# Air Pressure between two air tanks

• System1459
In summary, the main tanks may have less pressure than the starter tank due to the one way check valve. The check valve has a spring with some force and the pressure in the main tanks may be higher than the pressure in the starter tank due to the need to overcome that force.
System1459
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?

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?

+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?

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.

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.

jbriggs444
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.

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.

I agree. But bosses don't see it as necessary. "Just plug an airline into it."
:-/
Thank you all for your thoughts.

Does the air line connect to the main tank or the starter tank?

rbelli1
CWatters said:
Does the air line connect to the main tank or the starter tank?

System1459 said:
hook an aux line to it
System1459 said:

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

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

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

NTL2009
rbelli1 said:
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

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.

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?

Yes the starter only requires 90psi. Possibly less. All the trucks are 95-98 models. All with the same quirk.

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.

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?

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.

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?

Yes they have them per DOT. Set to 150 I believe as most trucks are.

I would assume that the tanks are maintained at 120 psi by a truck mounted compressor when the truck is operating, so is there any difference in the number of starting problems related to how long the truck has been idle without the engine running between starts (i.e. sitting shutdown overnight) or is it intermittent during the truck's normal operating periods.

System1459 said:
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.

Have you run the above test?

The trucks won't start semi randomly. Basically when the truck is off air leaks cause the tanks to lose pressure. My concern is why they fill at different speeds. Starter and main tanks. The only thing separating them is about 5ft of air line and a check valve that protects the starter tank.
Yes you are correct pressure is maintained by engine mounted compressor.
I have performed my test since I'm on Leave and happy to be away. Haha.

First, if the observed pressure in the supply tank is 120 psi at the time of the failed start and the starter fails to turn at all and, it is assumed that there is no issue with the air starter or engine that requires an excessive (>90 psi) pressure to start the engine, then it would appear to me that the source of the problem is most likely to be an issue with the check valve possibly sticking closed and preventing the supply tank from maintaining the pressure in the starter tank if that tank and it system have any leakage.

On the other hand, it may be a possibility that there is an issue with a combination of flow restriction of the check valve and the size ratio of the starter tank to the supply tank. Since you stated that the starter tank is larger than the supply tank, if the starter tank pressure is low at the point that the truck compressor is shut down with 120 psi in the supply tank and it is still filling the starter tank, then the volume of air in the supply tank at 120 psi alone may not be sufficient to raise the combined volume of the larger starter tank to its required 90 psi or above before the supply tank equalizes its pressure with the starter tank. For example, if the the starter tank is twice the size of the supply tank then their combined volume is 3 times that of the supply tank alone.

Spring loaded check valves always throttle the flow through them and the rate at which they will allow flow is based upon the differential pressure between the inlet and outlet sides, so the closer the starter tank pressure gets to the supply tank pressure the slower the flow through the check valve and therefore the longer it will take to to equalize the pressures of the two combined tanks.

For now, that is about the best possibilities I can offer.

That sound very plausible. I appreciate your help. I think this is as close to knowing for sure as I will get. Thanks again.

## 1. What is air pressure?

Air pressure is the force exerted by air molecules on a given area. It is caused by the weight of the air above the surface and is measured in units of pressure such as pounds per square inch (psi) or pascals (Pa).

## 2. How is air pressure between two air tanks calculated?

The air pressure between two air tanks can be calculated using the ideal gas law, which states that the pressure of a gas is directly proportional to its temperature and the number of gas molecules present. It can also be calculated by measuring the volume and temperature of the air and using the equation p1V1/T1 = p2V2/T2, where p is pressure, V is volume, and T is temperature.

## 3. What factors can affect air pressure between two air tanks?

The factors that can affect air pressure between two air tanks include the temperature and volume of the air, the number of gas molecules present, and any external forces such as gravity or compression.

## 4. How can air pressure between two air tanks be adjusted?

Air pressure between two air tanks can be adjusted by changing the temperature or volume of the air, adding or removing gas molecules, or applying external forces such as compression or pumping air into the tanks.

## 5. Why is understanding air pressure between two air tanks important?

Understanding air pressure between two air tanks is important because it allows us to accurately measure and control the amount of air and pressure in a system. This is crucial in many scientific and industrial applications, such as in pneumatic systems, scuba diving, and air compression processes.

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