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why do we LSGI for two minutes prior to shutdown.....I've know "save the turbines," "prevent clogging up the fuel nozzles" and "to eliminate coaking" Are all these equally valid reasons? If so what exactly is taking place; beside the obvious 69-75.5, bleed air valves opening etc. I don't understand how LSGI prevents "coaking".

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When shutting down from NGI, a lot more fuel is trapped in the fuel nozzles. This then runs

out, evaporates and the additives then coke up the air holes. Also I've found an engine run

at LSGI 2minutes, is quicker to continue work on than if shut down from normal. Other

consideration is oil temperature and oil scavenge.

LSGI has better cooling than NGI. I haven't yet found an explanation why. Also the oil

scavenge system is most efficient at LSGI. This can often be verified by watching the oil qty

gauge during up-shift/down-shift. Shutting down from normal may leave more oil in the RGB

and ADH, leading to over-servicing the oil tank later.

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why do we LSGI for two minutes prior to shutdown.....I've know "save the turbines," "prevent clogging up the fuel nozzles" and "to eliminate coaking" Are all these equally valid reasons? If so what exactly is taking place; beside the obvious 69-75.5, bleed air valves opening etc. I don't understand how LSGI prevents "coaking".

In LSGI the fuel nozzles are being cooled by bleed air coming into the burner section. I know it sounds confusing. The bleed air in LSGI is cooler than normal GI and the temp where JP-8 cokes is just higher that the air coming in. So if we don't allow the fuel nozzles to cool then they are more likely to coke than with the 2 min cool down. Hope this answers your question.

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In LSGI the fuel nozzles are being cooled by bleed air coming into the burner section. I know it sounds confusing. The bleed air in LSGI is cooler than normal GI and the temp where JP-8 cokes is just higher that the air coming in. So if we don't allow the fuel nozzles to cool then they are more likely to coke than with the 2 min cool down. Hope this answers your question.

Hey...no, the last two post are exactly what I'm looking for....thanks; so would you say that this procedure is done in part because of the specific use of JP8. It seems that we would want the temp in the combustion chamber to be hotter in order to prevent "coking" or carbon build up on the fule nozzels and not cooler. lastly, is there any credence to the claim "it saves the turbines?"

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It will extend turbine life, I am not sure just how much but it does have positive benefits.

When the airholes get plugged or partially clogged by coking it will change the burn pattern, and in extreme cases you can get downstream burning if the air pattern is bad enough (read that as flame into the turbine wheels), which is bad and will cause the damage (mostly) to the second stage rotor.

Dan

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The bulletin from Allison has been around for quite a while. It does not blame the procedure on any specific type of jet fuel. It does specifically state that the temps need to be reduced to prevent coking, so we need the two minutes at LSGI prior to shutdown.

When we get clogged fuel nozzles, we can get "burner can" damage as well as burned turbine vanes and blades. It is a bad deal all around.

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I asked for more information on this from then Allison years ago and the answer I got was the temp. going into the burner section in LSGI is 357 degrees f .and in normal ground idle was 558 degrees f. There by 200 degrees f. cooler on the fuel nozzle. This was for a -15 engine. Furthermore nothing is going to prevent coking long term without maintenance. It's the jet troops cleaning them (fuel nozzles) more frequently that stops the coking from becoming a problem. Talk to anybody that was around when the AF went from JP-4 to JP-8 prior to that time normal ground idle was the way to shut them down. It's just that JP-8 is a dirtier fuel that is cheaper and but increases range.

Edited by Old Talon 1 F/E
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The compresor output at HSGI is aprox 125 at 600 degrees due to compression....

At LSGI only a portion of the air is compressed and by fewer stages so it is cooler....

The TIT is Hotter at LSGI than HSGI due to the compressor being unloaded and the "cooling air" being reduced. However as noted the air temp is cooler entering the combustion section due to the reduced compression...

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so would you say that this procedure is done in part because of the specific use of JP8.

I believe, yes.

Talk to anybody that was around when the AF went from JP-4 to JP-8 prior to that time normal ground idle was the way to shut them down.

We always used to LSGI prior to shutdown, we just didn't wait 2 minutes. Downspeed, stabilize, shut down. I only remember this because of the NTS lite - if we didn't get it on shutdown from LSGI, we had to crank it back up and shut down in NGI to see if we got the light.

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I believe, yes.

Agree on JP-8 mucking up the fuel nozzles.

We always used to LSGI prior to shutdown, we just didn't wait 2 minutes. Downspeed, stabilize, shut down. I only remember this because of the NTS lite - if we didn't get it on shutdown from LSGI, we had to crank it back up and shut down in NGI to see if we got the light.

Who’s we? Our(16 SOS) normal taxi to parking configuration was 2up/2down and that’s how we shut them down. It was pretty rare to taxi with all four inlow speed.

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Agree on JP-8 mucking up the fuel nozzles.

Who’s we? Our(16 SOS) normal taxi to parking configuration was 2up/2down and that’s how we shut them down. It was pretty rare to taxi with all four inlow speed.

AMC "WE" - JP8 came out shortly before I upgraded to AFSOC. ;)

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If we read both the Commerical Service Letter and the service bulletin put out by Allison (Rolls Royce), we can see that the information about the advantages of using low speed ground idle was submitted both military and civilian aircraft using the T56 and 501D engines. It was rare if not impossible for civilian engine users to get JP8 at that time that those letters came out. Allison did say in their information about the need to clean and flow check the fuel nozzles that, while they recommend it be done every twelve hundred hours, that those who use JP8 have changed the intervals to 600 hours.

