Simple safety question: Can I leave a portable air tank pressurized for long periods of time?

General Questions
#1

Super basic safety/prudence question here. I have a compressor and this portable air tank.

I fill the tank up every time I want to use it (and don’t want to lug the compressor upstairs), then empty it and put it back in storage. Can I safely I fill it, then leave it filled/under pressure in the garage for months at a time? Or could that weaken it or lead to some sort of fault?

Why yes, I did just top off my tires before heading out this evening. If nothing else, take this thread as a reminder to check your pressure. And the batteries in your smoke detector. And call your mother. And put on a sweater, it’s cold outside…

Thanks~

#2

what about clean underwear when you go outside the house.

i knows lots of people with decades old tanks that are kept full. i keep one full (within the safety range on the gauge) or partially.

#3

I have a little 3 gallon compressor. Its instructions say to empty it after use. However part of that may be to drain moisture out of it.

I wouldn’t think that a storage tank would be much good if you couldn’t store air under pressure for a long time.

#4

Most auto shops leave theirs always pressurized.
If you are talking about very rare usage (filling your own tires) I wouldn’t worry about it.

The biggest concern IMHO is making sure that you drain the condensate. A frequently used tank will accumulate quite a bit of condensate (a few tablespoons), and you don’t want that to be sitting in the tank for a long period of time.

I imagine they coat the inside to prevent rusting, but I remember in one shop I worked in that whenever we drained it the water always came out rusty.

#5

a compressor tank will get water in it. it will blow out the drain.

if you fill a storage tank from a compressor tank then the storage tank will be dry.

#6

I have a 60-gallon compressor/tank in my garage; I don’t think it’s been below about 90 psi for perhaps seven years now.

Fill up your portable tank and store it that way; everything’s fine.

#7

Just don’t use it for target practice when you are mad. ( mom )

#8

I would think that this would not be correct. Unless you have a ‘moisture trap’, (in-line drier? not sure of the correct term) the moisture in the compressor tank (albeit, a very small amount) is just going to be transferred to the storage tank. :confused:

#9

Missed the edit window. :frowning:

I would think that this would not be correct. Unless you have a ‘moisture trap’, (in-line drier? not sure of the correct term) or manually drain the tank, the moisture in the compressor tank (albeit, a very small amount) will blow out with the air being released and is just going to be transferred to the storage tank.

#10

Heh, heh. True that! :stuck_out_tongue:

#11

I think leaving the tank pressurized would be the standard practice. I typically do.

About moisture in the tank, this is interesting. Water will condense out of air when the pressure of the water vapor in the air is higher than the pressure water develops by evaporating at the same temperature. The mixture of gasses all keep track of their own independent pressures, so to speak, and know when to condense on their own. Compressing ambient air multiplies the pressures of each of the gasses there, including the water, and often raises the water’s partial pressure above its condensation level, so the excess water condenses out.

You might typically figure that if air is compressed much, especially in humid conditions, it will equilibrate at a saturated condition with liquid water at the bottom of the tank.

When you fill a portable tank from the main tank, whether you saturate the air in the portable depends on several things, like how much you lower the main tank pressure, and like whether you then take the portable tank outside where it’s cold after filling it indoors where it’s warm.

They generally design things so the liquid water in a tank does not come out any of the openings except for a dedicated drain opening.

#12

I do not see any DOT information on this tank.
Transporting it filled may put you in jeopardy.

#13

Water inside any compressed gas cylinder is bad. Really bad.

The worst case scenario was tested in a Univ. of Rhode Island study. 500ml of salt water was placed inside steel compressed gas scuba cylinders. It corroded two thirds of the cylinder wall thickness making the cylinder dangerously unsafe within 100 days. Aluminum cylinders held up longer. Salt water was more corrosive than fresh water. Of course these operate at MUCH higher pressures than a shop compressor in your garage.

The first page of this Scubaboard thread has relevant discussion. The PDF of the cited study is archived there as well.

A moisture trap really is an essential safety device on any compressed gas cylinder. Shop compressors have MUCH thinner cylinder walls due to the lower pressures involved.

