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Steel Pipe pressure limits

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Steel Pipe pressure limits

Unread postAuthor: DYI » Sun Aug 19, 2007 7:01 pm

In a few weeks I will have capabilities for pressurizing my launchers as high as 500 psi with nitrogen. I was wondering, what is the highest pressure that I can run my galvanised steel pneumatics (made with hardware store variety fittings and SCH 40 welded pipe) at, while maintaining a degree of safety?

I have heard of people using these pipe and fittings on 800 psi unregulated CO2 systems, but that just doesn't seem safe with parts that are only rated to 150 psi. The biggest fittings are 2" (I'm not really worried about the pipe), and I wanted to know if anyone has any experience using them in relatively high pressure (300+ psi) pneumatics.
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Unread postAuthor: Fnord » Sun Aug 19, 2007 9:22 pm

A burst pressure calculation on 2" threaded steel ends up at about 1200 psi, so 500psi should give a reasonable safety factor. Most fittings I've seen are a bit thicker than the pipe itself, and should also hold up well. I've exposed 2" pipe and fittings to 400+ psi (hybrid) and have had no problems so far.

As always, it is best to remote fire for the first few until you know it can take the pressure.
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Unread postAuthor: Orpackrat » Sun Aug 19, 2007 10:05 pm

I have built many systems with regular galvanized pipe, all of mine run on unregulated Co2 and even the ones that are a couple years old function perfectly, The pressure your using should be fine.
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Unread postAuthor: SpudBlaster15 » Mon Aug 20, 2007 2:24 am

The threads are obviously the largest concern, as the pipe should easily hold at least twice what you plan to use.

Calculating the failure pressure of the threads is much more complicated than calculating the failure pressure of the pipe. Based on the best estimate that I can make using standard steel pipe thread dimensions, I generally assume that the threads will be safe to ~1/2 - 2/3 of the safe operating pressure of the pipe.
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Unread postAuthor: Orpackrat » Mon Aug 20, 2007 11:11 am

SpudBlaster15 wrote:The threads are obviously the largest concern, as the pipe should easily hold at least twice what you plan to use.

Calculating the failure pressure of the threads is much more complicated than calculating the failure pressure of the pipe. Based on the best estimate that I can make using standard steel pipe thread dimensions, I generally assume that the threads will be safe to ~1/2 - 2/3 of the safe operating pressure of the pipe.


True, you need tons of Teflon tape, plastic grocery bags cut into strips work just as well if you don't have some Teflon tape. I have used unregulated Co2 on galvanized pipe down to 1/8" without problems (haven't used smaller yet).

Theoretically, the galvanized pipe fittings, whether they are adapters, caps, splices, or just a inline connector, they are considerably thicker than the actual pipe. But again, the threads are the biggest weakness.

Hand tighten all of the pieces, give a turn with a wrench (try and use a wrench that will tightly grip the pipe, usually there is a ridge on the fittings, lettering, or on some pieces, a hex for wrenches). When it comes to tightening the pipe itself, a pipe wrench is usually the worst, if it is not tight the pipe will slip and the wrench with cut into the pipe removing shavings and weakening the pipe, putting the pipe wrench on tight enough for it not to slip can leave nice teeth marks on the pipe which can also weaken the pipe. I recommend putting a thin strip of hard rubber on the teeth of a pipe wrench or other wrench and then using it to tighten the treads. A heavy chain wrench with a strip of rubber works great too.

You will need to turn the pipe (when connecting to another) at least 3 full turns. I recommend wrapping the threads with Teflon tape or thin plastic strips from a grocery bag until the threads almost disappear.

Everyone can call me crazy but I have safely taken a regular air compressor quick disconnect and used it with unregulated Co2 (around 800 psi, please know that it was not used with a normal air line but connected to galvanized pipe and a paintball slide check was used to vent the pressure for easy connects and disconnects).
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Unread postAuthor: VH_man » Mon Aug 20, 2007 11:24 am

galvanized pipe can take unregulated CO2?

coolio. my next cannon: unregulated CO2 marble rifle, with integral supressor..............

neway, thats a little off topic but yes, galvanized steel pipe has HUGE pressure ratings. im pretty sure 500 PSI is safe for galvanized steel......
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Unread postAuthor: DYI » Mon Aug 20, 2007 5:09 pm

I know that the threads would be the weakest point, probably followed by the fittings (because of their irregular shape and iron construction), and then the pipe.

How can you know that your setup was safe, Orpackrat? It could have blown at 805 psi. Using a standard quick disconnect on any 800 psi system is, at the best, idiotic. And why did you use plastic bags? What advantage do they have?

I doubt that I will ever take the risk of pressurising the soon-to-be-redone SCTBDC up to 500 psi, even with a remote line. I just don't want to accidentally destroy my favourite creation.

GGDT shows a 1 pound projectile traveling at 490 fps at 400 psi, which should be plenty impressive. I will most likely use teflon thread sealant paste on any threads that haven't already been done, while the original 2" threads were done with the tape. The pipe wrench has already been used, but does anyone really think that the gouges it leaves would affect the pipe's ability to hold pressure very much? I'm not talking about 800 psi CO2, I'm talking about 400 - 500 psi.
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Unread postAuthor: Orpackrat » Tue Aug 21, 2007 12:25 pm

DYI wrote:How can you know that your setup was safe, Orpackrat? It could have blown at 805 psi. Using a standard quick disconnect on any 800 psi system is, at the best, idiotic. And why did you use plastic bags? What advantage do they have?


The standard QD is only being used to briefly transfer the Co2 to a galvanized pipe holding tank. Then a slide check valve is closed trapping the pressure in the holding tank and venting the pressure of QD and and fill line. With the pressure being released the maximun pressure the QD has is 120psi. I do know the a standard QD is not rated for that pressure and I use it at MY OWN RISK.

A note about using plastic bags, it has the same working qualities as Teflon tape. I use it in situations where I don't have Teflon tape (typically I'm out on the field miles from anywhere), I use it on my paintball guns and unregulated Co2 cannons when I make modifications and don't have access to Teflon tape.

Basically, plastic bags are an alternative to Teflon tape, it works just as well when you don't have it.
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Unread postAuthor: Collo » Sun Sep 02, 2007 7:39 pm

It all depend there are different classes of steel pipe with different amounts of iron in them class 1 pipe is the best as it is cast straight form the steel works and is capable of holding an easy 1000psi class 2 isnt near as good u can usualy see where is has been welded together and has a higher chance of splitting.
Collo :wink:
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