Today is...
Wednesday, May 24, 2017
Welcome to Control.com, the global online
community of automation professionals.
Featured Video...
Featured Video
Watch an animation of a conveyor stacking operation demonstrating the use of a move on a gear command.
Our Advertisers
Help keep our servers running...
Patronize our advertisers!
Visit our Post Archive
GT Starting Means
want to understand detailed operation of starting means

I am new gas turbine frame 6B.
Can anybody please provide detail functioning of scheme for starting means?

If it has been provided already in portal, kindly provide the link. i could not get the complete information of starting means all together on portal. We use Diesel Engine as starter.

0 out of 1 members thought this post was helpful...

khoriwal000,
So, the best place to look for information about GE-design heavy duty gas turbines is in the Operation and Service Manuals provided with the unit. There are sections for each of the unit's systems, including the Starting Means, which have a few pages which briefly describe the intent of the system and its components. I qualify 'intent' because GE typically writes generic system descriptions and supplies them with unit-specific manuals--and often the systems may vary slightly from what was written in the generic description.

If you--or anyone--want to learn about GE-design heavy duty gas turbines, the very best place to start is to get your hands on a set of the Manuals and make copies of, 1) the System Descriptions, and, 2) the P&IDs. Make the largest size copy of the P&IDs that you can--because as you read and learn about the systems over time, you should be making notes on the P&IDs for future reference, notes that help you make sense of the components and the symbols. Lately, the manuals have also been provided as a .pdf file on DVDs, and if you can get your hands on the DVD you can load it on a computer connected to a printer and make copies also. (It's a really GREAT idea (HINT! HINT!) to copy the DVD contents to the HMI, so that operators and technicians and operations supervisors can easily refer to the manuals when using the HMI.)

There should also be a document called the "Device Summary" which describes the settings for the various devices provided with the turbine and auxiliaries (NOT usually including the generator protection devices). This can be used to make notes next to the devices on the P&IDs with the settings of the devices (such as, INC 11 barg, etc.).

Units which were provided by GE USA usually had a document buried somewhere in the Operations and Service Manuals called the "Piping Symbols" drawing, which was a "key" or "legend" for the symbols on the P&IDs.

The starting means (be it a diesel engine, or an electric induction motor, or an expander turbine) is necessary because one can't just admit fuel into the combustor while the turbine is at zero speed, ignite the fuel and let the flow of hot gases through the turbine section accelerate the unit from zero speed. There has to be some method of breaking the unit away from zero speed (which usually requires a LOT of force due to the weight of the shafts (compressor, turbine and generator) and the design of the bearings (journal bearings, and/or floating pad bearings).

And, because the starting means will usually operate at a constant speed when developing the maximum amount of torque to be applied to the turbine/generator shaft and because the shaft will start from zero speed and accelerate to purge speed and firing speed and then have to accelerate to rated speed (Full Speed-No Load, FSNL) there needs to be some kind of "variable" torque transmission device between the starting means and the turbine/generator shaft. That device is most commonly a torque converter, which is a hydraulic pump (driven by the starting means) and a hydraulic motor (which is connected to the turbine/generator shaft). So, the output torque/speed is different from the input torque/speed which is how the starting means runs at constant speed(s) and the turbine/generator shaft can rotate at speeds from zero to approximately 60% with the assist of the starting means.

It's important to note that GE usually considers the cooldown mechanism (usually the "hydraulic ratchet" on a Frame 6B unit) as part of the starting means. It is necessary to prevent the shaft from bowing after a shutdown or trip when the shaft is hot, and it's also necessary to help break the shaft away from zero speed during starting--the starting means (even a diesel engine or an induction electric motor) can't usually break the shaft of a Frame 6B away from zero speed without the assist of the hydraulic ratchet.

When a diesel engine is used as a starting means, it's usually for one of two reasons. Either the grid can't provide sufficient electrical power to operate an induction electric motor as the turbine starting means, or, there is a perceived need for the turbine to start completely independently of the grid--what's commonly referred to as "black start" (when the power plant is "black" because there is NO power from the grid for lights or motors). It could also be a combination of the two that necessitates the need for a diesel engine starting means.

During starting, the diesel is usually started and allowed to "warm up" for a couple of minutes at a low "idle" speed. And, then after the diesel engine warm-up is complete the engine is accelerated to a much higher speed which causes the torque output to increase significantly. The hydraulic ratchet is also energized to help get the shaft turning (even if it's just a fraction of a turn--it still helps overcome the inertia of the rotor at rest!) and the torque being transmitted through the torque converter (from the diesel engine) helps to accelerate the unit to purging speed (sometimes called "cranking speed" which isn't always exactly correct depending on the unit and the starting sequence).

