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Is safe to buy a used air compressor?

My buddy had one of those cheap POS 120 volt garage units blow apart years ago- wasn't 10 yrs old. Blew a hole in the sheetrock and put a huge dent in the fender of his 70 Superbee.
My shop compressor is a 2 hp Wayne I purchased quite used in 1995 at an auction. I have no idea how old it is, but it keeps on lickin', and hasn't blown up...
 
I may hold the record for the oldest working compressor in captivity. I have a Henke Pacemaker (Kellogg American) built in 1965. It's been in the shop over 30 years. When I purchased it I also bought a couple rebuild kits thinking I was going to need them at some point in time.

The compressor doesn't get used on a daily basis but more as a backup when times are busy. It gets regular maintenance and draining, and so far, hasn't needed anything more than some valves and a new unloader.
See post 53 above.
When I contacted IR hoping for paper work or parts the nice lady said "Oh Honey, all i can say is from the serial # it's from the early 30's. We have nothing for it. Put it in your museum".........Bob
 
I’ve got my grandfather’s old 1939 Kellog that I have mostly retired for safety concerns. Replaced with a nice Quincy a few years back. I will still fire it up everyonce in awhile though, but I keep it tucked behind the mill and lathe to hopefully act as a shield in case anything does go boom.

I will say, an air compressor is not where I would be skimping and buying used. Never know how they were treated and they are extremely dangerous if they explode.
 

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The cheap units sold in hardware and auto parts ,the air tanks are made with the same metal as aerosol cans ......the steel is maybe 1.6mm thick...........the old units ,two men couldnt lift the tank ,it was around 3/8 thick ,and not hard steel ,but low carbon boiler plate...........i got some very nice units on govt tender as seized....each one was from a military uniform marking unit ,and the compressors were run from new without any attention until the oil got so low they locked up......most of them had the airtank completely full of water and oil........All freed up nicley ,and sold as new units.
 
You can sandblast with any size compressor as long you size your patience, nozzles, and media right. I have many, many hours of sandblasting with 20 cfm from a 5hp compressor. It works fine, but wouldn't be great for blasting away coatings except for spray paint. I use 140cfm for our large blaster and I find 40cfm to be a sweet spot for a gun type blaster before you can't hold the gun and the part very well.

The key is to use the lowest pressure, finest media, and highest airflow that will work for your application. The goal is 100% coverage of the part, which requires an abrasive impact at every point on that part before you are done. If you use coarse abrasive, each hit will be harder but there will be fewer of them. You will waste time if your media delivers a coarser profile than you need or if it does more than strip the coating or rust you care about. We get a solid 2-3x increase in productivity over my old employer because we found the correct parameters that work for our parts.

To be clear this does not mean find the finest media available.
 
Be sure to let the air out and drain the tank before hauling off your found compressor. Safer that way. If it rolls and breaks off a pipe fitting no loud whistle of air to scare the driver.
Neighbor put their old, 30 gallon, water heater in the alley, standing up. I tried tipping it to see how heavy it was. It seemed much heavier then a new one. So I opened the drain valve and about 20 gallons of rusty water and grit drained out. That made it much lighter so I could walk it around without too much straining. Did I say the neighbor was not too smart?
Within the week I got a bucket and drained some water out of my heater. Got a few table spoons of sand and grit out.
Bill D
 
My buddy had one of those cheap POS 120 volt garage units blow apart years ago- wasn't 10 yrs old. Blew a hole in the sheetrock and put a huge dent in the fender of his 70 Superbee.
My shop compressor is a 2 hp Wayne I purchased quite used in 1995 at an auction. I have no idea how old it is, but it keeps on lickin', and hasn't blown up...
Picture of the car before or after?
 
if you have ever blasted with a 1/2" nozzle and 1200cfm ,then the 5hp is so painfully slow you just give up......another difficulty with small nozzles is water /condensation.....a single drop of water will block a tiny nozzle ,a stream of water in the grit wont affect a 3/8 nozzle.........Styoyovskis point is also important ......dont be decieved by a coarse grit ........fine abrasive dust actually cuts fastest on metal ,but the dust is very costly on removal and on equiptment wear......and more important ....it doesnt leave a "profile ",which is critically important with high build low volatiles coatings.
 
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if you have ever blasted with a 1/2" nozzle and 1200cfm ,then the 5hp is so painfully slow you just give up......another difficulty with small nozzles is water /condensation.....a single drop of water will block a tiny nozzle ,a stream of water in the grit wont affect a 3/8 nozzle.........Styoyovskis point is also important ......dont be decieved by a coarse grit ........fine abrasive dust actually cuts fastest on metal ,but the dust is very costly on removal and on equiptment wear......and more important ....it doesnt leave a "profile ",which is critically important with high build low volatiles coatings.
We use I think 220 grit aluminum oxide and the wear on the equipment is insane. We use a Clemco pressure pot we got second hand and blast inside of a couple of shipping containers with supplied breathing air from a regenerative blower (getting breathing air from the compressor is dumb IMO) and three large fans exhausting outside.
At first we were having to do repair every 10 hours of blasting. The bearings in the belt drive fans seized, the pulleys and belts wore beyond usability, the metering valve ate itself, all of the blowoff hardware failed multiple times, the inlet valve stuck, the pilot valve failed after the original unit failed, the hoses ate themselves, the couplers ate themselves, the wires got blasted, etc.

