Draining Air From a Compressor

[Ben]

What do you guys think the maximum time between draining air from a compressor tank a guy can go that is still "best practices"?

I find on my 30gal compressor, that I am currently using it like every other day, sometimes 3-4 days go by. Seems like it's a waste to always be opening the valve and then having to refill the tank the next day or in a couple of days. When I got it, even draining after a day still got me slightly rusty water, so I'm not sure that's a good indicator.

When I know I won't be using it regularly, like in Winter, obviously I'll drain it after each use. Right now though, I'm wondering if once a week or so is okay.

A lot of the jobs are small, so maybe I should just be using my little 2gal compressor more, but it doesn't give me the volume I need for stuff like blowing out the tractor air filter, which is a pretty regular deal. Then I have to fire up the big one anyway.

[dogmush]

It really depends on temperature  and humidity.   In FL, it's like every 4 hours or so. In Idaho, I could see it being fine to go longer.  You don't  want a lot of water build up.  You also don't  need to drain the tank completely.  Just crack the drain, wait till water stops coming out, then close it back up.

If it keeps you up at night you can always put in a timed auto drain, and never consider it again.

[Ben]

Hey, that doohickey is interesting and cheap. I'm going to investigate further.

You know, I never thought about not draining it all the way. That's kinda stupid of me. If I didn't get the doohickey, I'd have no problem just cracking the valve for 10 seconds or so to get the wate buidl up out. I don't know why I thought I would have to completely drain it.

I already took your previous advice and got one of those extension hose valves, which makes it a piece of cake to get to the valve, especially compared to how DeWalt placed the stock valve, which required me to elevate the compressor to get my hand to the valve.

[dogmush]

I'm in the process of building a sound deadening enclosure for my compressors, and I'll put a remote auto drain in when I finish. That one's ben on my Amazon list for a year. They are pretty handy. the one we have at work is wired into the motor controller so that it powers on and opens the drain for 15 sec on power up, then 15 sec every hour that it's on.

[K Frame]

I have a small portable compressor. It gets drained after every use, and the drain cock is left open. Not sure if that's a good practice or not.

[dogmush]

I have a small portable compressor. It gets drained after every use, and the drain cock is left open. Not sure if that's a good practice or not.

Doesn't really matter.  While, yes, you don't want the inside of your pressure vessel rusting away, in practice those tanks take quite a bit to rust through.  What you are actually trying to avoid by draining the tank is running water through your lines, air tools, or oiler.  That will *expletive deleted*ck *expletive deleted*it up right quick.  And the water (in the tank anyway) comes from the cooling of the hot compressed air as it expands into the tank or goes through the air cooler (if equipped).  That's why my first answer mentioned location.  My compressors suck in hot muggy air, heat it more, then suddenly drop the temp quite a bit.  I'd wouldn't be surprised if I had full on thunderstorms in that tank.  Ben may well get much less water, just because he has much less water per cu ft of air.


Because it's the act of heating and cooling the air while the compressor runs that actually causes the condensation, you are better off with quick drains frequently during operation, then just draining it at the beginning/end of the day.  For all practical purposes your compressor isn't getting more water in it sitting at one pressure, either 0 or 125 psig.  It's the cycling that does it. I'm horrible about remembering to drain my tank (hence the planned auto drain), but I also run all my air through a big honking air dryer right after the tank, which helps a lot.  Ideally you'd run it through a cooler/dryer between the compressor and the receiver*, but that would make for an expensive system.  For smaller homeowner portable compressors, you are fine just leaving it open for storage, running it for a minute or two to blow out any dust/insects/stuff, then closing it and doing your thing. On my small nail gun compressor, I crack the petcock and drain water every time I load more nails in the gun.  That seems to be a pretty decent compromise of time spent vs water in the air.  For "shop air" that you might use off and on for a full day, or several days of projects, a more involved dryer/drain plan might be warranted.   


*In point of fact that's exactly what ships that used compressed air for controls and engine starting do.  several compressors running in parallels, then the air comes together and goes through a cooler/dryer, then into receivers.  The main line out of the receivers will usually have the regulator and oiler.

