# Getting a better understanding of gas turbine control

N

#### Neo

CSA.

And if you are busy,just let me know. You know waiting is miserable.
I hate to quit and i wanna to go on this journey and finish it.

Thank you again for your help.

---------------------------------------------------------------------------------------------------------------------------------
You missed this question below:
<pre>

A>B----|
| L14HSX L3TFLT
---------| |---------||-----------( )

</pre>

A=CAKCPD(35psi)
B=CPD
L14HSX=Auxiliary signal to L14HS

And i tried to find the differebce between L14HS and L14HSX.<pre>

L14HS L4 L14HSX
---------||--------------||-----( )
|
L14HSX L94X |
------||------|/|----

L94X=Startup check stop GT normal shutdown;
L14HS=HP operating speed signal;</pre>

As for L3TFLT,if CPD is less than 35psi when gas turbine is under normal operation,then L3TFLT is logic 1.

I was wondering the function of the L14HSX,and whether can L14HSX in the L3TFLT rung be replaced by L14HS.
I am confused about L94X.In the toolbox, its description is :Startup check stop GT normal shutdown.
----------------------------------------------------------------------------------------------------------

L3RS1 L3RS2 L5VPRO_LATCH L3RS
-------||----------||------------|/|----------( )

L3RS1=Startup check stop ready to start 1;
L3RS2=Startup check stop ready to start 2;
L5VPRO_LATCH=Protective VPRO card trip-latch;

As we just finished L3RS1,i think L3RS2 is my assugnment this time.
<pre>

L52ONLINE L3COMM_IO L3ACS L3RS2
------|/|---------||-------||------( )
L52ONLINE= Unit online ;
L3COMM_IO=VCMI state changes for <R>,<S>,<T> OK;
L3ACS=Auxiliary check servos;

L3GFLT L3LFLT L62TT2 L3ACS
----|/|-----|/|------|/|--------( )
| |
L84TL | L84TG |
-----||------||----
</pre>
L3GFLT=Gas fuel control fault;
L3LFL=Liquid fuel control fault;
L84TL=On total liquid fuel;
L84TG=On total gas fuel;
L62TT2=startup check stop mutiple start counter;

Timer_sec _Counter
L2FZ(log_in) (log_out)L62TT-----------L62TT(INC) (AT_CNT)L63TT2
K62TT(pu_del) K62TT2(MAX_CNT)
L1XY(RESET)</pre>

L2FZ=startup check stop multiple starts permissive;
K62TT=startup check stop multiple start time delay(5s);
L62TT=startup check stop multiple start and atuo refire;
K62TT=startup check stop multiple start count(2);
L1XY=startup check stop aux to master contr-stup perm;

When the number of pulse(L62TT signal) is less than 2(K62TT2),than L63TT2 is logic 0.
There are several combinations which will make L3ACS logic 1 when L63TT2 is logic 0:
1.L84TL=1,L3LFLT=0; The gas turbine is on total liquid fuel,and the liquid fuel control is not in fault.
2.L84TG=1,L3GFLT=0; The gas turbine is on total gas fuel,and the gas fuel control is not in fault.
3.L3GFLT=0,L3LFLT=0; The fuel control system is not in fault.

CSA,I am confused about L62TT2,and can you explain it briefly.
According to the above combination,the gas turbine can start up when one of the fuel control systems is fault,and is it reliable?

#### CSA

Neo,

I'm not quite sure what the question is, but let's try this. The comparator is checking to make sure the median value of CPD is greater than 35 psig; if it's not then when the unit is above 14HS and running (we'll get to that in a minute) then L3TFLT is a logic "1" and usually, these days, that means a turbine trip--especially for machines with DLN combustors (because CPD is very important for exhaust temperature control--VERY important).

L14HSX is a unique L14HS in that, as you can seem from the rung once the unit is running (L4 is a logic "1") and the unit gets above 14HS (usually 95% TNH), then L14HSX latches in (through the L94X/L14HSX parallel part of the rung). L94X is a very important signal--a 94 device is a shutdown device, not a TRIP, but a normal, orderly shutdown. As, in an operator selects STOP and the unit starts unloading, the breaker opens, and the unit goes into a fired shutdown until is reaches Cooldown.

Some other operating conditions can also initiate an "automatic" shutdown (sometimes high-high inlet air filter differential pressure, or similar) can do the same thing--through L94AX which drives L94X (have a look at the application code in your machine).

So, if the turbine trips, L4 goes to a logic "0" and at whatever speed it's at then L14HSX will go to a logic "0". BUT, if the unit goes below 14HS (usually 94.5% speed) when it's operating (that means the grid frequency is more than 5.5% below normal!!!) then L14HSX will remain a logic "1", and in the case of L3TFLT the rung can still activate if CPD drops below 35 psig. Kind of confusing, but not really. (I hope!)

L3RS2 would be the next logical progression; yes.

And the important logic signal in L3RS2 is L3ACS--another signal which should be relatively straightforward but which has become somewhat perverted over the years.

