We have a Frame VI GE Gas turbine duel fuel on NG and HSD. Recently we are facing problems on one of our machine, in which atomizing air temperature quickly drops to around 60 degC from 105 degC and pressure starts hunting between 12 to 15kg/cm2 whenever fuel is transferred to HSD or NG/HSD mix. Precooler flow regulating valve is robertshaw make which we operate manually to adjust AA temperature, but when this valve just slightly opened/closed there is an unusually huge drop/rise in AA temperature, say for eg: 20degC drop. We checked the check valve parallel to booster compressor to see if the air flow in circuit is restricted because of it but the check valve was found working fine. Another interesting observation is that the discharge temperature of main AA compressor was measured using a temperature gun and found to be around 170degC, about 30degC higher than on our other machines.

 

Someone please reply, if some data is missing/needed please do let me know. Recently one of the AD-8 piping from compressor discharging casing got ruptured in the said machine.

 

GTG,

It's very difficult to relate Main AA Compressor inlet temperature differentials to AA pressure fluctuations on your Frame <b>6</b> GE-design heavy duty gas turbine.

There is an air-actuated bypass valve around the Main AA Compressor on dual-fuel Frame 6B that is controlled by a solenoid. You didn't mention having checked the operation of that valve and/or its actuator or the solenoid. Have you reviewed all the components in the AA and Purge Air systems, per the Piping Schematics (P&IDs)? If so, what were the results of the review/analysis/testing?

60 deg C is VERY low for Main AA Compressor inlet air temperature. The temperature should be above boiling (to prevent condensation) and below approximately 135 deg C (which is the alarm setpoint; the regulator should be controlling the temperature to approximately 107 deg C when operating properly). A slug of water from the Precooler entering the Main AA Compressor won't do the compressor any good. And, if there is moisture condensing in the AA Precooler shell, you can tell by opening the Precooler shell drain when the unit is operating and seeing if there is water/moisture coming out of the shell drain (again--refer to the Piping Schematic (P&ID) which depicts the AA Precooler).

And large swings in temperature would be also be very difficult to understand, since a properly operating Robertshaw valve is very slow to respond. Very slow. Manually operating the valve could be part of the problem, but it's difficult to conceive how unless the actuator was devoid of fluid and/or the spring was broken.

So, there are just several things here which don't seem to add up (be related). Which might explain why you haven't had a response before now.

And, it's also difficult to understand how an extraction line could rupture, unless perhaps there was liquid fuel trapped in it and it "exploded".... When did the rupture occur? After a maintenance outage? After a fuel transfer? If after a fuel transfer, was it from liquid to gas, or gas to liquid?

Lastly, this is a free forum. The people who post replies here get paid exactly the same amount you paid to post your query here: Nothing. This forum works because people want to provide information to others, not because people want answers. And, sometimes, there isn't any response possible given the information provided. And, in this case, there seems to be a lot of information that is missing or isn't clear to those who aren't on site. A ruptured extraction line is definitely a cause for concern, but you didn't describe the rupture; was it a failed fitting, a failed piece of tubing? Was the tubing damaged (dented/bent) prior to the rupture?

Please try to remember that we aren't there on site with you every day. We don't know the nuances of everything that has happened. You could post pictures on a Web hosting site and then provide a link to the site, but, still--providing written details would be best.

You also haven't described what is happening to the power output of the unit when you are experiencing the AA pressure fluctuations. Is the power fluctuating also? Is it pretty stable?

So, there's just not a lot of information here to work with, and what there is seems to be unrelated. Help us, and perhaps we can help you.

 

CSA,

Thank you for responding. I had read previously several of your posts in control.com and all of them were very impressive and that's the reason I thought of posting our GT's problem here.

