PPS. A much smaller vessel will also be required, so that would be the place to start; probably around 200mm diameter x 200mm length.

 

I just don't know if 50psi is considered child's-play as far as pressure is concerened. (I know car tyres are usually around 30-40psi, so a properly engineered chamber shouldn't be too difficult...)

50psi is more than you think and can be VERY dangerous. It is no child's play. 50psi can send a piece of steel flying as fast as a bullet.

 

Using flanges would work. I am basing that on the fact if it's good enough for 600 ( or more ) natural gas compressors... it'll be darn good for 50psi.

Im not sure how big in diameter you are needing, but some pipe and weld on flanges would do the trick.

** Standard Disclaimer applies... ***

 

Hmm, thanks slamdvw, come to think of it there should be any number of ready-flanged pipes out there specifically designed for high-pressure stuff, most probably with bolt-on end pieces available. Just a matter of tracking them down for a decent price...
Gives me a new direction to look in, thanks!

 

8" NPS Sch40 black pipe (200mmID is close to the ID of the pipe) with weld-on flanges. Gaskets and rated blank covers on each flange. Penetrations can be done in the wall using weldolets or half-couplings. Be careful about putting penetrations in the flange blanks. Good for much, much more than 50PSI (the hoop stress will be in the ballpark of 600PSI. A53 pipe is is good to 40000PSI minimum for yield.)

 

You could think about using PVC as well - I have a potato canon that uses 100psi, no problems.

Here is a chart with pressure ratings:

http://www.harvel.com/tech-specs-pvc-pipe-40.asp

- John

 

Nyet, NO, nope on the use of PVC with compressed gases. Especially with temperature extremes or thermal cycling, or hydrocarbon (grease/oil/etc) exposure. Search here on PVC air lines, or look over on OSHA's site, or search the web. The use of PVC for compressed gas, even at a 'lowly' home/shop pressure level around 100 psi, is UNSAFE. PVC often has a sudden and catastrophic brittle failure when it has been used in compressed air systems, sending shards of plastic flying.

PVC pipe is generally rated and used for LIQUIDS, not compressed or pressurized GASES. Also notice the big temperature derating (from the Harvel website) that the PVC pipe has at even slightly -warm- temperatures (100F is really not hot, heck the -AIR- temperature around many places recently has been 100F+ this summer!).

Even at 100psi or 50 psi the ENERGY in the compressed air is pretty darn BIG. Big enough to cause all sorts of grief, pain, and damage (property and personal) if the vessel or pipe or fittings fails in a not-nice manner. Leaks are an annoyance, brittle fractures and shrapnel can be deadly.

Use metal pipe like enlpck suggested. Standard black iron pipe and pieces (Sch 40 stuff?) typically come right from the supplier with a 100+ psi rating. No fuss, no muss, no problems.

 

Remember that it is not just the pressure that is dangerous, but the volume of the gas at that pressure. For comparison, there is much more stored energy in a 2-liter bottle at 50 psi vs a small water bottle at 50 psi. So, I would recommend that you make the vessel not much bigger than the minimum size you would find usable. This might also help you get to pressure faster, but might not help if you have a leaky vessel.

 

Ditto on the no PVC, and Runchman, careful with that spudgun. When I was about 16 we built a pnuematic. We cleaned, primed, and glued the joints. Shot a few hundred spuds, then one day while filling the end cap broke off at about 60psi. The gun went one way about 50', the end cap squared me in the chest. Now I'm the type of guy that pretty much refuses to show pain, but it felt like a ball bat at full swing. I could barely breathe, laid on the ground for about 10 minutes gasping and squeaking out "I'm OK" to my friends. After a couple of days I finally went to doctor, I had 3 broken ribs. I'm 31 now, and they still hurt from time to time.

 

Hi, complete noob here. I need to make a resealable pressure vessel for casting resins, and need some info / oppinions before blowing myself up! :blob2:

I won't be doing any of the making, a friend of mine is an experienced welder, and he's experienced at making air tanks as such, but is a bit unsure about making a resealable system. SO trying to gather info to make sure a) it's doable, and b) it's done safely.

Basic info:
- pressure will be no more than 50psi, more likely 30-40psi. Regulator etc will be in place.
- needs to be long but thin; about 1200mm long, maybe 100mm diameter.
- need to open one end to insert and remove the piece being cast.

My uneducated thought was to get a thick-walled steel tube, weld plate over one end, weld a flange to the other, and then use high-tension bolts to secure the removable plate to the flange (sandwiching a gasket). And possibly encase the body of the tube in reinforced concrete just in case...?

Any problems or tips people can point out? I've been searching the web for weeks, but most results are dealing with pressure of over 300psi, and not recommended as DIY.
The chamber would essentially be a longer, narrower version of the pressure pot listed here: http://www.hobbyengineering.com/H4139.html

PS. I'm generally more savvy than I may sound here. I do know the very real risks of working with pressure, hence being a bit cautious about the whole idea. I just don't know if 50psi is considered child's-play as far as pressure is concerened. (I know car tyres are usually around 30-40psi, so a properly engineered chamber shouldn't be too difficult...)

One of my first jobs in the world of aerospace was doing hydrostatic testing of rocket engine pressure vessels. They hooked them up to a water pump that was capable of enormous pressure. Generally they would test the items at two to three times the design working pressure.

