Compressed air safety

[Piros1]

From what I have read and been told by experienced compressor technicians is if a tank cracks or develops a leak it is scrapped not repaired. When new they are certified for a specific maximum pressure. Always keep your tank dry as possible, never raise the pressure up to the maximum working pressure for extended periods, and never push your tank pressure to the maximum rating on the tank.

All said I recommend making sure your pressure relief valve is in proper working order and it’s limits are lower than that of the tank. Also ideally you should invest into an auto drain valve, this will keep moisture from building up in the tank.

 

[TonyM]

Rhetorical question but how many people with a compressor, test or even just inspect their air receiver ever let alone every few years.

 

[Mike Henry]

Rhetorical question but how many people with a compressor, test or even just inspect their air receiver ever let alone every few years.

I was required to inspect the compressor tank at work annually. Even small pressure vessels on research equipement also required annual inspection.

I've never inspected the tanks on any of my 3 personal air compressors but might do so with a small camera on a tether one of these days.

 

[Mike Ginn]

Tony. You are spot on with your observation. Thinking about pressure testing a tank, I am not sure where I would start. I know no one who has ever done it and I certainty haven't over 50 years and 5 compressors. An endoscope could be used but what conclusion would you come to when you see the rust?

Realistically I now drain the tank very frequently and only pressurise when needed. I also intend to fit an automatic drain.

Mike

 

[Toymaker]

Granted that regular safety inspections and draining water from the tank are key to longer, rupture-free tank life, but how does tank metal affect tank life and corrosion?

Of the two common metals used for tanks, Aluminum and Steel, which is better for home compressor use? Which metal has the lower failure rate?

The consensus according to Google favors Aluminum, due to it's corrosion resistance and lighter weight. Of the several videos I've seen on YouTube, it appears it's always a steel tank that failed.

 

[TonyM]

The failure rate shouldn't be much different if both are designed to the material limits and maintained correctly. Although Aluminium may not rust, it still can corrode, and it would certainly suffer stress cracking unless the thickness of the shell was dramatically increased over a steel one. So I am not sure the weight of aluminium is comparable.

 

[Toymaker]

The failure rate shouldn't be much different if both are designed to the material limits and maintained correctly. Although Aluminium may not rust, it still can corrode, and it would certainly suffer stress cracking unless the thickness of the shell was dramatically increased over a steel one. So I am not sure the weight of aluminium is comparable.

A little more than a year ago, I built a small air compressor for my shop using an aluminum tank sold primarily to be used on large diesel trucks that use air brakes; there's a short discussion about my air compressor here: DIY Shop Air Compressor. I never weighed the tank, but I can tell you that it's VERY light. The tank is rated for 225 psi but I have the auto-shut-off set to 120 psi, so the repetitive stresses should be well under design limits. Air-brake-tanks on big trucks take a real beating during their daily use, undergoing pressure cycling every time the driver hits the brakes, as well as enduring continuous engine and road vibration and jarring from pot-holes. I've never heard of a truck's air-brake tank rupturing; has anyone?

My thinking when I decided on an aluminum tank was, if these tanks can handle the beating they get on trucks, than it should easily last a very long time in my little home shop

 

[Ken I]

A simple way to hydraulically test your tank is to use your mains water pressure, assuming it's high enough - it often is.
Otherwise beg, steal borrow or hire a hand water pump and gauge.

My own house pressure reaches 125psi late at night when the bulk of use has ceased. Alternatively, if you can access a fire hydrant line, they often have higher pressure.

Be sure your gauge is correct - preferably use another known correct gauge to calibrate your normal gauge.

Be sure the tank is completely filled with no entrained air then perform the test - if using the mains, apply slowly so as not to accidentally over-pressurise the tank.