L62TT2 stands for "Turbine Tried Twice to Start". It's a "relic" (old) bit of logic that causes a LOT of problems. It was put into GE logic way back when a lot of GE-design heavy duty gas turbines were being used for peaker units (during times of high electrical demand) and natural gas compressor drive, and a lot of these sites were unmanned. So, if a START was initiated from a remote location (say 250 km away) and something not too critical interrupted the START sequence, a second START would automatically be initiated by the Speedtronic panel without any operator intervention. (This was long before the days of SCADA and Ethernet and HMIs, so alarms were grouped into critical and non-critical bunches and the only alarm the remote operator would get would be a "Group1" or "Critical" alarm, so it wasn't much help.)

If the turbine still failed to start and accelerate to FSNL on the second, automatic attempt L62TT2 would go to a logic "1" and then someone would have to go to site to check on the unit. Sometimes, the remote operator could just initiate another START if it failed the second automatic start, but usually, once L62TT2 picked up starting was prevented until someone went to site and initiated a MASTER RESET--which was the mechanism for requiring a human to reset the Speedtronic in the event of a critical problem before the turbine could be re-started. So, that's a little history of the signal.

It has, unfortunately, remained in the logic even for combined cycle power plants--even though it has caused a lot of problems. For example, if a START is unsuccessful for some minor reason and a FAILURE to START alarm is annunciated, while the operators are scratching their heads about what went wrong the Speedtronic will automatically--without operator intervention--initiate another START sequence, at which point everyone is shocked and surprised. And this is because of L62TT2 being left in the control scheme even for combined cycle power plants--which are almost NEVER unmanned.

Another problem that occurs is when the turbine is STOPped before the generator breaker synchronizes (which is usually Complete Sequence these days), which increments L62TT2 once, and when the unit gets back down to zero or minimum firing speed (depending on the machine) it will automatically START again--which also causes quite a lot of surprise and consternation.

I wish GE would stop shipping application code with that logic to manned sites, and only add it if the site is converted to unmanned, or block it with a NO 'false' or something like that, because it really causes a lot of grief and can be very scary. But, since most of the engineers at GE don't understand how the signal came to be and aren't capable of critical thinking they just leave the signal in--and, it usually results in at least one problem during commissioning, and again afterwards--when no one is expecting the unit to automatically re-start, but it does.

So, except for certain unfortunate circumstances you can presume that L62TT2 is always a logic "0". And, unfortunately, without being able to see all of the application code in the machine at your plant it's impossible to say precisely what those certain unfortunate circumstances are.... Thank you, Belfort, <b>yet</b> again. AAARRRGGGHHH!!!

The L3COMM_IO is one of the ways that the Speedtronic checks to make sure that all three control processors (<R>, <S> and <T>) are healthy and communicating with each other. I have, unfortunately, heard of people being able to force this logic to be able to TRY to start the machine when there is a problem with one of the VCMI cards and then when the unit won't start they are furious, to say the least. Because of the way the Mark VI (and VIe) are configured, it might be possible for a start to actually be successful--but it shouldn't, and I believe that most Mark VI firmware has been "fixed" to stop this from being even an unlikely possibility.

So, write the sentence for L3RS2--and for L3ACS, Neo.

Yes; if the unit is not on Total Gas (L84TG is NOT a logic "1") and there is a fault with the gas fuel system the unit can still be started. Will it be "reliable"? What's your definition of "reliable"?

N

#### Neo

CSA,

Thank you for your detailed and enlightening comment on L14HSX, L62TT2 and L30COMM_IO.
<b>
When the selected fuel control system is not in fault(L84TL=1,L3LFLT=0 or L84TG=1,L3GFLT=0) and L62TT2 is logic 0, then L3ACS will go to logic 1.</b>
<b>

When the unit is not online(L52GX=0),all three control processors are healthy and communicating with each other,and L3ACS is logic 1,then L3RS 2will goes to logic 1.</b>
-----------------------------------------------------------------------------------------------------------
<pre>
L4S L94T L4T L4
-----| |-----------|/|----------|/|------------( )
|
|
L4 |
-----| |------</pre>
It seems that we have just finished L4S,so what is my next assignment,L94T or L4T?
-----------------------------------------------------------------------------------------------------------
1)
<pre>
Timer_sec _Counter
L2FZ(log_in) (log_out)L62TT-----------L62TT(INC) (AT_CNT)L62TT2
K62TT(pu_del) K62TT2(MAX_CNT)
L1XY(RESET)</pre>
As we can seen from the above rung,if L62TT2 is logic 1 after a second startup failure ,it will reset to zero when L1XY picks up, then means L1X goes to logic 0.

TO my understanding, if the unit stops,L1X goes to logic 0(Is it right?),then L62TT2 goes to logic 0.But you said: "Once L62TT2 picked up starting was prevented until someone went to site and initiated a MASTER RESET."

Should i post L2FZ rung and L1X rung? Maybe it helps.

2) You said if the turbine fail to start for some minor reasons,the unit will start up automatically,and if it
fail again then starting was prevented until someone went to site.

My question is how can i deduce this from application code,which part of application code should i read to confirm this?
-----------------------------------------------------------------------------------------------------------
As we can see from L2ACS rung,if the unit is not on Total Gas (L84TG is NOT a logic "1") and there is a fault with the gas fuel system the unit can still be started.

