Not logged inAmerican Welding Society Forum
Forum AWS Website Help Search Login
Up Topic Welding Industry / General Welding Discussion / Sandblaster/blasting cabinet pressure
- - By Mathius (**) Date 02-01-2004 22:03
I borrowed my friend's sandblaster, and he and I were talking about a leak it had in the fittings, just a small one. I had talked about how basic the setup was and how I thought I could weld up my own. He said he'd be afraid to weld anything that held that kind of pressure. I'm MIG certified (3/8 vertical (3G)), and I've got an arc welding certification that qualifies me up to 4" thick material(1G tho). My friend is just a diy'er and has never taken any classes on welding, let alone have any certifications. Sandblasters run at what? 120 psi max? It doesn't seem to me like there's that much of a risk in designing one. There would be no pressure in it while welding on it, and it wouldn't be holding enough pressure that it would explode or anything, right? I shouldn't have any problems welding material thick enough to make a blasting cabinet or tank out of, but then I won't have equipment to x-ray the welds like I would on the job or something...

What do you guys think?

Mathius
Parent - - By mikew (*) Date 02-01-2004 23:49
Mathius,
I believe the pressure you are speaking of is on the hose itself, not on the cabinet. Most cabinets are made are nothing more than "tin cans" but they arefairly easy to build after you get a decent compressor , hoses and fittings.( habourfreight.com sells these parts pretty cheap)
Back to your question, you should not have a problem buiding one, I have never seen a cabinet that is under pressure but you would want to make sure the access door and windows seal so you dont have blasting dust all in your garage (and lungs).
hope this helps-
mike wright
Parent - By flatjwl (*) Date 02-02-2004 04:22
There are a couple of different processes used for delivering blast media. All the small blast cabinets, I have seen, use a syphon feed system and have venting and vacuum evacuation for the air/dust. In this system the pressure would be confined to the hoses within the system, and there would be no "pressure" developed inside the cabinet. This cabinet structure need not be overbuilt, with 16 ga being adequate for a hobby unit. In larger, more powerful, units the media is loaded inside a tank and pressurized to be forced thru the hose to the nozzle. In this system, the tank must be constructed to withstand the internal pressure and the weight, plus wear and tear from media. The cabinet or "blast room", of the pressure feed system, can be subjected to a more severe pounding from the media, so it too must be of much heavier construction than the syphon system cabinet, however the design of the cabinet/room is such that minimal interior pressure is involved. There are also systems which use spinning wheels to hurl the media at the parts. These must be constructed of very heavy, abrasion resistant materials, but these are not found in the average garage.
Parent - - By Mathius (**) Date 02-02-2004 05:07
I'm more concerned with the sandblasting unit itself than the cabinet. The only design I've seen involves air from the compressor going through the lines into the tank, where the pressurized air builds up until the trigger opens on the siphon gun, where the pressure (and sand) is released. At least I believe this is how it works. I was thinking if I went ahead with this project, 1/4" cold rolled would be thick enough to build a tank out of, probably with a big industrial sized tube, capped off at the bottom, and a small opening in the top for sand and a lift handle.

Mathius
Parent - - By - Date 02-02-2004 05:56
You really don't want to use a pressure pot type blaster in a cabinet, unless the cabinet is big enough to hold a car. The volume of incoming compressed air will turn anything smaller into a cloud.
For a small cabinet, use one of the cheap Harbour Flotsum siphone type guns. If you're planning on heavy use, get a commercial gun that uses ceramic nozzles.
If you want to maximize abrasive life and minimize dust in the cabinet, line the wall behind the target with sheet rubber.
For small units, Kirby Vacuum cleaner motors with bags make excellent dust collectors. Kirby fans are replaceable.
Parent - By dee (***) Date 02-03-2004 20:41
Franz,
For general advice I'll point out that the PLASTIC fan will give many more long years of service in abrasive atmospheres than the metal fans, but the proper plan of attack is a cyclonic filter as used in many central vac systems where centrifugal force seperates the dirt from the air before the fan ever gets to it...

regards
d
Parent - - By CHGuilford (****) Date 02-02-2004 14:38
I tend to agree with Franz in that you don't really want a pressure pot feeding into a blast cabinet. Blast cabinets such as Harbor Freight sells are more for hobby purposes. The air/abrasive siphon system give you good control over smaller or delicate items. The cabinet itself doesn't build up any appreciable amount of pressure due to sufficient venting.
If you build a cabinet and don't allow enough venting, then it very easily could rupture.
Pressure pots are built to the ASME code and actually use a "carburetor" to throttle the abrasive flow into the blast hose. They operate at 90 - 150 PSI and are intended for large volume blasting.

I would caution you on constructing your own pressure pot. 120 lbs per square inch doesn't sound like much until you add up all those square inches. You really might be better off buying a product made for the job.

Chet Guilford
Parent - - By Lawrence (*****) Date 02-02-2004 18:05
Under pressure!

Did anybody catch the episode of "Myth Busters" that focused on the compressed air Chicken Cannon?

When it came down to it, the chickens reached *terminal* velocity of over 100 mph with pressures varying between 120 psi and lower than 70. If the volume is sufficient in the holding tank and the valve is reasonably quick you can splat the heck out of a chicken at a surprisingly low pressure.

Sandblasting... Chickenblasting... is there really any difference?

Harborfrieghtchickenblastin!

