Good day friends,

We are facing a problem with our Gas turbine (Frame 5, TMR, Single shaft, Speedtronics MARK 5 control)from long back.

Problem is P2 pressure is hunting about 10 to 15 psi and load also hunting about 0.3 to 0.5MW due to this problem.

One year before, we have changed the new SRV/GCV full valve assembly (refurbishment) and calibrated by vendor. After 10 months, we have faced the same hunting problem about 2 to 3MW. We have changed the servo valves both SRV/GCV and calibrated. After that fluctuation was reduced, and we found some dust particles inside the servo filters. We changed the reservoir oil and again same problem came after 2 months. When we are changing from gas to liquid fuel, we found there was no hunting and load also stable. Again changeover to gas, P2 pressure was stable for some 2 to 3 days and again it will start to fluctuate. Till date we have changed SRV/GCV servo valves 3 times and GCV cylinder also changed one time.

Vendor from GE has been analyzed all the control parameters and finalized to change the whole valve assembly. But as per our records, we have changed everything which is involved in this problem including FPG2. we did FSR gag and checked the GCV performance. It is stable. Vendor told it’s SRV problem and adjusted the control gain from 1.9 to 1.71. SRV is responding good at offline. At FSNL its started hunting again and same fluctuation repeated.

Give me a solution friends. what all parameters to check?

regards
RK..

 

From your description, I would first check your gas fuel supply. Have you checked the supply pressure? Have you taken samples of the gas fuel (both when the instability occurs and when it is running stably) and had them analyzed? What other gas users are upstream of your plant - is something going on there when you experience the instability?

 

RKSGE,

Hmmmm..... So, the oil was replaced. Was the reservoir cleaned when the previous oil was removed, and a flush of the lube oil and hydraulic systems performed with the clean oil? Or even with the dirty oil prior to replacement? Was any flushing of the oil systems/piping done when the oil was replaced--prior to or after replacement?

Does the site perform periodic sampling of the oil, sending it our for analysis and then reviewing the analysis and recommendations and taking corrective action?

From your description, the problem always seems to return shortly after the servos are replaced, and shortly after the oil was replaced. There's a pattern. And, servos don't just go bad--bad oil causes servos to need to be replaced.

All other things being equal on a unit with a digital control system, it's not likely that servos are failing with any regularity. And, there isn't likely and Control or I/O Configuration Constants which can be changed to prolong the life of servos when the oil is bad or isn't being properly maintained.

The other possibility which doesn't seem to have been investigated or covered is that of contamination build-up on the gas valve internals which could lead to some control issues. A lot of natural gas suppliers, in the lack of any contractual requirements to do so, are not treating their gas at all before putting into the pipeline. And, a lot of the newer wells (think fracking) have some pretty dirty gas, which can collect anywhere there are pressure drops (control valves; nozzles; pressure regulators; etc.).

Again, servos and digital regulators don't go bad or drift (go slowly out of calibration). For servos, it's bad oil. For digital control systems, it's something external which changes which causes adjustment, or tuning, to be necessary (presuming tuning was done correctly to begin with).

Failing rings in the hydraulic actuators--again, most likely caused by dirty oil--could also be another indicator of poor oil quality maintenance practices.

 

Hi otised..

>From your description, I would first check your gas fuel
>supply. Have you checked the supply pressure? Have you taken
>samples of the gas fuel (both when the instability occurs
>and when it is running stably) and had them analyzed? What
>other gas users are upstream of your plant - is something
>going on there when you experience the instability?

Our inlet gas pressure is almost stable and all other turbines loads are stable with the same pressure. We have online gas analyzer for gas to monitor the condensate and other particles and we are not faced this load fluctuation problem in other turbines when gas pressure was unstable.