air compressor tank rusty, danger?

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  #1  
Old 06-29-04, 08:25 AM
19joey58
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Unhappy air compressor tank rusty, danger?

please help

i am currently overhauling a 30 gal air compressor. i heard rattling in the tank when i shook it so i drained it with a garden hose. i saw little rust pieces come out (about a handful) now the water drains clear. everyone says you should drain your tank regularily to prevent enevitbale rust but i received this tank used and am not certain how often, if at all, it was drained. the overall tank seems in good condition and when i peeked inside i saw minimal surface rust on the top of the tank and it was more rusty at the bottom with a slight creatored texture. my concern is safety !!!

the web has several stories of EXPLODING air tanks and rust is often the culprit, many sites state that most people do not take proper care in draining their tanks and therefore there are several air "bombs" just waiting to detonate because of the rusty thin walls of the tanks. i called local air compressor builders here in toronto and they said ALL air compressor tanks are rusty inside because of the condenstaion. they also said they have never heard of a tank blowing up and what usually happens is the rust deteriorates resulting in small pin hole leaks in which the tank is deemed unworthy and is the replaced. they also explained that air tanks are usually good for about 30 years

if all air tanks are rusty and rusty tanks are "bombs"...we have a global crisis on our hands!!!. most tanks are not new, and new ones will soon be all rusty because it is normal for then to rust, and further, they will rust rather quickly since MOST people do not drain properly.

i am just concerned about the safety of my house and family around this supposed time bomb. how can i verify the tank is OK without sending it out for a pressure test thay is worth more than the tank itself. more imortantly do these tanks really EXPLODE from rust? or just start pin hole leaks? if all tanks are rustly inside what determines when they should be retired?

i think for a tank to explode it must experience a SUDDEN change of some sort, like one website mentioned the "backdraft" of the hose used for painting caused the chemicals to spark and explode the 30 gal tank killing 1 employee. also faulty pressure regulators that allow the tank to overinflate could cause a sudden explosion. but it seems logical that as the rusty walls thin it will rupture at the weakest point causing a rather small opening (or even pin hole as mentioned above) and not an explosion... unless the rupture pressure sort of rip the metal like a sardine can and a very large hole could suddenly happen sending the tank airbourne

please ease my conscience, and rest my worries

JOE
 
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  #2  
Old 06-29-04, 09:45 AM
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I have worked around air compressors for many years and have never even heard of one exploding due to rust out. I have seen poorly maintained tanks develop pinholes and leak but thats all. One thing you might want to try is tapping the tank with a hammer and see if you can "hear" any drastic change in the sound,indicating a thinner area of the tank.Theres no way of stopping the moisture in a tank but draining keeps the moisture out of your tools. You may also want to put a low amout of pressure in the tank and spray the tank with a mixture of dish soap and water and look for bubbles.Hope this helped a bit. Good luck
 
  #3  
Old 06-29-04, 09:58 AM
19joey58
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thank you for your reply, i will "tap test" the tank tonight. what should i listen for? ...because the bottom of the tank is certain to sound different than the top becasue that's were the rust sediment is, so there is a layer of crud making it as thick as the top where little rust exists.
i dont care if it will eventually leak (i'll just by a new one then) i just became paranoid after searching the net and reading all the "explode" stories and of course safety before any dollar amount.
 
  #4  
Old 06-29-04, 05:22 PM
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Listen for a "different" sound as you move around the bottom of the tank. That would indicate a different thickness.Maybe...ping....ping...thunk. God luck
 
  #5  
Old 06-29-04, 09:08 PM
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I certainly wouldn't loose any sleep over this.

19joey58,

You are right about the crud making the tank sound dead at the bottom.
The layer of crud will hasten corrosion and should be removed.
If I perform major maintenance on an air compressor I will put some low foaming soap in the tank and wash it out through the fitting holes with a pressure washer.
I had hydro-tested a tank I though was pretty rusty but it held 250 psi which is 2 1/2 times the operating pressure.

Every tank I've seen leak has done so at the very bottom and there was always a layer of rust and flakes.
I've had a couple of instances where I did a mechanical inspection in a building with an air compressor for the hvac controls and drained a complete tank full of water.
In each case the compressor had an auto-drain valve that failed.
 
  #6  
Old 09-02-04, 12:56 PM
familyguy
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Question pinhole leak in air compressor tank

Does anyone have a suggestion as to how to repair a pinhole leak in the bottom of an air compressor tank? Thanks
 
  #7  
Old 09-02-04, 01:59 PM
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Welcome familyguy

A pinhole leak in the bottom of an air tank is the first sign that indicates the tank's life has ended.
If you have air coming out of a small hole you can be sure that the material around the leak has thinned to the point where it will be unsafe to use.

Depending on how much you paid for it and what quality this unit is you could buy a new tank and move the pump and electrics over................................or, sell it as parts and buy a new one.

