tripped and that IPC control program is not in service any more.

The BPVs are used as a protective device at our plant. During typical operation, their setpoint is around 300 psi above the current MS header pressure. That setpoint will vary quite a bit depending on what conditions are on the unit. In a shutdown, the setpoint moves a lot closer to the value of the MS pressure. During the sequence, They are opened at ~580 psi while line pressure is slightly below that, maybe 570-575ish.


> If the initial pressure is set for 750 psig, do you know the regulation, i.e., the amount of decay to fully close the CVs?

I'm not sure I understand your wording here. But the CVs pinching off are not particularly effective (or they don't seem so). In the trend, the HPCV has to reduce itself by 60% before pressure rises ever so slightly. There might be another explanation for this however. If I am not mistaken, the position feedback for that valve is not linear. In other words, if the valve is actually half way closed, the feedback isn't necessarily 50%, it might be something else. Either way, once you get the trend, you can see for yourself.

Forgive any mistakes I have made, I myself am a young engineer stationed at this plant learning how it operates. I have tried to be as correct in everything stated and I'm confident that I'm pretty close but there might be slight misinterpretations or wrong conclusions drawn along the way, my apologies. Hope this helps, let me know if you have more questions.
 
THANK YOU for the time and effort to help this old man learn a little something about what is happening in the new world of power generation. Your explanation was clear and makes sense to me. However I will study it in detail and I might have a comment/question after that

>> Hope this helps, let me know if you have more questions.
 
Thanks everyone for the input, it appears that there is more than one way to shut down a block.

Like I said I was taught to bring the steam turbine offline prior to shutting down the last Combustion Turbine. We would take the CT down to minimal MW then reduce load on the steam turbine until it fell out on reverse energy. Keeping the CT online allowed us to ensure we had good steam seals until we broke vacuum on the condenser at 2/3 steam turbine speed. It also kept us from hitting the steam turbine with cooled steam.

Now we give a stop to both Combustion turbines and they reduce load and come offline, the steam turbine MSVs pinch back as we lose HP steam pressure due to IPC control. The CT’s will flame out and of course the steam turbine will continue unloading. Once the Steam Turbine reaches 10MW we punch out (trip) the unit.

Both ways seem to work fine I was just wondering what the most common way was.
 
Top