(I have this feeling I'm going to regret this.)

If you have a GE Speedtronic turbine control system on a Frame 6 it's mostly likely a Mark II or a Mark IV. If you have a GE Speedtronic turbine control system then the name of the solenoid that controls the IGV position (for a two-position "bang-bang" (no pun intended) IGV system) is most likely 20TV-1. I would presume 20TV-1 is to be energized when the IGVs are to be in the open position, and de-energized when they are to be in the closed position.

When the unit is operating at approximately 20-25 MW, what is the IGV angle?

Can you determine from the Speedtronic elementary drawings (again, presuming the turbine control panel is a GE Speedtronic) when 20TV-1 is to be energized?

This may be a little complicated because at one time GE used servos (yes, servos) for solenoids and applied 125 VDC to the servo coil, reversing polarity to change the direction of flow through the servo, which is akin to opening/closing a solenoid-operated valve. So, the servos were always energized, but the polarity of the applied voltage changed with the intended operation of the device it was controlling the position of.

The questions I'm trying to get answers to are:

1) When should the IGVs be open and when should they be closed?

2) When the unit is operating at about the load it is experiencing the bangs and flames, what position are the IGVs at?

In other words, is it possible the IGVs are either not open when they should be, or are open when they shouldn't be? I'm asking this because <b>from the information provided</b> it seems there may be a compressor surge/stall problem, but it's difficult to know for sure without being on site.

The only way to know the answers to the above questions for sure is to examine the Speedtronic elementary drawing to determine when the IGVs are to be open and closed, and then to determine if they are open when they should be and closed when they should be.

I still think you should have someone come to site (barge) and help with resolving this issue instead of continuing to start and load the unit and experience the bangs/flames. At some point, something in that machine is going to break, and because it's rotating at approximately 5100 RPM when something breaks (like a compressor blade or a turbine bucket) there's going to be a significant amount of damage, and even more lost revenue whilst trying to repair the damage at great cost.

 

I'm in agreement with you, otised. All Frame 6s <b>should have</b> modulated IGVs. I've had people tell me that IGVs were two-position because at closed they were at 34 DGA, and at maximum they were at 84 DGA, but, in fact they were modulated to intermediate positions during normal operation. (They were just reading from a Control Specification, and not reading the entire section.)

I have a feeling this unit was relocated to a barge from its original site, and in the process may have gotten a new control system. I do recall there was a company producing PLC-based turbine controls called Speedtronic<b>s</b> or Speed<b>o</b>tronic.

Again, it's still a credit to GE that some of these turbines survive as long as they do. Some turbines would never have survived this kind of ... operation. And, this one keeps starting (though it can't seem to be loaded past a particular point at this time).

 

When the unit is operating at approximately 20-25 MW, what is the IGV angle? – 840

Can you determine from the Speedtronic elementary drawings (again, presuming the turbine control panel is a GE Speedtronic) when 20TV-1 is to be energized? – No

We are not on Servo – 20TV

The questions I'm trying to get answers to are:

1) When should the IGVs be open and when should they be closed? – @ 95% Turbine Speed it should be open while on shutdown or trip it should be closes (though faster at trip than at normal shutdown)

2) When the unit is operating at about the load it is experiencing the bangs and flames, what position are the IGVs at? - – Cant even tell because we weren’t there to confirm physically and unfortunately the limit switch in control cant sense level/position because it is preset to sense signal @ fully open or fully closed (–– 340 and 840 respectively)

 

Oluwafemi Ogundaisi,

It is virtually impossible for me to be of much more help to you, or to Solomon Osayi (who has also opened a thread for this same site), because we can't see the Piping & Instrumentation Diagrams (P&IDs, or Piping Schematics) for the turbine, and we can't see the sequencing that is running on the turbine control system. <b>From the information provided</b> and without being able to examine the documentation and/or to witness the problem it is going to be very hard to help with your problem.

As otised has said, it would be very unusual for a GE-design Frame 6 not to have modulated IGVs--not impossible, but very unlikely. Even if the IGVs are NOT modulated, there has to be an actuator of some sort (hydraulic) that has to have high-pressure hydraulic oil ported to it to open or close the IGVs (or it could be an electric actuator, but that wouldn't likely be original equipment). So, we would need to know what that device is that is porting the high-pressure hydraulic oil to the actuator to open and close the IGVs, and then to understand if it is working correctly.

