Is safe to buy a used air compressor?

[adammil1]

I was wondering what the general opinions are on buying a used air compressor?

Pretty much all of my tools are bought used as my preference is to buy quality used tools versus cheap new ones.

I need something primarily for sandblasting which I think puts me in the 5hp 60+ gallon tank range if that sounds about right?

When it comes to air compressors I can find a lot of nice ones in the area at reasonable prices that meet my needs, however tank condition always makes me nervous.

I see all few of the older USA made brands available locally which one would think may have thicker tanks than our modern value engineered ones. I even have a brand new 5HP compressor duty motor lying around in case I need it. However it all comes down to the tank being safe. Any thoughts on inspecting one? Maybe it's time to get a boroscope too will they be useless looking inside at determining how bad any likely rust is?

 

[CarbideBob]

Not a worry on the tank.
Head valves and pistons/rod/crank/ bearings.. Those die.

 

[Milland]

I'm no expert, but if you're doing serious sandblasting 5hp sounds low - I'd shoot for ten or more if you can handle the electric costs.

I do think tank soundness is important, a soft-faced hammer and a couple of taps on the tank will give you some idea of its condition. Tap on the top surfaces, then on the bottom - does it sound the same? Listen for rattles or other noise of debris moving at the bottom, which is a bad sign.

You can DIY a hydrotest, but having it professionally done is better.

 

[dalmatiangirl61]

Look at the date on the ASTM tag, if no tag pass, more than 25yo and you might consider passing. Open drain valve on bottom of tank, if gallons of water come out you might want to pass. If you can get a large plug open, usually requires a large pipe wrench and 6ft cheater bar, you can visually inspect interior, to a degree. If possible, and concerned, mount it (bolt it) to a slab outdoors. As mentioned above, hydrostat it.

 

[john.k]

Yes ,5 hp would be hopeless for sandblasting ........its minimal for glass bead blasting,(the compressor will run flat out all the time) ..........about 40 cfm would be bare min ,and for that you will need 10hp +.

 

[dundeeshopnut]

I've bought both my QR series Quincys used. The 5120 [1981] with 120 gallon tank had a fresh pressure test and all I did to it was a valve service and clean the crankcase just because I didn't know it's history. Been perfect since and will outlast me. The 325 [1965] I did the same, and mounted it on another 80 gallon tank that I opened and inspected [no pitting or corrosion to speak of], pressure tested it and it now is my shop air. For high demand jobs it is also plumbed to unload and run continously. I do sandblast small projects with it with a small pressure pot when it doesn't warrant hooking on to and starting the bigger one. Quiet, smooth, 100% duty cycle and it too will probably outlast me.

 

[Superbowl]

5hp is too small for sand blasting. 7.5 hp will work and won't make the house lights blink like the in rush on a 10 hp. A rusty tank rupturing is always a possibility but extremely rare so I would not worry too much so long as the tank looks reasonably clean.

 

[Gordon Heaton]

If you must stay with 5hp, plan on blasting nozzles 1/8" or smaller and sustained pressures of 60psi or less. Ceramic nozzles wear so fast with hard media that they double in area quickly. Boron/carbide, etc is the way to go.

 

[john.k]

Im told you can buy a Asian ultrasonic steel thickness tester on the net for around $100........these things certainly work reliably with boat hulls ,so a tank wouldnt be a problem.

 

[adammil1]

Im told you can buy a Asian ultrasonic steel thickness tester on the net for around $100........these things certainly work reliably with boat hulls ,so a tank wouldnt be a problem.

The problem I've seen with those is don't you need to calibrate them on a local piece of material. How do you find a piece of the same host material to get a micrometer on the tank. Or if it set up to run on carbon steel would it be just good enough to use against the tank through the paint etc?

 

[adammil1]

Look at the date on the ASTM tag, if no tag pass, more than 25yo and you might consider passing. Open drain valve on bottom of tank, if gallons of water come out you might want to pass. If you can get a large plug open, usually requires a large pipe wrench and 6ft cheater bar, you can visually inspect interior, to a degree. If possible, and concerned, mount it (bolt it) to a slab outdoors. As mentioned above, hydrostat it.

I always thought that the old ones were made thicker heavier walls than the new stuff we're getting and he won't know if that's true?

