Air Compressor Tank - Leaking Drain

[GraterFang]

Hi all, new to the forum so hopefully I'm posting in the right place. I'm a hobbyist fabricator, but relatively new to air compressor service. I have recently acquired an older 80 gallon tank and compressor (unknown USA made) that I am getting back in working order. The pump required service but now works excellent. For the most part, the tank also appears to be in great shape too. However, there is a leak around the fitting for the drain at the bottom of the tank, and so I intended to replace it. However, when I went to remove it I found that the elbow fitting there simply spins without any significant resistance or axial travel (as an uncaptured bolt and nut would). I do not feel any stripped threads that are trying to bite, nor does it seem loose enough to just to be removed it even though it can be easily spun by hand. I was expecting an NPT thread fitting of some sort, so I was a bit surprised to find whatever this is. The bung that the fitting is in is rather mediocre as well, so while the possibility of tapping it out to a larger NPT thread exists, I worry that the bung may not be robust enough for that. I don't want to ruin a good pressure vessel by doing something stupid, so I figured I would see if this is typical behavior for a drain fitting on an air compressor tank, and if there were specific recommendations for how it would be best to proceed. I can try and get some pictures, but being on the bottom of the tank its hard to visualize right now. I'll almost surely need to address that to work on it, but have left it on the ground for now until I have a better idea of what to do next. I'm comfortable doing almost all operations, except welding on pressure vessels.

 

[RC Mech]

A tank that looks serviceable on the exterior could be scrap when the interior is inspected.

Typically the bottom bungs are NPT. Don’t discount ham-fistedness on the count of the PO. I had a relatively new tank from an auto shop that some absolute muppet sealed with a wheel stud and half a tube of RTV. The bottom fitting had to be cut out to restore the 2” NPT.

When the fitting was out, we could do a thorough inspection of the tank.

You say it’s an elbow fitting, pry against the tank with a small pry bar or big flat head whilst turning the fitting. May just need to “bite” to get moving.

 

[GraterFang]

Thanks for the advice. I'll give that a try. Once out, other than using a scope to look for internal damage is there a way that I can effectively test the tank at home? My understanding is that pressure testing is typically done with water, but I don't have the equipment for this. The best I could do is fill the tank 80% or so with water, then pressurize with air up to a certain point. As damage is typically at the bottom of the tank, this theoretically seems "mostly" safe but again I'm a little out of my element with this stuff. I know unsound tanks can explode, though, so I do want to make sure the tank is safe. If testing at home with my means is not acceptable I'm willing to have the tank tested somewhere or buy a new one, but not sure what would be most cost effective typically. I find joy in making old equipment useful again, but perhaps this could be one of the exceptions?

 

[technocrat]

Thanks for the advice. I'll give that a try. Once out, what is the SOP for inspection of the tank internally? Is this something a relatively experienced hobbyist can do or should it be evaluated by a professional? I'm well aware tanks can explode if in bad shape.

A bit of general surface rust inside is ok. If it looks pitted anywhere you may have a future disaster. New tanks are probably cheaper than inspection and test.

 

[Rollerman]

Is your tank National Board coded? There should be a steel tag welded the the tank shell with the manufactures name, date of construction, maximum allowable working pressure and other information. No tag, no go. To test the tank, fill it completely full of water. You can pressurize the tank with compressed air. Since water is relatively non compressible if the tank fails you have a leak and a little splash. If the tank is pressure tested with air a failure is disastrous, there will be damage and injures, possibly fatal

 

[technocrat]

Is your tank National Board coded? There should be a steel tag welded the the tank shell with the manufactures name, date of construction, maximum allowable working pressure and other information. No tag, no go. To test the tank, fill it completely full of water. You can pressurize the tank with compressed air. Since water is relatively non compressible if the tank fails you have a leak and a little splash. If the tank is pressure tested with air a failure is disastrous, there will be damage and injures, possibly fatal

yes, that is how they are tested, but do you really want to do that yourself? Tanks are cheap. Re-threading an outlet or a bung in fairly safe and whilst possibly requiring some new underwear if things go wrong, that is about the limit of damage. Serious internal corrosion may pass test today, but will it pass tomorrow? again, tanks are cheap and one single replacement gives you years of peace of mind.

 

[metalmagpie]

Please post link to cheap tank.

 

[jim rozen]

Cheap, compared to the damage that a catastrophic failure would cost. [1]

[1] granted most don't fail that way. Do ya feel lucky?

 

[GraterFang]

 

[GraterFang]

As posted above, yes the tank has a National Board plate. I'm assuming wp 200 means working pressure of 200psi, which is more than I would plan on running.

I also posted a picture of the leaking (and corroded) fitting.

 

[steve45]

You may have a place nearby that can test the tank for you. Hydrotesting is required periodically for welding gases, etc.

I did test one of my own air tanks years ago. I filled it completely full of water, used a pressure gauge and a grease gun to pressurize it.

 

[CarbideBob]

Cheap, compared to the damage that a catastrophic failure would cost. [1]

[1] granted most don't fail that way. Do ya feel lucky?

