Air compressor question ?

The risk of damage to air tools is important. Equally important is the internal rusting in the compressor's air tank if water is left in it. I recently bought a used 8hp gasoline speed-air compressor used. It wasn't until I got it home that I discovered a rust-through pin-hole in the tank. The previous owner did not properly drain the tank. In theory, I *could* weld the pin-hole, but then I would wonder about the integrity of the rest of the tank. My next smaller compressor has aluminum tanks, which have less of a risk of rust through. I still drain it as well.
 
There are numerous threads on here and around the web regarding moisture control in compressed air systems.
I utilized an air conditioner coil from a residential furnace (A-coil) to remove most of the water before the tank.
 
addertooth said:
The risk of damage to air tools is important. Equally important is the internal rusting in the compressor's air tank if water is left in it. I recently bought a used 8hp gasoline speed-air compressor used. It wasn't until I got it home that I discovered a rust-through pin-hole in the tank. The previous owner did not properly drain the tank. In theory, I *could* weld the pin-hole, but then I would wonder about the integrity of the rest of the tank. My next smaller compressor has aluminum tanks, which have less of a risk of rust through. I still drain it as well.
Click to expand...
Drill the hole out, tap it NPT, and put a brass plug screw up it. That said, the pinhole where it came through would be only the deepest rusted spot in a generally larger rusted, hence weaker, area. I would think there is not much you could do by way of welded repair that would be weaker than a place where water collected, and already corroded through. Cylinders mostly have a weld seam anyway. I would consider cutting a big hole, enough to inspect the nearby insides with a small stalk mirror. Then decide. Maybe a circular plug, welded in, covered by a reinforcing patch, welded all the way around. You can make it so that if it was ever going to fail, it would have to be somewhere else.

In my original post, it was about the practice of letting the air out, and that I said that I thought it was not beyond our wit to contrive some way to let the water out. Of course, let us look after the compressor by draining it. My first thought was just use two taps, with a tube between. Open one, let the water into it. Close it, and then open the outer tap, and lose the water.

I guess some like to blast the water out. OK then, but once the water is out, close it off, and keep the pressure. I am sure we can come up with more. Beyond that, one might use an electrical trick to inhibit the corrosion process.

Water vapour is in the original air when it gets compressed. There is a point if later it gets cold enough to dew, then it comes out, but this point is harder to reach if it is kept under pressure. When it is released in normal use, the air gets very cold from expansion, and is ending up at atmospheric. The water will come out, and if you don't want it there (say for spray painting), you have to use inline dessicators. Depending on the compressor type, you may have to remove oil as well.

Letting the air out all the way, inevitably drops the temperature in the tank, and the water gets below the point it can remain as a vapour, and collects in the tank. By all means, switch off when done. Yes, of course, drain the water. I was just trying to discover if there was some other good technical reason for letting go the compressed air. The energy it took to get it there was lots.

[EDIT : I do like it. @llamatrails provides us with a solution from HF. Apparently "operates from any interrupted air signal". I am not sure what this means, but anyway
--> https://www.harborfreight.com/automatic-compressor-drain-kit-68244.html
]
 
Last edited:
graham-xrf said:
Where I have been, always at the end of the day, they opened the valve, and let out the compressed air.
Maybe dumb question day - but why do it?

One has expended the energy in getting it compressed, It will surely keep until it is needed, or until maybe it has slowly leaked away before a top-up could hap[pen.
Click to expand...
you didn't ask why people just drain the water out of their tanks like everyone does, you asked why places you worked always drained their tanks completely at the end of the day.
your answer to why some companies completely drain their compressor storage tanks answer is in #13 and #18. mainly for safety.
Dave
 
graham-xrf said:
Drill the hole out, tap it NPT, and put a brass plug screw up it.
Click to expand...
forgive me if I am off base here, but this does not seem like a very safe way of solving this problem. Again, I don't have any formal experience with repairing compressed storage tanks but two things come to mind that might be worth looking into before following this recommendation.
1. If one area is rusted through, then I would think it likely (at least until proven otherwise) that there are many more areas that are close to rusting through and any patch of any kind will be short lived.
2. Adding mass, in the form of an NPT Plug could become a projectile if the area around the plug (that is likely weakened by rust) gives way.

I think there are many ways to save $ in a hobby shop, but taking chances with a pressure tank that is known to have rust issues, is beyond my personal limit. Hope you get this worked out.
 
I put a 90 degree fitting on my drain (60 gal upright tank) and brought it out to a ball valve that I can operate with my foot. I also have a ball valve right at the pressure outlet to my distribution system so that when it's closed the tank will stay at pressure indefinitely. At the end of every day when I shut the pressure ball valve for the night I crack the bottom drain ball valve with my foot to clear the condensate out of the bottom of the tank.
 
maspann said:
So, what triggers this one? I like the idea of a drain that opens for a few seconds every time the compressor kicks on.
Click to expand...
Been a few years since I installed it.

From one of the HF comments,
Hoyt - April 10, 2021 :
" .. use the tee to come off the line to the pressure relief line, that pulsates as the compressor runs, and that pulsation activates the pump. When the tank is charging, the pump is pushing water out, and when the compressor shuts off, it lets out a final big squirt... "
 
Boswell said:
forgive me if I am off base here, but this does not seem like a very safe way of solving this problem. Again, I don't have any formal experience with repairing compressed storage tanks but two things come to mind that might be worth looking into before following this recommendation.
1. If one area is rusted through, then I would think it likely (at least until proven otherwise) that there are many more areas that are close to rusting through and any patch of any kind will be short lived.
2. Adding mass, in the form of an NPT Plug could become a projectile if the area around the plug (that is likely weakened by rust) gives way.

I think there are many ways to save $ in a hobby shop, but taking chances with a pressure tank that is known to have rust issues, is beyond my personal limit. Hope you get this worked out.
Click to expand...
Not at all off base, and we do agree on pretty much everything except how well (or poorly) a threaded plug can perform.
In context, I did follow with "that said.." and addressed the question of a possible larger weakened area.

A fix it hole has to be either in a thick enough wall, or have it's own base, but In my experience, it would normally still be intact when the whole cylinder breaks. That is a tame event, normally failing while under test full of water. A NPT fitting won't become a projectile by itself, although I think you meant the NPT plug, along with a whole lump of weakened, rusted steel around it, pushed by compressed gas. Try not to misunderstand. I feel the same way about "rusted stuff".

It was part of my job in the past to be getting involved with (very) high pressures, and temperatures, and sometimes, I would test to destruction.

There are BSPT (NPT) fittings of all kinds to allow getting stuff onto walls of pressure cylinders. They are rated well beyond the fail pressure of the cylinder, and do not normally become projectiles. When I said "drill it out", that gives opportunity to discover the condition of the wall. I have tried this, and I was so unhappy that I went on to cut a big enough hole to see inside. The sides of the hole reveals how much the nearby area is thinned by rust. In our case, almost none, but the repair fitting was overkill, looked ugly, and was only needed for a short time before we junked all the kit. The hassle it took to test it might have exceeded the price of a new compressor, but it was a question of delay.

In my place, I don't share the space with compressed air cylinders, and none of the tubes can rust. There is the argon cylinder, but I think that is OK.
 
Last edited:
You must log in or register to reply here.
Back
Top