Trying to start up a turbine after a long while. Turbine fails to reach FSNL always stops at 92-95 TNH. the turbine is a frame V GE nuovo pignone gas turbine.

 

How about some more information. Alarms would be useful. fuel type, control system?

 

Trying to start up a turbine after a long while and the turbine fails to reach FSNL always stops at 92-95 TNH could be due to several reasons. My first thought would be to ask if you have performed a fuel valve calibration. Was any alarms associated with the unit stopping at that speed level? Did the unit light off properly? Has anyone changed FSR constants for FSNL operation?

 

Let's take a guess that this is happening when 14HM Speed Relay picks up.

1) Inlet Guide Vane problem?
2) Aux/Main Lube Oil problem?
3) Aux/Main Hydraulic Oil Problem?

Please write back and we can help.

 

Check P2 pressure. Speed/Ratio Valve Calibration.

>Trying to start up a turbine after a long while. Turbine
>fails to reach FSNL always stops at 92-95 TNH. the turbine
>is a frame V GE nuovo pignone gas turbine.

 

> Check P2 pressure. Speed/Ratio Valve Calibration.

Sorry but P2 pressure is not going to stop you getting to FSNL. If it is, you are going to get a "Loss of flame " alarm. Definitely not SRV Calibration.

Remember, you need very little fuel to get to FSNL

 

glenmorangie,

I wholeheartedly agree it's not a problem with P2 pressure. SRV control and SRV LVDT calibration aren't likely to be the cause--it's most likely a problem with the fuel supply--plugged strainers, low fuel supply pressure, something like these are most likely the cause. And that goes for liquid fuel as well. Though as you noted, the original poster didn't provide what fuel was being burned during these start attempts.

> Remember, you need very little fuel to get to FSNL

However, the average GE-design heavy duty gas turbine usually reaches FSNL at approximately 17-20% FSR (based on factory calculations using expected fuel parameters and gas valve characteristics). Base load is usually achieved at approximately 70-75% FSR, so it requires approximately one quarter of rated fuel file-rate to achieve FSNL (18.5% is approximately 25% of 72.5%--approximately). The axial compressor is a huge load on the gas turbine taking slightly less than two of every three horsepower produced by the turbine section at rates load (it takes most of the power at FSNL).

Just have a look at the 'Expected Fuel' section of any Control Spec-System Settings document for more specific information, including mass flow/rate data at various speeds and loads. It's very interesting reading, though all the numbers are based on calculations derived during the requisition process using expected fuel characteristics provided by the purchaser. And, the expected FSRs are rarely equal to the actual FSRs, and LVDT calibration accuracy is also a large factor in differentials. But, the percentages are still usually accurate (the percentages of expected values are usually fairly close to the actual percentages in a new and clean unit).

Steam turbines, on the other hand, take very little steam flow (at rated temperature and pressure) to maintain rated speed with no load.

It's doubtful we'll ever hear anything back from the original poster. I applaud you, sir, for trying! If the unit has a Speedtronic turbine control system it probably had at least one alarm that a technician could use to troubleshoot the problem.

 

> Trying to start up a turbine after a long while. Turbine
> fails to reach FSNL always stops at 92-95 TNH. the turbine
> is a frame V GE nuovo pignone gas turbine.

Well, you've had a lot of help, are you ever going to reply or are you just wasting our time?

 

deeply apologize for the delay in response. was off station for a long while due to some challenges. how-be-it i have noted your response and that of others. will notify you on my progress when i return and the progress on the job.

 

> How about some more information. Alarms would be useful.
> fuel type, control system?

MARK V control system. The fuel is natural gas fired DLNox burners.

the unit accelerates slowly taking unusually longer time (about 20mins) to get to 95% TNH. then when the 14HS picks up the TNH drops down to 92%, and comes up again to 95% and again drops. this cycle continues like that without attaining FSNL. However at that 95% TNH, the alarm "LOSS Of Compressor Discharge Pressure Bias" comes up. at this point the Compressor discharge pressure is <= 4bar. The FSR gets stuck at approx 27% at this point, and sometimes dips and returns to 27%. P2 is stuck at about 10bar at d 95% TNH point while dipping and returning also as the TNH drops to 92% and comes up again.

