During shut down of the gas turbines the shaft is turned by the turning gear at low speed. This turning gear Permissive on GE Make turbine at Zero speed only. But Siemens make Gas turbine which is started from 120 rpm.

Please any one could clarify, why the GE Make turbines are started at 0 rpm. is there any reason?.

 

Saravana

There are lots of cooldown mechanisms in use on GE-design heavy duty gas turbines. There are turning gears (AC electric motors with gear mechanisms to keep the shaft turning slowly at several RPM while the axial compressor rotor is cooling); there are slow-roll mechanisms that use Aux. L.O. Pump Discharge pressure/flow through the starting means torque converter to maintain 30-50 RPM (nominally) during cooldown; there are hydraulic ratchet mechanisms that rotate the turbine shaft approximately 45 degrees (1/8th of a revolution) approximately every three minutes; and others, as well.

What kind of cooldown mechanism does the turbine-generator at your site have?

The starting speed is characteristically defined by the type of cooldown mechanism, so if you can tell us what kind of cooldown mechanism you have we can tell you more about why and how the starting speed permissive is configured.

 

Saravana,

I neglected to ask what kind of GE-design gas turbines you are referring to--and what kind of Siemens gas turbines you are referring to.

But, I may be able to answer your question about why some (NOT ALL) GE-design heavy duty gas turbines are not permitted to start above zero speed. It's usually because of the type of starting means (including a torque converter) and how the starting means is coupled to the turbine shaft. GE and their packagers quite frequently employ jaw clutches to couple the starting means to the turbine shaft. In this case, it will damage the jaw clutch halves/teeth if the turbine shaft is spinning at any speed above zero speed and the clutch mechanism is engaged.

GE and their packagers do offer a SSS (Shifting, Self-Synchronizing) clutch as an option for replacing jaw clutches and permitting starts at speeds above zero speed (sometimes called a "flying start").

So, the answer to the "GE part" of the question is most likely because of the type of coupling used between the starting means (electric motor and torque converter; diesel engine and torque converter; expander turbine; etc.) and the turbine shaft and not damaging the clutch/coupling mechanism during "flying starts."

Hope this helps!

 

Saravana,

We believe GE-9E too permit cool down sequence at 120 RPM but in GE-6B is not. Because 6B uses ratchet mechanism whereas in GE-9E equipped with turning gear motor.

Take care
G.Rajesh

 

thanks for your reply.

we have two GE make Frame 6B machine's and one Frame 9E Gas turbine and also one GE Make Non reheat condensing mixed pressure impulse Steam Turbine. these all GE make machine's are turning Gear started permissive at 0 RPM only.

I can agree your explanation of Gas turbine systems. but the same philosophy mismatching with Steam turbine. don't have starting means system on STG. don't have SSS Clutch or torque convertor also. but Siemens make V94.3 and V94.2 series Gas Turbine Turning Gear Started at 120 RPM.

 

Saravana,

Your statement/question is baffling.

If you want to know why some machines can only be started at zero RPM and others can only be started from some higher RPM, then you need to analyze the types of starting means (if any, because you are lumping steam turbines and gas turbines together in your statement/question) and how they would be engaged and the effects of engaging them at some other RPM than the manufacturer has specified.

Other than that, even the same turbine by the same manufacturer may be equipped--for one reason or another--with different types of starting means and engagement mechanisms and so have different starting procedures, and starting speed (including zero speed).

Some gas turbine buyers order a Frame 6B with a diesel engine starter; others with an electric motor starter; others an expander turbine starter (which may, use, for example, the pressure from compressed natural gas from a nearby pipeline as the energy for the expander turbine).

As was suggested, a different type of clutch can be substituted on an existing turbine for a jaw clutch--which must be engaged at zero speed--to allow starts at speeds above zero speed. If you want to have all the turbines at your site start from the same speed, all you need is time and money and someone or some firm can make it happen for you.

It's simply not reasonable to think that all turbines would be started in the same way, using the same equipment or procedures--even from the same manufacturer. One can't just put fuel in a gas turbine while it's at zero speed and energize the ignitors ("spark plugs") and accelerate it to rated speed and load. It requires some means of breaking the shaft away from zero speed, purging the turbine of combustible gases before admitting fuel and energizing the ignitors, and even to provide a torque assist during starting.

Gas turbines have to provide torque to the axial compressor which is a huge load and weight to accelerate.

Steam turbines, on the other hand, are considerably lighter machines with no axial compressor to drive. Steam (at the proper temperature and pressure) is admitted to the steam turbine to accelerate it to rated speed and load. They don't require any purging of combustible gases; they don't require a means of breaking the shaft away from zero speed other than the admission of steam through the stop/control valve(s); they don't require a torque converter or engagement mechanism (clutch) because steam is used for breaking the shaft away from zero speed and for a torque assist during acceleration.

Steam does all of that, and, again, because there's no axial compressor required to run a steam turbine there's no need for torque to help accelerate that huge mechanical load.

When one uses an electric motor or diesel motor to break a gas turbine shaft away from zero speed and accelerate it to purging and firing speed and provide a torque assist to accelerate towards rated speed the shaft of the electric motor or diesel engine cannot be coupled directly to the turbine shaft. A means of converting the speed and torque of the starting means (electric motor or diesel engine) which is usually running at a constant speed while producing torque is required because the turbine shaft starts at, basically, zero speed (for a Frame 6B for example) and it's speed changes (increases) as the starting process is under way.

So, a (hydraulic) torque converter is the means of coupling a starting means running at a constant speed to the variable speed of the turbine shaft, using a clutch of some kind to disengage the starting means (and torque converter) when they are no longer required.