In my last reply i doubted its reliability.I just wondered if we changed to gas fuel during operation,and gas fuel system is in fault,that will be a problem. Why not make sure both of the fuel control system is OK?

N

#### Neo

CSA,

"As for the comments about how did I know the unit burned low-BTU gas fuel, well, from the Longname descriptions you provided which included the Inert Purge term."

Is the Inert Purge term special for low-BTU system? If one unit burns natural gas,purge system is still needed,right?

I happened to see a test bench application code.

The test bench application does't have IGCC module,and its GAS_LIQ_PURGE module has LIQ_NO_DLN and GAS tasks,while our application has IGCC module,but only LIQ_NO_DLN task in GAS_LIQ_PURGE module.

So i think you deduction "that portions of the purging sequence in your machine were likely copied-and-pasted from the IGCC unit control schemes as they use purging quite extensively in the gas fuel system(s)" is right.

I wondered that if an IGCC unit has an unique purge system?
And for our unit that burns low-BTU gas fuel, should its purge system be different from those that burns natural gas?

Maybe these questions are very boring, but i am still curious.
May it not bother you.

Best regards!
Neo.

#### CSA

Neo,
Usually, whenever a GE-design heavy duty gas turbine burns multiple fuels (even two different gaseous fuels) there are fuel purge systems. This primarily to ensure that there is no fuel in the system which is not in service which can accidentally ignite, and also one of the primary purposes of the purge system is to provide a flow of cooling air through the fuel system not in service. Another benefit of this flow of air through the fuel system which is not in service is that it prevents the backflow of hot combustion gases into the fuel system which is not in service and which can cause very serious damage to piping and components (especially fuel nozzles and flexible piping section).

Some fuels are very dangerous if exposed to oxygen, even atmospheric oxygen, so the purge is used to ensure that no unburned fuel is ever present in the piping.

A fuel purge system is a fuel purge system in my opinion. I don't think one fuel or process has a purge system that is that much different than any other--except when some inert gas is required to purge the fuel to avoid coming into contact with oxygen. These systems will be more complicated, necessarily so, because of the additional piping and regulators and valves and solenoids and such. I don't know if the low-BTU fuel at your site qualifies as such, but it could. Gas turbines are burning a lot of different fuels these days as people are looking to prevent flaring off-gases to reduce emissions--and to increase the efficiency of the process by utilizing available energy from hydrocarbons instead of just burning it or venting it.

Hope this helps!

Next rung in the L4 Journey, please?

N

#### Neo

CSA,

Thank you so much for your comment on fuel purge system.You are always patient to answer all kinds of questions.

I think you missed another reply of mine.
--------------------------------------------------------------------------------------------------------
This time i will examine L94T,and i think it is very important too.
Here comes L94T:
<pre>
L94XZ L94T
------||---------|----()
|
L28FD L83RB |
-----|/|-----||---
|
L2CANT |
------||--------- </pre>

L94XZ=Startup check stop fired shutdown
L28FD=Flame detected
L83RB=startup check stop ramp to blowout selected
L2CANT=Trip on can flameout timed out

I confronted withmany signal name descriptions that start with "startup check stop",while i think these signals has nothing to do startup check,just like L94XZ.

As we can see from above rung,there are three kinds of conditions that will lead L94T to logic 1.
----------------------------------------------------------------------------------------------------------
<pre>
L28FD L83RB L94T
-----|/|-----||-----------( )

L60RB L94SD L83RB
---------|/|--------||----------( )</pre>
L60RB=startup check stop above ramp to blowout speed
L94SD=startup check stop shutdown with breaker open

I am a little confused about the term "blowout speed",and i think "ramp to" can be "increase to" or "decrease to",right?

<pre>
(TNH>K60RB)----
| L28FDY L60RB
------------| |-----|/|--------( )</pre>

According to the above rung,when TNH is less than 30% rated speed or Flame is not detected,then L60RB is logic 0.And while L94SD is logic 1,then L83RB is logic 1.When l83RB is logic 1 and Flame is not detected,then L94T is logic 1.

It seems that whether TNH is less than 30% rated speed or not doesn't matter,for when flame is not detected no matter what value of THN is the L94T will be logic 1(with l94SD=1).
------------------------------------------------------------------------------------------------------------
<pre> L94XZ L94T
-----------||---------------------( )
L52GY L94X L94XZ
-----------||--------||----------(T)
8min</pre>

If STOP command was sent for more than 8min and the breaker is open,then L94T is logic 1.
------------------------------------------------------------------------------------------------------------
<pre>
L28CAN1 L28CAN L2CANT
-------||--------------(T)----------(T)
| 1s 5s
L28CAN2 |
-------||------
|
L28CAN3 |
-------||------
|
L28CAN4 |
-------||------

L28CAN=Any can flameout
L2CANT=Trip on falmeout timed out

Take L28CAN1 for example:
L28FDF1 L28FDA L28CAN1
------||-------|/|----------()
L28FDF1=Flame Detector 1 Funtional
L28FDA=Flame Detector Channel #1

