What about fridge compressors?

[john bunsenburner]

That would be a really stupid thing to do. The compressor has a safety factor built in for a reason. SAFETY.

If the resevoir ruptures at 12 Bar it'll likely kill anybody within a few feet.

If you swap out the stock pressure switch for a higher pressure one then what happens ten years from now after the tank has been setting with condensation it?

If you check the tank frequently and keep the pressure at 12bar then you should be ok. Most of the time the "max pressure" is the maximum working pressure not the failing pressure.

[Ragnarok]

That would be a really stupid thing to do. The compressor has a safety factor built in for a reason. SAFETY.

In most cases, it's to prolong motor life. Few air tools need more than 120 psi or so, so they set it to shut off there, giving a longer compressor life.

It's not about reservoir safety. The reservoirs are up to more than the pumps put out.

then what happens ten years from now after the tank has been setting with condensation it?

Last I checked, preservation steam engine boilers have a ten year service life. That's at higher pressures, higher temperatures, a fairly high service cycle, and a heck of a lot more water.

Those need far higher safety regulations because if one of those blows, you wouldn't want to be within a hundred metres of it - and if you were, you aren't any more. Basically that 10 year licence period has to cover a period where you can be sure of it will not go.

If those are certified "incredibly damn safe" for 10 years under harsher conditions, I don't see why a compressor should because it's had a dribble of condensation in the base - and besides, a good owner would clean that out regularly, making it fairly moot.

I was actually thinking of lowering the RPM to get more torque (you know gearing it down (or up??))

Gearing it down would work, assuming there is already a gear train you can replace.

I am happy with my fridge compressor so I am not really interested in this personally...

Fridge compressors won't be enough for one project I have in mind. I'm looking for ways to get a LOT more pressure, and faster too. I think I can get to 150 bar...

[john bunsenburner]

@rag: What exactly are you planning to do? 150bar are alot, nearly as much as commercial scuba compressors puts out. It is of coarse great for a gun to have such pressures but I doubt you could get any where near that with either a fridge or a work shop compressors. Maybe i misunderstood you, but i would love to know what exactly you are trying to do simply because I too would like to achieve such pressures and over time bulk tanks do sum up. I am not sure if you mentioned it in an earlier post if so, i am sorry for the question, if not please do explain what you are planning to do, most things you do are very interesting and are maybe i could adopt the idea, because having 150bar at hand without the need for a bulk tank would be really great. Thank you in advance for an answer to my question

[Ragnarok]

150bar are alot, nearly as much as commercial scuba compressors puts out. It is of coarse great for a gun to have such pressures.

It is a lot, and no I don't plan on using it directly into a cannon... yet, at least - those pressures are not to be trifled with. I might not even go that far, I might use only 100 bar.
The plan is mostly using it to pressurize a high pressure tank which I will then regulate back down again to about 500-600 psi for use. I could get this end result direct from a fridge compressor, but I want a lot of portable air available pretty quickly. Fridge compressors take a lot of time.

I have a lot of work to do on this at the moment. Basically, I will be building a pump powered by a pneumatic ram, using force multiplication to reach pressures of the order of 150 bar.
However, building such a pump will take time and resources I can't spare for a while, so I'm not spending too much time on the design yet.

[john bunsenburner]

Ok this sounds like a really great idea, i hope it works out, and i really want to see some pics and vids once your done. This is one of the things that i would love to have and i was thinking about it for hours on end. Please inform me(and all the others that are interested) about your ideas and progresses. I am trying to build a pump powered by an electric motor, i am aiming to get about 20-50bar from it, i have never dared to think about building something that would supply more power, due to the fact that i am unsure of how to acieve this. I have to do some reasearch into pneumatic rams because i am unsure of how exactly they work, but i prefer your idea over mine, simply because of the power that yours could theoretically supply.

[psycix]

That would be a really stupid thing to do. The compressor has a safety factor built in for a reason. SAFETY.

If the resevoir ruptures at 12 Bar it'll likely kill anybody within a few feet.

If you swap out the stock pressure switch for a higher pressure one then what happens ten years from now after the tank has been setting with condensation it?

1-The compressor is brand new. Bought it a few months ago.
2-Everything is rated to at least 12 bar, and ratings are always with a safety factor.
3-Tank has a rating for even 15 bar.

12 bar wouldnt be much of a problem. The 8 bar limitation is there as a safety factor, even while this is kinda double as the ratings itself also have a safety factor.
They cut it at 8 because most air tools are made for 8 and 12 isnt needed.

[jimmy101]

The "safety factor" is there for a reason. It's isn't just some engineer blowing smoke out'a his ass. Nor is it some paranoid corporate lawyer trying to cover the companies ass. The fail statistics are based on sampling data, not actual data for individual tanks. There will be some small number of tanks that will fail at pressures not too far above the rated pressure.

The tank is made on an assembly line. No two tanks are identical. The tanks may be tested to the spec pressure but they certainly are not tested to their fail pressure. They also aren't tested to the spec pressure then abused by bashing, thermal cycling, being allowed to sit with a small amount of water in them, or other real world events that may compromise the tank's integrity.

No engineer worth a damn will ever purposely exceed the spec's of a device. Engineers don't look at "Safety margins" as just suggestions. Nor do they ever figure they can stray into the 2x (or whatever) margin built into the spec.

It doesn't matter if the tank is brand new. It doesn't matter if you "know what you are doing". What you are doing is probably illegal and is certainly dangerous. What happens if you forget you've swapped the pressure cutoff switch to a higher rating and ten years later someone fires the compressor up? You are responsible for not only yourself but for everyone that ever uses the compressor.