Again, it's unreasonable to think every turbine will be started by the same means or procedure--even turbines manufactured by the same company, much less turbines manufactured by different companies.

If you still have a question, or a clear statement, we're here to try to help. But, please--clarify your need or your question or your statement. And do so after taking into consideration the differences between equipment. A turbine is not a turbine. Heck, all Frame 6Bs are not exactly the same (contrary to popular opinion). Even Frame 9Es are different (shocking, but true.)

Hope this helps!

 

There are many types of turning gear/cooldown control operations for GE like CSA said. When the GE 7FA gas turbines at my facility shutdown, cooldown control turns on when the low speed relay picks up (42RPMs) on the coast down, engaging the turning gear. This gives us time to work on the turning gear assembly if an alarm comes in during the engaging process. As the turbines RPMs roll down the turning gear is running and keeps the turbine at 6RPMs. This is pretty common with the 7FA gas turbines.

Other types of turbines i have seen utilize a rotor barring system to push a racheting fork against the turbines shaft to keep the turbine just above 0RPMs (if the turbine is not windmilling already).

 

Hi CSA,

Very well explained as usual. I worked with GE 9E engines which uses a TG motor, a cranking motor, and a TC with an AUMA Motorized actuator. additionally to engage and disengage (supply oil and drain into TC) a Directional valve controlled by a solenoid. My Question is how the TG and Cranking motor coupled to the TC? How do both the motor independently transmit the power to TC (I don't know how Cranking motor is idle while TG motor rotates at 120 RPM, Vise verse)? What is the mechanism between TG and Cranking motor?

Could you please advise.
Thank You

 

4_20 mA,

The answer to your question can be found on the Starting Means P&ID.

I have never seen a GE-design Frame 9E with a turning gear--which I define as an AC motor-driven gear mechanism that rotates the shaft at a constant speed when on Cooldown. I've only seen ratchet mechanisms, and "slowroll" mechanisms. Slowroll is accomplished by porting a portion of the Aux. L.O. Pump discharge (using a solenoid-operated valve, 20TU-2) through the "motor" part of the torque converter with the Torque Adjustor Mechanism (usually an Auma or Siemens device) to maintain 30-50 RPM.

In the above slowroll configuration the Starting Motor is directly coupled to the "pump" part of the torque converter and the torque converter is directly coupled to the Accessory Gear Box/turbine-generator shaft (no clutches on either side of the torque converter).

I have been told that some Frame 9Es have a Turning Gear coupled to the collector end of the generator using a SSS (Shifting, Self-Synchronizing) clutch.

Again, the answer to the question can be found on the Starting Means P&ID.

The answers to <b>MANY</b> similar questions can be found on the gas turbine P&IDs. The P&IDs are <b>critical</b> to understanding and troubleshooting GE-design heavy duty gas turbines. I am completely dumbfounded when I ask for the P&IDs in a gas turbine- or combined cycle power plant only to find everyone staring back at me with the "deer-in-the-headlight look." (For those of you who have never driven an automobile at night and encountered a deer on the road, the deer's eyes suddenly open very wide in fear, shock and amazement, and they are unable to move--making a collision nearly unavoidable. The expression 'deer-in-the-headlights look' refers to an expression of complete and utter lack of understanding and comprehension--and is usually accompanied by a lack of movement because of a lack of knowledge of what to do next, or where to turn for the answer to the question.) Without having a working knowledge of the P&IDs, and without ready access to them in the control room, a <b>LOT</b> of SWAGs (Silly Wild-Arsed Guesses) are made by many operators, technicians, and Plant Managers.

The really good news about GE P&IDs is that they are extremely similar for most GE-design heavy duty gas turbines. Familiarizing oneself with one set of P&IDs will help one if they change jobs to another plant with GE-design heavy duty gas turbines. And, most people who do take the time to familiarize themselves with GE P&IDs usually have multiple "Aha!" moments when things become clear or misperceptions (and guesses) are cleared up with knowledge and understanding that comes from reviewing and understanding the P&IDs. <b>EVERY SINGLE POWER PLANT</b>, regardless of the type of prime mover, should have a laminated (plastic-covered) set of P&IDs in the control room. Any self-respecting operator or technician should have a personal bound set of P&IDs with notes they have made on them while familiarizing themselves with the P&IDs.

 

Just a minor clarification here on P&ID's for GE gas turbines. What the rest of the world call P&ID's, GE calls Piping Schematics. This is mainly because the drawing format does not follow industry standards for P&ID's.

 

Hi CSA,
Good day , the configuration which you were described is exactly same as the machine which I worked with. there is no ratcheting mechanisms. there are FLEXIBLE COUPLINGS (FC). FC are there either side of torque converter, either side of 88CR and Driven end of 88TG (AC Motor to rotate the GT and compressor during the cool down. yes there is NO so called Turning Gear ).

May I ask you how the coupling and decoupling of 88CR and 88TG according to the required operation of gas turbine?

Yes this Torque converter speed is adjusted by a AUMA motorized valve and the filling and draining of the TC is done by a solenoid that you have mentioned.

Your advise is much appreciated !Thank you

Regards
4-20 mA

 

We have Siemens Westinghouse gas turbines W251B11 of 50 MW capacity which are operated on mix fuel (LBTU + natural gas). We have operated the machine on turning gear for 3 ~ 4 days before putting on it on standby conditions due to fuel shortage.

Is there any further requirement for turning gear operation if we do not start the gas turbine for next few weeks or even for more time? What will be the minimum time to operate on a turning gear before startup of gas turbine so that bearing vibrations remain normal during startup? We don't have any electricity monitor for turbine shaft.