If your tank will hold hydrostatic test pressure at 150 psi or so, it should be more than safe at only 50-60 psi.

Good luck.

 

As a nubie to it all..

Are you nuts??

...zap!

 

The indicated dimensions are such that, if built with code fittings and pipe, and the dimensions are such that it an easily be done, it will be safe, overbuilt, and, in many places, not require any construction stamp, other than that required for piping.

Items of concern are nozzles, proper bolting material, max service temperature, gasketing, overpressure protection (key key, key!), pressure relief provision (a MUST, and should be set up to be verifyable. Nasty case a year or two ago. Polymerization tank, and a process error caused the material to set up in the vent, and in the pressure indication device. Workers pulled the head, and several injuries/deaths when the hot unpolymerized material flashed).

Nozzles/ports should be done with certified commercial fittings with the proper reinforcement built in to them, Care should be taken with where they go (a commercially available flange blank can be supplied with threaded ports in it for test lines, gauges, etc, and all paperwork showing that it is properly engineered for a given pressure and temperature)

To save the need to weld, in fact, thread-on flanges could be used. An 8", 300PSI black steel (NOT cast iron) blank is in the ballpark of US$100 to US$150. The flanges for th pipe side are a it less. These are usually 12-bolt.

Now, as for a rule of thumb: 40gallons of air compressed to 100PSIG has the stored energy equivalent of one stick of straight dynamite.

 

Now, as for a rule of thumb: 40gallons of air compressed to 100PSIG has the stored energy equivalent of one stick of straight dynamite.

Geeee, I'm glad you cleared that up! After all, everyone is so familiar with "dynamite", and how much damage one stick can do...


Sorry, I always have a problem with references like that. It is extremely vague. Especially since "dynamite" doesn't refer to any explosive device used in recent history, and a "sticks" of TNT come in various sizes.

Makes me wanna watch some Roadrunner cartoons to find out how dangerous my compressor is.


I don't think he compressing air anyhow.

 

As a note: the vessel will be filled probably 90%+ with the mould, polymer, heating system (only 50-60 degrees celcius, or 120-140 fahrenheit), etc, so the actual volume of compressed gas will be around 10% of the volume of the entire chamber.

 

I have experience with pressure vessels, both design and fabrication. Here is the best piece of advice you will ever receive, DO NOT SCREW WITH PRESSURE VESSELS WITHOUT THE PROPER ENGINEERING.

Can a "good" welder adequately weld a pressure vessel to withstand 40 or 50 psi, yes. How do you know your buddy is a good welder? Have you conducted mechanical testing of his welds to ensure he knows what he is doing? Is there a weld procedure written based off a PQR that was tested to ensure the weld being made by the welder was sound in the first place? I could go on but won't.

Pressure vessels are best left up to those who know how to design them. Welding of a pressure vessel is best left to a welder who has been tested and certified for the particular weld joint employed in the fabrication of the particular vessel.

Can 50 psi kill you, you better believe it, 1psi can kill you. If you are doing this in an effort to save money, ask yourself what your life or eyes or arms or any number of other body parts that could be destroyed are worth just to save a few bucks.

Not worth it in my book. Oh and I have seen a pressure vessel explode with 5psi on it and watched the shrapnel penetrate the surrounding structures.

Don't mess with it!

 

As a note: the vessel will be filled probably 90%+ with the mould, polymer, heating system (only 50-60 degrees celcius, or 120-140 fahrenheit), etc, so the actual volume of compressed gas will be around 10% of the volume of the entire chamber.

ASME section IX welder and procedure qualification for boilers and pressure vessels, and I believe it's ASME section VIII for design criteria. Might want to look into it, other wise you might end up looking like this guy:blob2: .

 

Thanks to those who gave input, just a note to say I've been informed by some professional resin moulders that using vacuum will solve the problem (air bubbles in castings), and only in extreme cases would I need to resort to using pressure. I was previously going to vacuum in the mixing stage and then pressurise during casting (as per a few articles I had read), but apparently using vacuum at both of these stages will do the same trick, and be a hell of a lot easier to construct (PVC piping, 1atmosphere of vacuum for 1-2mins).

So thanks again, and hopefully I'll be able to get away with not using pressure.

 

Much safer for sure..


What's 1 atm of vacuum??? It's usually measured in inches of mercury..

 

The figure 27inches rings a bell, but don't quote me on it.

(just did a search, looks like officially it's 29.92inches of mercury. I think 27 might have been quoted to me as a figure to aim for)

 

just my not so humble opinion on the matter. spend the money and get an engineer to design a vessel for you. you can dictate the outside dimensions and he will tell you what, where and how. you can then take the prints to any weldor you desire to weld it up for you. preferable one that is experienced in pressurized vessels.

 

1 atmosphere = 14.7 psi = 760 torr = 760 mm of Hg = 29.92 inch of Hg

Vacuum is trickier in some ways than pressure, because a good vacuum pump is harder to make and more expensive than a mediocre pressure pump. For really low vacuum levels and such, that is.

Vacuum degass your resin before casting to get rid of entrained air. Watch out for foaming and the volume expansion of the foaming as you are degassing. The first time you overflow the resin/goop out of the container and into your vacuum chamber, you'll curse the mess and hope you didn't get goop into your lines or vacuum pump.