 

Thanks for that. I wasn't being critical just curious to see if you found a solution. Good Luck in your search, keep posting.

 

Obiloko4,

The 'Loss of Compressor Discharge Bias' alarm is VERY bad. DLN combustion systems rely very heavily on compressor discharge pressure signals for proper control and protection, so without a compressor discharge pressure signal the Mark V is "reluctant" to allow the start-up process to continue.

I would have to say there is likely something amiss with the isolation valve(s) for the compressor discharge pressure transmitter(s), either the one at the compressor discharge casing is not fully open, or the valve(s) at the transmitter(s) are not in the proper positions. (Generally, because CPD (the compressor discharge pressure signal) is so important for DLN combustion systems, there are multiple (3, usually) redundant sensors for CPD. Sometimes, there is only one--but of course that means if that sensor is not working the unit won't be running.)

As for why it's taking so long to reach 14HS (95% speed), you need to monitor TTRX and TTXM, the exhaust temperature control reference (which is also the maximum allowable exhaust temperature) and the average actual exhaust temperature control, respectively. If TTXM reaches TTRX during starting, then fuel will be limited and acceleration will be slow. There can be many reasons for why TTXM is too high--including low torque from the starting means (electric motor; diesel engine) or poor torque transmission through the torque converter.

Also, what are the exhaust temperature spreads during starting: TTXSP1, TTXSP2 and TTXSP3?

What is FPG2 and FPRG at 95% speed? (FPG2 is the gas fuel valve interstage pressure, commonly called P2 pressure. FPRG is the gas fuel valve interstage pressure reference. FPG2 should be equal to FPRG.) P2 is a function of TNH; as TNH increases, FPRG--and FPG2--will increase. As TNH decreases, FPRG and P2 will decrease. When the unit is at rated speed (100% TNH), FPRG and P2 will be stable and relatively constant.

What are the values of FSGR and FSG when the unit is at 95% speed? (FSGR is the Stop-Ratio Valve position; FSG is the Gas Control Valve position.)

But, that "Loss of CPD bias" alarm is very telling. And needs to be resolved before any further troubleshooting can be performed. Without CPD feedback, the Mark V is most likely not going to allow turbine operation past 95% speed.

Please write back to let us know how you fare in resolving this issue.

 

Thank you for your reply. will look into the CPD bias alarm, and observe the other variables during the next startup. the space will be open to let you know on my progress.

 

Thanks to all for your responses. During our last start-up and troubleshooting, we had issues with a part of our diesel starting means (Over-running clutch). so the unit startup is suspended pending the purchase of the faulty part. I will surely leave this channel open and post updates on the job when it commences.

 

So we discovered that the control constant FPKGNG for determining the intervalve pressure reference was changed and was referencing 12bar at FSNL instead of 14bar. hence the slow acceleration and stalling at 95%. Apparently it was due to insufficient gas pressure to accelerate it to FSNL. So the FPKGNG was changed to 0.1585 while the FPKGNO remained at -1.38.

This sikved5the solved the problem as the next start up went smoothly all the way to FSNL. only that we observed some serious SRV hunting between 80 - 86% TNH after which it stabilized and went on to FSNL.

<b>Moderator's note:</b> Have no idea what sikved5the should be.

 

Obiloko4,

Thanks for the feedback. I think....

So, I have a very difficult time with this--all of it. How did the Control Constant get changed--and why did it get changed (the change which caused it to be less that 14 barg)?

For a GE-design Frame 5 heavy duty gs turbine the axial compressor discharge pressure (CDP) is approximately 100 psig, or about 7 barg (for a new and clean machine with clean inlet filters at ISO conditions). For fuel to flow into the combustors it only has to be at a pressure slightly higher than CPD. Of course, to get it properly atomize and to develop torque it has to be at a higher pressure--but NOT even 12 or 14 barg. Those P2 pressures are what is required to overcome CPD when the unit is at Base Load and CPD is much higher (but not much higher than approximately 150 psig, or approximately 10 barg).

