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
|Drew Lawson | Mrs. Tweedy! |
| | The chickens are revolting! |
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
I'm also thinking when/if it rusts through, it will go pfsssssh instead
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.
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
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
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.
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.
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
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
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..
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.
"The potential difference between the top and bottom of a tree is the
reason why all trees have to be grounded..." -- Bored Borg on
"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.
"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
I would be more worried about the lost capacity in the tank if it not
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
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.
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...
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.
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
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.
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.
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
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.
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