If it leaks at any seam or pinhole or actually splits - junk it! This is a non-repairable issue. (You can ignore leaky pipe joints or gaskets those can be repaired / replaced). Don't forget to test that the safety valve relieves at test or lower pressure - test to at least 1.1X

Don't lose any sleep over a failure - consider it a blessing - far better to have it fail under hydraulic test rather than like the explosive example at the opening of this thread, failing with massive, compressed energy realease.

Regards, Ken I

 

[Mike Ginn]

Thanks Ken - if we are lucky we get 15-20psi on a good day. Out street pipes are very old (cast iron) and internally corroded so a pump would be needed. It can be done but loads of work. Since my tank is around 2 years old I'll wait a bit (3 years?) before reconsidering and in the mean time I will take sensible measures.
Mike

 

[Badhippie]

Just make sure you use a tank that has been certified. All quality air tanks are certified and have a tag attached to them or they are stamped. I also would recommend putting an auto spitter on the tank that way you don’t have to worry too much about moisture in the bottom of the tank. Are far as material goes if you are using certified tanks and a auto spitter valve or even a auto drain valve then the material won’t matter and it becomes a personal preference

 

[Steamchick]

Even certified receivers should be examined for leaks (commonly a rust patch on the paint, blistering, etc.) And re- pressure tested annually.
Drain the receiver every time you finish and blow-down using the residual air after switching OFF. It is false economy (due to condensation rusting) to "save the pressure for next time". Blowing down removes the moisture as well as can be performed. And usually the receiver + contents is warm then as well.
K2

 

[Mike Ginn]

Guys - I have a strategy moving forward. I have always run my main compressor to full pressure of 120psi but I realise that is not necessary. For most use I use a blow gun so 10psi is fine. I now fire up the compressor to around 10-20psi and that lasts me the whole day of occasional blowing chips. I only let the tank up to 125psi when I am sand blasting which is not a daily occurrence. I have recently purchased a flexible drain extension pipe to aid draining and the next step is an automatic drain. Up to now, and before these posts, I kept the tank permanently at 125psi but no more.
Thanks everyone for their thoughts and advice.
Mike

 

[ajoeiam]

Guys - I have a strategy moving forward. I have always run my main compressor to full pressure of 120psi but I realise that is not necessary. For most use I use a blow gun so 10psi is fine. I now fire up the compressor to around 10-20psi and that lasts me the whole day of occasional blowing chips. I only let the tank up to 125psi when I am sand blasting which is not a daily occurrence. I have recently purchased a flexible drain extension pipe to aid draining and the next step is an automatic drain. Up to now, and before these posts, I kept the tank permanently at 125psi but no more.
Thanks everyone for their thoughts and advice.
Mike

(Not really directed at you mr Mike - - - you're just the last in this line of conversation - - - sorry!)

Hmmmmm - - - I'm not so convinced that pressure cycling your tank is as much of a problem as some have been warning here.
A tank that is held at in a fairly narrow temperature range, fed by air that doesn't have widely varying levels of humidity won't have near the levels of internal moisture generated by a system where such conditions just can't be met.
Here this should be a major issue judging by the writings yet I remember a system where I doubt it was ever blown down still had the tank last around 30 years and that's with uses in an over 50 C (over 120 F) range. Blown down would have extended that tank's life but for the life of me I don't remember a valve on that thing.
There seems to be a consensus that an automatic valve is a must. What ever happened to organizing your shop departure? Do you regularly walk out the door leaving all the machines you used running and the lights on and your big doors open? If you don't - - - then what's so difficult about adding in hitting the drain valve for the 10 seconds that remove 99% of the internal moisture. In my experience automated systems that fail are a far more common occurrence that might be expected. That failure is often accompanied by catastrophic failure of other parts of the system. If I should be checking an automatic valve on a regular basis (recommended behavior by the way) - - - how in the sam snot is that easier than spending significantly less $$$$ and using a manual drain on every use of the equipment?
(I am not even trying to suggest that an automatic valve is a terrible idea - - - I just don't think its as wonderful an idea as has been presented. Something like all the driving assist tools on the newish vehicles - - - I find them a royal pita - - - I do know how to drive and carry license to drive most everything on a road (one type and 2 sub types not included) and think that relying on these kind of tools is going to really incentivize the complete idiotification of driving - - - - not that its that far off generally as it is - - - too many people learning how to drive using computer games rather than actually having to practice the art - - - accurately!!!)

