6850 GC Flame does not stay lit

[jenseeli]

My department has an HP 6850 GC with FID that is used only once or twice a year for an undergraduate teaching lab and is shut off completely the rest of the time. It was last used in January of 2014. I turned everything on two weeks ago to get ready for this year's class: checked for adequate gas supply, lit the FID flame, let the instrument run for ten minutes before shutting off, and thought all was well. No parameters had been changed since last year; no one has touched the instrument since last January.

On the first day of class last week, as I was explaining the instrument operation to the students, the FID flame went out for no apparent reason after about 10 minutes. I could not get it to re-light. I tried changing the proportion of gases, with no effect. According to the ChemStation readings, gas flows for H2, air, and He came up to their set points (H2 30, air 300, make-up He 30 mL/min) with no problem, as usual. I heard the "pop" of the igniter in the FID, and I could see the element glowing hot. Just no flame on.

I thought to check the column for breaks or poor connections to the inlet/detector, but this turned up nothing.

A few days later, when I had time to go back to this, I was able to light the flame on the first try (using the original settings). The flame stayed on for exactly 14 minutes and then went out, again for no apparent reason. The instrument automatically tried to relight and succeeded. The flame stayed on for about 1 minute and then went out again. Again the instrument did its automatic relight procedure, but this time nothing. It tried maybe four cycles (each time I heard the "pop" sound and saw the igniter glowing) with no effect. I tried turning the flame off and then back on "manually" (by clicking the command boxes in the software) but that didn't work either.

We were using an H2 generator to provide hydrogen, so I thought perhaps there was a problem with that. I disconnected the generator and substituted compressed H2 from a cylinder (air and helium have always been from cylinders). Again, no trouble lighting the flame on the first try. Stayed on for 14 minutes, then off for no reason. GC went into its reignite routine and was able to relight and stay lit for about a minute, then not after that.

I don't know why the flame goes out after 14 minutes. If I can light it in the first place, why doesn't it stay lit? I'm not getting any kind of error message or indication of any problem at all until the GC gives up trying to reignite (then, of course, it automatically goes into a safety shutdown). I would be grateful for advice and suggestions.

[Consumer Products Guy]

The Lit Offset for the FID may be set too high (default of 2.0, I believe). If the signal falls below that setpoint, then FID will go out, will try to re-light itself, etc. So maybe make that offset 0.5; we had to do this on a 6890 GC, real similar.

[Peter Apps]

First thing is to check that the flame really is out by holding a cool shiny surface above the FID and looking for condensation. If you see condensation than the flame is burning and there is a problem with the flame out detection.

If the flame really is out, then the 14 min of burn sounds as if either the hydrogen or air flow is slowly dropping. This might be a problem with the flow control but it can also be due to a restriction in the supply lines that chokes the flow to a bit less than the detector needs. When the FID is off the line pressure builds up and when you start up the flow is adequate. As the FID runs the line pressure drops until the flow is inadequate and the flame goes out. Check flows and pressure immediately after the flame goes out. If you let the gasses flow for 15 min with the flame off, can you light the flame immediately afterwards ?

It is also conceivable that the make-up flow is increasing until it lifts the flame off the jet, but very unlikely.

Peter

[jenseeli]

Thank you very much for the quick suggestions!

I guess I don't know for sure that the flame was really out, because I didn't specifically check that, but the GC certainly acted like it was out. I did not see any change in gas flow that would lead me to believe there was a restriction. I was actually thinking along a similar road when I switched from the H2 generator to the compressed cylinder: I thought maybe the generator wasn't producing enough H2 to keep up with the FID but could build up a little reserve pressure when the detector was off. Switching to the cylinder did not eliminate the problem, so I figured it had to be a problem with the GC itself, rather than the gas supply.

I tried decreasing the Lit Offset to 0.5 and the flame stayed on! I let it go for 20 minutes and then injected some methanol (to go through the whole data collection process) and the flame stayed on throughout. This morning I tried again, and had the same positive result. I don't know if I would ever have thought to look at the raw FID signal without your help, but both of these times I did watch it. Immediately after lighting, the signal was about 2.5 pA and then it slowly decreased with time. I would guess that previously, it took 14 minutes for the signal to drift below the default offset (2.0 pA) and cause the shut down/reignition cycle to start.

Thank you, thank you!

[Peter Apps]

Thanks for the feedback.

[Consumer Products Guy]

The Lit Offset for the FID may be set too high (default of 2.0, I believe). If the signal falls below that setpoint, then FID will go out, will try to re-light itself, etc. So maybe make that offset 0.5; we had to do this on a 6890 GC, real similar.

I tried decreasing the Lit Offset to 0.5 and the flame stayed on! I let it go for 20 minutes and then injected some methanol (to go through the whole data collection process) and the flame stayed on throughout. This morning I tried again, and had the same positive result. I don't know if I would ever have thought to look at the raw FID signal without your help, but both of these times I did watch it. Immediately after lighting, the signal was about 2.5 pA and then it slowly decreased with time. I would guess that previously, it took 14 minutes for the signal to drift below the default offset (2.0 pA) and cause the shut down/reignition cycle to start.

Thank you, thank you!

Winner, winner, chicken dinner !

Obviously, we had this same issue in the past, just on one of our GCs. We figured it out ourselves, so "the fix" must've stuck in my little brain.