Gas turbine starting device bogged down trip

K

Thread Starter

kofiappiah

In my power plant where I work in Ghana, we have 4 Frame 6 GE gas turbines. Recently, one of them underwent a major retrofit, ie the turbine blades were changed and was started normally. However, during the second start up after this maintenance, the turbine started normally but after firing, the acceleration could not take place. the diesel engine speed remained at 1800rpm and turbine speed remained at 41.6% for about 3 minutes and then turbine speed began to decrease. it is worth noting that the fuel control status changed to temperature control because at that turbine speed the exhaust temperature was already 567 degrees Celsius.

what could be the possible causes and how do we solve this issue?
Thank you
 
B

Bob Johnston

Firstly, the diesel at 1800 rpm is too low, it should be around 2100 once you have established flame. I would check out your diesel control solenoids (if you have them) 20DA-1/2 or your electronic controller if that is what you have. Have any control constants or calibration been changed during the outage? You have definitely got into a temperature control "hung start" and the speed is exactly where it happens. Get the diesel fixed and try again and let us know what happened.
 
It sounds like there is some sort of mechanical problem with the turbine itself. The starting device should be able to accelerate the turbine to around 50% speed where the turbine is able to then be self sustaining and accelerate to operating speed.

The fact that the turbine stops accelerating at around 40% indicates to me that the turbine is not able to extract the working power from combustion of fuel to accelerate itself. The fact that fuel control is being limited by temperature control pretty much confirms this for me.

It sounds to me like a boroscope of the unit would be needed to identify if there are problems in the turbine compressor or hot section. The fact that the unit started "normally" once tells me initially everything may have been ok. But how normal is "normal". Were wheelspace temperatures, compressor discharge pressure and temperature, etc. all normal as before the major? I would suggest a good review of any trend data available before and after the major to look for anything obvious that does not look normal. But I still suspect and would look first at the mechanical aspect of the machine by performing a thorough boroscope. As always please report back on what was found so we can all learn from your experience.
 
To me, i think there is a problem with the rotor. possibly the rotor seems to be rubbing which has resulted to excessive exhaust temperature, because the maximum permissible exhaust temperature is about 485 degree Celsius. A boroscope inspection should be conducted.

Thanks!
[email protected]
 
To can help you, I need the following information:

The model of your Diesel Engine.

What is the maximum speed in Crank Operation Mode?

Your Control, is Speedtronic?, Model?

I don't think that your problem is in your rotor. I seems to me, that your problem could be in your Diesel Engine or in your fuel control.
 
While I don't claim to be a frame 6 guy... I bet the reason that the diesel is running a little slow is that it is loaded heavy. I would put my money on bleed air device stuck closed or inlet guide vanes open too far or stuck causing the compressor to put too much load on the diesel engine. At that critical point in a turbine start everything has to work perfect to get the thing to accelerate correctly. 300 RPM droop in the diesel speed is an indication of excessive load - not a malfunction. Excessive load is going to come from trying to compress too much air... Good luck if you have not already found the problem.

John
 
We don't know the age of the diesel, how recently it was refurbished (if ever), or the age of the torque converter or how recently it was refurbished, if ever.

Most older diesels used a hydraulic solenoid/cylinder arrangement to set diesel speed. The control was entirely open loop (meaning, that although the diesel speed was fed back to the Speedtronic it was not used as feedback for a regulator or control loop to change fuel flow). The "stop" positions of the hydraulic cylinders were manually set to hold particular speeds or to provide acceleration (depending on the type of control and number of solenoids).

These hydraulic systems are usually rarely, if ever, maintained. Any filters are usually rarely replaced, if ever. The solenoids and the cylinders do develop leaks over time. The diesels do "get tired" and require maintenance, also. The fuel filters for these diesels also get plugged and the fuel can be contaminated with water, which can cause the filters to restrict fuel flow. Fuel racks can develop significant wear over time, as can fuel injectors. Normal maintenance and normal wear should be expected of these engines and they do need to be maintained and refurbished from time to time.

I've even seen starting diesel air filters that hadn't been changed in twenty years.

Same goes for the torque converters. They wear out, as do any solenoids used for controlling flow through the torque converter. This has been found to be a problem on several previous threads on control.com.

If the compressor bleed valves were closed it would likely represent a higher back-pressure on the turbine, BUT one of the start-check permissives is to ensure all compressor bleed valves are fully open before allowing a turbine start (though many sites blatantly "bypass" such protective logic).

If the IGVs were open more than normal it would also represent a higher load, and it might not be evident if there are no LVDTs on the IGVs (some older machines did not IGV LVDTs). And on machines without LVDTs there was usually a limit switch to indicate the IGVs were at "minimum" angle as a further start-check permissive (these days that's done via a check of the LVDT position). If the IGVs were open more than they should be that should reduce the exhaust temperature, in theory.

My money's on one of three things: diesel starting means and/or its hydraulic control components; the torque converter and/or it's associated solenoids; or some problem with turbine fuel flow, that's not very likely though since it would seem the Speedtronic put more fuel in to try to get the unit to accelerate, resulting in the exhaust temperature reaching the isothermal limit.

And, has happened many times in many places, it could be some combination of the above.
 
One more thing about diesels: The color of exhaust smoke can be very telling about the condition and/or the load of the diesel.

White smoke means incomplete combustion, as in poor injector condition, or bad rings, or low compression.

Heavy dark smoke indicates excessive load.

This quite often gets overlooked when trying to troubleshoot diesel starting motor problems.
 
Recently we faced same problem on the GE frame 5, that the GCV valve was not calibrated well and the LVDT's were giving high signal but actually the GCV opening is less.

For your information that during start-up the FSR is limited not to exceed the 26.9%.

try to notice the GCV opening during start-up or check your old trend, normally at about 41% of TNH the FSR should not be more 22%
 
Dear Kofia,

It could be in your case that the FSR is restricting the diesel to increase and if the Diesel doesn't exceed 14HC (43%) within specific time, the trip L3SMT will be generated.

Do not focus on the diesel as the acceleration (20 DA)had taken place otherwise would not reach 1800 rpm.
 
V
Please, check the following item:

There is a control pump coupled to the diesel. This pump passes the oil to the governor in order to get it accelerated. This pump is coupled to the diesel by means of a teflon coupling who, with every start it starts to slip over the axis who drive the pump. Some diesel motors have this coupling who slides with the use and others have a rigid coupling that do not slide. Check what type of coupling do you have in you machine. If you have the sliding one it is almost sure that replacing this coupling will fix your problem.

The control pump should be locate close to the governor and the acceleration valve(20DA), by the side of gauge panel.

Regards,
Oscar
 
Top