(Further wonder if a comparison of air receiver or pressure tanks and their fabrication over the last 50 to 60 years would show the same thickness of material
from previous to now - - - I'd bet not. The bean counters or the liars run the company - - - - very very rarely is it a technically competent person!!!)

 

[ShopShoe]

Building on what ajoeiam said above:

I've owned several compressors over my life and have gone full circle on how I take care of them.

I had a 2 HP, 20-gal portable that I used when I started working on cars and I used it in a portable way. Taking it out for a work session and draining it to put away a couple of hours later, for a few years. Tank stayed OK when treated that way.

I moved up to a 5HP, 60-gal compressor and it served me well. I installed extended plumbing and easy-to-reach drain valve because I hate those *&#@* petcocks. Then I decided to leave the tank full on shut-down and drain the condensation mid-work-session. Tank soon was leaking. I stripped the pump, motor, etc from that and scrapped the tank. My local recycler required the tank to be cut in half to avoid potential re-use, where I found the rust was only around the threads for the drain valve, proving my little plumbing project to be partly responsible: I would think one could replicate that harmful situation in the installation of an auto-drain valve.

My new large compressor still has the petcock and I shut it down and drain the tank after every use, including just an hour or two. I am much older and getting up and down is harder, but I still go through the pain and hassle of getting to the drain every time, as I am determined this will be the last large compressor I buy.

For the using the air gun to clean parts and inflating tires I have a Harbor-Freight "hotdog" nailgun compressor that I also drain after every use. It's quicker to fill and quicker to drain and gets used for those small uses when starting up the large one would be more work. It's cheap and I've already had to put new brushes in the motor, but I consider it disposable and another useful tool in the shop. It was cheap to buy with accumulated H-F discounts and credits.

 

[Badhippie]

All I was saying was the installation of either a spitter valve or an auto drain valve is another way of doing something. I have personally had a spitter valve on my 10 horse 120 gallon compressor for well over 25 years and knock on wood never had a problem with it. Some embrace technology and some don’t. I am also fortunate enough to have an air dryer on the outlet side of the compressor. Was lucky enough to find both the compressor and the air dryer at the same auction and only paid 500 dollars. Both was only a year old

 

[Toymaker]

Prior to the 1990s, most automotive exhaust systems were fabricated from mild steal, which rusted out and needed replaced after a few years. In the 1990s nearly all auto manufacturers started using 409 stainless steel, resulting in exhaust systems that mostly lasted the life of the car.

So, if you want an air tank that will last pretty much forever, whether or not you remember to regularly drain the condensed water, buy a stainless tank, and never worry that it might rust.

 

[H Pearce]

Prior to the 1990s, most automotive exhaust systems were fabricated from mild steal, which rusted out and needed replaced after a few years. In the 1990s nearly all auto manufacturers started using 409 stainless steel, resulting in exhaust systems that mostly lasted the life of the car.

So, if you want an air tank that will last pretty much forever, whether or not you remember to regularly drain the condensed water, buy a stainless tank, and never worry that it might rust.

Pressure vessels are made out of plain steel for good reasons. Predictable failure is one of them. I’m not aware of any SS air receivers but for sure someone makes them. The problem with SS is it can corrode badly and unpredictably in certain circumstances. Not something I would trust.

 

[Toymaker]

Pressure vessels are made out of plain steel for good reasons. Predictable failure is one of them. I’m not aware of any SS air receivers but for sure someone makes them. The problem with SS is it can corrode badly and unpredictably in certain circumstances. Not something I would trust.

