I have seen this Minimum value gate for controlling GT's and Booster Compressor in couple of CCPP.

Why we are using so? what is the basic theory behind this?

I really don't know why are we using this, I surfed in the net and i could find only one article regarding this, but that was also really not convincing.

Someone here likes of CSA, MKVGUY could help me understand this basic GT and Process control principle.

Thanks in Advance

 

4_20 mA,

The Minimum Value Gate is used when there are two or more possible references for a particular parameter, say fuel control valve positioning.

The concept is that in order to protect the turbine and axial compressor from overload and/or overtemperature (exhaust overtemperature) the "competing" reference with the lowest value is used to determine how much fuel, in this example, is to be admitted to the turbine. This can apply to IGV position/control, and load compressor control, and many other control and protection functions for turbine and driven devices.

Using this logic, when one or more reference values are not to be used in the determination they are usually set to high, or maximum, values to keep them from adversely affecting the determination. Examples for fuel control would be Shutdown Fuel Control and Start-up Fuel Control during normal loaded operation; they are usually "biased" to be much higher than when they should be controlling the fuel valve so that they will not abnormally affect unit operation.

If a reference value is too low (lower than it should be) and becomes the controlling reference, that is deemed to be less injurious to the machine than if an abnormally high reference were being selected for the same parameter. If an abnormally low reference is limiting the machine output it can be investigated and resolved without causing much damage to the machine.

Whereas if an abnormally high value is being used to control some aspect of turbine operation or protection the unit might be at risk while investigating and resolution is being done, and in some cases, the unit might even trip before investigation and resolution might be completed. (Of course, the unit can be tripped on an abnormally low value of controlling reference, too, but not generally.)

By understanding how various control references are determined and how they affect unit operation using a Minimum Value Select gate or function to choose the lesser of multiple possible references for one parameter the unit can be better protected against overload or overtemperature (exhaust overtemperature).

Hope this helps!

Could you share the URL for the other reference you found, please?

 

Hi CSA,

Thanks for the advice. how the load selection and control selection (EGT Temp or Speed/droop control) considered when choosing minimum value?

We can't say NO to your requests, you are helping many people here.

following is the URL,

http://www.pondlucier.com/Company/TIPS/2004/May 2004.pdf

Thanks a lot CSA

 

For digital Speedtronic turbine control systems, as an example, the inputs to the FSR Minimum Value Gate are:<pre>
FSRSU FSR Start-up
FSRACC FSR Acceleration Control
FSRN FSR Speed Control (Droop OR Isochronous)
FSRT FSR Temperature Control (Base, Peak, Peak Reserve, if so equipped)
FSRSD FSR Shutdown
FSRMAN FSR Manual

---------
FSRSU | |
FSRACC | MIN |
FSRN | VALUE |---> FSR
FSRT | GATE |
FSRSD | |
FSRMAN | |
---------</pre>
(NOTE: FSRMIN, FSR Minimum, is unique and well outside the scope of this brief explanation.)

During normal, loaded operation, FSRSU is 100% (because start-up is finished) and FSRSD is also 100% (because the unit is not on fired shutdown during deceleration). FSRACC is is mirroring, but just slightly higher than, FSRN (to protect against sudden load loss, such as generator breaker open event, which would cause the unit to accelerate very quickly). So, the Min Value Gate would see this:<pre>
---------
FSRSU 100.0% | |
FSRACC 44.8% | MIN |
FSRN 44.6% | VALUE |---> FSR 44.6%
FSRT 88.7% | GATE |
FSRSD 100.0% | |
FSRMAN 100.0 | |
---------</pre>
That leaves FSRN and FSRT as the two "competing" values being compared by the Minimum Value Gate. FSRT is always the maximum fuel control reference and is always being calculated based on the relationship between axial compressor discharge pressure and actual exhaust temperature. By definition, it is ALMOST always the higher of FSRN and FSRT under normal loaded operated (more and the ALMOST always bit in a moment).

