Troubleshooting front inlet flow problems

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. 

  • Hi mj.hazlett,

    Can you tell me a little more about your inlet parameters? Column dimensions and approximate temp that the inlet shuts down would be useful. Oven ramp rate would be beneficial as well.

    Usually a shut down at higher oven temps indicates that you may have marginal total flow set for the inlet. Split/splitless inlets are designed to work with a minimum of 20ml/min. You can sometimes get away with lower total flow with isothermal or small rates of temp increase.  You are correct that one flow channel (inlet) shouldn't affect the other as they are on separate EPC modules. A supply of 95 psi should be fine as long as its 95psi to the back of the instrument and not meeting a restricted gas trap that would lower the pressure under flow conditions. 

  • Ok, this is interesting as you are working in constant pressure and not constant flow as I assumed. Usually constant pressure takes oven temp and pressure changes during the run out of the equation.What shutdown message are you seeing on the front panel of the GC or in the logbook when it stops during your run?

  • The flow decreasing behavior you're noticing is in line with expectation. If the inlet is set to constant pressure, then the column flow will indeed decrease with increasing oven temperature. The total flow values you list in your method are really only applicable when you first start the run, when the oven is only 50 degrees C. Over the course of the run, that total flow value will decrease precipitously down to about 54.5 mL/min by my calculations. Besides the linear velocity being too high for Nitrogen (and therefore a not optimal Van Deemter efficiency), the GC can have trouble maintaining that low a flow.

     

    So, the question is -- does the mode have to remain in Constant Pressure or can the method be changed to Constant Flow? If you're able to change this parameter, your flow will remain stable (as will the linear velocity) and the carrier gas requirement will go up instead of down.

     

    You can use the GC Pressure/Flow Calculator (found here: Agilent | GC Calculators) to determine the column flows, inlet pressures, and linear velocities at different temperatures and for different column dimensions and gases.

  • When the instrument is used in constant pressure mode the column flow rate is not controlled, but rather will go up or down based on the temp of the oven and changes in temperature . Is it the the total flow that is dropping off of the set point? The reason I ask this is that it looks like your method is Simulated Distillation and its possible this is an issue with restriction in the split vent line , split vent cartridge or both. 

  • If you have the same symptom during constant flow mode, I would first guess that the pressure at the source (95 psi) is not sufficient. Do you have Nitrogen tank feeding this system? Are there any other lines (either for other instruments or any other device that consumes Nitrogen) being fed by the same source? Try increasing the delivery pressure from 95 to 105 psi but don't exceed 120 psi. If you do that, does the oven temperature at which the shutdown occurs increase?

     

    The other possibility is that you have an inlet leak. You hinted at the fact that you tried an inlet test. If you did the "Inlet Leak Check," can you specify where in the process the inlet flow went to zero? I'm including the document for reference:Manual Steps for Inlet Leak Check, Pressure Decay Test, and Split Vent Restriction Test for 6890/7890.

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