[Ragnarok]

The fail statistics are based on sampling data, not actual data for individual tanks. There will be some small number of tanks that will fail at pressures not too far above the rated pressure.

Yes, but TINY numbers. Few tanks will fail even getting close to twice the rated pressure - even after repeated use and fatigue.

Rated pressures typically come with a safety factor of usually at least 3 slapped on it (I can't be arsed to look up the exact SF for a compressor tank) against the fail or yield pressure.

No engineer worth a damn will ever purposely exceed the spec's of a device.

Actually EVERY engineer worth a damn will do just that. It's called product testing.

However, outside the engineering world, when on this forum we spend our entire lives pressurizing plastic pipes in circumstances that they weren't designed to handle (PVC is only rated for non-shock water pressure in a stationary application), their ratings essentially then mean precisely dick.
The compressor tank rating is for what it's doing - sitting still, holding air pressure - so it means a crap load more than the rating on PVC air cannons. If psycix says the tank has a 15 bar rating, and everything else a 12 bar rating, I cannot see any greater hazard from using it to 12 bar, (particularly as compressors are stationary units, unlikely to see any jarring forces while under pressure) than most of what we do on this forum on a daily basis.

You might as well say we should shut the forum down, because even the rated pressure on my high pressure copper launchers means crap because I've made it into what it is.

Like I said before, the 8 bar limit on most compressors is because most air tools use no more than 8 bar - and also to cut the risks of high pressure hose whip.
If the compressor only needs to do 8 bar, then you make it stop at 8 bar - saves the motor life, means users don't have supply line pressure they don't need. Also means you can slap a longer warranty on it without having to fret about the motor burning out.

Not because the 15 bar rated air tank will explode.

It doesn't matter if you "know what you are doing". What you are doing is probably illegal and is certainly dangerous.

You've just summed up most of spudding there.

[john bunsenburner]

I could not agree more to rag. If someone decided to switch the 2hp motor for a car engine and then decided that he should pump the tank up to 200bar, then i would say he is mad(not if he replaced all the parts for 400bar rated stuff though). But in this case it is most probably safer than most pvc combustion guns...(not saying they are unsafe). Spudding involes a little bit of danger, alot if you are a moron who has no clue of what he is doing. It is important not to over estimate one self but otherwise spudding is relatively safe. I think it is well worth the try to crank the compressor up a bit, after all nearly all of us have one and it would be great if we could fully use all of its power.

[POLAND_SPUD]

as rag pointed out, using a PVC for spudguns is dangerous since PVC is not to be used with pressurised air... this basically means that all PVC cannons are dangerous and anyone using & building them is irresponsible


however, safety is not about sticking to some set rules without acctually understanding them nor thinking about them... it's about pushing the limits where you can while not doing this where it would end up bad...



while I don't think it's 100% ok to exceed the rating of a compressor... I do belive that psycix is inteligent and responsible enough to take some basic safety precations if he decides do try it... (like for example moving the compressor somewhere where it won't do much dammage if it fails... or something like that)

[Daltonultra]

If you're going to go over the pressure ratings for your compressor or it's tank, do yourself and everyone in your neighborhood a big favor and build a proper blast enclosure for it.
All you need is a heavy box of either steel or concrete. The walls should be very heavy, enough so that if the tank fails, the walls will be unaffected.
The top, however, should be very weak, thin plywood or something equivalent.
With this, if your tank blows, all the force goes UP, blowing off the roof, but otherwise harmlessly dissipating into the air.

Rag, you're right that every engineer will test a new design to destruction, but an engineer will also take great pains to ensure that if/when the item fails, it will cause no unintended harm to anyone or anything.

[VH_man]

If you're going to go over the pressure ratings for your compressor or it's tank, do yourself and everyone in your neighborhood a big favor and build a proper blast enclosure for it.
All you need is a heavy box of either steel or concrete. The walls should be very heavy, enough so that if the tank fails, the walls will be unaffected.
The top, however, should be very weak, thin plywood or something equivalent.
With this, if your tank blows, all the force goes UP, blowing off the roof, but otherwise harmlessly dissipating into the air.

Actually, In all Technicalities, The top only has to be the weakest part.......

[Ragnarok]

do yourself and everyone in your neighborhood a big favor and build a proper blast enclosure for it.

You make it sound like it's several pounds of C4.

At 12 bar, in the unlikely event it does fail, and considering the ways it's possible it will, almost certainly the results will be no more than startling anybody that's too close.
Metal pressure vessels don't fail in the same way as PVC ones.

For it to violently explode, you'd have less than one in a million chance - and that's a real world one in a million chance, not a hollywood one in a million - and even then the results would be localised.

[Daltonultra]

It's not just the pressure vessel itself you're dealing with. The pump and fittings can be rather violently separated from the tank and sent flying.
Believe me, my Father is an EPA man, in the Emergency and Remedial Response division. I've gotten to look at a great many failed pressure vessels, from small manufacturing reservoirs, to failed rail cars, on up to failed LNG storage tanks. Even a small tank can throw fittings for hundreds of feet.

EDIT, BTW, a compressor tank failing catastrophically at 5-600psi is also pretty much guaranteed to get the cops called out around here...lol

[Ragnarok]

EDIT, BTW, a compressor tank failing catastrophically at 5-600psi is also pretty much guaranteed to get the cops called out around here...lol

12 bar is not 500 or 600 psi, not even close.

It's 174 psi. I've force failed things at those kind of pressures, and I don't suddenly find that my garden looks like I've set off dynamite in the middle of it.

I wouldn't condone 500 or 600 psi - but 174 psi in a vessel rated for it is a different matter.