When the turbine is NOT on Exhaust Temperature Control, it IS on Speed Control (usually Droop Speed Control when it's synchronized to a grid in parallel with other generators and their prime movers), meaning FSRN is ALMOST always less than FSRT (except when the Speedtronic is being commanded to run at Base Load, or the Speedtronic determines it should be running on exhaust temperature control, Base Load). The Min Value Gate would see something like this when the unit is being commanded to, or has reached, exhaust temperature control.<pre>
---------
FSRSU 100.0% | |
FSRACC 64.3% | MIN |
FSRN 64.4% | VALUE |---> FSR 64.0%
FSRT 64.0% | GATE |
FSRSD 100.0% | |
FSRMAN 100.0 | |
---------</pre>
During a unit START in FIRE or AUTO mode (on newer digital Speedtronic systems), FSRSU is used to control the amount of fuel during initial firing and for about a minute after flame is detected. FSRT is 100% and FSRN is usually around 25-100% during initial starting, and FSRACC being calculated based on the actual acceleration rate versus an acceleration rate reference, and the actual acceleration is usually less than the reference so FSRACC is higher than FSRSU. A properly tuned start-up might look like this at about 85% speed:<pre>
---------
FSRSU 100.0% | |
FSRACC 19.4% | MIN |
FSRN 37.9% | VALUE |---> FSR 19.4%
FSRT 90.1% | GATE |
FSRSD 100.0% | |
FSRMAN 100.0 | |
---------</pre>
When the unit is a fired shutdown, decelerating after the generator breaker has opened, a typical shutdown might look like this at about 60% speed:<pre>
---------
FSRSU 100.0% | |
FSRACC 15.9% | MIN |
FSRN 51.7% | VALUE |---> FSR 15.1%
FSRT 100.0% | GATE |
FSRSD 15.1% | |
FSRMAN 100.0 | |
---------</pre>
So, the Minimum Value Gate is just a "comparator" looking at the values of multiple inputs and choosing the lesser of all the inputs and writing that value to its output; in the examples above, the output is FSR.

Hope this helps!

 

The explaination is fine but I have a specific query. During startup when Gas Turbines breaks away at one point of time FSR increases to overcome critical speed and then decreases to continue in fire mode. Now my question is how this is controlled in mark VI? I have checked the Application codes but could not find the satisfying answer.

 

fluidflow,

Please list the events you believe occur during a start from zero speed, including when you think fuel and ignition occurs.

The list doesn't have to be detailed; here's an example:

-- Select AUTO
-- Operator-initiated START
-- Shaft breaks away from zero speed
-- Unit accelerates to purge speed
--
--

Complete the basic list to FSNL.

There is something you perceive to be happening at times when it's not happening. When we know how you perceive start-up and acceleration to FSNL we can provide more assistance.

 

Dear CSA,

thank you for showing interest in my question.
major events up to FSNL in my opinion are (I am mentioning chronologically, but only relevant to my query).

start up check permissive ensured


AOP started

Ratcheting (88HR) starts

Disel Engine (Our prime mover) initiates

14 HR, turbine breaks away

Turbine accelerates to minimum speed(20% of MCS)

Exhaust line is purged

14 HM displayed

Turbine vent timer is out

FSR is raised to 22.5%*, ignition transformer, energised, Firing timer starts

Flame is detected

FSR is lowered to 17.5%* of warmup level

warmup finished after 60s.

FSR is raised to acceleration limit uptill 33.1%*

14HA is displayed

Speed reference is raised

14 HS is displayed

turbine has reached FSNL

startup sequence L3 is completed.

these are the major events in my opinion.

Now after warmup as you can see FSR is raised gradually to incorporate the acceleration. At about 45% of rated speed turbine requires additional torque to overcome Critical speed. this I have calculated from design consideration of rotor. Actually there are two critical speed first comes at 8% of MCS and second at 45% of MCS. (Based on torisional criteria).

Now what I am unable to understand is how my control system Mark VI understands it and how FSR is altered in logic and sequence ladder to achieve it.

Regards
fluidflow

 

Wow, i think it is a good question, and i am also interested.

In DEH, we just increase speed increase rate to overcome the critical speed zone as soon as possible. But in gas turbine, the output of the starting means is constant, and the FSRSU increase rate is constant too before it reaches about 33.4%. How does the gas turbine overcome critical speed?

