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I know that most of McMaster's galvanized and stainless fittings are only rated for 150psi, but I am pretty sure that they can withstand brief bursts (hybrid ignition) of around 3000psi. However, can these low pressure fittings withstand a rapid rate of fire at high pressures, for example, 2-3 rounds per second (working on an automatic design)?

Also, will welding stainless or galvanized pipe weaken it in any way like with aluminum?

Using fittings at pressures in excess of what they're rated for never leads to really simple answers. In this case, the answer depends on the desired safety factor, the design, where the operator is standing and how close, how the heat is being sunk, and what kind of peak pressure you're actually expecting.

From my understanding, most of these pipes are already annealed, and welding them will make little difference to their pressure ratings - many are designed to be welded without any special heat treating process. As I'm sure you're aware, welding galvanized anything without proper precautions is a recipe for disaster (poisoning and/or death in this case).

I wouldn't recommend using pipe outside its ratings on an automatic design - standing back a suitable distance would take all the fun out of it. Although I've certainly had my share of operating outside the ratings, for a device that's supposed to entertainment, you're better off doing it right the first time.

First off, it is actually going to be a very small cannon, preferably in a rifle form factor, so yes, I will be holding it. One specific fitting I was looking for was a 1.5" x .5" reducer. I could not find one anywhere, galvanized or stainless, that would be able to withstand more than 300psi (rated pressure). And even those ones were at ridiculous prices ($35+). I don't care whether it is threaded or unthreaded, I am probably going to be getting a welder this Christmas. Do you know of any place where I could get a high pressure reducer of this size?

Short bursts do generally result in higher strengths, but you still have fatigue to worry about. Unless you're very familiar with the characteristics of the material (i.e. you know exactly what you are doing), do not exceed the pressure rating. As far as I know no one here is qualified in this respect.

Testing could reveal some useful information if you want to go that route. But note that fatigue is important. You won't see a material fatigue unless you do a great number of hydrotests for example.

Welding will weaken the material IMMENSELY. You will anneal the material, and if you would like to use it at any kind of pressure afterward you should temper it. Assuming the material isn't too thick and there's no zinc or other contaminates left it can be done with a blowtorch and oil.
Tempering. Google for more information.

Before picking the vessel/pipes you should do a quick calculation using a program for calculating burst pressure of the pipes. Use a fair safety factor and if you don't know the exact temper or alloy of your material then use the LOWEST you can find to prevent over-estimation. (or increase the safety margin).
Clicky.
Using the ultimate tensile strength and a factor of zero will give the burst pressure, while using the yield strength will give the point at which deformation will begin.

Lastly, the welds. Unless you are a VERY experienced welder DO NOT trust your welds with significant pressure. What kind of welder will you be using? I'd try find someone who is certified to weld pressure vessels or is a boilermaker to do the welds for you. Unless you are certified to weld pressure vessels, this could be very illegal as well. I'm very good at welding, and I've been welding for a long time (and we also have a very good welder) though I'd never trust any weld I can do for HPA.

you can use stuff for hydraulics

I guess I am not an extremely experienced welder, so I probably won't weld this together.

Thanks for the link to that burst pressure calculator! It is very handy. I only have one question - what is the safety factor? Is it how safe you want it to be?

Yes. A safety factor of zero will give the burst pressure. A safety factor of 3 is often the absolute minimum, though if you are beginning to add cycling mechanisms (automatics), impacts etc then a higher rating will be better. I'd use a higher factor if your face will be continually close to it. Also don't overestimate the properties of your material and be sure to use the right numbers.

It's good, but does not simulate it being dropped, fatigue etc. It's no substitute for hydrotesting.

Well, with SCH80 stainless and a safety factor of 8 it predicted a burst pressure of about 20000 psi, and a max working pressure of 2400psi.

Coincidentally, HGDT predicted a max combustion pressure of 2400psi. However, I am estimating that it will be a little higher since I will be using pure oxy.

Also, I browsed the web for stainless pipe, and without dropping $40+ on a small pipe, I would have to use unthreaded, which means my only option would basically be screws and o-rings to connect the fittings.

Screws and o rings are good. Make sure to leave length between the bolt and edge of the pipe to be secured (to prevent shearing of the pipe) and find bolts with decent properties, calculate the amount of force exerted and then add in your safety factor.

Assuming you used the correct numbers I'd be happy with a safety factor of 8. Still try and get hold of a good grease gun or pressure washer and hydrotest it first.

If you have a lathe or can get access to one then you can cut threads with the lathe (not an easy task).

If it's suitable, you could buy an old CO2 or HPA bottle and find a way to adapt it to your needs.

I do have access to a lathe at my school, but I am not really that experienced on the lathe, the best I can do is cut o-ring grooves into a piston.

An old HPA bottle probably wouldn't work, as I will be using a complex blowback bolt that I designed, and it will only work in pipes of a certain length and width.

I do not exactly know what kind of screws I should use. I was thinking of sheet metal screws, but they might not be strong enough.

inonickname wrote:Welding will weaken the material IMMENSELY. You will anneal the material, and if you would like to use it at any kind of pressure afterward you should temper it.

Depends on what you're dealing with. Many fittings will be sold pre-annealed. As such, welding does not affect their strength.

Using the ultimate tensile strength and a factor of zero will give the burst pressure, while using the yield strength will give the point at which deformation will begin.

Yes and no. What you're saying is true for a given piece of material, however ultimate tensile means very little once you start dealing with assemblies. More to the point, once deformation begins, a threaded pipe joint is most likely no longer sealing. So while the pipe may not burst, that doesn't mean you aren't venting gases in unconvenient places. Or worse, the entire joint could just rip apart.

Result? Unless you REALLY know what you're doing, one should treat the yield stress as "ultimate" as once you excede yield it can become very difficult to predict what's going to happen with an entire assembly.

I'm very good at welding, and I've been welding for a long time (and we also have a very good welder) though I'd never trust any weld I can do for HPA.

The first part of that sentence and the last part of that sentence do not agree with each other. Honest opinion: If you can't do HPA, you aren't a good welder. No offense intended, I am a *TERRIBLE* welder (I don't even try anymore...fortunately, I know several good welders.).


One other point aimed at the original poster: NOT ALL FITTINGS ARE WELDABLE. Cast fittings don't weld for crap. So if you're buying threaded fittings and planning on welding them for "extra strength" or something, you may be doing a lot more harm than good.

I am going to be buying stainless pipes and fittings. For the larger ones I will use screws and o-rings, but on the smaller ones I will be using taps and dies to create threads. Can this stainless threaded part be welded to increase strength?

The first part of that sentence and the last part of that sentence do not agree with each other. Honest opinion: If you can't do HPA, you aren't a good welder. No offense intended, I am a *TERRIBLE* welder (I don't even try anymore...fortunately, I know several good welders.).

I'm good at structural welds- though not welds to hold pressure. I'd trust welds I can do for lower pressures, though nothing like HPA. The consequences for making a mistake at 3000 psi are severe.

Not being able to weld pressure vessels doesn't make you a 'crap' welder- pressure vessel welding is completely different to anything structural. It needs more certification etc.

kjjohn wrote:I am going to be buying stainless pipes and fittings. For the larger ones I will use screws and o-rings, but on the smaller ones I will be using taps and dies to create threads. Can this stainless threaded part be welded to increase strength?

Depends on the alloy involved. Some stainless steels are quite weldable. Others are not (or at least, not unless you're a very good welder...but if you were you wouldn't be asking us these questions).