After the purge is complete (purging is done to ensure there are no combustible (explosive) gases in the combustors and the turbine and the exhaust prior to energizing the spark plugs/ignitors to establish flame), fuel is then admitted and flame is established. After the combustors and hot gas section of the turbine is allowed to warm up for approximately 1 minute, the fuel is increased to help with accelerating the unit--but all the time during purging and firing and turbine warm-up the starting means is still providing torque to the turbine-generator shaft to maintain or even slightly increase speed. There is not enough torque from the hot gases even after flame is established to accelerate the turbine-generator without the assist from (the torque output) of the starting means.

Usually, at about 60% speed the torque output of the starting means of a Frame 6B unit is no longer required to help accelerate the unit to rated speed. This is called "self-sustaining speed", when the unit can produce enough torque to accelerate itself without any assist from the starting means. There is usually some kind of "clutch" which is used to couple the output of the torque converter to the turbine/generator shaft during starting/acceleration. When the unit reaches self-sustaining speed, the clutch disengages, and that's usually the signal to shut down the starting means. For a diesel engine starter, the fuel to the diesel engine is usually reduced to the "idle" speed and the diesel goes into what's called it's "cooldown" for a minute or a few minutes (depending on the vintage of the machine).

Again, when the unit reaches zero speed during a normal shutdown or after an emergency trip it's important to not let the axial compressor shaft sit for very long in the same position--it will begin to sag or bow under its own weight. So, the hydraulic ratchet mechanism periodically (usually about every three minutes) rotates the turbine/generator shaft approximately 45 degrees (one-eighth of a turn) to prevent the shaft from bowing. This has to go on for about 24 hours (sometimes less; sometimes more) before cooldown can be turned off (usually by an operator) and the auxiliary L.O. pump can be shut down. (The Auxiliary L.O. pump must continue to run during cooldown operation to provide cooling oil flow to the bearings AND to provide lubrication for the ratchet mechanism when it's turning the shaft every three minutes.)

Hope this helps!

Get your hands on the manuals and/of the DVD of the manuals--and make your own copies of the documents listed above. It's the ONLY way to learn how GE gas turbines operate. If you have questions, we can try to answer them, but you NEED those P&IDs to help with your understanding. And, the System Descriptions, while not always 100% unit-specific, provide a LOT of useful information about how the systems are/were intended to work and some of the components of the systems. If you--or anyone--is serious about learning how GE-design heavy duty gas turbines operate the educational journey starts with your own copies of the P&IDs and the System Descriptions and builds from there. It doesn't begin with the HMI, or the Mark IV, or Mark V, or Mark VI or Mark VIe. It doesn't matter what kind of operator interface or control system is used--they still have to control and operate and monitor the devices and components on the P&IDs. So, get thee a copy of the P&IDs, make notes, and then make sure you can go out and find every device on them (okay; some of the devices are located in the L.O. Tank and aren't accessible--but one should still know where they are, and if possible, have put eyes on them). Again, it doesn't make any difference what kind of operator interface (HMI) or control system (Mark* or PLC)--they all have to use the systems and devices and components on the P&IDs. And, the P&Ids are almost NEVER generic--they are supposed to be unit-specific and almost always are. (Many systems on GE-design heavy duty gas turbines are similar--but the P&IDs are ALWAYS supposed to be unit-specific, while the System Descriptions are almost NEVER unit-specific, but are still the best place to develop one's understanding of what the systems are supposed to do and in some cases how they do it.)

We can answer most questions about GE-design heavy duty gas turbines and operation here on control.com--but we're not very good with assuaging doubts. (Look up the definition of 'doubt' in your Oxford's English Dictionary; it's not an appropriate word for questions or when clarity is required or being requested.)

dear CSA,

i have gone through all the P and IDs and understood also with the help of manual. but starting means part of manual is missing. description you provided is basic operation.

can your please help me understand or get me some explanation of "scheme for starting means" in frame 6B machines with diesel engine?
I am confused with the operation of electrical operated valves included in this scheme, i.e. 20CS, 20DV, 20DA1, 2, etc.

if you have explanation regarding this in your manual, kindly mail it to me at following mail ID:
himanshukhoriwal00@gmail.com

khoriwal000,

I don't have any manuals myself, and if a portion of the Manuals is missing one should contact the packager of the equipment to obtain the missing section.

I don't have access at this writing to provide exact details of what the various solenoids do, but from the information provided it would seem the unit is an older one because many of the newer units with diesel starting means use digital electronic controls for the diesel, which get very limited signals from the Mark* turbine control system.

