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Troubleshooting front inlet flow problems

mj.hazlett

I require technical support for a 6890 GC.  When running a method with a temperature ramp, at the higher temperatures there is an inlet flow shutdown and the GC shuts down.  The upstream carrier pressure is 95 psi which should be high enough.  I notice, when setting up a method, that when I change the pressure for the back inlet, the front inlet pressure changes and may not reach the right setpoint depending on the pressure set for the back inlet.

 

I tried an "unobtrusive" leak test that I saw on some troubleshooting guide, which suggested going to a splitless flow and seeing if the flowrate in the front inlet was higher than the setpoint which would indicate a flow -- when I followed this procedure the front inlet had zero flow.

 

I'm wondering since the pressure from the back and front inlets aren't independent and I was not able to get front inlet flow from this test, if that means there is an issue with the flow module, or if anyone has any other ideas. 

  • 0

    Just a quick suggestion on method as well, you would benefit from using gas saver on your inlets. Saves gas and money. Simple feature that can be turned on in the method under inlet parameters. You don't need that full total flow for the complete duration of your method run. Typically after the first two minutes you have transferred your sample to column and really only need to manage inlet pressure and column flow. 

  • 0 in reply to mj.hazlett

    6890/7890 Split Vent Restriction Test

    1. Set the mode to split
    2. You can either have the column installed or the inlet capped.
    3. De-configure the column from the inlet.
    4. Set the inlet temperature to a constant value.
    5. Set the inlet pressure to 0 PSI (this is not the same as turning the pressure OFF).
    6. Set the inlet total flow to 400 ml/minute.
    7. Monitor inlet actual pressure.  Typical Values for a new, clean GC are, for split liner 1-2 PSI, and for a splitless liner, 3-10 PSI.  Much higher values indicate condensed sample contamination in the injection port split vent tube, the copper split vent line or the split vent trap, or possibly a defective EPC.

     

    There shouldnt be an issue with setting headpressure to zero and total flow to whatever you like, are you doing this from the keypad of the GC?

  • 0 in reply to valentinrusu

    valentinrusu,

    Thanks for including the document for reference, I did try that inlet leak check; after I complete step 6 the total flow rate is zero. 

     

    The GC is the only instrument connected to the nitrogen at the moment.  I used to have the source set to 80 psig and increased it to 95 psig when I noticed the issue and it did not seem to impact it.  The max pressure on the FID and TCD are 100 psig, so I don't feel comfortable going much higher...

  • 0 in reply to mj.hazlett

    It sounds like from both of your feedback maybe I should try the split vent restriction test.
    For this instrument I have the front column with manual injection to split/splitless, and the back column is attached to a gas sampling valve.  The instructions say to deconfigure the column from the inlet, this would only be for the front column, correct?   I will try this next.

     

    If it is the split vent line or trap that is restricted, is there a procedure to follow to clean these out?

     

    Just as a bit more background, this GC has been sitting dormant for months due to a lab move.  The septum has been recently replaced, and I checked the liner o-ring and it was intact.

  • 0 in reply to james_jenkins

    Thanks.

    Is this issue split vent restriction issue related to why the pressures on the front and back inlets seem to depend on eachother?  (ie the back inlet pressure needs to be higher than the front inlet pressure to reach the setpoint pressure, but if the back inlet pressure is set too much higher the inlet pressure overshoots and can't get low enough)

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