Welding two pressure vessels together

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Hello AllI am a Aerospace Engineering student at arizona state university, and I am a member of a rocket propulsion group at the school.  We need a pressure vessel to test our propellants at the chamber pressures at which they would be subjected to inside of a rocket motor.    Here is what we are planning on doing:  We want to cut two paintball co2 or compressed air tanks in half and weld them together.  We will use one of the valves to fill the tank with an inert gas.  And the other valve will be replaced with a similar sized/threaded plug that will allow test wires, to measure burn characteristics to enter into the vessel.We only plan on filling the vessel to around 500 PSIG and the propellant grain is small so it wont significantly increase pressure in the vessel during combustion.  We would like to have a factor of safety of about 2000 or 3000 PSIG.  I know the unaltered tanks factor of safety's are much higher than this because they are routinely filled to 3000+ PSI.  Obviously we would be hiring a certified welder to carry out this task.  I have a few questions as I am not a welder.  1) Is this even possible to do safely?2) How much would a certified welder charge for this type of operation?
Reply:I don't think you are on the right track.   CO2 is only 600 psi in the liquid state.  A piant ball gun cylinder is not going to be rated rfor 2000 or 3000 psi.DavidReal world weldin.  When I grow up I want to be a tig weldor.
Reply:I have read of some modern air rifles which do work at those pressures, but don't know what kind of cylinders they use. It would probably be easier to fabricate something from solid material of your own design. I would assume the School has a shop where such things could be made.For some information on airguns, you might try:http://www.pyramydair.com/.
Reply:I would not recommend doing it using these cylinders. Modifications are a risk, especially if you arn't dead certain of the materials.  There also may be regulatory or institutional policy issues with pressure vessels of the size and rating you are looking at, in particular due to the combustion inside the vessel.Food for thought:Your school probably has a policy covering this work. See your groups advisor and go through the proper approval channels. Check with the EHS dept (policy manual: http://www.asu.edu/aad/manuals/ehs/) and with the Research Office (seems to be where the appropriate compliance knowledge may be. Any prof or grad student in physics or engineering should be able to tell you for sure.... again, see your groups advisor) There may be an on-campus person that can do the fab to meet requirement (if small enough, the requirements are greatly reduced or removed for pressure vessels, but this may be considered a potentially explosive device and have other regulations) for much less than the commercial price. In terms of construction issues:1) if the test chamber can be made in a size where standard pressure pipe can be used, use it. For example, 2"NPS SA106 sch80 pipe (wall thickness: 0.218", 2.375OD, 1.939"ID) has a nominal yeild pressure of approximately 7500PSI (assuming 35KSI yeild), giving a safety factor of 3 at 2500PSI. 2) Smaller volumes present LESS risk from the energy stored by COMPRESSED GAS (less gas at a given pressure: less stored energy), and MAY present GREATER RISK from the energy liberated by combustion (Less leeway for error: An error if the predicted combustion product volume will have a greater effect in a smaller vessel) Smaller diameter pipe has relatively heavier wall, though (1"NPS: 1.315OD, 0.957ID, 0.179 wall in Sch 80, for a nominal yield pressure of about 12500PSI)3) If the temperature increase is is significant, you will need to consider the strength of the material at temperature (See ASME BPVC, SecII, part D, which gives ALLOWABLE stress at temperature, for a specified safety factor)4) Standard forged steel fittings are available rated to 3000 or 6000PSI. These ratings MAY not apply for threaded applications, especially when there may be shock loading.5) Is there a prototyping shop on campus to service researchers? If not, can you maybe arrange for one of the other Arizona (or region) schools with a shop to do the fab? Next would be approach a local shop (see The National Board web site for shops holding authorization to do pressure vessel work) that meets the requirements of university policy and state regs for this application (which you will need to determine). How much it will cost is a wide open question, as it will depend on what certifications are required, what regulatory requirements need be met, whether engineering needs to be done, and if you an get a deal as a favor.You may also want to take a look at http://www.jlab.org/ehs/ehsmanual/index.html ... see the 6000 section (6151 specifically), and http://www.lbl.gov/ehs/pub3000/CH07.html to get an idea of how this works 'in the real world' of research. Note that LBL requires evaluation of the available energy to determine the level of hazard, which influences safety factor requirement and if additional protections (such as fixed shields) are needed. Note that I have no affiliation with either Jefferson or LBL... I have used their materials in preparing and evaluating safety policies in my work, and have used them as examples when teaching but make no claim of suitability or applicability. I reference them only as examples. Since your schools EHS program doesn't appear to cover your work, they may come in handy for you as a guideline, especially if there is friction ('I;ve looked into the types of design and documentation procedure used elsewhere, and....')
