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Troubleshooting front inlet flow problems

Question asked by mj.hazlett on Oct 2, 2017
Latest reply on Oct 6, 2017 by james_jenkins

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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.

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mj.hazlett Oct 2, 2017 12:51 PM
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which would indicate a leak** typo
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james_jenkins Oct 3, 2017 5:07 AM
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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.
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- mj.hazlett @ james_jenkins on Oct 3, 2017 10:18 AM
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  The parameters are below; the flow issues start when the oven reaches 200C where the flow is half the setpoint, and the shut down occurs shortly after when the oven is around 220C.
  
  OVEN
     Initial temp: 50 'C (On)               Maximum temp: 300 'C
     Initial time: 4.00 min                 Equilibration time: 0.50 min
     Ramps:
        # Rate Final temp Final time
        1 20.00      250        6.00
        2   0.0(Off)
     Post temp: 0 'C
     Post time: 0.00 min
     Run time: 20.00 min
  
  FRONT INLET (SPLIT/SPLITLESS)           BACK INLET (SIM DIST)
     Mode: Split                            Pressure: 15.00 psi (On)
     Initial temp: 260 'C (On)              Gas type: Nitrogen
     Pressure: 10.00 psi (On)
     Split ratio: 50:1
     Split flow: 115.7 mL/min
     Total flow: 120.5 mL/min
     Gas saver: Off
     Gas type: Nitrogen
  
  COLUMN 1                                COLUMN 2
     Capillary Column                        Capillary Column
     Model Number: Agilent 19091J-413       Model Number: Agilent 19091P-Q04
     HP-5 5% Phenyl Methyl Siloxane         HP-PLOT Q
     Max temperature: 325 'C                Max temperature: 290 'C
     Nominal length: 30.0 m                 Nominal length: 30.0 m
     Nominal diameter: 320.00 um            Nominal diameter: 320.00 um
     Nominal film thickness: 0.25 um        Nominal film thickness: 20.00 um
     Mode: constant pressure                Mode: constant pressure
     Pressure: 10.00 psi                    Pressure: 15.00 psi
     Nominal initial flow: 2.3 mL/min       Nominal initial flow: 2.3 mL/min
     Average velocity: 38 cm/sec            Average velocity: 43 cm/sec
     Inlet: Front Inlet                     Inlet: Back Inlet
     Outlet: Front Detector                 Outlet: Back Detector
     Outlet pressure: ambient               Outlet pressure: ambient
  
  FRONT DETECTOR (FID)                    BACK DETECTOR (TCD)
     Temperature: 270 'C (On)               Temperature: 250 'C (On)
     Hydrogen flow: 40.0 mL/min (On)        Reference flow: 20.0 mL/min (On)
     Air flow: 450.0 mL/min (On)            Mode: Constant column+makeup flow
     Mode: Constant makeup flow             Combined flow: 7.0 mL/min
     Makeup flow: 45.0 mL/min (On)          Makeup flow: On
     Makeup Gas Type: Nitrogen               Makeup Gas Type: Nitrogen
     Flame: On                              Filament: On
     Electrometer: On                       Negative polarity: Off
     Lit offset: 2.0
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  - james_jenkins @ mj.hazlett on Oct 3, 2017 10:46 AM
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    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?
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    - valentinrusu @ james_jenkins on Oct 3, 2017 11:07 AM
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      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.
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      - mj.hazlett @ valentinrusu on Oct 3, 2017 11:33 AM
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        valentinrusu,
        
        I understand that the flow will decrease over the temperature ramp, the method compensates for this by adjusting the flow rate setpoint as the temperature ramp increases... the issue is that the instrument cannot reach this decreased setpoint and does a front inlet flow shutdown when it can't reach this setpoint. I have tried also with a constant flow program and the same front inlet flow shutdown occurs as the temperature increases.
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        valentinrusu @ mj.hazlett on Oct 3, 2017 11:52 AM
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        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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        mj.hazlett @ valentinrusu on Oct 4, 2017 9:29 AM
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        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...
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        mj.hazlett @ mj.hazlett on Oct 4, 2017 9:43 AM
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        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.
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        james_jenkins @ mj.hazlett on Oct 4, 2017 9:51 AM
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        Replacement cartridges in a two pack G1544-80530, for flushing, I have just adapted a syringe and forced solvent (your choice) through once the lines have been removed.Flushed into a beaker in a hood. Part of the lines are copper and can be replaced if needed.
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        mj.hazlett @ james_jenkins on Oct 4, 2017 10:00 AM
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        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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        james_jenkins @ mj.hazlett on Oct 4, 2017 10:07 AM
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        That's the interesting part as they shouldn't depend on each other, two flow channels that are separate with seperate EPC modules. You have good supply pressure to the instrument so unless there is a restricted external gas trap its interesting that you see the effects that you do.
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        mj.hazlett @ valentinrusu on Oct 4, 2017 10:56 AM
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        I tried the Split Vent Restriction Test... I am not able to set the pressure to zero without the flow being automatically adjusted to zero. Please advise.
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        james_jenkins @ mj.hazlett on Oct 4, 2017 11:11 AM
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        6890/7890 Split Vent Restriction Test
        Set the mode to split
        You can either have the column installed or the inlet capped.
        De-configure the column from the inlet.
        Set the inlet temperature to a constant value.
        Set the inlet pressure to 0 PSI (this is not the same as turning the pressure OFF).
        Set the inlet total flow to 400 ml/minute.
        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?
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        mj.hazlett @ james_jenkins on Oct 6, 2017 9:19 AM
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        Hello James,
        
        I did this from the GC keypad this time. The max flow I can set is 200, and the pressure of front inlet is set to zero. The actual pressure is still dependent on the back inlet. When I put the pressure of the back inlet to zero as well, the pressure in the front inlet is 0.8. When I put back inlet to 25, the front inlet pressure reading goes to 9.3.
        