[K Frame]

What? You mean the water in my compressor doesn't spontaneously generate there? 

[Ben]

Dogmush, your previous post was enlightening. I never get a lot of water out of the tank, but there is almost always some (though I am going to check the difference between what I get out of it now and in Winter, when humidity often drops to the teens). I also wasn't thinking at all about water in the tools, etc.

Previously, all I ever had was a small compressor, and probably 90% of the use was topping off car tires. Hence it got turned on, tires filled, tank drain and compressor stored until "next month" or whenever. Now I do all kinds of things with a compressor and much more frequently. I need to adapt to a completely different procedure, and I think that's going to be either quick, ten or so second drains more frequently, or just getting one of the auto-drain valves.

[bedlamite]

We have two screw compressors at work. Once a week we drain the water trap, and a maybe an ounce of oil from each, then top off the oil.

[Brad Johnson]

Sounds like your setup is a prime candidate for a water trap between the compressor and tank. Easy install, mostly just a few feet of tubing and mounting the trap body.

If your compressor is mounted high enough, maybe a water catch and draincock on the drain plug. Something you can valve off and isolate for draining without having to depressurize the whole tank.

Brad

[Cliffh]

Dogmush, your previous post was enlightening. I never get a lot of water out of the tank, but there is almost always some (though I am going to check the difference between what I get out of it now and in Winter, when humidity often drops to the teens). I also wasn't thinking at all about water in the tools, etc.

Previously, all I ever had was a small compressor, and probably 90% of the use was topping off car tires. Hence it got turned on, tires filled, tank drain and compressor stored until "next month" or whenever. Now I do all kinds of things with a compressor and much more frequently. I need to adapt to a completely different procedure, and I think that's going to be either quick, ten or so second drains more frequently, or just getting one of the auto-drain valves.

Try doing some spray painting using your compressor, you'll notice the water most ricky tick.

ETA:  Without a dryer, that is.

[Kingcreek]

Whoa, you’re supposed to drain the air from an air compressor?
What is this, some kind of a prank?

[JTHunter]

You might want to leave pressure in those tanks.  Letting all the air out causes the air to get cold and condensation forms on the inside of the tank.  This can cause unseen rusting that will result in a possible catastrophic failure.
Letting air out and refilling it frequently can cause "metal fatigue" because of the metal flexing in and out as the pressure changes.  With air in the tank all the time, the metal doesn't flex as much and the air is hot going in because of the compression.  Yes, as it cools down, moisture that was pumped in can condense.  If any runs down to the drain valve, a short release will get that out without letting the air inside getting too cold.  Draining a small amount of water out every morning (if any) before using the tank keeps the tank charged and dry.

[HeroHog]

When I mechaniced, we drained the shop tank daily AND had water separators on the lines that were drained as well.

[230RN]

I'm not sure on why you want to completely empty the tank down to ambient pressure.

Seems to me that rusty water coming out contains oxygen from the air aleady in the tank in the form of the rust in the water.

Therefore there is less oxygen to cause rusting in the remaining air.

So it seems to me you should drain the water and when it stops, close the valve.

If you empty it completely, you start all over in providing fresh air containing a full 20% oxygen, in addition to using power and time in refilling the tank completely.

Terry

[Bogie]

We had black pipe lines (DO NOT USE PVC - it blows up), and had them at an angle.
 
FWIW, if you use the "squeeze tube" pipe dope instead of the teflon tape stuff, you will have a LOT less leakage. You might be surprised.
 
(tell the guy at the home depot that you want the stuff for installing gas lines, not water lines)

[HeroHog]

you should drain the water and when it stops, close the valve

[230RN]

I never paid that much attention to the problem since the air's pretty dry here.  Besides, don't they design the tanks to spring  a goodly leak instead of catastrophically rupturing?  Seems to me if were an engineer designing a pressure vessel I'd slip something in there besides the safety valve to avoid an explosion.

Thanks, HeroHog, that's what I was saying, too.

Terry