In the previous days I have been working on Gas Turbine. And in the next days I will be also working on steam turbine.
Our plant is Combined Cycle with 3 GTs Exhausting into 3HRSGs and the HRSGs supply One Steam Turbine. Our steam turbine is GE D11 and the control mode is IPC (Inlet Pressure Control).
There are many concepts that needs to be cleared. Unfortunately, The level of the manual is not suited for new learners. So If any one can break down the concepts, I will be more than happy.
First, Why the turbine is rolled through the IP turbine (called reverse flow)? I think the turbine will switch into foreword flow @ approximately 10 MW.
Second, During this roll up, what is happening to the control valves of HP, IP, LP?
Third, what is the deciding factor of when to switch from reverse flow into forward flow?
Fourth, what the control valves of Each Turbine (HP, IP, LP) do When Switching from reverse into forwards?
Fifth, there is something I believe called lube oil temperature hold, what does it exactly mean and why it is used?
Sixth, Can you talk and shed some light on Thermal Stress Control and Shell Expansion Control? And why there are necessary not like GAS TURBINE?
Seventh, are the control valve of HP, IP, LP controlled using position loop?
Eighth, usually I see the Steam Turbine of our plant is running on the highest load ( around 260 MW). Is this necessary?
I hope someone knowledgeable in ST like Otised can break down these concepts.
I strongly suggest you use this as an opportunity to use the Trend Recorder (Trender) function of ToolboxST to gather data on a properly working machine so that it can be used in the event of a problem or question later. Having trends of good start-ups of any turbine is just as important as taking trends of bad start-ups--because the two can be compared when there is a problem or perceived problem or question.
Trend Recorder (Trender) is your friend--and a very valuable tool to know how to use. Find the signal names for the position feedbacks from the various valves and trend them along with speed and then study them. And save them for future reference. If you want to know what should be happening the best way is to "capture" data from a good start-up using the best tool in your arsenal and study them to understand what is happening--and sometimes that makes things which weren't so obvious clear.
otised is a VERY good resource, but I don't think he posts as frequently as he has in the past, and probably because he doesn't check in as often as he has in the past. (Retirement can have that effect on people--and I wish him a long and happy, and easy, retirement!). Hopefully he will see this post and respond as only he can.
But in the meantime, learn to use Trend Recorder--and now is as good a time as any! Using Trend Recorder WILL NOT cause an inadvertent turbine trip--and supervisors and managers need to learn and understand this. You don't need any special ToolboxST Privilege to use Trend Recorder, you just need to use it. And practice will help in the future. And, again--having data from good start-ups is just as important, if not more so, than collecting data from a suspected bad start-up. And, this will give you practice in using the very powerful feature.
To be specific, I should have said to trend both speed AND load all the way to full ST load to get the best data on all the paramaters.
You can save the .trn (trend) file to the HMI's hard drive and review it at ANY time in the future. You can "hide" some parameters from view to make it easier to see others--and then you can "un-hide" them later. So, capture LOTS of data for LOTS of points--it makes the file large, but the HMI hard drives are HUGE and you can store them for later review, AND with the ability to hide and un-hide points you can "drill down" to only the information you want. And, if you record it at a pretty good clip (at least once-per-second, or even faster) you can have good data for reference.
You can scale every parameter as you wish. You can change the pen color AND/OR the pen width to make it easier. You can zoom in, or out.
Trend Recorder (Trender) IS the absolute BEST thing about Toolbox and ToolboxST--far and away. AND, having trends of GOOD start-ups to refer to when troubleshooting problems--or perceived problems--is priceless.
Learning to use Trend Recorder--especially to gather data for NORMAL operations (start-ups; loading; unloading; etc.)--cannot be understated. Having data from normal operations to refer to when there is a real problem--or a perceived problem (which many "problems" are!) is invaluable, and can be an excellent learning aid and teaching tool.