L28FDA L94SD L4 L28FDF1
------||-----||------||------()
|
L28FDF1 |
------||-----</pre>

L94SD=Startup check stop shutdown with breaker open

When STOP command sent and the breaker is open,if any one of the Flame Detector doesn't detect flame, L2CANT
is logic 1.
------------------------------------------------------------------------------------------------------------
I wondered why there are three kinds of condition that will lead L94T to be logic 1.
If STOP command is sent by operator,it will go though which rung?
And the gas turbine fail to start after ignite,it will go through which rung to stop?
------------------------------------------------------------------------------------------------------------
CSA,you should respond when you are not busy.
May not disturb you to make money:.
Just a joke

#### CSA

Neo,

This thread is getting pretty large/long and I'm not sure I'm seeing all of the replies in a timely manner. So, if I miss a message/questions just let me know.

L62TT2, is, again, kind of an "artifact" of code that GE just refuses to remove from manned sites. L1X is not usually "cancelled" until the operator selects STOP or the unit synchronizes (I think that's the final permissive in the automatic drop-out of L1X these days; it used to be just complete sequence, L3, but it's been modified over the years. No use in starting a generator-drive turbine unless the generator breaker closes, right? (Except for testing purposes.))

L62TT2 was put in place so that if the unit did not fire/accelerate to FSNL when a "remote" start was initiated that it would automatically attempt a second firing attempt <i>provided the reason for a failed start was not critical--which would have set the Master Protective Lock-out, which would have required a Master Reset to be initiated ON SITE</i> (Master Resets were not allowed to be initiated from a remote location). Typical reasons for not starting were air in the liquid fuel supply piping, air in the gas fuel supply piping, borderline start-up parameters, etc. So, it was felt that--unless the turbine tripped--a second automatically-initiated start attempt was warranted on remotely started and -operated machines.

Again, since people making these decisions about application code today weren't around then, and since there isn't really any documentation in GE that explains some of the older sequencing these people just are loathe to make any changes because they don't understand if there will be any knock-on effects--and worse, they can't work through the application code to see what the signals do and if there will be any knock-on effects and how to mitigate them if there will be.

Without being able to see all of the application code at your site I can't say for sure how it has been "modified" and what it does today. Back in the day, yes; when L62TT2 picked up, it was latched in by a Master Reset. Someone had to go to site to observe the start and resolve the issue, rather than just continuing to pump fuel into a turbine (or not as the case may be). A lot of the older peaking units were liquid fuel-only machines, and there can be lots of issues with liquid fuel machines that aren't operated regularly and frequently on liquid fuel.

You just have to look at the L1X, L1XX, L1XY and L62TT2 rungs and contact usage and determine what, if anything, will happen at your site if the unit doesn't fire on the first START. If the turbine trips on a START, then L62TT2 doesn't come into effect (usually). It's only if there's a failure to fire--which is NOT USUALLY a trip (though Belfort seems to think differently about that, and on occasion; they are as consistently inconsistent as other divisions of GE can be).

So, let's say there was too much air in the liquid fuel supply piping and the turbine failed to fire on a START attempt. The Mark VIe should annunciate a "FAIL TO IGNITE" alarm, and the unit should continue to crank. Under the old L62TT2 logic, it would re-initiate the purge timer and the unit would go through another purge and then re-try another firing attempt. If the second attempt failed, the unit would just go back to cooldown (ratchet) after annunciating a second FAIL TO IGNITE, and the operator would have to take appropriate action. That would be typical of the L62TT2 action of the past. Again, without being able to see all of the application code in your Mark VIe, it's very difficult to say what function L62TT2 actually serves--if any.

As for the reason for allowing a start on one fuel if the other fuel is "locked-out", you may need to start and run the machine for some reason while the other fuel system is "unavailable." Your machine might not have that capability, but many multi-fuel machines do. The liquid fuel might be very expensive--but if someone needs the power, they might be willing to pay that if the turbine can't run on the other fuel for some reason.

Again, GE is all about trying to make the machines as reliable as possible, under the widest set of circumstances possible. Try to imagine other sites and circumstances when working through logic, remembering that the same application code is used for LOTS of machines (modified as necessary for the different types of auxiliaries) so while it may not seem logical for your machine, it may be for others. And, leaving it so probably doesn't hamper operation of your machine (though it may; a careful analysis is best when you find something like this).

Hope this helps.

#### CSA

Neo,

Yes; L94T is very important. L4 will drop out (goes to a logic "0") when either L4T or L94T picks up (goes to a logic "1"). L4T is from the application code trip detection (Low-low L.O. Pressure; High-high L.O. Temperature; High-high vibration; etc.--we'll cover them all!). But these trips are only the ones detected by the application code running in <R>, <S> and <T>--not <P>.

L94T is the way fuel is shut off during a normal shutdown--either an operator-initiated STOP or an automatically-initiated STOP. A '94' device is a normal shutdown device. This is not to be confused with a trip--an emergency shutdown.