When I Googled, "what corrodes Stainless Steel" I could not find any conditions which you suggested are a severe potential problem. It would be a benefit to all members reading this thread if you could tell us what conditions you believe cause Stainless Steel to corrode so badly and unpredictably that it should not be used as a pressure vessel for air compressors.

 

[Steamchick]

I worked in a car factory, where there was a laboratory with salt-spray chambers.... One big enough for a whole car. The chambers used for accelerated corrosion testing were virtually condemned, and needed all the stainless fittings (lights etc.) replacing after 10 years as they were badly corroded. Electrolytic corrosion in aggressive wet atmospheres (like the inside of a compressed air tank) simply attack the "stainless" steel despite the Chromium, etc.
I live on the coast - North sea. Where the salt winds blow in and there is a lot of humidity. I have to buy the expensive corrosion resistant stainless steel outdoor household door furniture so it doesn't rust. Unlike the decorative "indoor" stainless steel stuff.
A Professor of Materials Engineering always started his first lecture thus "The electrolytic corrosion of ferrous materials in an atmosphere containing oxygen and water can be expressed easily in 2 words . Steel rusts".
A Doctor of Civil Engineering plagiarised this into "Concretions containing Calcium products in an acid water medium corrode and fail (aka ageing) and this can be described in 3 words. "Wet concrete corrodes". - Think of that one next time you are in a steel framed building or concrete building in an Earthquake, or driving at high speed beneath or over old concrete bridges on the highway, - or don't. Because the corroded brake lines on your 15 year old truck are more likely to get you. (There is water in the brake fluid that attacks the inside of the brake pipes!).
AAAARGH! Where does it end?
As someone else said.. "Acid to magic, Rust to dust".
I learned some simple lessons as a lad.
K2

 

[Toymaker]

I worked in a car factory, where there was a laboratory with salt-spray chambers.... One big enough for a whole car. The chambers used for accelerated corrosion testing were virtually condemned, and needed all the stainless fittings (lights etc.) replacing after 10 years as they were badly corroded. Electrolytic corrosion in aggressive wet atmospheres (like the inside of a compressed air tank) simply attack the "stainless" steel despite the Chromium, etc.
I live on the coast - North sea. Where the salt winds blow in and there is a lot of humidity. I have to buy the expensive corrosion resistant stainless steel outdoor household door furniture so it doesn't rust. Unlike the decorative "indoor" stainless steel stuff.
A Professor of Materials Engineering always started his first lecture thus "The electrolytic corrosion of ferrous materials in an atmosphere containing oxygen and water can be expressed easily in 2 words . Steel rusts".
A Doctor of Civil Engineering plagiarised this into "Concretions containing Calcium products in an acid water medium corrode and fail (aka ageing) and this can be described in 3 words. "Wet concrete corrodes". - Think of that one next time you are in a steel framed building or concrete building in an Earthquake, or driving at high speed beneath or over old concrete bridges on the highway, - or don't. Because the corroded brake lines on your 15 year old truck are more likely to get you. (There is water in the brake fluid that attacks the inside of the brake pipes!).
AAAARGH! Where does it end?
As someone else said.. "Acid to magic, Rust to dust".
I learned some simple lessons as a lad.
K2

Any idea what chemicals, and at what concentrations, were mixed in the water that was sprayed onto test subjects inside the accelerated corrosion testing chamber you mention? Since the chamber was intended to accelerate the corrosion process, I'm guessing it wasn't normal sea-water.

One of the most common mistakes people make when working with stainless is using common steel fasteners (nuts, bolts, pipes, etc), which causes galvanic corrosion to occur; watch out for this little problem in your outdoor stainless furniture, as many manufacturers will use common steel and aluminum screws, bolts, and rivets. The Electrolytic corrosion you mention requires an external electrical source such as a battery or an AC power line, in order to occur.

We can look to the salt water marine industry to see what works over extended periods of time, and what doesn't last very long. As one example, nearly all salt water propellers are made of stainless, and they typically last longer than the boats they're mounted on.