I am looking forward to master like CSA and someone else can help us.

Best regards!
Neo

 

Neo,

What the heck is a "DEH"?

 

fluidflow,

Pretty good description; but I would be surprised if FSR actually goes to 33% during acceleration. If it does, then something is wrong with the starting means/torque converter or the unit is being accelerated very fast (much faster than it should be) and somehow exhaust temp control is being bypassed--or the fuel system isn't "calibrated" properly, or the fuel doesn't have the heat content it was originally estimated to have had.

But, I have never seen a machine critical speed as low as 8% or even 45%. It's not clear where or how you arrived at those numbers. Critical speeds of a GE-design heavy duty gas turbine are never published by GE or its packagers, and are only "visible" by watching vibration profiles during acceleration. The first critical is usually around 30-40% speed and the second critical is usually around 60-80% speed (it varies depending on the machine)--and it usually doesn't require more fuel to get through the critical speed. GE heavy duty gas turbine control philosophy specifically does NOT allow manual control of turbine speed below 95% just to ensure that the turbine does not ever get "stuck" (paused or stopped) at a critical speed when high vibrations can damage the machine.

Acceleration of a GE-design heavy duty gas turbine with a Mark V, Mark VI or Mark VIe Speedtronic turbine control system after warm-up is complete is done by using an acceleration rate reference and comparing the actual acceleration rate to the acceleration rate reference, adjusting the fuel (FSR) to maintain the desired acceleration rate. If more or less fuel is required to maintain the desired acceleration rate then FSR is varied accordingly.

As torque from the starting means tapers off then usually more fuel is required to maintain the desired acceleration rate. As the IGVs begin to open and air flow through the machine increases then usually a little more fuel is required to maintain the desired acceleration rate.

When there is more torque available from the starting means (during initial acceleration after warm-up) it may not be necessary to increase fuel so much as the acceleration rate may be okay or it may even be excessive--in which case sometimes FSR is reduced to minimum which can also result in loss of flame and high exhaust temperature spreads during starting and acceleration.

But, in my experience I've never seen FSR be increased to counter the effects of critical speeds/high vibration. As long as a GE-design heavy duty gas turbine is accelerating and does not stop at a critical speed during acceleration--which is when the damage from a critical speed can occur--there is no need to worry about the effects of critical speed on the turbine--or on the amount of fuel required to accelerate past the critical speeds.

GE-design heavy duty gas turbines are not like steam turbines or other rotating equipment when it comes to critical speeds. They are usually pretty well balanced and because of GE's control philosophy of not allowing manual control of speed below 95% there is virtually no chance of a GE-design heavy duty gas turbine pausing or stopping at a critical speed during acceleration--presuming that the control parameters are not "adjusted" or forced during starting and acceleration.

The concept of avoiding critical speeds is to prevent damage caused by high vibrations if allowed to operate at such speeds for any period of time. Again, normal GE-design heavy duty gas turbine control philosophy prevents that from occurring by maintaining a programmed acceleration rate which will not allow stopping or pausing at any speed below 95%.

Hope this helps!

 

CSA:

I use DEH to refer to steam turbine control.

I have one question that is related to this topic:
As i know,during startup,the gas turbine is accelerated to FSNL. How is the unit being kept in FSNL state as the govenor mode is not Isochronous Speed Control.And when the unit is in FSNL state,is FSR is determined by FSRN? How does control system detects that the unit reaches FSNL?

As i learned from one thread, FSR is about 18.4% when unit reaches FSNL,but FSRSU is increased to about 33.4% before L52GX is logic 1. So when does the FSRN take charge of FSR during startup,and when does FSRSU lose control of FSR during startup.

What should i read application code if i want to figure out this question?

May be it is a poor question.

Best regards!
Neo

 

Neo,

I believe the 33.4% value you are referring to is an upper limit on acceleration control--not the actual value of FSR achieved during acceleration (at least I hope it's not the actual value achieved during acceleration control!).