During a normal shutdown from load, the unit load is ramped down at some uniform rate until the generator breaker opens on reverse power, at that point the Speedtronic goes into what's called 'fired shutdown' mode, where the fuel is reduced <b>in steps</b> to decelerate the unit until it reaches (ramps <i>down</i> to--you are correct in that a ramp, in this case, is a decrease in speed) and then after a certain time below that speed (K60RB) the L4 will drop out and fuel will be shut off, and the unit will continue to decelerate to Cooldown.

Before the advent of digital control systems, shortly after the generator breaker opened fuel was cut off. This represents a large thermal stress to the hot gas path components, and so when the control systems became more capable it was decided to reduce the fuel slowly while the turbine was decelerating until such time as it was felt the fuel could be shut off without introducing too much thermal stress to the turbine.

Somebody thought a good name for this speed level would be "blow-out" speed--who knows why; it's not a very good or appropriate name. It is intended to mean that if fuel were to be further reduced the air flowing through the machine would be insufficient to maintain flame (the fuel-air mixture would be too lean) and the flame would be blown-out.

There is also logic that says if the unit doesn't reach that blow-out speed before flame is lost in any combustor with a flame detector for a second or so that fuel would be shut off at that point. The thinking here is that if the air flow is so high for the fuel flow at that point that one or more combustors lose flame that this would result in a high exhaust temperature spread condition which is also very damaging to hot gas path components.

So, fuel is just cut off if flame is lost for a brief time during shutdown--and the alarm 'CHAMBER FLAMED OUT DURING SHUTDOWN' is annunciated, and the unit continues to decelerate to Cooldown.

This alarm simply means that flame was lost "prematurely"--before the unit reached "blow-out" speed (again, that poor name). It means that if the condition continues on subsequent shutdowns that someone should be investigating changes to the shutdown FSR to keep flame on longer to help reduce the thermal stresses. This is especially true if the condition happens at speeds above, say 70% or 80%--and even more critical if it happens when shutting down on natural gas fuel.

It is VERY important to note that it is VERY difficult to maintain flame while shutting down when liquid fuel is being burned to speeds below approximately 50% <i>without having lots of white smoke in the gas turbine exhaust</i>. This is because of the lack of atomizing air (usually the Booster Atomizing Air Compressor is NOT running during a fired shutdown, and only the main AA compressor is running), resulting in poor, incomplete combustion which is the cause of white smoke in a gas turbine's exhaust when running on liquid fuel.

So, it's very common for there to be two "blow-out" speeds for units that can burn natural gas and liquid fuel--one for natural gas (around 20%, usually; sometimes as high as 30%), and one for liquid fuel (usually somewhere around 50-60%, sometimes even higher, because white exhaust smoke can be very bad during shutdown while burning liquid fuel because of the lack of atomizing air).

Finally, there is logic that says if the unit doesn't reach "blow-out" speed within a certain time after a normal STOP is initiated without losing flame in any combustor that fuel will just be shut off. Why? Sometimes, the fuel is excessive during a fired shutdown and the unit doesn't decelerate past a certain point and just kind of "stalls" at some speed. No point in continuing to pump fuel into the machine if it's not going to decelerate.

Again, if this happens someone should be looking at adjusting the shutdown FSR constants to maintain deceleration all the way down to "blow-out" speed. Some units, newer units, have some alarm to "alert" a conscious operator to the excessive time on fired shutdown. I don't recall the Process Alarm text message, and I've seen at least two different text messages for the same condition, so it seems there is no "standard" for the alarm text.

So, them's all the conditions for L94T.

Yes; some of signal names can be very misleading, VERY. Again, it's best not to just inherently trust every signal name, but, rather to analyze and form one's own opinion and definition. But, the one thing it does do (besides making things difficult for many) is it forces one to use their critical thinking skills and to investigate and analyze. That's why I like to make hand-written notes on printed copies of application code--especially as I'm working to understand what a particular signal's function is.

I have also objected to the use of the word "trip" in shutdown logic over three decades, but it just fell on deaf ears. Makes it really hard to get people to properly call a trip a trip and stop a shutdown when the longname descriptions are misleading. But, I'm only one person, and while I never gave up hope that someone might listen and understand and make the changes, I'm also a pragmatist. Just not a defeatist. I'm still trying--just in a slightly different way.

;-)

Hope this helps!

So, what's your sentence for L94T?

N

#### Neo

CSA

I have to say your comment on L94T is great, great, great!
It must not easy to get where you are now.

So during a normal shutdown, when the generator breaker is open, if the speed of the HP ramped down to K60RB, or any one of the flame detector detects no flame, or it has been more than 8min since the generator breaker was open, L94T will go to logic 1,and that means fuel will be cut off?

Just a few questions:

1) "During a normal shutdown from load, the unit load is ramped down at some uniform rate until the generator breaker opens on reverse power, at that point the Speedtronic goes into what's called 'fired shutdown' mode, where the fuel is reduced in steps to decelerate the unit until it reaches (ramps down to--you are correct in that a ramp, in this case, is a decrease in speed) and then after a certain time below that speed (K60RB) the L4 will drop out and fuel will be shut off, and the unit will continue to decelerate to Cooldown."

