Compressors - drain every day or leave pressured?

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Um, no. There isn't much air in water, but there is a hell of a lot of oxygen. Water contributes to rust so well because water is self-ionizing (something to do with the shape of the molecule). A small fraction of the water is always free oxygen and free hydrogen.
Beyond that, I'm completely ignorant about the details of the steel that is used for these tanks. Maybe the typical tank-steel rusts very slowly.
http://en.wikipedia.org/wiki/Self-ionization_of_water
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Drew Lawson wrote:

Good point.

Thanks for the link, but I got to tell you, it was way over my head.:-) I couldn't figure out if it addressed say a nail submerged in water vs a nail kept constantly wet/damp but not submerged. I'm thinking that a compressor is always wet on the bottom whether or not you drain it. What do you think? I don't have a clue myself, but I know I rarely have drained mine in over 30 years, and it still has no leaks, and the last time I drained it, about 6 months ago after a similar discussion here (where I learned the tank doesn't have a glass liner) I drained several pints of water into a glass container, no sign of rust, and no sign of oil.
I'm also thinking when/if it rusts through, it will go pfsssssh instead of boooom.:-)

Maybe they use, or used to use, or some use, a rust resistant, high nickel or something type of metal. I know mine is over 30 years old, bought it used, and my brothers he bought used when I was 12 years old, really old then (50 years ago), and it still holds air fine, no signs of leakage (he drains his though) I remember painting cars with it and it sounded like it was going to knock itself apart. My brother said if breaks, he'll buy a nice 2-3 stage compressor... still running still a knocking. I painted lots of cars and trucks with that thing, and it proves if you want something to break, it never will.

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wrote:

I'm thinking you're right. And the reason I think so is that the relative humidity (RH) inside a tank charged to 135 PSIG (factory shutoff setting on my compressor) will be about 10 times greater than the ambient RH. So anytime the compressor runs when the RH is somewhere above 10% you're going to get condensation inside the tank by the time the 135 PSIG (~150PSIA) shutoff pressure is reached.
If the shutoff pressure is below 135 PSIG, then the ambient RH necessary to cause condensation inside the tank is correspondingly higher.
Tom Veatch Wichita, KS USA
An armed society is a polite society. Manners are good when one may have to back up his acts with his life. Robert A. Heinlein
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Jack Stein wrote:

I used to have a small home air compressor. Over time the air tank developed a couple of holes. These were in the bottom and would vent air and water when in use. The pump eventually gave out so I replaced it with one from HF.
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Is an iced tea glass only wet on the bottom? It condenses moisture on the outside of the glass every where the glass is cooler than the ambient temperature. The compressor tank works the same way. HOT compressed air goes into the tank and the moisture in the air condenses every where on the inside tank walls. Basically the moisture is all over the inside walls untill the droplets become large enough to run down to the bottom of the tank and collect.
I firmly believe that draining the tank helps to slow rusting but more importantly it maintains tank air capacity and helps to keep moisture out of the air hose. A tank that has a 20 gallon capacity and has 4 gallons of water in it will recycle 20% more often, or something like that.

I had an 80 gallon unit fail that way, it was a slow death. With relatively low pressure and the fact that there is/are weakest points in the tank pin holes developed and leak. As they rust they become bigger and leak more. I suppose if you ignore that situation the tank could eventually explode or blow a larger hole.
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wrote:

Think away.

Maybe.
Would you go into a disco with an epileptic...when the epileptic had a loaded gun pointing at your head? It has similar risks to an uninspected uninsurable pressure vessel.
(answers such as I can't dance and I wouldn't go into a disco are not permitted)
:-)
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Actually I think a closer comparison would be the epiletic hoalding a loaded gun to your head and an eliletic pointing a loaded gun at a pressurized container that you are setting on. As story hungry as the media is I don't recall having ever heard of an air compressor exploding and I have worked around compresssors for my entire professional career.
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On Mon, 18 May 2009 16:54:25 -0500, "Leon"

And I doubt that you will hear of one caused by rust through. Failure of a pressure relief valve along with simultaneous failure of a pressure shut off valve could cause overpressurization to the point of catastrophic failure in a tank in good condition. Rust through will weaken a tank wall to the point that pinhole leaks will develop at the weakest points. Those pinholes could grow due to the escaping airflow, but in doing so, would act as a pressure relief valve reducing the tank pressure. Catastrophic, shrapnel producing tank failure due to rust through is a very low probability occurrence.
My opinion, unsupported by any indepth analysis, is that being brained by a meteorite is about as likely as being injured by a rust through failure of a shop compressor tank.
And no, I'm not saying rust through failures don't occur and people have been hit by meteorites. I'm sure someone's second cousin thrice removed has a neighbor who knew someone who'd heard of about a pressure tank exploding due to rust through..
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Water would tend to collect in the bottom of the tank first, and that's where the rust would occur, right? So by the time the bottom of the tank has rusted out, the sides and top of the tank would still be in good shape. Should the right conditions occur, I think it more likely a compressor tank act as a rocket and not a bomb.
Puckdropper
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reason why all trees have to be grounded..." -- Bored Borg on
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"Puckdropper" <puckdropper(at)yahoo(dot)com> wrote in message