What make, model and hp is it?
 
  #8  
Old 09-03-04, 09:28 AM
familyguy
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Thankas for the rapid reply to my leaking compressor problem

Given the dire warning of impending tank failure, I returned the $20 dollar air compressor to the guy at the yard sale who assured me that it was in perfect working order. Sometimes, those yard sale bargains are for real, sometimes they are really more problems than they are worth. In this case, my $20 panacea didn't pan out. Thanks again
 
  #9  
Old 09-04-04, 09:57 PM
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I should have read this earlier... I started my overhauled compressor for the first time this evening. It had been sitting for many years when I got it and the tank was built in 1978. The pressure switch said it's set at 175 off/ 140 on and It was kinda creepy when the gauge started getting around 165 and I was waiting to see if it was gonna shut off or explode
All went well and it ran for 12 minutes before shutting off at 170. The tank is rated at 200. Does that time sound right for a Kellogg 325 on an 80 gal. tank?
I rebuilt it with new valves, springs etc and fresh rings.
 
  #10  
Old 09-05-04, 07:03 AM
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hoffman,

Unless you have a need for 175 psi, it is a waste of energy and puts undue strain on your compressor to run it that high.
The higher the pressure, the less efficient the pump becomes, the hotter the head becomes and the longer it will take to shut off.
Also, the pump will run more efficiently after it's broken in.

If you set the cut-out at 125 and the cut-in at 90 you will have plenty of pressure to run anything you plug in, the unit will cycle off in a reasonable amount of time and it will run much cooler which will extend its life greatly.

Having just rebuilt the head you should change the oil after about two hours of running, then again at about ten hours of running.
Check with Kellog on their recommendation for oil type, but almost all piston compressors can use 30 weight non-detergent motor oil.
You can get this at most auto parts stores and is the same oil that is labelled for compressors which is usually more money.

Good luck and have fun!
Let us know how it all turns out.
 
  #11  
Old 09-05-04, 08:26 AM
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Thanks Greg. Do I screw the pressure switch in or out to decrease the cut off pressure? It's a Square D switch.
I put 30 wt compressor oil that ACS advised. It's clear synthetic stuff that cost $6 a qt. They said never to put anything else in it and they talked me out of spending more $$ than I needed to and even gave me a few odds and ends so they weren't trying to sell me any unnecessary stuff.
It may be OK to run other types of oil but those guys REALLY believe that there is a difference in oils. It's a big operation and they do overhauls on some pretty big industrial equipment and they also build compressors.
I build engines as a hobby and I never run synthetics at break-in so I thought it was a little strange...
 
  #12  
Old 09-05-04, 10:15 AM
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Oil selection is pretty much determined on a "belief based system"

If you want to start a riot, just ask "which oil is better" in a different forum!

There are so many facts out there, I'm sure alot of them good, that I honestly don't know how one would be able to make an intelligent decision.

As far as using synthetics go they do have improved wear capabilities and stability, but I "believe"the need for this has to be put into perspective.

I service smaller compressors used in building control systems, build and repair smaller ones for myself and work around very high hp reciprocating and screw type compressors when I repair refrigerated air driers.
My perspective on this is that the company that recommended synthetic for your compressor is probably correct in recommending the use of synthetic oil in all applications for the simple fact that they don't have the time to be worried about a $4.00 price difference in a quart of oil.
Compressors that operate 24/7/365 are at an extreme end of performance and need all the help they can get to maintain cleanliness and lubrication.
I have watched a local saw mill do an oil change on a high hp screw unit and thought they were taking road tar out of the pump!

Is synthetic better.....yes, do you need it.............I don't think so.

My personal experience is that in using non-detergent, 30w in the summer and 20w in the winter and only changing 4x/year, the oil is near as clean as when it went in and the discharge line has no sign of fouling.
My use would be harder that most as it is always on, is installed in an unheated enclosure in sub-zero temps with only a magnet heater on the compressor, has an 80 gal air tank with a 40 gal auxiliary tank and I have a habit of regularly doing projects where the compressor runs for sometimes six hours straight.

So you decide and let us know what you think.

Oh ya, I kinda got goin' on the oil thing.
What model of Square D is the control?
 
  #13  
Old 09-05-04, 05:57 PM
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I'm with you on the oil deal. I bought a 1.5 qt bottle and the compressor only holds a little over .5 qt so I'll use that first.
I only run a few small air tools and occasionally a small pressure type sandblaster. My old Porter Cable 60 gal. had trouble keeping up with the sandblaster so I'm hoping this one will do a little better.
The one I just rebuilt makes a "pinging" sound that sounds like someone tapping on the inside of the tank when it's running. It's sort of rapid and irregular and not that loud. I started it up before hooking up the discharge line and it sounded smooth as glass without any pinging. I know what a bearing knock sounds like but this is different. I am thinking that it's the one way valve on the discharge line because it's screwed into the tank but I don't know much about compressors.
I'll have to look for a # on the pressure switch. It has two sets of contacts in it but I'm only using one side.
Thanks a million for the advice!
 