If you have the P&ID for the Starting Means, it shouldn't be too hard to sort out what each component does. There is a solenoid that controls a hydraulically-operated fuel rack (the hydraulic system is self-contained on the diesel and is NOT part of the turbine hydraulic system), and some units have a second solenoid on the fuel rack that increases diesel speed/torque when accelerating after flame has been established (I believe these are usually designated as 20DA-1 and 20DA-2, respectively). There is also usually a solenoid that shuts off the diesel by forcing the fuel rack to the zero or very low flow position (20DS?).

20CS is usually the solenoid that is used to engage a hydraulically-operated jaw clutch so that torque can be transmitted from the starting means and torque converter to the turbine/generator shaft. During ratcheting 20CS is usually energized, and after the turbine/generator shaft breaks away from zero speed during starting 20CS is usually de-energized--the shape of the jaw clutch teeth keep it engaged as long as the speed and torque from the starting means is providing an assist to the turbine/generator shaft during starting and acceleration. There is usually a limit switch to sense when the jaw clutch halves are engaged (33CS); the switch contacts should be closed when the jaw clutch is engage and open when the jaw clutch is disengaging or is disengaged.

Once the turbine/generator shaft speed is above the speed of the output shaft of the torque converter the jaw clutch teeth disengage and a very large spring on the clutch mechanism fully opens the jaw clutch. The jaw clutch engagement limit switch contact should open as the jaw clutch is disengaging and this tells the Mark* that the turbine/generator shaft is now or should now be capable of sustaining its own speed by virtue of the expansion of hot gases through the turbine section without the need for any torque assist from the starting means.

Usually, the diesel starting motor runs for a minute or two or sometimes longer at idle speed to cool the engine and then the Mark* energizes the diesel stop solenoid to move the fuel rack to the position that shuts off fuel and stops the engine.

Most of the problems with the starting means systems are the result of 1) problems with the diesel not providing sufficient torque during starting to get the unit to self-sustaining speed; 2) misadjustment of the jaw clutch limit switch, and, 3) torque converter problems. The diesel engine problems can be and are usually related to poor or inadequate maintenance of the diesel and its fuel system (injectors; fuel pump; filters). Also, diesels do wear out--specifically the rings and valves, and when they wear out the diesels don't make enough torque to get the unit accelerated to self-sustaining speed. Dirty fuel filters can limit fuel flow, also.

When the diesel isn't making enough torque during starting and acceleration the jaw clutch will disengage early which can cause the turbine-generator shaft speed to "stall" or even decrease. Sometimes the newer Mark* control systems will try to increase fuel to maintain acceleration--but because the air flow through the machine is very low during low-speed operation and because the IGVs are also closed the exhaust temperature is high and the exhaust temperature limit may be reached which will limit fuel flow which can also cause speed to stall or even decrease.

The speed control mechanism which relies on solenoid signals from the Mark* can also be misadjusted after diesel maintenance, or because the locking screws on the shafts loosen and slide. Unfortunately, there is no guide or specification for the hydraulically-operated fuel rack mechanism to dictate what speeds the diesel should operate at for each of the solenoid settings. That's why it's very critical for maintenance personnel to work with controls technicians to measure and record the diesel speed at the various settings when the diesel is working correctly so that the settings can be checked or adjusted properly in the future. Some diesel engine service companies have never encountered a mechanism like the one used on GE-design heavy duty gas turbine diesel engine starters (before the advent of digital engine management systems), and so adjusting them is not very precise--especially when the owner can't provide proper specifications (because they usually weren't supplied by the equipment packager). Again, this is where is critically important to check and record as-running settings when the diesel is in a new and clean condition.

The torque converters can also become worn and not transmit sufficient torque even if the starting means is working fine. The results will be the same: early disengagement of the jaw clutch and stalling or even decreasing turbine/generator shaft speed.

The jaw clutch limit switch can vibrate out of adjustment, or can be misadjusted during a maintenance outage during reassembly. Again, if the Mark* was configured correctly during commissioning the contacts of 33CS are to be actuated and closed when the jaw clutch is engaged, and should deactuate and open as the jaw clutch is opening. The discrete (contact) input L33CSE is normally NOT inverted, and is a logic "1" when the jaw clutch is engaged, and a logic "0" as the jaw clutch is disengaging and when it is disengaged. (That's how the limit switch and input should normally be configured for proper operation.)

If you're having problems with the starting means, perhaps if you can describe the problems we can try to help. If some other reader can scan and email you a copy of their GE-design Frame 6B heavy duty gas turbine Operation Manual 'Starting Means' section, that would be great (I don't have a copy at this writing).

Hope this helps!