<b>I think your above comment refers to the rung below.</b>

<pre>
L28FD=Flame detected
L83RB=startup check stop ramp to blowout selected

L60RB L94SD L28FD L94T
---------|/|--------||--------|/|------( )
L60RB=startup check stop above ramp to blowout speed
L94SD=startup check stop shutdown with breaker open

(TNH>K60RB)----
| L28FDY L60RB
------------| |-----|/|--------( )</pre>
According to the above rung,during a normal shutdown,if TNH ramps down to blowout speed while flame is detected(L28FD=1),the fuel will not cut off. That seems unreasonable.

If i were the programmer of application code, i would like to replace the above with that below:<pre>
(TNH>K60RB)----
| L60RB
------------| |------( )

L60RB L94SD L94T
---------|/|--------||------------( )</pre>
Is it Ok?

2) There are four flame detectors while there are 18 can combustors. so if one of the can that not installed with flame detector blows out during a normal shutdown, fuel will not cut off, and this hot gas path component will suffer high thermal stress, right?

Best regards
Neo

#### CSA

Neo,

Thank you for the kind words.

Yes; I agree with your sentence describing the L94T rung.

1) Yes; my comment refers to the L60RB. Remember, the comparator says if TNH <b>is greater than</b> than K60RB, then L600RB will be a logic "1". And, remember, we said that sometimes signal names are not chosen properly. L60RB is a logic "1" when the turbine speed is above K60RB AND flame has not been lost for some period of time during shutdown, and L60RB is a logic "0" when either TNH<K60RB OR flame is lost before TNH<K60RB. It would have made more sense if L60RB were a logic "1" when TNH<K60RB OR flame was lost for more than a couple of seconds during shutdown--but nobody consulted me.

Why would you eliminate the L94SD?

2) Yes, there will be increased thermal stress if flame is lost in any combustor which doesn't have a flame detector during shutdown because fuel will not be shut off. But, there are tradeoffs--do you want to pay for 18 flame detectors? And maintain 18 flame detectors? And remove and replace 18 flame detectors during maintenance outages? With all the wiring and tubing (I'm presuming the flame detectors are water-cooled at your site)? But, in general, this "low cycle fatigue" logic works very well because it keeps fuel on much longer during deceleration that was previously possible (remember--flame was shut off at 95% speed during every shutdown).

Can we start on L4T now, please?

N

#### Neo

CSA,
Here comes the L4T!

L4T(1)
<pre>
L4PST
-----||-------
|
L4PRET |
-----||-------
|
L4POST |
-----||-------
|
L3SMT |
-----||-------
|
L4IGVT |
-----||-------
|
LFPAUXG2LLX |
-----||-------</pre>
L4PST = Protective status trip logic 1
L4PRET = Pre-ignition trip
LPOST = Post-ignition trip
L3SMT = Starting means device trip
L4IGVT = Inlet guide vane control trouble trip
LFPAUXG2LLX = Auxgas P2 pressure low low delayed

According to the above rung, the trip signals are classified into six categories,and i will start with L4PST.
<pre>
L4PSTX1
-----|/|-------
|
L4PSTX2 |
-----|/|-------
|
L4PSTX3 |
-----|/|-------
|
L4PSTX4 |
-----|/|-------</pre>
L4PSTX1 = Protective status trip logic 1
L4PSTX2 = Protective status trip logic 2
L4PSTX3 = Protective status trip logic 3
L4PSTX4 = Protective status trip logic 4

<pre>
L63QTX L45FTX L5ESTOP1_FZA L86GT L63ETH L63CSHH L4PSTX1
------|/|------|/|------|/|-------|/|-------|/|-------|/|-----( )</pre>
L63QTX = Lube Oil Gen Prssure Low Low Trip
L45FTX = Fire Indication Trip
L5ESTOP1_FZA = VPRO E-STOP PB Circuit Open
L86GT = Generator Protective Trip
L63ETH = Exhaust Duct Pressure High Trip
L63CSHH = Flow Inlet Duct Diff Pressure GT Trip

1)L63QTX
L63QTX is related to L63QT(Low L.O. Pressure TRIP). So when te L.O. pressure is too low to protect the bearings, L63QT will be logic 1.

2)<pre>
l45ftx1
-----||------
|
l45ftx2 |
-----||------|
|
l94f1b false|
----||---||-- </pre>
l45ftx1 = fire protection aux/turb/load compt fire detection
l45ftx2 = fire protection exciter compt fire detection
l94f1b=fire protection exciter compt CO2 release

If fire is detected in specified space,l45ftx1 or l45ftx2 will be logic 1.

I will continue next time, and i need to time to refer to background information.
------------------------------------------------------------------------------------------------
I would like to know why there should be such a trip (like L63QTX), and what will happen if the there were not such trip signal (when L63QTX is logic, the unit doesn't trip). And what does the signal name stands for (l63QTX, 63 for pressure switch, Q for oil, T for??, X for Aux?).

It seems that L4T is much more complicated than those signals we confronted with before, do you have any suggestion for how should i examine L4T. i need your guidance.