NO! Like a glass of ice water the water would collect on all cooler surfaces, basically every square inch of the interior surface of the tank. Compared to the very hot compressed air going into the tank, the tank is quite cool in contrast. Then as the moisture condensed more, it would run down to the bottom and collect, but the whole tank is going to be wet.
So by the time the bottom of the tank

No, see above.
Should the right conditions occur, I think it more likely a

No, Pin holes will develope all over the surface. I had an old 80 gallon compressor, that I inherited, develope pin holes near the center of the sides of the tankfirst, none were at the bottom.
The above is true if the compressor is use regularily. If you store it for years on end with water in side and bring up the pressure the bottom may fall out then.
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Ah, I see there's more to the story than I had originally thought. Would a light-use (that doesn't cycle often) compressor tend to act like the stored tank?
Puckdropper
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"Puckdropper" <puckdropper(at)yahoo(dot)com> wrote in message

I was referring 10-20+ years with water stored inside. If it simply sat the condensation would eventually settle to the bottom. Then it may fail in a particular area. If it sat long enough that there was considerable rust around the perimeter of the stored water and you then added pressure there might be a more dramatic failure, if the large rust area suddenly gave way. Typically however the weakest point will be a pin point leak that could grow in size over time
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I would be more worried about the lost capacity in the tank if it not drained regularly.
I have never seen a tank go south and that includes reservoirs on ships that were 40 years old.
I lean toward small pumps but like to mount reservoirs in the system for pressure stability.
I have two small compressors selected for their noise level and kept in clean areas in the basement.
They have automatic float type drains but small tanks.
Since water does not compress like air, I would be more worried about loss of air stowage.
I don't think I have ever had a pump outlast a tank.
Piping is a different issue especially with all the oil free pump ends that are out there nowadays.

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Mike wrote: Jack Stein wrote:

Does that mean you think it will do something different? If so, what and what makes you think so? I've never heard of one going boom but even then, I would think it would be less than spectacular.

Well, what do you think makes tanks that are constantly wet last for 30 - 50 years and more?

I've been around stuff, and doing stuff that OSHA would have cardiac arrest over for my entire life. So far, even though I may have been lucky a time or two, I'm still alive and well, no serious damage. I feel somewhat comfortable with my judgment so far. As for insurablity, I've never needed a sure thing to survive, and wouldn't want to live that way anyway. When I got my first mower that the mower deck shut off when you went in reverse, I immediately disconnected the dammed thing... I like living on the "edge" and don't need no stinking insurance company, or government, to force me to their levels of safety. I've been using table saws without a guard for close to 50 years, and have no plan on sticking one on now. They look downright dangerous to me...

Well, I can dance with a sufficiently loose definition of the word dance, and sufficiently large enough consumption of alcohol, and there is a fine line between being able to "dance" and being unable to walk to the dance floor...
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Mike wrote:

Let gravity do your work:
Take out the existing drain valve, add about three feet of large diameter air hose with appropriate fitting where drain valve was; put the drain valve on the end of the hose and run hose to convenient location.
Easier to drain, and the air hose, which doesn't rust, will now be holding a good deal of water that will not be standing in the tank.
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What do you mean by "drain"? If it's drain the condensate, yes, drain it daily. If you mean depressurize, then no, leave it pressurized for two reasons. First, it's simply a waste of energy to store it in the tank, then, for no good reason throw it away. Second, there is more fatigue damage to the metal tank from cyclic stresses than there is from static stress, and the deeper the stress cycles, the greater the damage.
IOW, it introduces more fatigue damage to the tank to go from 0 stress to maximum operating stress back to 0 stress than to leave the tank pressurized. Granted, the stress levels under normal pressures are small enough that you aren't likely to see a significant difference in tank life in either case. But, leaving the tank pressurized is less damaging than cyclic pressurization/depressurization.
Tom Veatch Wichita, KS USA
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I suspect the manual means "drain" in the sense that any accumulated water in the tank should be removed as opposed to dumping out all the compressed air. Just open the valve on the bottom of tank long enough to let out any water, and then close it up again. I try to do this on my compressor about once a day, but it winds up being more like once or twice a week when I remember.
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Just drain any water out.
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Make it as simple as possible, but no simpler.

Larry Wasserman - Baltimore Maryland - lwasserm(a)sdf. lonestar. org
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Mike wrote:

SFWIW:
Replace drain petcock with a 1/2" ball valve and some pipe so that the ball valve can be kicked open or closed with your foot without bending over.
The final pipe is pointed towards the ground.
Drain tank on a weekly basis.
For me that was Sunday night.
Would kick the drain valve open and forget it.
When the compressor was started the next time, the drain valve would start "singing", reminding me to kick it shut.
Lew
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Mike wrote:

I installed one of the Harbor Freight "Automatic Compressor Drain Kit" on mine. It opens momentarily each time the compressor starts or stops.
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?ItemnumberF960
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