        Sent from my iPhone
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        james_jenkins @ mj.hazlett on Oct 6, 2017 9:38 AM
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        By chance is your TCD plumbed to your FID, non destructive to destructive detector configuration? Or using a splitter device of some type? A picture from inside the oven would help. Have you tried the split vent maintenance yet? I would do this even without the result of the testing. Especially if you do not know the maintenance history.
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        mj.hazlett @ james_jenkins on Oct 6, 2017 9:42 AM
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        James, here is a photo of inside the oven.
        
        I ordered the filter and I am trying to clean out the split vent, honestly I am new to the instrument and finding the instructions on the service disk very confusing as I don't know which is the split vent line or where the trap is..... Is there some schematic or picture of this you can send me?
        
        Sent from my iPhone
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        image1.jpeg40.2 KB
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        james_jenkins @ mj.hazlett on Oct 6, 2017 10:13 AM
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        Ok, I think we know why one inlet is affecting another. This is not a straight forward basic single inlet to single detector configuration . This is why when you pressurize or change flows one inlet causes a change on the other. That mystery is solved . The GC has been altered and the Sim Dis comment from the method makes sense. I am not familiar with that configuration but can enlist help of a co-worker if needed. Are you going to run this as a simulated distillation application? If not you may benefit from making some changes to the GC as configured. That would go beyond the scope of forum or phone support and would need an Agilent representative onsite. For the time being, the split trap maintenance should take place. Pic above should help, to see the trap canisters, remove the top back cover that stretches across the entire top rear of the GC. Filters are left rear. I pasted a video link that should help. This is a newer style GC, but very similar. https://www.youtube.com/watch?v=vIoHnW5NKBE
        Change the Split Vent Trap - GC Troubleshooting Series - YouTube
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        mj.hazlett @ james_jenkins on Oct 6, 2017 10:26 AM
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        Hello
        
        Can you elaborate more on this configuration? So I want it to be single inlet to single detector for the purposes of these experiments... I have one liquid sample I want to analyze on front column with FID and gas sample on back column with TCD. Is there a way I can make it like that?
        
        The application for liquid injection is that I want to get concentration of methanol in methanol water solution... I have been diluting this with acetone for analysis. I was using kind of a blend between ASTM method D7768 and the Agilent Alcohols I method.
        
        I washed out the copper split vent line like you suggested. For the trap, while I wait for the new one, should I rinse that as well?
        
        Sent from my iPhone
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        mj.hazlett @ james_jenkins on Oct 6, 2017 10:47 AM
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        Hi James,
        
        I was previously unfamiliar with simulated distillation. Can you elaborate on the issues of using the GC for applications other than this and why you might recommend reconfiguring?
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        james_jenkins @ mj.hazlett on Oct 6, 2017 10:57 AM
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        Without knowing the complete configuration this is a hard question to answer. On the top of the oven, is there a valve or valve oven? Typically the unmodified setup is column from inlet to detector . I assume this GC was taken out of storage or was not in use for awhile. If modified by Agilent, with a serial number I may be able to find a diagram if its valved.
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        mj.hazlett @ james_jenkins on Oct 6, 2017 11:06 AM
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        Yes this GC has not been used in awhile. There are two switching valves on the top (photo attached) and the serial number is US10451032.
        
        Sent from my iPhone
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        image1.jpeg45.0 KB
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        james_jenkins @ mj.hazlett on Oct 6, 2017 12:38 PM
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        I had a look at this photo and this is a custom analyzer rather than a basic GC. Reconfiguration of what was an expensive custom analyzer goes beyond the scope of the Agilent community. We were able to find the original configuration and this was special order. I would not modify this GC for simple applications. I also see evidence that it may have been modified as the inlet labelling has been changed possibly by another company. If you wish to modify I would suggest contacting your local service Agilent | Contact Us .
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        mj.hazlett @ james_jenkins on Oct 6, 2017 12:43 PM
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        Thanks. Is there any reason that I can't use it for the simple application as is?
        
        Sent from my iPhone
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        james_jenkins @ mj.hazlett on Oct 6, 2017 12:57 PM
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        You can certainly try, I just dont know how the valves and plumbing will play into this. With some of these analyzers the back inlet was meant to only provide flow rather than be an active part of the injection path. There maybe someone else who is following this thread that maybe able to add value.
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    - mj.hazlett @ james_jenkins on Oct 3, 2017 11:46 AM
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      James,
      The shutdown message just says "Front Inlet Flow Shutdown", and sometimes has said "Back Inlet Flow Shutdown" in addition. The front inlet flow rate is always below the setpoint on the instrument display when it happens. I have noticed that if I increase the split ratio at the front inlet, the temperature where the shutdown occurs is lower. I have also tried with constant flow to see if this helps, however it does not. I notice when setting up the method, the back inlet pressure always needs to be about 5psig higher than the front inlet in order to be able to get the right flow rate.
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      - james_jenkins @ mj.hazlett on Oct 3, 2017 12:16 PM
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        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.
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        mj.hazlett @ james_jenkins on Oct 4, 2017 9:22 AM
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        Yes its the total flow dropping off.
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        james_jenkins @ mj.hazlett on Oct 4, 2017 9:45 AM
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        It sounds like you have some issues with the split lines and split vent cartridges. I would suggest cleaning or replacing the lines, but more importantly, the split vent cartridges should be replaced. In one of the above posts, valentinrusu has a link for manually checking for a split vent restriction. You could start there.
        James
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james_jenkins Oct 4, 2017 10:43 AM
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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.
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Troubleshooting front inlet flow problems

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valentinrusu,

6890/7890 Split Vent Restriction Test

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