Tell me more about "catastrophic failure" in yours or your employers air tanks?
Been there, seen that or just dreams or net stuff you read?
Bob

 

[Limy Sami]

Don't believe it needs testing? ...it's safe, etc etc ........we've heard them all on PM (nd a few more)

FYI https://www.youtube.com/results?search_query=air+compressor+explosion

I don't give a fuck if some knuckle head blows himself up, it's the others who get hurt that concern me.

 

[EPAIII]

Err, I am reading "1977". That's 47 years old. I would think about getting a new tank.

View attachment 425104View attachment 425105

 

[jim rozen]

"...Tell me more..."

Probability of failure compared with loss from failure compared with cost to prevent failure.

Very low, very high, pretty low.

It's a standard legal evaluation. Probable failure is the loose drain fitting gets spit out and shoots around the room. Ping!

And yes I did see that happen once.

Cost to repair the leaking drain combined with the trouble of hydrotesting means just buy a new damn tank. He's not in the compressor business, he wants to run a shop. Some rabbit holes, best to not go down.

 

[GraterFang]

Well, thanks all for the responses. There's sure a lot of feelings in this thread, but maybe there's some history behind that.

I'm not sure what is directed at who, really, but to clarify I don't intend to just run the tank as is. Yes, I know its 40+ years old and may be unfit for duty. No, I don't plan on running it without testing it, let alone not fixing the loose fitting. As I mentioned early on, I plan on either testing or replacing as either necessary and/or practical. I'm not opposed to buying another tank, but since I haven't been through testing a tank before I didn't really have a good idea of whether or not its more economical to professionally test vs. replace. However, I'm getting the idea that replacement is likely more cost effective given the odds the tank has issues, which is fine. I may go that route anyways after I do my own inspection, but would still like to find a use for what I've got here.

So, some advice I'd be interested in would be
a) Are there any general recommendations on purchasing a replacement tank? Would most of you just buy a new tank and compressor, or just replace the tank? Yes, I know it depends on the state of the compressor itself but assuming its good (aside from being old) what would you do? I'm feeling inclined to purchase the whole shebang, and just keeping the extra compressor and motor as spare parts to spare myself the work of immediately remounting an old compressor on a new tank.

b) Any recommendations for good brands where you are paying for quality and not the laurels of a name? Cost isn't a concern to me, but on principle I don't like paying double the price just to have "that" name brand either. I'm always happy to pay more for better quality, though.

c) I hate the idea of scrapping this tank, even if its not fit for high pressure duties. Any recommendations for repurposing it that would supplement a shop? Assuming its leak free under an at-home hydrotest, could it be repurposed to a kerosene tank or something of that nature? I'm guessing the knee-jerk reaction to this question is "no", but I still want to hear what people have to say about it. And finally, if there aren't any good ideas for repurposing it to supplement the shop, what about outside the shop? A smoker? Seems like it would work for a water tank too but probably too small to be of any real value for that.

 

[Superbowl]

I can't see why you don't test it yourself instead of going on and on. Others have told you how to do it safely. Do you not have water?

 

[GraterFang]

I can't see why you don't test it yourself instead of going on and on. Others have told you how to do it safely. Do you not have water?

Haha, sweet jesus, insert huge exaggerated eye roll here

Well first and foremost, this is in support of my hobbies. The tank is one of my many irons in the fire, and I work ER, travel, and do long extended shifts. I don't get to play every night or every weekend, so while I was waiting for when I had time to work on it I ordered parts and wanted to see if there was any useful advice before I force the old drain out. After that I brought up testing it at home before anybody else did just to have a number of people bicker about why somebody would even bother to do that in the first place. I got some useful information from the first couple posts about inspection, but barely anything about testing it or if my proposed method was sound except for a single reply. And going on and on? How so? By asking what people would recommend next since the majority basically said testing it was a waste of time?

But you're right, that's exactly what I was planning on doing all along. I'll get time next weekend to move the tank outside, work on the drain, and test it then. I have a scope so I'll look for deep pitting once the drain is out too. If it looks exceptionally good and passes hydrotesting it'll probably go into service in a small purpose-built shed out behind the shop. Even if it grenades the shed at some point I'll likely just have a mess and a good story. If its even moderately bad I'll repurpose it, but I don't know what for yet.

Stay cool Superbowl, don't know what this forum would do without you. You made me laugh for sure.

 

[???]

Tell me more about "catastrophic failure" in yours or your employers air tanks?
Been there, seen that or just dreams or net stuff you read?
Bob

Company I worked at decided to up the pressure by fiddling with the compression ratio. Blew a hole through the wall. Luckily the compressor was outside. The boss his partner and secretary both smoked a lot of weed so I don’t think much deep thinking going on.

 

[???]

Don't believe it needs testing? ...it's safe, etc etc ........we've heard them all on PM (nd a few more)

FYI https://www.youtube.com/results?search_query=air+compressor+explosion

I don't give a fuck if some knuckle head blows himself up, it's the others who get hurt that concern me.

From some of the questions and advice I’ve seen on here I think the posters misunderstood the meaning of PM and thought it meant Practical Moron and that they had finally found their home.