We have a Hitachi H-25 gas turbine (similar to GE Frame-V machine). There are 03 Cpd transmitters but one sensing line from compressor. this tubing has developed a crack and normal cpd pressure at around 21-22 MW has dropped from 1.1-1.2 MPa to 0.89. We did some temporary fix on tubing by rubber and clamp but leakage is still there. cpd around 0.95MPa as a result of repairs.

My questions:
Can i replace this tubing online with cpd sensing tubing replacement using isolation valve at tubing outlet from compressor? Will the turbine change control mode to temp control ? Any other impact on turbine operation ? I cannot force the analog cpd values and median in logic ( only toggle access)..... Using forcing on digital signals can i prevent mode change ?

 

srh,

Without being able to know what kind of control system and without being able to see the logic/application code running in the control system it's pretty hard to say what's going to happen if you lose CPD feedback.

The most worrisome part of your post, though, is the comment about replacing the tubing on-line--while the turbine is running. It's very hot and dangerous in the turbine compartment.

And, usually the fire extinguishing system is armed and active while the turbine is running. If someone's in the turbine compartment the fire extinguishing system should be disabled/blocked, and that's not very safe, either.

If the unit has DLN combustors I believe the unit will trip if CPD feedback is lost. Not being familiar with Hitachi's application code (I believe they use Mark VI turbine control systems), I don't know what the turbine control will do if all three CPD feedback signals are lost at the same time.

Certainly the unit will not be able to run on CPD-biased exhaust temperature control (Base Load) without CPD feedback. It should have back-up exhaust temperature control, but I couldn't venture a guess if it's FSR- or MW-biased back-up exhaust temperature control.

The fact there are three CPD transducers leads me to believe the application code is designed to have some CPD feedback at all times--but again that's just a guess.

Sorry; there's just no way to tell you what will happen if you run without CPD feedback. Unless someone else is much more familiar with Hitachi's H-25 control philosophy, I can't be of much more help--except to say it's dangerous to send someone into the turbine compartment, even to close then re-open the isolation valve. Even using a long stick to close then re-open the isolation valve.

Please write back to let us know how you resolve the problem.

 

My apologies for having forgot to mention the control system. This is Mark-VI Control System. I will try and share the scanned printouts of the soft logic.

The reason for attending online is that these machines provide power to Petrochemical site and shutting down the machine would lead to shutdown in process area as well.

There are no DLN combustors. The machine is 25MW rated and the temperature in the compartment approaches 100 deg. Yes, it's hot but with known temperature hazard, we are still looking into possibility of attending online through proper safety precautions.

The F&G System would obviously remain defeated during the job. From what i know, it is FSR biased back-up exhaust temperature control.

I am now looking into possibility of isolating the transmitters and ensuring that they hold the pressure, so that the mark-VI Control system and logics get the same pressure signal for the duration of job (which i think with all necessary tubing and fittings prepared should not take more than 20 mins).

I will share the observations if we go ahead with replacement online or otherwise; along with soft logic scans.