Best regards!
Neo

#### CSA

Neo,

Let's back up just a minute, please, and explain why you wanted to remove L94SD from the L60RB rung.

As for L4T--yes; it's much more involved as it has most of the critical protection for the turbine and auxiliaries in it, so it's going to take a while. That should be okay, though, because there's a lot to learn here--a LOT.

As for the question about L63QTX and the need for a low-low L.O. pressure trip, it's very important to protect the machine (turbine, reduction gear, generator) bearings from being "wiped" (destroyed) because of lack of lube oil pressure and flow. And, I would have thought you would have been all over this one--because, when the turbine is being started from zero speed when it's off Cooldown (and the Aux. L.O. Pump is not running) the turbine should be tripped! But, to get a ready to start the turbine has to be NOT tripped, and to get L4 to pick the turbine has to be NOT tripped. So, let's put up the L63QTX rung and see how it works so we can explain how the turbine is not tripped when there is no L.O. Pressure during starting when the Aux. L.O. Pump is not running. It's pretty important, wouldn't you say?

As we go through L63QTX you will see why there is a "auxiliary" signal to low-low L.O. pressure (a '63' device is a pressure switch; 'Q' generally stands for Oil, but sometimes stands for 'Flow', and the 'T' stands for Trip, and yes--the 'X' stands for auXiliary.

Also, we are gonna have some fun (NOT!) with that LFPAUXG2LLX logic signal, too!

Yes; L4T is (or at least it's supposed to be) divided up into several categories--namely:

<b>Protective Status Trips</b> - If any of these trips are detected at any time (turbine running or not) then a trip will be initiated. The purpose is to protect the turbine and auxiliaries, running or not, from potential catastrophic damage.

<b>Pre-Ignition Trips</b> - As the name implies, these are conditions which, if detected before flame is detected ("ignition") will result in a trip of the unit.

<b>Post-Ignition Trips</b> - As the name implies, these are conditions which, if detected after flame has been detected will result in a trip of the unit.

<b>Starting Means Trip</b> - This is (was) intended to be the way the turbine was tripped by the starting means if a problem with the starting means was detected.

<b>IGV Trips</b> - This is (was) intended to be the way the turbine was tripped if a problem was detected with control of the IGVs.

<b>Interesting Named Trips</b> - Yes; this is another poorly implemented trip (probably should have been in Post-Ignition Trips) as well as an extremely good example of an extremely poor choice of a signal name. We're gonna have a good time with this one.

It's, again, critical to remember that these trips <b>ARE NOT</b> all of the possible trip conditions. There are trips connected to the TREG (<P>), as well as the overspeed trips and the dreaded "Composite Trip" that are generated in the PPRO IO Packs, <b><i>AND</b></i> the Emergency Stop ("E-Stop") pushbutton circuit.

One of the most important things any technician (and operator) can do to learn how the turbine at their plant operates is to develop a list of "Runbacks, Shutdowns, and Trips". We are going to be working on the trips that are detected by the control processors (<R>, <S> and <T>) as we work through L4T.

We might be able to open another thread (when we complete this one) that can investigate the other trips everyone should know and understand--the ones connected to and generated by <P>.

And, if you're really interested we might be able to open still another thread (when we complete the other two) about Runbacks--conditions that result in a load reduction to some setpoint or to FSNL (with the generator breaker open) to protect the unit. (At that point, it's up to the operator to understand the condition--which should have a Process Alarm to indicate the cause of the Runback--and take appropriate action, either to resolve the problem and re-load the unit, or to shut the unit down until such time as the problem can be corrected and the unit re-started and re-loaded.)

Traveling can be such fun!

B

#### Batman

Dear CSA,

I am a mechanical engineer by qualification, trying to understand GE 9FA control system. I have a question on L4 & L4Y commands. How both of them can be a logical state "1" simultaneously? As you mentioned L4 needs to be "1" for getting start permissive and apparently L4Y too needs to be "1" in L4S loop. This is a bit confusing. Sorry in advance if it's a stupid question.

N

#### Neo

CSA

Thank you again for this exciting journey,and i am willing to continue this journey until the end of doomsday.

I was wondering how i should examine L4T,as there are so many signals involved.Should i examine those signals one by one? And what should i foucs on?

And this thread is getting very long. I think I'm going to make this part the beginning of a new thread next time with the same name called part 2. I will put a note in the beginning of the threads stating that it comes in two parts along with the URL of other part.

I would like to know your opinion.

I am a little exhausted today, and i will try to reply to your previous respond tomorrow.

BTW, it seems that there are more than one CSA in this forum
It will confuse me whether i am talking to "the CSA".

Best regards
Neo

#### CSA

Batman,

Good on you for expanding your knowledge; it's very helpful for mechanical engineers to understand how turbines operate, too, not just to know how to turn a wrench or orient an orifice plate.

I believe you found a typographical error in my post (something I had just chided Neo for making... and then I made a serious one!). The rungs in questions are:<pre>
L3RS L1X L33CSE L63QT L4SX
-----| |-----| |-----| |-----|/|------------( )

L4SX L14HR L4Y L4S
-----| |---------| |---------| |------------( )</pre>
The first, L4SX, is an "auxiliary" to L4S, made necessary because a sequence editor can't have more than eight (8) elements, so it was broken into two rungs by making the first an "auxiliary" and then including the auxiliary in the "actual" rung.

