We have two frame-5 gas turbines powering a fertilizer plant. It is common belief not to run GT on FSNL for a longer period of time. Is it a myth or it has some consequences?

 

There is no problem with running the machine at FSNL for as long as you want. The machine is operating under the "speed" control algorithm as it will be under most load conditions other than when it is on temperature control. Some may complain about the cost of burning fuel with no power output, but there is no detrimental effect to the machine by operating it at FSNL.

 

adeel imtiaz,

I agree wholeheartedly with MIKEVI--there is no problem running the unit at FSNL for "long" periods of time.

I might suggest, though, that the unit be synchronized and lightly loaded (1-3 MW)--this way some electrical power is being produced to offset the cost of fuel (somewhat, anyway!), and the exhaust temp won't be too high. (I presume the reason the unit is operated at FSNL for long periods of time is that there is no need for steam--but that's just a SWAG (Scientific Wild-Arsed Guess).)

About the only reason this might not be a good idea is if the grid frequency is unreliable and fluctuating then the unit might intermittently trip on reverse power....

If this is tried, I would highly recommend <b>NOT</b> using Pre-Selected Load Control to hold 1-3 MW. At such a light load, if it's not tuned properly then it might cause exaggerated MW swings. But, if the grid frequency isn't too unstable it might even help the grid (a little bit). It surprises most people to know that the unit will hold load just fine, thank you very much, without Pre-Selected Load Control. Droop Speed Control is proportional to the error between the Turbine Speed Reference (TNR) and the Actual Turbine Speed (TNH). As long as TNH is pretty stable (meaning grid frequency is pretty stable), then when the operator uses the RAISE- and LOWER SPEED/LOAD buttons to get to some load then TNR will not be changing--so the error between TNR and TNH will not be changing, and therefore the fuel (FSRN) will not be changing.

And, when TNH does change because grid frequency is unstable, then the unit will respond as it should--increasing load when the grid frequency drops, and decreasing load as the grid frequency increases. (That's the benefit of Droop Speed Control--and when Pre-Selected Load Control is enabled and active it causes the Speedtronic to do just the <b>opposite</b> of what it should do. A little gift courtesy of the Speedtronic designers!). Now, that may mean the load is not stable--but it's NOT SUPPOSED to be stable during grid frequency excursions. It's supposed to vary in order to try to help support the load and grid frequency (also, contrary to popular belief). I wager that if you asked any grid regulator they would definitely say the load should vary when grid frequency is not stable because it helps to support grid frequency excursions and therefore helps to stabilize the grid. Pre-Selected Load Control does just the opposite--and it's only because people use it when it's not really necessary. It's great for changing loads automatically, but when the desired load is reached then the operator should cancel Pre-Selected Load Control and just let the unit run on Droop Speed Control.

(Just as an aside--it's also <b>NOT</b> good to use a Pre-Selected Load setpoint above Base Load to load the unit to Base Load. That causes the unit to take much longer to lower load when it is desired to do so, either with a lower Pre-Selected Load Control setpoint, or with a STOP.)

Anyway, some ideas and some tips which may prove helpful. (Though I've never succeeded in convincing an operator or operations supervisor to not use Pre-Selected Load Control, it doesn't mean I won't stop trying! Except for a certain poster, I refuse to give up.)

 

I would agree with this. One thing I had heard many years ago was that the unit should be loaded slightly-the reason given was that with out some excitation being applied, the brushes tended to glaze faster(esp the rotor grounding brushes) I have no experience to validate this--maybe an old operator's Tale.

I was involved in some 7FA compressor tuning several yrs ago, an GE didn't want the unit at FSNL for any longer than necessary for the testing--but then again, the 7FA's are delicate little things--nowhere near the robustness of a Frame 5 or 7B.

 

sggoat,

GE-design F-class heavy duty gas turbines are, indeed, delicate "little" machines. They were designed for base load operation; day-in, day-out base load operation. They were not designed for light loads, or even for load following applications, or for daily--or sometimes, twice daily--cycling. The power generation business changed during the time the F-class machines were being designed and originally produced--greatly--and is still undergoing quite a lot of change, requiring gas turbine-based power plants to be operated much differently than power plants were operated even 25 years ago.

And, B/E-class GE-design heavy duty gas turbines have MUCH more margin built into them--they can take a licking and keep on ticking (meaning, they are robust machines and can handle just about any operating conditions and just laugh off most abuse--the exception being 9E units with DLN-I combustors which seem to be the recipient of huge amounts of abuse and misoperation, and yet, other than combustion hardware problems, just keep on running in spite of the abuse they continue to receive).

Most GE-design heavy duty gas turbines have their exciters energized some time during acceleration; some earlier than others. But most have their exciters energized during FSNL operation, and that doesn't really cause any problems because the brushes are not transmitting that much power (for brush-ed synchronous generators). My experience has been that some machines have a rather difficult time maintaining speed/load when lightly loaded because the fuel control valves are at the bottom end of the controlling range of operation (they are not open very far). Older valves which are worn seem to have difficulty with FSNL/low loads, oscillating (hunting). Also, some liquid fuel systems have difficulty with stable liquid fuel supply pressure during acceleration/FSNL/light load conditions, causing some oscillating and instability.

But, if a little oscillation at FSNL is acceptable, or if a little load oscillation is acceptable at low loads it would seem to be more "economical" to synchronize the unit and either sell some power or offset the purchase of some power while the unit is operating. Remember--it usually takes about 15-20% of maximum rated fuel flow just to maintain FSNL (that axial compressor requires a lot of energy to compress the air used for combustion).

 

CSA--

You are certainly correct

The 'robustness'(if that is a word) has dwindled. I remember pulling apart a 7B for a Major, and finding two 1st stage buckets missing-sheared off at the root--about 60 degrees apart. The unit never exhibited any indications of this- no vibration- no nothing, in fact, it was one of the better running units out of the 12--we never found any evidence of the failure downstream except the normal pitting and dings---go figure.

 

very nice contribution everybody has made. CSA i really appreciate your explanatory contributions. I like all the comments. They have really been of help to me and has served as an eye opener.

 

A very nice contribution by you CSA and everyone in this forum.

Thank you very much.