Coming back to issue, we did test the AA recirculation/by pass valve VA-18 actuated by solenoid 20AA. The operation of VA-18 was found satisfactory. Whenever Machine is in NG 20AA energizes and air pressure opens VA-18 so that Atomizing air is recirculated back to precooler inlet. 20AA operation was also checked and found working fine. Initially in this machine we used to face Atomizing air pressure hunting issues whenever VA-18 closes (as indicated by a spindle on the valve head) ie:- when machine is taken in 100% HSD or NG-HSD mix. Also there were several instances when the air line to 20AA/20PL used to get choked with porous stone filter particles. However even after cleaning the air line the same problem is observed.

The problem of temperature lowering accompanied by AA pressure hunting started recently a few months back. At that time a lot of condensate was found coming from AA-1 drain (precooler drain) almost continuously because of which we suspected that somewhere the precooler tubing would have got ruptured. But this was proved wrong when machine was shutdown taken and water stopped coming from AA-1 drain even when cooler was kept charged, the condensate we believe could be the result of extreme high humidity levels during that period.During that shutdown we replaced VA-18, AA temperature regulating valve Robersthaw make and also did cleaning of AA precooler tubings. During GT start-up we found a lot of steam and water splashing from Main AA compressor's atmosphere discharge vent pipe which stopped after sometime.

But the problem didn't get solved. After that shutdown we face another problem of adjusting AA temperature using VTR-2 (as i had already mentioned we do manual adjusting), even when we slightly rotate the gear mechanism there is huge drop or rise in AA temperature. This we suspected could be because of low air flow in AA circuit and hence the check valve parallel to booster compressor was checked but it was working fine too.

The power output of the unit is stable even when AA pressure hunts and spread levels are also within limits in HSD firing.

Coming to AD-8 extraction rupture, yes it did happen a few days after we did HSD firing but HSD firing was carried for a few minutes for checking HSD system healthiness. The tubing was not damaged nor was there any bent. There was air leaking from flange joint before AA precooler and also from another AD-8 extraction joint. I believe that something could be obstructing the air flow in circuit or could it be a damage to main AA compressor?? Anyways hope to get an answer from this forum. Please if you require any other data I shall fetch it. Once again thank you for your time.

 

Still difficult to understand how the AA Precooler outlet temperature can change so drastically so quickly, it just doesn't seem possible unless there is an unusual combination of very cold cooling water <b>and</b> a greatly oversized cooler.

What is the temperature of the cooling water entering the AA Precooler tube nest?

It's also not normal for the outlet air from the AA Precooler to be less than was stated above precisely to prevent condensation and water from entering the Main AA Compressor.

My best guess is that the Main AA Compressor has been damaged by the water and that in combination with the extremely low inlet air temp it's not working properly.

Fix your Robertshaw valve; adjust it to maintain the proper temperature. And, get that Main AA Compressor looked at. It spins at a very high speed and if it comes apart it could be fatal.

Simply because the unit runs on liquid fuel with low exh temp spreads does not mean all is well.

 

Yes that's what I am also guessing a damage to main AA compressor. But one doubt still haunts me, WHY AA PRESSURE HUNTING AND TEMPERATURE DROPS ONLY WHEN VA-18 CLOSES?? when VA-18 is open everything is fine like temperature within acceptable limits and pressure steady,and only problem is AA temperature control through VTR-2.

I don't suspect an over sized Precooler since it was running fine with the same cooler for a long time and other machines in our plant are also doing fine with the same precooler design. And water inlet to precooler is around 29 degC which is also same for our other GTs. But one starkling difference in this machine is the temperature of water outlet from precooler which is around 70 deg C or sometimes higher.

I also suspect that somewhere air flow is restricted in the AA circuit. Will check out in next maintenance outage.

 

Couldn't a compressor which isn't flowing rated throughput be similar to a flow restriction?

And when the bypass is closed that is trying to increase the flow through the compressor, and if the compressor can't handle the throughput (remember the suction side of the compressor is fed with CPD) then that could be why the pressure fluctuates.

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

 

CSA

Guess that should be the answer to our problem and also the Robertshaw valve. anyways we will need to wait for another couple of months for machine shutdown since other machines CCI are underway. Anyways I will let you know of our findings and actions. Thank you once again for your feedback and suggestions.