#14

Isn’t this pretty rare? I’ve seen filters that include a passive moisture trap on compressors and other systems, but I don’t think I’ve ever seen a moisture trap on just a gas cylinder, ever - and I’ve been around them for 50 years. Am I missing something?

#15

Thanks. So here I’ve been filling, then emptying both compressor and portable tank after every use. I’ve followed the steps to let the water drain out (there’s a plug at the bottom of my compressor), but have yet to notice any actual water leaking out.

So, I’m going to fill the tank, shut the valves, then leave it like that in the garage. Heck, I may even go hog wild and leave a tire chuck with it (but come to think of it, not attached, as the fittings could leak air over time).

It sounds like I can even run/fill the compressor, then unplug and store it with air in it.

To be safe, I’ll add ‘check compressor’ to my daylight savings list (smoke detector batteries, grease garage doors, etc.) and empty it and any possible water a couple times a year.

How wonderfully convenient, thanks !

#16

Do you live in very dry climate? If there’s very little moisture in the ambient air (say, less than 10% relative humidity), then when you compress that air to ~125 psi, the partial pressure of the water still won’t exceed its saturation pressure, so you won’t see any condensation.

Also, since the air heats up when it gets compressed, the water will remain in vapor form until the air in the tank cools back down to ambient temperature. If you fill your compressor tank and then empty it before that air cools off, you probably won’t see any condensation then either.

Yes, SCUBA tanks are filled to somewhere in the neighborhood of 2000 psi about 16 times what a typical household air compressor sees. At that pressure, there’s a lot of oxygen available, so anything known to facilitate corrosion at ambient conditions (e.g. salt water) can be expected to really rot things in a hurry inside a tank like that.

#17

I opened up a new air storage tank at the store yesterday and read everything included.
The only relevant safety information was that tanks like this have a “Destroy By Date” stamped onto a leg. It looks to be like 3 years on the ones sold by this store.
I also didn’t see any codes or regulations to sustain this information.

#18

(bolding mine)

Well, that’s interesting information, and something I wasn’t aware of until now.
FWIW, I know at least two people that own (what I think of as typical, 10-15 gallon, ~125 psi max pressure ) ‘air storage tanks’. IIRC, and to the best of my knowledge, they leave them pumped up all of the time and have never had any problems.
As for the ‘destroy by date’, WTF? I can’t even think of a reason why you would have to destroy one of those types of tanks, unless of course it incurred damage of some sort. IMHO, those don’t get filled to a high enough pressure to even be a serious danger.
‘High pressure’ tanks such as oxy-acetylene tanks and scuba tanks on the other hand, yeah sure, those should (and for that matter, have to) be tested every so often.
IIRC, the requirements for scuba tanks are: visual inspection (unscrew the valve and look inside) every year, and a hydrostatic test every five years. Not sure of the requirements for other types.
Aluminum scuba tanks are filled to 3000 psi, steel scuba tanks = 2250-2500 psi.
These figures are what I remember from when I did a lot of diving back in the 90’s. (There have been recent design changes with increased pressure ratings for newer model tanks.)

#19

I run a machine shop, we have a 200 gallon tank and compressor that has been more or less on and energized to 150psi or so, for about 5 years now.

It replaced a tank compresser unit that had been used here by the owners’ grandfather for about 20 years and it was used then. The tank on this old one did eventually rust out from the inside from condensation. That was after what could be millions of cubic feet of air had been run through it.

Even if your tank did eventually start to fail, it would be a small soft spot and the pressurized air will weep out. I can’t possibly see any sort of catastrophic boom.

#20

There isn’t enough pressure and volume to worry about. I’ve had one for 20 years, I leave pressure in it frequently. I do drain the air compressor, when I think about it. Generally, the pump on consumer grade compressors will fail before the tank rots out. I worked in a factory for 30 years. They had huge compressors, feeding twin 48 inch lines, maintaining a minimum of 150 psi. It was stepped down to 90 psi for most lines. They still needed dryers on machines, water in the air was just something to be managed, you couldn’t totally eliminate it.