Reply:Hello Bgood44,So is it mainly that you need a reservoir with at least two threaded ports?Enlpck's pressure pipe suggestion sounds good.Also many stock hydraulic system components could safely handle the pressures you're talking about without worry. Possibly an accumulator, or maybe a hydraulic cylinder barrel fitted with two end caps.Also you could probably find a CNG cylinder similar to your description.Best Luck
Reply:enlpck;   I hope you didn't just scare '44 into changing majors to something dangerous like PolySci.!Your points are a good introduction into the real world of rules, regulations, and liability which were somewhat fewer in my more inventive days. I was assuming an official School adviser was overseeing the project but maybe that's naive on my part.Similar "bombs" already exist for accurately measuring the heat content of compounds and high pressure models like what the group wants to make are probably used by companies like United Technologies Corp. to evaluate their propellants so it may be worth contacting them for information and advice.
Reply:Your points are a good introduction into the real world of rules, regulations, and liability which were somewhat fewer in my more inventive days. I was assuming an official School adviser was overseeing the project but maybe that's naive on my part.
Reply:Originally Posted by bgood44I am a Aerospace Engineering student at arizona state university,
Reply:Note also that detonation/explosion or even rapid combustion add additional design and engineering considerations over and above 'standard' pressure vessels.And all of the above mentioned cautions still apply.  The best laid schemes ... Gang oft agley ...
Reply:Compressed air tanks for paint ball guns are available in 4500# pressure ratings. My grandsons and some of their friends play paintball out in my pasture and that is what they are using. Would I cut a couple of them up and weld them together? Nope, and I carried an ASME Code 9- G 6 card for years. Welded up a lot of production systems that were hydroded at 10,000# psi and I still wouldn't use paint ball tanks. I wouldn't hesitate to build a tank to do this for myself but I'm not going to elaborate. Don't want the responsibility.Craig, I'm not getting on your case or arguing with you but I would like to know where you got your information about the 200# cap. I'm just curious as it is something new to me.
Reply:bgood44,I think you might get some good input if you were to post this question on the forum listed belowhttp://www.eng-tips.com/
Reply:"Craig, I'm not getting on your case or arguing with you but I would like to know where you got your information about the 200# cap. I'm just curious as it is something new to me."I'm scratching my head on this one too....I routinely weld 2000 psi pressure vessels.....KbnitI r 2 a perfessional
Reply:Thanks for the spelling lesson Craig.....I used to play paintball and the compressed air tanks come in two varieties, those that can be filled to 3k and 4.5k.  So you guys don't think this approach is feasible/safe?Probably another crazy question but would it be possible to just get a steel pipe with fairly thick walls, cut some deep threads in and put some caps on it?  Like I said before, the operating pressures would be around 500PSI.
Reply:bgood44, it's done every day in the oilfield. If you live close to an oilfield supply store just walk in and tell them what you want to do. They will have threaded connections rated all the way up to 5000 psi.
Reply:Originally Posted by Craig in DenverI'm not the assigned spelling cop, but sometimes ya gots ta pay attention. My apologies to them what I offended.
Reply:Originally Posted by OldtimerMy grandsons and some of their friends play paintball out in my pasture and that is what they are using. Would I cut a couple of them up and weld them together? Nope, and I carried an ASME Code 9- G 6 card for years. Welded up a lot of production systems that were hydroded at 10,000# psi and I still wouldn't use paint ball tanks. I wouldn't hesitate to build a tank to do this for myself but I'm not going to elaborate. Don't want the responsibility.Craig, I'm not getting on your case or arguing with you but I would like to know where you got your information about the 200# cap. I'm just curious as it is something new to me.
Reply:Thanks Craig. My first thought when I read your post was: "He must be thinking about air compressors". Great minds think alike, huh?
Reply:I'm not sure on the cap on compressor tanks but I do have one that operates at 200psi. I do know the material used to manufacture the tanks is fairly light and I certainly wouldn't want to try and run it any higher. ASU is a much larger school than the one I attended (ENMU) and every student project there had a faculty advisor overseeing the project. Granted the oversight wasn't particularly tight on many projects but as soon as we moved into the shop with machine tools, power tools and welding equipment it got much tighter and safety became the main concern. So much so that one safety violation could get you booted from the class. I think the best advice was to check with your advisor and go from there. Don't blow your career before it even gets started by making a dumb mistake.I use 3000# tanks on my paintball gun, weld on pressure vessels regularly and really think there might be a much better way of doing what you want to do.The difference between art and craft is the quality of the workmanship. I am an artist.
Reply:Originally Posted by OldtimerThanks Craig. My first thought when I read your post was: "He must be thinking about air compressors". Great minds think alike, huh?
Reply:As stated above there are paintball tanks that go to 3K and 4.5K but (at least when I was playing) those tanks are wrapped with some kind of composite.Its not rocket scienceYup
Reply:Burnit,I've welded pressure vessels out of aluminum that go on spacecraft.  They, too, are carbon fiber wrapped.It IS rocket science.    Another case of things developed for the space program that find their way into the commercial sector.Best regards, KbnitI r 2 a perfessional
Reply:Thank you for the help everyone, I've found a solution that doesn't involve welding on pressure cylinders.  Its essentially a homemade Crawford strand burner that i found on an amateur rocketry site.