You seem to have discounted the possibility that there are liquids of some sort being sent to the fuel nozzles/combustors, even though you have said you found condensate in the condensate system of the gas fuel system. Okay; that's fine.

The conditions you are describing seem to be slightly contradictory--to me, anyway. For there to be loud bangs and explosions in the exhaust, it would seem that there would need to be some accumulation of unburned fuel that was suddenly being ignited. This would seem to mean that flame was lost in one or more combustors without flame detection (unless the loss of flame detection was being "bypassed") allowing unburned fuel to flow into the exhaust where it was subsequently ignited causing the loud bangs and flames.

If slugs of gas fuel liquids were making their way into the combustors then load would spike very high, the exhaust temperature would also spike, the compressor discharge pressure would also spike while the liquids were burning. When the liquids had finished burning the load would reduce, probably below the previous steady-state level, and compressor discharge pressure would also probably decrease below previous level. These kinds of conditions are very difficult to trace without high-speed trending capability to learn what's occurring first. But, again, you seem to be discounting the possibility of liquids.

When flame is lost compressor discharge pressure will decrease; the more combustors that lose flame, the higher and more sudden the decrease in compressor discharge pressure.

Not ever having personally experienced an axial compressor stall/surge condition at rated speed under load, I don't know what would happen to compressor discharge pressure--though I would imagine it would decrease rather quickly (the "stall") and then pulse (the "surge") until the unit tripped, possibly on high vibration or exhaust overtemperature or something similar. But you say you aren't experiencing any high vibrations, so that would seem to rule out any axial compressor surge/stall conditions. The only surge/stall condition I witnessed caused vibration readings to over-range the input scaling, and the ground shook, too. The turbine had already tripped and was coasting down from rated speed, and fortunately there was no damage to the turbine.

So, I'm completely at a loss to explain what's happening <b>based on the information provided</b>. Sorry; but nothing adds up with the descriptions. Either there's something wrong with the air flow through the compressor, or there's something causing an accumulation of unburned fuel in the exhaust. An axial compressor problem would most likely cause high vibration; I would expect at least one of the combustors with flame detectors would be indicating loss of flame.

You say you've "rejected" exhaust T/Cs, so it's difficult to say what the actual exhaust temperature spreads are. You also said the rejected thermocouple readings were abnormally high, which is kind of unusual for a turbine running on gas fuel because unless there are fuel nozzle problems (excessively worn orifices or leaks) the amount of fuel going into each combustor should be relatively the same (within, say +/-10% even for a poorly matched set of fuel nozzles).

The loss of compressor discharge pressure and exhaust overtemperature are also somewhat contradictory--based on the information provided. Of course, if fuel continued to flow when compressor discharge pressure was low and then compressor discharge pressure suddenly increased, that might account for the exhaust overtemperature.

I suppose it's possible that a loss of speed signal feedback to the turbine control system could cause a momentary spike in fuel flow command and fuel flow-rate. But, why would that only be occurring at one load at what should be a relatively stable speed? Although you have said the national grid frequency--which directly affects your turbine-generator speed--is not very stable when these events are occurring. Could be a poorly tuned control system, but, why would these problems start occurring now? What has changed recently? Some "tuning" of the control system?

I don't know if we've asked, but are you sure the compressor bleed valves are fully closed? That shouldn't cause the kind of problem you are seeing, but, in conjunction with other conditions it may be aggravating the problem(s).

The only other question I can think to ask is: When did this problem start? After some maintenance outage? After fuel nozzles were replaced? After another emergency trip condition?

Sorry; I'm at a loss here. Hopefully you'll write back to let us know what you ultimately find the cause(s) to be, or someone else here may have another idea.

 

CSA_MVP,

Investigation completed, the compressor was lifted and the consequential effect of the under frequency and bang noise, lead to bent and broken blades mostly on stage four and five. This contributed to the reason why the unit cannot be load above 20MW target. We are going to renew the rotor and get back online ASAP.

Thanks all for your effective contribution on this forum.

You guys are the best.

 

Oluwafemi Ogundaisi,

Thank you very much for the feedback. It's unfortunate about the damage; best of luck for a speedy return to generation!