We now do maintenance every three months. These are the key things that allow us to do this:
All Clemco valves, except for the metering valves, are removed. The inlet valve is replaced with an oversized solenoid. The outlet valve is made of a custom pilot operated diaphragm valve, that the diaphragm can easily be changed out, as it does wear, but there are no moving parts to stick. Because the pilot is handled by a solenoid switching clean air into the back of the diaphragm, there is no debris from the outlet that can get into any of the small ports. The orifices in the blowoff valve that drains the tank pressure are 3/16" in diameter to prevent clogging.
Our compressor has a refrigerated air dryer. This greatly reduces clumping and plugs.
We removed all quick couplers in the blast line. Only barb fittings into the hose. The nozzle holder is upgraded to metal, and we have a process to cut the hose exactly square so it butts to the nozzle perfectly. The nozzles only last about 6 months on 30 hours/week blasting until the orifice is too large for the compressor to keep up.
We have a high flow regulator set to 50 psi. No need to have the pressure any higher than that for our stuff. It just gets louder and makes a rougher profile that is hard to cut.
We have 3d printed or metal bearing shrouds, and greatly oversized belts and pulleys to keep the fans running as long as possible. We run the motors off of a VFD at 50% of their rated power.
The control cable is routed away from the base end of the hose, which is the first part to blow through. We cut the hose back every 3 months if it has soft spots, which it usually does. I used Soow cable which has lasted 9 months so far but I will need to replace it next service interval. I'm considering using 1/4" hydraulic hose to armor it, and I want to make a new deadman switch anyway.
All of our paddle and switch endeavors failed and we just use a milspec waterproof switch from McMaster Carr that was very expensive. It's not a deadman switch, which I don't like, but its very uncomfortable to hold a paddle for several hours, as you really need to be able to switch your grip up a lot. Anyone in the blast room is well protected by PPE, and power and air can be shut off externally so it's unlikely to lead to injury. The whip is securely hooked to the ceiling so it can't flail around too far. (It can be easily unhooked for large parts).
We also have to replace the expanded steel every three months, as it gets blasted into nothing. Might switch to a formed plate table.
Breathing air is handled by a regenerative blower (same thing as a pond blower). Use pond hose, which is smooth on the inside. Corrugated hose screams in your ear. We bleed off excess pressure and the operator can control how much airflow he gets. It ranges from nothing to sticking your head out the window at 85mph. It pulls clean air from far outside and doesn't heat it up much at all. The inlet is raised and shrouded to protect it from rain, dust, exhaust, and pranks.
The floor is armored with steel plate.
We are very controlling about what we do and do not allow into the booth. Spray painted or powder coated items are okay, but never anything greasy or with sealant. At an old job they patched holes in their makeshift booth with RTV silicone to keep dust contained. It caused huge fisheye problems for weeks and had to be stripped completely. Silicone is bad for powder coat.
Cheap LED lights need replacing. About 2 of 12 go out every three months, but the kind with switches on the cord die very quickly from sand ingress.

I probably forgot a ton of stuff, but yeah, the machinery gets eaten during operation big time. We'll likely have to replace our pressure pot soon due to wear, at which point I'll probably use another regen blower to make a suction lift so sweeping and shoveling sand becomes unnecessary. I could probably also just put a door in the bottom of a suspended shop vac.
 
A good 'compressor danger ' story.......Col bought a used Sullair 1200 for the refinery job........the Golden Ones always wound the pressure up to just under safety valve blow .....the compressor had a small safety valve on the wet discharge pipe to the oil separator..........the boys went off to break leaving the compressor running ,pressure crept up ,and the small safety blew off ......... oil foam blew onto the red hot exhaust of the GM .....instant inferno .......at an oil tank farm.........I dont know how the boofheads talked their way out of that one,but they did.
 
I bought a used compressor about 10 years ago. It was branded Gilbarco but the pump was a Quincy 325. The tank is quite thick compared to today's tanks and it has not skipped a beat since I have owned it.

My opinion on the stories of compressor tanks blowing up is the safety valve was stuck or not installed. Sure, anything is possible but I cant see a tank exploding if the safety valve is installed and working properly.

For me, the Gilbarco/Quincy compressor is the best compressor I have ever owned and I certainly could not afford have afforded to purchase a new one so the risk of buying used was worth it to me.
 
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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.
I agree. If your hearing is good you should be able to tell the difference between sound and rusty metal as long as the tank is completely drained of any water and sediment.

I have reproofed a couple of antique muzzle loading shotguns and the first test is to hang the barrels from a string and tap gently with a small brass rod. If they make a ringing sound they are good enough to proceed with additional tests.
 
Another thing to consider is the 3 prong ground plug. Some guys use the plug instead of the machine switch and so can wear the ground prong making any such machine unsafe..My grandson was almost electrocuted because of this...
He (my grandson) told me they were getting a shock from the machine (air compressor)..I saw that the plug was shot and told them I would go get a new plug, and don't run it until I get back.
They plugged it back in and then my grandson got electricity stuck the steel door.
His dad, my son-in-law gave a football-type block and knocked him free or he would be dead (110V @ 30 amps can kill you). I Have no idea whose idea it was to plug it back in..really dumb.
That is why I don't help at the deer blind shop anymore..
I have many years of tooling/machining and engineering and my son-in-law who has next to no shop experience ignores much of what I say...I was working there for free but had been away for a time so did not know the air compressor was giving shocks.. ...Go figure,
 
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