Does draining the tank actually help prevent rust at all?
Here's my thinking...
No matter how often you drain a tank, the inside of a compressor is ALWAYS going to have moisture on it. That's what compressing air does.
Rust doesn't form under water. It needs air too.
How would a damp surface steel tank with plenty of air in it be LESS prone to rusting, just because there was a smaller quantity of water in it?
Seriously- I'm kinda curious.
That must have scared the E36 M3 out of someone.
SVreX wrote: Rust doesn't form under water. It needs air too.
rust -does- form underwater.
mad_machine wrote:SVreX wrote: Rust doesn't form under water. It needs air too.rust -does- form underwater.
I seem to recall the Titanic is just a little rusty. 
SVreX wrote: Does draining the tank actually help prevent rust at all? Here's my thinking... No matter how often you drain a tank, the inside of a compressor is ALWAYS going to have moisture on it. That's what compressing air does. Rust doesn't form under water. It needs air too. How would a damp surface steel tank with plenty of air in it be LESS prone to rusting, just because there was a smaller quantity of water in it? Seriously- I'm kinda curious.
H²O when FE meets the Oxygen on an atomic scale rust happens. I have a cut open rusty tank i can get a photo of if anyone wanfs to see it. The rust is much more flaky and prominent at/below the water line. There were just odd patches of rust on the above water sections.
Oxygen is one of the components of water
MOLECULAR oxygen causes rust... the oxygen in water is tied up with hydrogen and can't combine with iron. However water does absorb free oxygen, it's how fish breathe after all.
I knew someone who did a weird experiment or something. Not sure what the goal was, but he had an aquarium that he filled with very salty water and put a piece of steel in it so it could rust. What he found was after a short period of time, the steel stopped rusting. It had used up all of the oxygen in the water. He agitated the water a bunch to stir air into the water and the steel started rusting again.
stuart in mn wrote: I once saw the aftermath of a home air compressor where the pressure switch that controlled the motor failed, plus the pressure relief valve was stuck shut. It just kept running until the tank exploded. It made a mess of the car that was parked next to it, fortunately no one was in the garage when it went off.
This scenario is my guess.
Wait a minute- I worded that really badly. Half asleep.
The question wasn't if it wouldn't rust. The question was if it would rust less by draining the tank regularly.
Seems to me there is plenty of water AND air to rust, whether or not you drain the tank.
I recently had one that was at least 15 + years old. I went to drain the tank and it had a fair amount of water in it.
I rocked it up and down and I could hear either more water or rust rolling around in the tank.
It is now outside my garage in the scrap pile.
This is why I do not have just one valve on my 80 gallon tank but two. Both are temp and pressure valves so if the tank is charge and there's a fire they will open before the tank blows up killing a fire man yes the added air might feed the fire but tank should not rupture. 3/4 pipe fittings on it in the tank the other in the supply line as the motor and compressor are up stairs tank is in between the garage doors.
Not sure when it changed but tank used to be ASTM cert pressure vessels but some time in the last 20 years that changed hence tanks got thinner and cheaper. I got mine from a Dental office that was getting rehabbed.
Don't take a chance w/ a suspect tank. Not a total loss though, think lemonade, they make great smokers and bbq's.
We run 180psi into a pair of 80 gallon tanks at work. One is the old compressor tank and its near 50 years old. Im nervous all of a sudden. We drain once a week.
In reply to 44Dwarf:
Hmm... interesting thought... Could the emergency relief vent be piped to outside? Then the air wouldn't feed a fire. 
You know, I should probably drain my shop compressor very soon. It's a horizontal 21 gallon Craftsman, but the drain broke off. I really need to get around to fixing that.
After you drain, do you leave it open or close? My little PC pancake compressor seems to put out quite a bit of black water everytime I use it.
Sure could be I had not really given that much thought but yes easy piped away and out. thanks
Ian F wrote: In reply to 44Dwarf: Hmm... interesting thought... Could the emergency relief vent be piped to outside? Then the air wouldn't feed a fire.
I've got a monster 100-gallon compressor that was made in the 50s that I'm going to scrap due just because I don't want something like this to happen. I don't use air anymore thanks to advances in cordless technology. I've got a 2-gallon compressor that fills tires, that is it.
I googled "air compressor explosion" once and wished I hadn't. Apparently the oil from the compressor can make it into the tank and coat the walls. If someone welds on the tank it can go boom. The first few pics I found were just like the one in this thread, then came the one from a shop accident where a young guy was welding on a small tank mounted to a work truck. He was missing half his head and the tank was turned inside out. Supposedly it had been rinsed out and wasn't under pressure, but the oil residue still went boom.
My Dad's shop had two huge 150-gallon monster compressors that shook the warehouse. I always made a point to be out of the building when they drained them at night because it sounded like an air raid siren.
Enyar wrote: After you drain, do you leave it open or close? My little PC pancake compressor seems to put out quite a bit of black water everytime I use it.
I typically leave the drain open, but I also don't use mine nearly as frequently as many people at the moment. I rarely if ever leave it with any air/pressure in it overnight and always depressurize it, open the drain, and leave it open after I'm done for the day.
Looked like the compressor lived in the bathroom. So was it the compressor.....or the methane?
A 90 degree pipe fitting screwed into the bottom of the tank with a with a quarter turn ball valve screwed into the fitting and a section of rubber tubing on the exhaust side of the ball valve make the tank very easy to drain into a can. I happen to use an old one gallon mineral spirits can. If you open the valve slowly until all the water is drained it doesn't make a mess. Like everything else in life if its easy to do it gets done. Easier then laying down trying to reach the drain on the bottom of the tank while tired after a long day in the garage. I live in very humid conditions and am always surprised how much water can accumulate in a short time of the compressor running.
I bought a cheap 220v solenoid valve for my air compressor. It's installed in the drain fitting between an elbow and a barely cracked open ball valve and wired in parallel with the motor. Every time the motor runs the solenoid valve opens and the tank drains. I'm pretty sure it was a $10 project.
For people worried about future tank explosions... An idea that I really like for multiple reasons is eschewing the use of a tank entirely and just run really large copper tubing. Put the compressor wherever is convenient, and run say 2" copper pipe in the roof, down stringers to convenient points, to a chuck outside (ahem), and so forth. You can easily get 20+ gallons of volume this way, and no steel tank to worry about rusting.
Say 2" pipe is 3 square inches inside, 231 cubic inches per gallon (just like a Buick 
) 20 gallons is 4620 cubic inches or 1540 inches of pipe needed. That's 128 feet, which you could do in a largish shop if you were generous with how you routed things.
Now, you do admittedly have to factor in how much 128 feet of 2" copper will cost...
The tank on my compressor is dated 1971. It was pressure tested to 500psi in 71 and is rated at 300psi working pressure. I don't see any rust colored water when I drain it every couple of months. I'm running it at 125psi, or 25% of it's tested pressure. There is a rubber hose between the compressor and the air plumbing to act as a fuseable link, it should blow first, if the pressure switch and safety valve fails. There is more steel in my 80 gallon tank, than there are in some car bodies. Even though I leave it on 24/7, I'm really not worried about it blowing.
Some of the HF stuff would make me nervous. I've seen the pressure valves on two of those stick and burn up the motors. I would imagine their relief valves are pretty cheap as well.
I have been thinking of moving the compressor to the other side of a cinder block wall from my work area just for noise reasons. This will accelerate that plan i suppose.
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