Diffusion Mode Not Cut Out in Gas Turbine 9FA


Thread Starter


Dear All,

We are facing a problem in our plant gas turbine GE 9fa. The issue is when we started machine and reached to TTRF1 setting 2300 F to cut off D5, GCV start closing. PM1 also closing little bit, and TTRF1 reduced up to 2020 F. And again D5 cut in and machine continue with diffusion mode.

Please can any one help me to solve this problem?

Thanks to all
Dear Msalem_73,

Could you tell please ist combined cycle or just a GT? Which DLN is't 2,6+ isn't it? At which load the incident happened? Also I wonder, during transfer mode when TTRF1 get decreasing is their any Grid issue affecting the frequency? Load?

What about D5 purge line is their any issue like opening delay? vent closing issue?. In our case 9FB reaching that TTRF1, with purge issue will activate PPM Mode (piloted premix) which means D5/PM1/2/3 all are in service and none of them is in purge mode. in some cases with purge issue.

If the issue disappear within 15 min, you can master reset. With decreasing and after increasing the load, we could solve the problem. Otherwise the issue is latched with 28FD loss of flame, which means you have to manually shutdown the unit, check 20pg/va13 D5 purge and then restart your unit.

Hope this help.
Dear Chemsouhd,

Thanks for your reply and sorry for late in reply.

Our GTs 9FA working in combine cycle power plant with DLN 2.0. The incident happened at load approximately 120MW, approximately at TTRF1 2300F. I did leak test for all purge line D5, PM1 and PM4. Leak test passed except D5. I found it is passing from VA13-2 gas side. I replaced the valve, check solenoid 20pg-2, positioner, and adjusting all switches for valve. And now we are going to start machine. I will update you what is the final.
Well, this is interesting. It looks like you're suspecting D5 purge line. Did you receive any alarms before and during the incident? Didn't you? About DLN purge or relevant to the incident.

Share the alarms please..

What about the unit did you manually shutdown it? Or it was just tripped?

Keep sharing info.
Dear boss,

After adjusting D5 valve VA13-2 and start unit again, same problem happened, and diffusion mode not able to cut out due drop in TTRF1 during D5 cut off. GE engineering recommended to replace PM4 gas control valve. They feeling it is not give a required flow cos during start leak test B failed. I strocked gas stop valve and SRV valve and felt some gas flow and noise appear in PM4 manifold. That is why they need to replace the valve. During change over, no alarm related purge valves. Only one alarm generated " HIGH C.R.T PILOTED HIGH EMISSIONS." We are going to replace the valve and start unit again.