For both the GTs and the STs!!! And, the EX!!! And the LCI!!!
Use the "tools" (in this case the software tools) available in Toolbox and ToolboxST. Trend Recorder is the BEST tool in the "box"!!!
NOTE: Trend Recorder (Trender) is also the tool to use to analyze the hourly log files, and Trip History files--so learning to use it helps when trying to analyze and understand those files (which are automatically stored on the HMI and can be very helpful in troubleshooting--and comparing to normal operation files). AND, alarms and events are stored in the log- and trip files!!!
Hopefully you can see that Trend Recorder is very, Very, VERY, VERY powerful and useful. It can make you a very valuable employee! Now and in the future....
I am not really a steam turbine expert. While I have some knowledge about how the steam turbine works, my expertise was more on the HRSG and BOP controls and about how the overall combined cycle plant control systems work together to operate the power plant. I retired from GE nearly 8 years ago, and there have been changes since then that I am not very familiar with. I can provide some general information that you are looking for, but I no longer have access to the GE drawings and specifications.
Taking your questions in order:
First, as far as why the steam turbine is rolled through the IP section. NOTE: I don't have any P&ID's available so I can't answer all of your questions. But, I believe one of the reasons for rolling off through the IP has to do with getting steam flow into the reheater so it is not run "dry" for an extended period of time. Another reason may have been to achieve a faster startup. Also note, the term "reverse flow" refers the flow direction through the reheater, not through the steam turbine.
Second, the IP intercept valves are controlling the steam turbine speed up through synchronization. I am pretty sure the HP control valves are closed until the switchover to forward flow. The LP control valve will remain closed until very late in the start up cycle - it takes a long time to build up LP superheated steam pressure and temperature to the point where the LP admission control valve can be opened.
Third, I don't remember what the deciding factor for changeover to forward flow is, but there needs to be sufficient HP steam production. HP Bypass valve position was one indicator of steam production, along with sufficient HP steam temperature (at least 100 degF above steam turbine 1st stage metal temperature and at least 75 degF of superheat).
Fourth, the IP control valve will remain open, the HP control valve will begin opening and admitting steam into the HP steam turbine section, the HP steam turbine exhaust will provide steam for forward flow through the reheater, and the LP control valve will remain closed waiting for sufficient LP steam production.
Fifth, I don't remember anything about a lube oil temperature hold. There is a minimum lube oil temperature start permissive, but any sequence hold on lube oil temperature is new to me.
Sixth, steam turbine stress control is a proprietary algorithm in the steam turbine control which limits loading rate based on calculated stresses in the steam turbine. It provides a better means of loading the unit than the old method of waiting a set time based on initial (pre-start) temperature conditions in the steam turbine. I can't help you with the Shell Expansion Control. As to why these are not needed with the gas turbine, they are different machines.
Seventh, HP and IP are controlled using position loop. LP I'm not sure. If it is, it will have redundant LVDT position transmitters.
Eighth, in a combined cycle system, the steam turbine will run with the control valves essentially wide open, swallowing all the steam produced by the HRSG's. That's how you achieve the high thermal efficiency.
It is great to have you here. Thank you for taking the time to respond to each question. It was HELPFUL
One thing I have to respectfully disagree with. The Reverse Flow refers to the direction of steam entering the HP Steam Turbine. During startup, there is a steam which enters the HP turbine in the reverse direction from last stages toward the first stages. This steam is called cooling steam to prevent overheating of the HP steam turbine. The direction of steam flowing in the reheater is always in one direction and this can be clearly seen when looking to the P&ID.
Again, thank you for responding to each question. It was helpful.
You are absolutely correct on the reverse flow through the HP steam turbine for cooling purposes. I did sit in on some design reviews on the controls for this shortly before my retirement, but I never had the chance to work on a project involving this design. All the projects I worked on during my last 3 years at GE were single shaft projects (gas turbine, generator and steam turbine all coupled together on the same shaft).