 

[jbacc]

I bought an old 5 HP Gilbarco Compressor about 15 years ago which turned out to have a Quincy 325 pump on it. The tank is much more substantial than today's tanks and other than putting a new motor on it, it has been trouble free. The Quincy pump is the bees knees of compressors. I wouldn't hesitate purchasing a used compressor but would most certainly check it carefully using some of the advice others have offered.

That's my .02.

 

[john.k]

Not only are the old tanks much thicker,but they are made of proper boiler plate .....which does not corrode in compressor service,provided a bit of tramp oil is left in the tank

 

[BT Fabrication]

under 10 years I most likely wouldn't worry, but 5HP is low. unless its a screw compressor and you are running the smallest sand blaster like 1/4" nozzle.
might only realistically get 10SCFM from 5hp.

 

[Overland]

The problem I've seen with those is don't you need to calibrate them on a local piece of material. How do you find a piece of the same host material to get a micrometer on the tank. Or if it set up to run on carbon steel would it be just good enough to use against the tank through the paint etc?

I've bought used compressors. Recently a Saylor-Beal 5hp for $350, deal !
Regarding calibration, I'd have thought the tank would rust at the bottom, and not much at the top. So maybe you could "calibrate" with several readings in the upper half of the tank, then check out the bottom. That should tell you if there's a significant difference.
You could also measure the motor mounting plate for another thickness reference.

I've used a 3 hp 60 gallon compressor to drive a Harbor Freight 3 or 4 gallon pot with reasonable results on smaller projects. So 5 hp might be fine, depending on what you're trying to achieve.
Bob

 

[adammil1]

I've bought used compressors. Recently a Saylor-Beal 5hp for $350, deal !
Regarding calibration, I'd have thought the tank would rust at the bottom, and not much at the top. So maybe you could "calibrate" with several readings in the upper half of the tank, then check out the bottom. That should tell you if there's a significant difference.
You could also measure the motor mounting plate for another thickness reference.

I've used a 3 hp 60 gallon compressor to drive a Harbor Freight 3 or 4 gallon pot with reasonable results on smaller projects. So 5 hp might be fine, depending on what you're trying to achieve.
Bob

Problem with calibration is the way I have seen it done is you go to a flat section of material usually with the paint removed and mic it then enter that into your meter and measure away. Maybe just setting it up for carbon steel and hitting a few spots with paint and all would be good enough? As for sandblasting plans are for small parts in a blast cabinet. Once again hopefully I can get something high quality used but I do see a lot of people getting good luck on a harbor freight cabinet with some mods is 5hp still a bit low for that?

 

[Karl_T]

I look for used quincy QR-325 compressors. These commercial quality units are made to run 24x7x52. A 20 year old one is just broke in.
I did have to do a head rebuild on my last one, parts on ebay just $150 or so. This unit will outlast whom ever buys it at my estate sale.

 

[steve-l]

The problem I've seen with those is don't you need to calibrate them on a local piece of material. How do you find a piece of the same host material to get a micrometer on the tank. Or if it set up to run on carbon steel would it be just good enough to use against the tank through the paint etc?

You should always recalibrate every time you use it anyway no matter who made the tool.

 

[adammil1]

You should always recalibrate every time you use it anyway no matter who made the tool.

Yes but how do you do it on a tank with out cutting a hole in it to mic thickness? Or maybe I am overthinking it and you can just calibrate it to some of the steel at the top where the motor and compressor mount? It probably is similar to the tank material but no guarantee and it may be painted on both sides which may also effect the readings.

Plus with ultrasound does it work with paint as I am not sure I would want a potential buyer removing spots of paint on a tank just to tell me if he's willing to buy it.

 

[steve-l]

Problem with calibration is the way I have seen it done is you go to a flat section of material usually with the paint removed and mic it then enter that into your meter and measure away. Maybe just setting it up for carbon steel and hitting a few spots with paint and all would be good enough? As for sandblasting plans are for small parts in a blast cabinet. Once again hopefully I can get something high quality used but I do see a lot of people getting good luck on a harbor freight cabinet with some mods is 5hp still a bit low for that?

First, compressed air tools are very inefficient. A 5 HP compressor will barely feed one air tool and even then at a reduced duty cycle. My compressor is 5500 watts 3 Phase and it will only pull one tool at a time with a 100% duty cycle. It will support a sand blasting cabinet for 10 minutes max at 60 PSI. and that is with a 600 liter air tank.