Just because the steam turbine governor has to be in Isoch when the breaker is open to control speed does not mean the Mark VI has to be in Isoch when the breaker is open to control the gas turbine speed. Speedtronic Droop Speed Control has very little "droop" when the breaker is open, and only exhibits droop when the breaker is closed. Don't ass-u-me that just because one turbine governor does things one way that all turbine governors should do them the same way--because you're going to find out there are many ways to accomplish the same task(s).

Finding out how the Speedtronic shifts from acceleration control to Droop Speed Control during start-up is very difficult to explain. It has to do with Complete Sequence and speed level, and it's never been 100% clear to me. Early digital Speedtronic turbine control systems weren't so good at the transition; newer ones are better, but the transition, while smooth, is very fast and difficult to detect.

 

Sir,

I checked it with my instrument department and then came to know that max. FSR achieved upto FSNL is 20%. For our GT, FSKRN1 is 20.1%, which is a constant. What I wrote earlier was 33% which is told to Plant operators and other maintenance staff. I regret to post an erroneous value.

Here at our plant site its very difficult to open tool box or Control logic as they are password protected and only a Senior Instrument Engineer know it.

On the other hand as far as critical speed is concerned, still the dilemma persists as now after much debate and brainstorming over it with my seniors and colleagues the value arrived is more or less the same.

However my seniors suggested to calculate the critical speed based on FEA (Finite Element Analysis) instead of Torsion criteria only as this take in account of mechanical aspects and many stress concentration points are ignored as good approximation.

Here are the results

Turbine base output: 24080KW
(ISO Rating)

At plant site (46°C, 60% RH): 18000 KW

MCS: 5355 rpm
OST: 5610 rpm
Critical speeds:
1. 900- 1000 rpm
2. 2300- 2600 rpm
3. 5500- 6000 rpm

Result is arrived with taking in account MOC of shaft as Cr-Mo-V steel. Structure is assumed as Overhanging Beam (Statically Indeterminate).

I remember that this is not a forum for discussion of such Numerical problems, but I really find it hard to explain things in lucid terms as you do with your usual aplomb. Perhaps, I am still figuring out the GAS turbines. I have just recently started or rather forced to learn Automation, leaving my comfort zone. So, I submit my apologies in advance as I know mistakes will happen again and again.

Regards,
fluidflow

 

fluidflow,

Yes; it's unfortunate that too many plants do not allow interested parties to open Toolbox (or ToolboxST) and poke around and learn. Many sites mark a Toolbox (or ToolboxST) file as 'Read-only' which makes making unintentional changes impossible. Combined with proper password administration this allows people to investigate, learn and become competent individuals. Alas, such is life, eh?

I don't quite understand why we are having any discussion around critical speeds. For most GE-design heavy duty gas turbines the subject is a moot point because they are well-balanced and the control system prevents pausing/stopping at a critical speed. The exact value of the critical speeds is therefore irrelevant and does nothing to add to our understanding of minimum value gates, FSR, or GE-design heavy duty gas turbine control philosophy and operation. On a properly operated unit, there is virtually no chance of encountering a critical speed or of any damage resulting from prolonged operation at that speed.

Any equipment that has the possibility of prolonged operation at critical speed(s) has instructions to alert conscious operators about the hazards, and any plant with such equipment should have written, standard operating procedures to prevent inadvertent prolonged operation at critical speed(s). It's not the speed level that's harmful, it's what happens when the equipment operates at or near that speed level for prolonged periods--that's why it's important to accelerate through the critical speed(s) quickly, to avoid harmonics which can lead to catastrophic damage.

So, you can continue to debate the exact value of any critical speed--but you need to take into account any load gear, coupling, and generator rotor influence on those speed(s). And while you might be able to pretty accurately calculate and predict a critical speed: What does that do to help understand minimum value gates and FSR and GE-design heavy duty gas turbine operation?

I submit that neither accurately calculating nor even definitively knowing the exact critical speeds of any GE-design heavy duty gas turbine adds nothing to the knowledge required to properly operate and control and protect a GE-design heavy duty gas turbine under virtually any circumstance.

And if you were referring to me (CSA) as 'Sir'--thank you, but that would be too formal. I put my pant legs on one at a time, and work for a living just like most people here following and contributing to this forum.