Ours is Gas turbine frame-5 machine. During start-up the machine can be started in crank/fire/auto mode or directly auto mode. During crank start-up, diesel engine speed is reaching 1800 rpm and slowly reducing to 1500rpm and hence turbine is unable to reach crank speed i.e 920 rpm or 18% of rated speed.

But once machine is started directly in auto mode, diesel engine is reaching 1800 rpm and after warm-up speed is raising to rated speed i.e 2100 rpm and hence turbine is reaching crank speed and machine could be started.

I want to know whether there is any difference between these two start-ups and how?

Looking forward for your reply

Thanks

 

When you start the Frame 5 in Auto, the unit will automatically come up to Cranking (Purging) speed, which seems to be 1500 to 1800 rpm, and purge the unit for a set period of time to get (usually) 4 complete changes of air through the turbine. This is to remove any residual combustible gas or liquid fuel from the turbine and exhaust system (including any waste heat recovery equipment). Then the unit will drop back to Firing Speed (920 rpm, I guess) which is required for proper ignition. Once flame is detected, fuel flow is reduced to warmup level, usually for 1 minute, and then the unit is accelerated to synchronizing speed.

When you start the unit in Crank, it will go to the purge speed and stay there until you change to Fire or Auto (as long as the purge timer has timed out). If you then change to Fire, the unit will Fire, and then stay at Warmup fuel until you change to Auto. When you do that, the unit will accelerate to synchronizing speed.

The Crank and Fire positions are generally only used for test or maintenance purposes; Crank can be used to work out a rotor bow, if ratcheting was interrupted, or to rapidly cool the turbine and exhaust for maintenance purposes, but read your manuals before doing this! Rapid cooling can be bad for the hot gas path parts (the ones that are really expensive to replace).

 

Ravivarma1910,

It would be really helpful if you could tell us which turbine control system is being used on your GE-design Frame 5 heavy duty gas turbine (and if it's SIMPLEX, DUAL, or TMR). I also want to clarify a few things in your post.

Some diesel starting means (NOT ALL diesel starting means) had a "purge" torque setting and a "maximum" torque setting--purge torque being slightly less than maximum torque. This was so that the unit could be CRANKed without using maximum torque from the starting means, and then once fired a slightly higher torque assist from the diesel could be achieved to help with acceleration. Without knowing a LOT more about the unit at your site and how it's configured and without being able to see the configuration and programming of the turbine control system (it's presumed it's a digital turbine control system), all we can do is provide the information below for a typically-configured GE-design heavy duty gas turbine control system and hope it helps with your understanding.

When being started in CRANK mode, the operator is essentially saying to the control system, "I'm not going to fire the machine, so there's no need to be able to admit fuel and energize the ignitors (spark plugs)." CRANK mode is just a means of using the starting means--in your case, a diesel engine working through a torque converter and clutch (jaw clutch or SSS (Shifting, Self-Synchronizing))--to spin the turbine-generator shafts (I'm presuming the gas turbine is driving a generator) either for testing or off-line water washing.

Selecting FIRE mode and initiating a START means the operator is saying to the control system, "I want to admit fuel and establish flame, but nothing more than that." The unit should go through a normal purge sequence/time period, then admit fuel and energize the ignitors, and once flame is detected the fuel will be slightly reduced and held at what's called warm-up value for as long as the unit remains in FIRE mode with an active START. This does mean the unit is not self-sustaining--and requires an active torque assist from the starting means to maintain this low, fired speed.

While there is flame in FIRE mode, the unit will accelerate slowly (both from the starting means torque assist and from the energy developed as the turbine warms up and more power is extracted from the hot gases passing through the turbine section)--but it should not accelerate much past approximately 40-50%, and all the while it remains in FIRE mode it will still be drawing torque from the starting means.

Selecting AUTO mode and initiating a START means the operator is saying to the control system, "I want to start the unit and have it accelerate to FSNL (Full Speed-No Load) in preparation for synchronization. The turbine control system should use the starting means for purging any combustible gases out of the turbine, exhaust and stack (including any HRSG (Heat Recovery Steam Generator)), then admit fuel, energize the ignitors, and once flame is established and after a 1-minute (typically) warm-up period the diesel starting means may be accelerated to full speed/torque power output to assist the unit with accelerating to self-sustaining speed at which point the energy from the fuel being burned will complete the acceleration to FSNL.

Now, it is possible to select CRANK mode and initiate a START and allow the unit to spin under the assist of the diesel starting means. One can then select FIRE mode (that's all that's necessary--just select FIRE mode while still CRANKing), and the unit, if it hasn't already completed a purge it should then go through a purge, after the purge is complete the turbine control system should then admit fuel and energize the ignitors and after flame is established the fuel will be reduced slightly and held at the warm-up value.

Then, one can simply select AUTO mode and the fuel flow-rate will be increased to help the unit accelerate to self-sustaining speed and then to FSNL.

Note that it's NOT possible for an operator to control the unit speed below 95% of rated speed. Once someone selects FIRE and establishes flame, one cannot select CRANK and have the flame extinguished while maintaining speed (still using the starting means). Nor can one select or re-select FIRE after AUTO mode has been selected. If there's flame in the machine and it's desired or necessary to extinguish flame there are only two choices: initiate a STOP, or push the Emergency Stop Push-button.

Hope this helps!

 

Depends on how your diesel control is set up. Usually on older Frame 5s the diesel will have two solenoids, 20DA-1 and 20DA-2. 20DA-1 will bring the engine up to cranking speed when set to "CRANK", 20DA-2 will energize when flame is detected and put the diesel into full speed to help with the acceleration, this will only happen when in "FIRE" or "AUTO".

I think you need to check your diesel governor settings when set to "CRANK" to see why the speed is dropping back to 1500 rpm, sometimes the governor mechanism can be a bit sticky and you are not getting the full diesel speed. When in "CRANK" try pushing the governor linkage and see if the speed changes and you can achieve 14HM.

Hope this helps, please report back.