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Air Pressure between two air tanks

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  1. Feb 8, 2017 #1
    So I am a heavy vehicle mechanic. One of my vehicles has an air starter with a separate air tank dedicated to the starter. This tank is fed from the main air tanks on the truck with a check valve to only allow air into the starter tank. The gauge on the dash only displays the pressure in the main tanks for the truck. We frequently notice the starter tank will have more or less pressure than the main tanks.

    I understand why the starter tank sometimes has more pressure than the main tank due to the one way check valve.
    I assume the check valve has something o do with why the starter tank might have less air than the main tanks.
    The check valve is a 3/4" and has a spring with probably not more than a couple of lbs.
    The starter tank is considerably larger than the main tanks, it that matters.
    My question is, why will the starter tank have less pressure than the main tanks?
     
  2. jcsd
  3. Feb 8, 2017 #2
    How much less pressure in the starter tank? (edit - And for how long?)

    The check valve works as you describe, air into starter tank when main is > starter. No flow the opposite direction.

    But as you notice, the spring has some force. That force must be overcome by the pressure in the main tank. So the starter tank could have less pressure than the main due to the need to overcome that spring. But that maybe just a few pounds?

    Other than that, a sticky check valve? The starter tank being drained faster than it can be filled by the main tank?
     
  4. Feb 8, 2017 #3

    CWatters

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    +1

    I would expect the pressure to be lower in the starter tank due to the small loss in the valve.

    It's not clear why the pressure would ever be higher in the starter than in the main tank. Possibilities include:

    Inaccurate pressure gauges (try swapping them over?).

    Temperature differences (starter tank getting hotter).

    Main tank leaking intermittently?
     
  5. Feb 8, 2017 #4
    The pressure in the starter tank is significantly lower than the main tank, more than a few pounds. I was hoping for a better explanation from some unknown to mortal men physics lesson. Haha
    The main tanks drop lower than the starter tank due to the check valve. The design theory is that your main system could have a leak but it wouldn't drain the starter tank.

    I was thinking due to the starter being significantly larger as air increased into the tank the pressure in the tank would also be trying to close the check valve. Since the surface area on the spring side of the check valve is a bit larger than the inlet side, would that make a difference?

    If I didn't mention before the starter tank does not have a gauge on it. The reason I know there is a difference is due to the main tank being filled to 120psi, the starter requires at least 60psi possible 90, to be able to turn the engine over. Sometimes the truck just won't start. We have to hook an aux line to it wait a little while then the truck will turn over without problem.

    Thanks again for looking into my problem.
     
  6. Feb 8, 2017 #5
    Without a gauge on the starter tank, you are in the dark. That also threw us off, we assumed there was, since you reported: "We frequently notice the starter tank will have more or less pressure than the main tanks".

    But you didn't "notice that", you just noticed you sometimes have a starting problem. Maybe there is some other starter problem, such that a higher pressure is needed sometimes?

    Never assume while troubleshooting. Measure. Question your measurement.
     
  7. Feb 8, 2017 #6
    I realize what you are saying and agree. I also realize their is information that I didn't provide. The system is maxed out at 140psi. I.e. The safety valves prevent it. Normal operating pressure is 120psi for the maintanks. If the main gauges show their is 120psi but the engine won't turn over and I add an aux line also limited to 120psi wait a few minutes then the truck starts it would imply that the starter needed more air.
    You could argue that the "120psi" shown on the gauge is inaccurate which is possible but the gauge does not control the aircompressor. The air compressor pumps until 120psi is reached at the governor. this is not the only truck that does it. I have 23 trucks with the same situation.
    I was trying to train a couple of my guys on the system and I couldn't explain why this happens to all of them.
    In 13 years in the Air Force everyone of these trucks I have encountered has this same quirk.
     
  8. Feb 8, 2017 #7
    With 23 trucks with the same problem, I would def consider taking the time/effort to add a gauge to at least a couple of the starter tanks.
     
  9. Feb 8, 2017 #8
    I agree. But bosses don't see it as necessary. "Just plug an airline into it."
    :-/
    Thank you all for your thoughts.
     
  10. Feb 8, 2017 #9

    CWatters

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    Does the air line connect to the main tank or the starter tank?
     
  11. Feb 8, 2017 #10

    rbelli1

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    Good catch.

    We all assume that "it" is the starter system. Possibly the OP sees the AUX line and assumes that the fact that the AUX port works the starter it connects to the starter system. We all have brain farts where we do the same thing over and over and make unfounded assumptions and when it doesn't do what we want the assumptions break our mental view of the system.

    BoB
     
  12. Feb 8, 2017 #11
    The aux line is connected to the main air system/tanks. Which then continues on to the starter tank as usual. The aux input was designed to allow supplemental air for braking while towing the vehicle
     
  13. Feb 9, 2017 #12

    rbelli1

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    It seems you have an issue with the main tank system. If your main tank is at 120psi and your aux supply is at 120psi then connecting it will do absolutely *nothing*. Either the main tank system is at less than 120psi but thinks it is higher or your aux system is at higher than 120psi. What happens to the gauge on the main system after connecting the aux pressure?

    BoB
     
  14. Feb 10, 2017 #13
    Depending on the pressure of the main tank. There is either no change in pressure if the main tank gauge reads 120. Which makes sense.
    If the main tank reads less than 120 it will raise until it reaches 120 but the vehicle still will not start.
    I'm going to try disconnecting the aux line as soon as the main tank hits 120. Then watch to see if the gauge drops. Maybe I have a restriction that's slowing the flow into the starter tank which would account for the pressure difference.
     
  15. Feb 13, 2017 #14

    JBA

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    Assuming the starter tank alone is expected to be able to start the truck engine if there is a loss of pressure in the main tank, then the required cranking pressure for the starter should be somewhat lower than the maximum 120 psi supply pressure in order to provide a sufficient air reservoir for a series of cranking rotations to the engine before the tank pressure falls below the required starter minimum operating pressure. Is this problem something you experience on new as well as used trucks; or, is it something that becomes more of a problem as a truck ages?
     
  16. Feb 13, 2017 #15
    Yes the starter only requires 90psi. Possibly less. All the trucks are 95-98 models. All with the same quirk.
     
  17. Feb 13, 2017 #16

    CWatters

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    Is there some sort of decompressor on the engine and is it working?

    I discovered my ride on mower wouldn't turn over because the decompressor was very sensitive to the correct valve rocker gaps.
     
  18. Feb 14, 2017 #17

    JBA

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    Do you have some type of dryer on your air supply to the main tanks and/or drains on the tanks to check for collected water?
    Also, does the problem occur more at low or high ambient temperatures?
     
  19. Feb 14, 2017 #18
    I'm not sure what a decompressor is. As far as I know the trucks do not have them.

    Yes the trucks all have Bendix AD9 air dryers and of course purge valves. This is fed from the compressor, then splits off to the main and starter tanks.

    I haven't noticed a specific issue with not starting correlating to a temperature range.
    Though we do have more air leaks when the temperature fluctuates. I mean to say, when it's cold at night then warms up during the day, we have more air issues. But once the wearer is cold all the time we have less issues, and the same for when it's hot all the time.
    North Florida and North Louisiana are the areas I've had the most experience in. The autumn and spring weater.
     
  20. Feb 14, 2017 #19

    JBA

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    Do your tanks have a pressure relief valve installed; and if so, what is the valve's set pressure? If there is a relief valve have you checked to if it is leaking on any of the tanks?
     
  21. Feb 14, 2017 #20
    Yes they have them per DOT. Set to 150 I believe as most trucks are.
     
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