I don't know what the units were for FPKGNG and FPKGNO. But, the P2 pressure reference formula is:<pre> FPRG = (TNH * FPKGNG) + FPKGNO</pre> So, presuming the units for FPKGNG are barg/% TNH, and the units for FPKGNO are barg, the calculation would look like this:<pre> FPRG = (100 * 0.1585) + (-1.38) = 15.85 - 1.38 = 14.47 barg</pre>At 92% speed, FPRG would be:<pre> FPRG = (92 * 0.1585) + (-1.38) = 14.582 - 1.38 = 13.202 barg</pre>And, at 95% speed, FPRG would be:<pre> FPRG = (95 * 0.1585) + (-1.38) = 15.075 - 1.38 = 13.678 barg</pre>All of these pressures are above 7 barg, which is about what the CPD should be at FSNL--so there should be plenty of fuel to flow into the combustors, at a much higher pressure than CPD.

So, I'm very confused. Even with P2 pressure at 12 barg at FSNL, that's still about 5 barg above CPD--so there should still be plenty of flow.

The previously provided facts are that the unit reaches 95% speed (which is usually the 14HS speed level), and then the speed drops to 92%. And, when the unit (which has DLN-I combustors and is starting on natural gas fuel) reaches 14HS the alarm "LOSS OF CPD BIAS" was annunciated. DLN combustion systems rely very heavily on CPD for proper operation. But, we never heard back on any findings with the CPD pressure, transmitter(s) or alarm.

It could be possible that the gas fuel that was being used last year did not have the expected heating characteristics. Other than some flow restriction issue (as previously described) it's difficult to understand how the change which was described (increasing FPKGNG) would solve the problem described. Now, if the unit is loaded and it can't reach rated load, then some kind of flow restriction is still the suspect. It could be that increasing the P2 pressure slightly allowed enough fuel to flow to be able to get to FSNL, but we don't know how the unit has operated once synchronized and loaded.

We don't know what the GCV position was during the problem starts, nor what they were during the successful start.

Sorry; just too many unknowns, and this doesn't seem to make much sense.

 

I hear you CSA and thanks for your educative comments. I certainly have just learned that P2 must be above CPD for effective fuel flow and atomization.

As regards when and why and how FPKGNG was changed to reference a lower P2, I dont have the information as I'm relatively new to the site but that's what was eventually discovered when the Control constants were checked. Let me give a brief of how that became the focus of our troubleshooting. After we took a break from the investigation and returned we change air inlet filters to take of Loss of CPD bias which used to get anunciated when the unit gets to 95% at which point the CPD Pressure Tx indicates about 4bar or 3.8bar. The CPD Tx (only 1 installed for our unit) was tested offline with 6bar service air & it indicated fine so Instrument fault was ruled out. After the air inlet filter change, the unit was restarted and CPD alarm didn't indicate at 95% TNH as the CPD was about 5 bar @95% TNH but the acceleration problem beyond that point wasn't resolved as the unit still stalled and had a very slow acceleration.

So we turned focus to the fuel system and on keen observation eventually noticed the P2 anomaly which was traced to the FPKGNG which after changing and restarting the unit there was a normal acceleration all the way to FSNL within our usual start up time of 15mins as opposed to the previous problematic stage where the unit takes about 20-25mins to get to 95% and stall there.

So your question as to why the unit couldn't accelerate properly to FSNL event though the P2 at the reduced FPKGNG was still greater than CPD, that I can't say for sure why. Perhaps you need to factor this as a new experience and update your previous knowledge about it. But I can tell you for sure from my first hand experience of the troubleshooting of this problem and start up trials on this unit on site that changing the FPKGNG to 0.01585 which references P2 at approx. 14bar at FSNL was the solution to the problem.

Thank you always CSA. Your posts are very helpful.

 

Obiloko4,

If this explanation works for you, that's all that matters.

Thank you for the kind words!