Is there a hazard with the 2nd tank having too much air in it? How does the 2nd tank not overfill itself?

by Marc Carraway
(Lexington, SC)

This is a question just for an explanation on how or why something works.

I'm sure theres not much danger adding a 2nd tank to an air compressor. But what my mind cant understand is how the 2nd tank doesnt fill up past what its rated for or is supposed to hold. In other words, if I have a 21 gallon air compressor, I add a 2nd 15 gallon tank or something like that, what makes sure the 15 gal tank doesnt end up with 20 or 30 gallons of pressure inside of it, possibly making it crack, break or explode or something?

See what I'm saying, or not understanding?

My mind isnt acting right for some reason. (and its not what some of you may be thinking) I simply really just cant understand it mentally. Thanks a lot for any explanations helping me 'get it'.
Thanks for a good question, Marc.

Ask yourself where the compressed air that is going to the second tank is coming from? It's either plumbed from the first tank or a tee from a line from the discharge coupler on the air compressor, right?

What stops the air compressor from overfilling the first tank? Two things, the pressure switch which shuts the compressor off when the first tank pressure reaches the cut out setting. The second is the PRV, protecting the air circuit by allowing air to blow off once the pressure inside the tank gets past the normal cut out (the pressure switch didn't work) and reaches the PRV cracking pressure point.

OK, now, if we are supplying the second tank with air from the first, there is no way that the compressed air pressure in the second tank can reach a danger level, as it's protected by the two devices in the first tank / air compressor.

Make sense?


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Feb 22, 2014
air on 2nd tank is at same pressure as 1st tank
by: marc carraway

Doug also explains it clearly. The 2nd tank has no extra motor or seperate compressor creating extra air. The air is coming from the same motor or compressor(piston maybe) that tank 1 gets its air from. At least that's sort of how I think Doug explained it. So once air starts building up, its got a pressure reading from all the air combined in both tanks at once, I assume anyway. Thanks for clearing up my brain confusion you all!

Feb 04, 2014
two tanks is really one in a way?
by: Marc

Ohhhh. I got it. (i think) I think it doesnt matter that theres a 2nd tank if I dont think of it as a 2nd tank. If I think of it all as one tank, it makes sense to me. Like all the pressure from the 2 tanks is combined together to give one pressure reading. Your 2nd tank isnt a 10 gallon or a 20 gallon or whatever you use, anymore. Its just combimed with the 1st tank and say that 1st tank was a 20 gallon and the 2nd add on tank was a 10 gallon, you really actually have one big compressor tank that holds 30 gallons. Maybe? So the 2nd tank isnt by itself really, its all one tank, the 1st and the 2nd, theyre all just one like 30 gallon tank now. I think. Thanks a lot for the input!! Big help.

Feb 04, 2014
Another thought on pressure
by: Doug in

I think maybe what is getting you is the idea that air, when pushed into a smaller space, gets to higher pressure than in a larger space.

And that is quite correct; but that only happens when something like a piston in a compressor forces air into a smaller space - some sort of mechanical force on the air.

Left on its own, air will only expand to evenly distribute the pressure within whatever holds it.
Key word here is evenly. So, as Bill said, the pressure can go down, but not up.

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How to drain the air compressor tank?

How to drain air compressor tank?

Thanks for writing in.

I've uploaded a picture for you to see what I'm talking about.

Under the tank, usually at the lowest spot on the tank, you'll find a valve, one of which you can see in the photo.

Turn this valve one way and the air and water can flow out of the tank. Turn it the other, and the valve closes keeping your compressed air in the tank.

When I'm finished using my air compressor I open this valve to drain off the collected water, and I leave it open until the next time I want to use the compressor.

Don't forget to close it before you turn on your air compressor. Most of them are so loud you won't be able to hear the air escaping out the drain valve is the compressor tries to fill the tank.

That the drain valve is open is one of the many reasons why your air compressor runs but doesn't build pressure in the tank.

One other thing. Expect the drain to have water in it, so when you open the valve you'll get water with crud and oil in it all over your hand. I wear a glove when I drain my compressor tank. :-)



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Jul 28, 2011
About draining my air compressor
by: Effie from TX

Hello Bill
I wanted to know if I should be concerned about rusty water blowing out of my air compressor?
And if I'm in danger of any sort..??
There has been times where I forgot to drain it.
Is it okay if I leave the valve open after every use??


I recommend that you leave the drain open after every use, yes.

The rusty water indicates that you have rusting taking place inside the tank, which doesn't surprise me, as they tanks, like all the rest of DIY import compressors are ordered from the lowest bidder.

One of the good things (if that can be said) about a rusting air tank is that, eventually, when rust eats away sufficient wall thickness, they will leak, and that's all.

I don't believe you are in any danger due to the possible rusting of the compressor air tank.

In terms of how to drain it, please see the page Drain the tank (how to).



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Convert an LP tank to an air tank

by John Lees
(Dahlonega, Georgia)

I have a 250 gallon LP tank that I want to convert to an air tank. The rating on the tank is 250 psig at 125 degrees F. My compressor produces a maximum air pressure of 125 psi. Do I have to use a conversion factor to compare psig to psi? And, the tank has a pressure relief valve installed. Will it work for air as well as lp gas?
John, see the pages on gauges that provide information on what PSIG versus PSI are.

While I suspect the PRV will work for compressed air too, since I have nothing to do with LP gas or storage, I cannot say unequivocally yes. Perhaps another visitor might comment?


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Feb 13, 2014
LP tank
by: Doug in

Air is a gas and so is (vaporised) LP.

Air molecules are smaller than LP, so if there is a small opening anywhere, it might pass air while being too small for LP to get through.

Having said that, but not knowing for sure, I think the existing PRV should work fine. You might want to test or just replace it if in doubt.

Finally, that's a lot of air. Depending on your compressor, you might run over its max run time filling from "empty" the first time, and possibly even from 95 or so cut-in pressure. You can do a rough calculation of the time based on you compressor output.

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