Parent - By jwright650 (*****) Date 02-02-2004 18:13
L,
Yeah, I saw that episode of the chicken cannon. I would not want to be the pilot of a plane that has a frozen chicken coming through the windshield at 120mph. That's gotta hurt!
John Wright
Parent - - By dee (***) Date 02-03-2004 20:57
Chet,
Picture a modest steel box, perhaps with a couple of fittings to hold some blasting gloves sealed into the side of the box, and a piano-hinged lid laid down over perhaps a 2'x3' hole to permit abrasives to be conveniently reclaimed via a funnel in the bottom of the box. Fankly I am confused but I am seeing this as the kind of gadget he wants to feed a commercially procured pressure blaster into... The only precaution would be to never latch the lid, and should it want to rupture inexplicably due to pressure, the gloves would fail first, hopefuly.

BTW:
There is an optimum angle for the bottom of the box, which should exceed the angle which the abrasive of choice will form naturally when poured into a pile on a flat surface- I believe it will come out to be about 27 degrees.
Parent - - By CHGuilford (****) Date 02-04-2004 17:50
Dee,
We have a blast cabinet on site, used for small items that would fall through the grating of our blast pit. The air line feeding it is only 3/8 hose. If a large commercial blasting were used in it, several things would happen. 1) the blast nozzle flows at least several hundred CFM so the cabinet would skate across the floor like a bottle rocket. 2) if you bolt it down, the high velocity, high volume blast media would take the back of the box out in short order. 3) the rubber gloves would inflate backwards and maybe even pop. That is if they don't pull away from the seal.
Of course, there is the problem of blast media coming out of the cabinet at 90+ mph. The idea of the cabinet is to contain everything in a small area and that the operator doesn't need to wear a fresh-air supplied bag hood.
If you fed the blast pot with a "home sized" air compressor you'd only get short blasts of air with a long intervals waiting for air pressure to build.

My point of the post is that it really is not a good idea to re-invent something unless the person thoroughly understands all the details. The end of a blast hose at short range is roughly equal to a shotgun blast.
(I know how blast media ricochets feel, even through 2 pairs of coveralls)
A pressure tank not strong enough for pressures put into it equals a potential hand grenade.
I'm not really trying to talk anyone out of being inventive and learning. But I would feel bad if someone got hurt, or worse, when I could have said something.
Very often the best model to copy is something that has already been built. Then you have to ask yourself if you can really make it cheaper that the factory did. If you can, or if you can make an improvement then great.

Chet Guilford
Parent - By dee (***) Date 02-05-2004 01:17
Chet,
I like bottle rockets!

I see your point. Your discretion in consideration of caution is responsible. Your advice is always sound. I dont think this equipment I'm envisioning will quite make the big-leagues though.

The little spray-gun looking sandblasters I have tried are really frustrating; I have found it difficult to obtain the correct etch and they are far too slow; they were never designed to handle the necessary volume and pressure, I think. There are various home style pressure pot blasters, and connected to a homeowners compressor feeding it through a tiny 1/4 inch connector fitting orfice (or less on a quick-connect- Lord only knows) the supply volumes and pressures (not to mention the compressor design) kind of stretch the definition of blasting nearly to hopefulness. They create quite a mess in a homeowners garage, and my neighbor is living that point down even now... I havnt seen the little homeowner blast cabinets in action, but I pesumed results similar to the (useless)hand gun I've tried and saw a logical progression from useless suctuon gun to almost useful pressure blaster "wannabe".

I'm still laughing... mental images of the blast cabinet turning inside out and flying across the room, jumping on top of it to hold it down... with a cloud of dust and a hearty Hi-Ho-Silllver! Ye-Gads... Slim Pickens ridin an A-bomb!

Thanks Chet,
d

Parent - - By billvanderhoof (****) Date 02-03-2004 04:42
Old style 20 lb propane tanks (no overfill prevention device) are easy to find around here for free since you can't legally fill them any more. That would be a good place to start for the pressure tank. Careful to really purge it out really well before you cut into it though.
Bill
Parent - - By dee (***) Date 02-03-2004 20:59
Jeeze Bill!
I can almost smell the burning hair and flesh...
regards
d
Parent - - By billvanderhoof (****) Date 02-04-2004 04:46
If you take the valve out and then let an air line run into the tank for 15 minutes or so there should be no residue. The boiling point of propane is so low that it will boil away completely.
Bill
Parent - - By - Date 02-04-2004 05:59
I've built the wrench for removing the valves, and I've made a few things from Propane tanks, so let me say this very clearly; Propane Tanks are DAMN Dangerous if you don't know what you're doing with them.
When I remove valves, I first evacuate the tank for an hour with a vacuum pump, Then I charge them to a positive pressure of at least 5psi with Co2 before I remove the valve.
Then, I leave them sit inverted for a few days.
Water won't wash out propane!!
PROPANE has a nasty habit of permiating into microporoscity in the steel and the welds, and will stay there with the valve removed and the tank exposed to the atmousphere.
I have launched a 100# cylinder that had the valve removed and sat open for over a month when I welded on it. That cylinder traveled about 300 feet till a tree stopped it.
Propane Cylinders are not something to treat casually.
You can take my word for it, or you can invest in life insurance so you leave a rich widow.
Parent - By boring (*) Date 02-17-2004 00:17
Unless you are blasting 24/7 like we do in my section of our plant, you DO NOT NEED a blaster with any kind of pressure vessel on it. A siphon blaster will work just fine......kinda like the ones in the rest of our plant that blast 16/5.
Up Topic Welding Industry / General Welding Discussion / Sandblaster/blasting cabinet pressure

Powered by mwForum 2.29.2 © 1999-2013 Markus Wichitill