The resulting rung (which I mistyped) would look like this:<pre>
L3RS L1X L33CSE L63QT L14HR L4Y <b>L4S</b>
---| |---| |----| |------|/|-----| |----| |-----( )</pre>
Hopefully this part was clear:

>L4Y is a logic "1" 1.0 seconds after L4 is NOT a logic "1".

So, the rung which I mistakenly put L4 over the coil, should have been L4S, and this should help clear your doubt.

The rung for <b>L4</b> is <b>always:</b><pre>
L4S L4T L94T L4
----| |--------|/|-------|/|----------( )
|
L4 |
----| |-----</pre>
Does this help? Sorry for any confusion--and there is no such thing as a stupid question when one is trying to learn something new. Just dumb answers.

#### CSA

Neo,

I think it will be best if we examine most every trip condition--not all of them--but probably most of them. They are very useful in understanding what conditions lead to a turbine trip from the controllers (<R>, <S> and <T>).

You are avoiding answering the question about why you think L94SD is unnecessary....

And, yes, poster H.A.R. has decided to use 'CSA' as his moniker. He only has a single star next to his CSA, so that would be your first clue as to who was responding to you. ;-)

And, good idea about starting a 'Part Deux' for this thread. The first post should be about L63QTX.

B

#### Batman

Thank you CSA for the quick reply in spite of your busy schedule. I'll summarize what I understand (about L4Y), then I'll ask you a question.

1) L4 is a master protective signal, which is normally "1", when everything is fine.

2) But, just before start up, it is "0", so that L4Y which is used in STCK2 is "1" which makes L4S "1" if all other conditions are met.

3)Once after L4S is "1", L4 changes its state from "0" to "1" if L94T & L4T are "0", and it remains "1" until L94T or L4T change their state.

Now I have a question (Assuming my comprehension is correct so far)

Why time delay for L4Y is only one sec, if it's used to block an immediate start attempt followed by a trip/shutdown? One second seems to be little impractical. I guess it's main purpose is to avoid a start attempt when L4 is "1", but not convinced with the time delay. Please correct me if I miss something here.

#### CSA

Batman,

L4Y is used in other rungs, not just L4S. The reason there is a timer at all is to prevent what's called a "relay race" during very "busy" times when L4 is changing state from "1" to "0". I honestly don't know why it's set for 1.0 seconds, but I know it works.

And I know that changing things like this without understanding all the knock-on effects (in all the other rungs that L4Y is used in) can have very bad effects.

A lot of the application code in Speedtronic turbine control panels are artifacts from the days of electro-mechanical relays. That's one of the things that makes Speedtronic panels unique--every time a new Speedtronic panel comes out GE doesn't reinvent the way it controls the turbine. And, for sites/Customers that buy new turbines with Speedtronic turbine control panels, or technicians with experience and knowledge from older versions of Speedtronic turbine control panels, it's very easy to troubleshoot and understand the way the turbine is being controlled.

There was one exception to this--Mark II Speedtronic--when GE went away from relay ladder logic and diagrams to a "logic gate" type of depiction. A lot of people really didn't like that format, even though it was "modern" and functionally exactly like Mark I (the turbine operated in virtually the same way). A LOT of Mark II-equipped turbines were sold around the world (and a LOT of Mark IIs were still in service), but a lot of Customers with older Speedtronic turbine control panels complained, and when GE developed the Mark IV it went back to relay ladder logic/diagrams.

The upshot of all this is: a time delay may not be necessary for today's digital control systems. But, why fix something that isn't broken? It may not seem that L4S needs a time delay, but other rungs/functions probably do.

#### CSA

Batman,

I likely didn't address your explanation of what's happening--but it's basically correct.

1) L4 must go to a logic "1" in order to start the turbine, and it must remain a logic "1" in order to keep fuel flowing to the turbine once flame is established.

2) When the unit is at rest or when it's on Cooldown (ratchet; turning gear; etc.) L4 is a logic "0". And it doesn't go to a logic "1" until an operator puts the unit into some mode other than OFF and initiates a START (and all the other permissives are met).

3) Yes; L4 remains "1" until L4T or L94T changes state. L4T goes to "1" when the control processors detect a condition that requires an immediate shut-off of fuel (and emergency trip). L94T goes to a logic "1" (during normal operation) when flame is lost during deceleration, or the unit drops "blow-out" speed for a specified period of time, or when the unit hasn't reached "blow-out" speed within a certain period of time after a STOP was initiated.

So, with a few qualifications, your comprehension is correct.

O

#### otised

> I honestly don't know why it's set for 1.0 seconds, but I know it works.

Engineering inertia is the reason the time delay on L4Y is set to 1.0 second. Back in the days of Mark I, 4Y was a time delay dropout relay (electromechanical), and I believe 1 second was about the minimum setting on the device. Basically, it's always been that way, it works, why change it?