  #14  
Old 09-05-04, 07:02 PM
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hoffman,

It likely is the check valve.
A few things can cause it to oscilate but the easiest to deal with is a weak spring.
Sometimes just taking it apart and cleaning it and while it's apart, check to see if the return spring is broken or just weak.
A gentle stretch of the spring is sometimes all it takes, if not they are not that expensive.

Another thing to check is that the air discharge line off the compressor is not restricted by sludge or a kink.
It can be harmfull if the discharge line doesn't have full flow.

There should be two adjustment screws on the pressure switch.
The one you want to adjust is the range screw.
If you turn the differential screw you will be extremely frustrated with the results.
Post the model of the switch and I may have a diagram.
 
  #15  
Old 09-06-04, 03:10 PM
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The discharge line is new 5/8 copper and the check valve is also brand new. I wanted to go with 3/4 copper but my flare tool only goes up to 5/8. I may install the old valve and see if it still makes the noise. The new one has a plastic deal in it and the old one is all brass. I'm in Mid. Ga and it looks like we're gonna be getting some serious rain for the next day or so. I'll post the pressure switch # when it dries out. If the valve rattling doesn't hurt anything I don't mind. I'm just worried because it's my first rebuild and I'd hate to blow the $300 or so that I have in it so far. I think it's gonna be a real good machine. A new one in the same class would be way outa my $$ range...
Thanks a million for the help
Oh yea, what do HVAC systems use air compressors for? Just wondering...
 

Last edited by hoffman; 09-06-04 at 03:39 PM.
  #16  
Old 09-06-04, 05:06 PM
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If the check valve is banging I wouldn't use it. It will eventually come apart and you don't want a blockage in the discharge line because it can cause overheating and the pressure switch and relief valve is on the other side of it.
There should be enough spring pressure to hold it open when the pump is running.
It is possible that if the discharge line is too small there would be no dampening effect and the pulsations in the line would be exagerated.

The largest compressor I would use 5/8" od tubing would be about 10 [email protected] 90 psi. And 3/4" od for 10 to about 18 cfm.
BTW, how big is your unit.? CFM, HP?

I work in hvac and have never had the need for a 3/4' flaring tool but where I had to connect something in a situation like yours I would use a short stub of 5/8" on each end and then braze in a 5/8 to 3/4 copper adapter and use 3/4" in the middle.

A larger discharge line helps the head run cooler which both makes it last longer and reduces condensation build up.

Air compressors are used in a lot of commercial buildings where the heat/cool control system is air operated. It is referred to as a pneumatic control system.
 
  #17  
Old 09-10-04, 05:45 PM
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Man, my phone service has been off and on for a few days.... I'm not sure about the cfm and displacement but it's a Kellogg 325 TV with a 3hp Baldor motor. I think the current Kellogg replacement is a 321. Funny, it's a lot bigger than the "7hp" motor on my Porter Cable compressor Wouldn't having 5/8 connectors at the end of 3/4 line kind of defeat the purpose? The discharge line does gets mighty hot...
here's a link to some tech data:
http://www.aircompeq.com/ka.htm
It's a 325. I notice that the displacement changes with the psi. I guess this has something to do with the valves??? I don't see how the displacement can change unless they're figuring out air leaking past the valves???
Weird
 

Last edited by hoffman; 09-10-04 at 06:12 PM.
  #18  
Old 09-10-04, 08:07 PM
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The specs show your unit to produce about 12 cfm @ 100 psi.

The line should be hot but not hot enough for the head to overheat.
The 5/8" fittings and 5/8" tubing is pretty close according to the specs as long as the line is very short.
If you were to increase the line size to 3/4" you would lower the temperature of the head, increasing it's life.
Ideally the fittings and line should be the same size but increasing the line and not the fittings will still make a big difference in the amount of pressure drop. The line could be cut fairly close to the 5/8" fittings and a 5/8" to 3/4" reducing coupling brazed in to make a smooth transition.

The reason that the displacement decreases is that the compressor looses some efficiency at higher pressures.
Also, you have to consider that when the specs show a lower cfm there will be more energy in the stored air due to the fact it is at a higher pressure.
Cfm ratings are used to ensure adequate capacity to be able to match air supply with a particular tool and to be able to compare units.

A common motor frame size for consumer compressors is a 56 frame. Small industrial units often use the larger 143 frame.
Also, if you look at the motor on the Porter Cable you might find that the hp figure on the nameplate is blank or has SPL or 7.5 spl or something similar to show that the 7 1/2 hp it claims, is something out of their imagination rather than the common standards motor mfr's use.
 
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