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  • 1 month later...

Running at LSGI for 2 minutes is strictly to cool the shroud tips of each fuel nozzle. Each nozzle is actually bombarded by hot, pressurized air from the diffuser. As mentioned earlier, the air temp is much lower (almost 200 degrees) in this part of the engine during LSGI as opposed to GI. In addition, the lower RPM of LSGI slows the fuel pump, thereby reduced fuel pressure/volume. So at shutdown, the cooler nozzle, combined with less fuel pressure doesn't give the residual fuel optimal coking qualities...

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  • 1 month later...

Running at LSGI for 2 minutes is strictly to cool the shroud tips of each fuel nozzle. Each nozzle is actually bombarded by hot, pressurized air from the diffuser. As mentioned earlier, the air temp is much lower (almost 200 degrees) in this part of the engine during LSGI as opposed to GI. In addition, the lower RPM of LSGI slows the fuel pump, thereby reduced fuel pressure/volume. So at shutdown, the cooler nozzle, combined with less fuel pressure doesn't give the residual fuel optimal coking qualities...

Accually your backwards about the nozzles.

2 min in LSGI is there to burn off any coking of the fuel nozzles.

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I still dont buy it all. Why is the TIT 100° cooler in GI than LSGI?

Dumping air off the compressor (5th and 10th open)...kinda like getting a TIT rise when turning on Wing and Empenage, take air off the engine TIT rises. You can see it on an APU also, run an AC pack off the APU, EGT rises, again taking air away from the enigne.

hope that helps

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Accually your backwards about the nozzles.

2 min in LSGI is there to burn off any coking of the fuel nozzles.

not trying to knock you, but can you offer more detail on this burn off of coking....my understanding of this whole LSGI thing is cooling the nozzles preventing "coking"..all the burning takes place downstream of the nozzles, they're in the path of compressor discharge air, which is hotter in Normal GI/lower at LSGI, the 2 mins of exposure at LSGI is to cool them off...

not saying this is your case, but I've heard people say "TIT is hotter, burns the fuel off the nozzle" however TIT is downstream of the nozzle, not sure the nozzle cares about TIT, however it does get whacked by CDP pretty good

just talking here

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Again back to the air requirements of the motor.

When we select LSGI we open 5th and 10th. Effectively removing the additional air that was used for cooling . The air from the diffuser is even hotter at this point along with the nozzle that changes it's spray pattern. The nozzles is two stages LSGI is 50 to 60psi , once NGI is selected the additional fuel pressure moves it to 250psi. Flame patterns are also different between LSGI and normal. So in affect the heat inside the cananular is hotter and removing build up of carbon.:cool:

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Not sure about Lockheed. But Allison/Rolls Royce published a unofficial guide to cuase and failure as a result of many situations.

I have a copy and will see if I can get the info posted.

The guide is given to Herc maintainers along with FE's and pilots as a eye opener . :cool:

Here is one of the earliest pamphlets it has since changed to RR with a blue cover.

http://www.aircraft-manuals.com/alt56aienopo.html

Edited by MAXTORQ
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While I was no longer in the 130 busness when my then unit converted to JP-8 in the 81/82 time frame, the conversion occured with no problems. The TF-30-P100A engine burned JP-8 without a hiccup.

JP-8 is cheaper primarily because it is jet A-1 with a couple of additives. Believe one was to prevent icing and the other was for microbes.

Not surprising the Air Force spends more on fuel than any other DOD component. In an effort to reduce fuel costs and get off oil from our arab friends:mad:the Air Force is spending big bucks developing synthetic fuel. Currently several MDSs have been certified to burn this fuel. Now it's a matter of getting the cost down. The Air Force had contracted with a refining company to build a syn fuel refinery at Malstrom, but the deal fell through. So, in the years to come, the young troops out there will most likely be fueling something other than JP-8.

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While I was no longer in the 130 busness when my then unit converted to JP-8 in the 81/82 time frame, the conversion occured with no problems. The TF-30-P100A engine burned JP-8 without a hiccup.

Not surprising the Air Force spends more on fuel than any other DOD component. In an effort to reduce fuel costs and get off oil from our arab friends:mad:the Air Force is spending big bucks developing synthetic fuel. Currently several MDSs have been certified to burn this fuel. Now it's a matter of getting the cost down. The Air Force had contracted with a refining company to build a syn fuel refinery at Malstrom, but the deal fell through. So, in the years to come, the young troops out there will most likely be fueling something other than JP-8.

The TF30 Pressurization and Dump Valve would dump the fuel from the primary Fuel Nozzle manifolds upon shut down which removed the source of coking. Hence fewer problems.

The Navy has designed and installed a bleed air system which blows air directly through the nozzles after shutdown on the R/R 501's installed in ships. They had a coking problem too.

The process is known as Fischer-Tropsch and dates back about 70 years.

We have plenty of coal, if we lose the ability to get oil from our "friends" it's better to have the capability to produce enough for defense.

I have no doubt the program will eventually come under the scrutiny of the ultra-left tree huggers and shut down due to the use of water during the process. Especially since it's a DoD program.

Supposedly the per barrel average is about $50.00.

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