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01-09-2005, 06:48 PM #1
Does anyone know the pressure capacity in psi for air in 3/4" schedule 40 pipe? Will it hold 400 psi in air pressure inside it?
01-09-2005, 06:54 PM #2
Schedule 40 pipe made from what material?
edited to add:
Barlow's formula for bursting pressure is
where t=wall thkns, S=tensile strength and O=outside diameter
01-09-2005, 07:49 PM #3
If I recall correctly.(I'm gettin old). sch.40 galv. or blk iron is rated to 375psi. If you are running close to this it will behoove you to step up to sch80 steel.
01-09-2005, 07:57 PM #4
If you're looking at pvc pipe, 3/4 shed 40 is required to have at least 1500 psi burst capacity, and maximum operating pressure is just under 300 psi. But don't use pvc for air since if it breaks, it will send shrapnel flying and possibly kill you or someone else. I don't know the ratings for steel but I have to assume it will have a maximum recommended operating pressure higher than pvc. A large factor is whether it's seamless or welded. My (limited) books don't show pressure ratings for welded pipe or tubing. Maybe that's because welded isn't the right stuff for pressure applications.
01-09-2005, 08:13 PM #5
3/4 sch40 can be used at 1000psi internal pressure and still have less than 9000psi tensile stress. 9000 psi stress @ 150*F is the lowest stress level for the weakest pipe made (butt welded, probably not even produced any more). Most domestically produced pipe for pressure service today is ERW and the weld is continuously ultrasonically monitored for soundness. In other words, yeah it'll handle the 400psi air pressure just fine and no, you don't need to spend twice or 3 times the money and buy seamless pipe, because the rated working pressures on seamless and welded standard steel pipe is the same for temperatures you'd normally have in compressed air service. Don't use any cast iron fittings. If it were me, I'd spend the money for forged steel fittings simply because the poor overall quality of malleable iron fittings, both foreign and domestic, can leave you chasing a lot of leaks at high pressures, both from fitting porosity and bad threads.
01-09-2005, 08:19 PM #6
from Pocket Ref by Glover
1/8 to 1" continious weld or seamless Sch. 40 STEEL pipe 700 psi.
1-1/4 including 3" continious weld 800 psi
1-1/4 including 3" seamless 1000 psi
2" to 12" electric weld 1000-1300 psi
3 to 12" seamless 1000-1300 psi
3-1/2 to 4" continious weld 1200 psi
01-09-2005, 08:24 PM #7
I appreciate your wisdom!
01-09-2005, 08:44 PM #8
Ray, I'd make sure and leave that reference in my pocket in the interest of living a longer life. 12" seamless ASTM A-53 Grade B pipe has a maximum safe working stress of 15,000 psi, and an ultimate tensile strength of 60Ksi minimum. 694 psi will produce the 15,000 psi stress and their 1300 psi would give near double that stress. Because of the way stress develops in relation to diameter and wall thickness in pipe, it is impossible to give one-size-fits-all numbers for safe working pressures. For example 1" sch40 has a .133 wall, and it takes 1575 psi to develop the 15,000 psi stress. 12" sch40 has a .406 wall, slightly more than 3 times as thick, yet 694psi gets it to the 15,000 psi stress level. To compare pipe with different diameters but equal wall thickness, 1" sch160 has a .250 wall and 4294psi=15,000 psi stress. 12" sch20 also has a .250 wall, but 364psi=15,000psi stress for that size.
01-09-2005, 08:51 PM #9
That is why I listed the source of information. I don't work with pipe specs often.
And this book was handy today.
01-09-2005, 09:04 PM #10
Out of curiosity, does the wall thicknesses talked about in a couple of places take into account the reduced wall thickness due to thread cutting?
Also could run of the mill plumbing fittings be the weak point?
01-09-2005, 09:44 PM #11
George, the numbers are for the pipe itself. The thread would obviously weaken the pipe, but a properly made up screwed joint will have the male thread pretty much buried within the female thread with the exception of the last couple threads which aren't cut deep due to the taper on the pitch line. The portion of the pipe within the fitting is reinforced by the fitting itself. Very little screwed pipe is run in sizes above 2", and the smaller sizes have the greatest margin of safety, so the minor weakening in these sizes shouldn't be a big concern. The majority of pipe sold today meets A-53 Grade B, so the 15,000 psi safe working stress is a good number. Of course, one should always pay attention to the markings on the pipe itself to verify the spec, particularly in higher pressure applications.
Run of the mill CI and malleable fittings are rated at 150 psi. 300 psi fittings are also readily available. I mentioned forged steel fittings earlier because they're available in 2000 and 3000psi ratings and the quality on them is consistently high as compared to CI and malleable fittings. They're available both in screwed pattern and socket weld pattern. The fitting itself is identical in both patterns with the only difference being one is threaded and the other is bored such that the pipe will slide into it and make a place for a fillet weld. 3/4 pipe can easily be gas welded, and probably the cheapest high quality installation would be achieved by using plain end pipe and butt weld fittings, thereby staying away from any concerns with reduced wall due to thread cutting. (It would also be good because I despise running screwed pipe ) We had a guy who worked for us for a long time when we still did contracting work who could run 1/2 and 3/4 pipe for air lines and such, and form the bends and offsets as he went by heating and bending the pipe. About the only places he would have a weld was at the end of a joint of pipe or wherever he had to put in a tee. It made a real nice looking installation. Although he was an excellent pipe welder using stick, he always did this small stuff with oxy-acetylene, because he said it was too hard to keep from running slag into a position weld on pipe with such small diameters when running a stick. TIGing the joints gives the ultimate in appearance, but is too slow and therefore too costly for this type work.
Copper would also work fine for air @ 400psi, as long as the joints are made with a hard solder such as sil-phos. The condenser side of HVAC systems often run at pressures in this range and at elevated temperatures, all done in hard soldered copper, and subject to loads of vibration. A copper system costs more for material up front, but is far faster to install and stays much cleaner over time. The downside to the copper is that it can't stand the physical beating that steel pipe can take in a lot of industrial applications.
01-10-2005, 12:24 AM #12
Where does one buy the forged steel fittings? I've only ever seen cast iron and brass one.
01-10-2005, 03:48 AM #13
Threads not only reduce the wall thickness, they are also horrendous notch stress-raisers. Pipe threading and fitting manufacturing tolerances are so wide, and the depth of engagement so variable, that the strength of a threaded joint is a complete crapshoot. Go for socket-welded fittings.
Bending is good (you can use an electrician's bender for rigid conduit) and a nice wide bend has much less flow resistance than an elbow.
01-10-2005, 06:56 AM #14
You could probably save some time and use Sch 80 fittings.
That will cleanup the fitting quality nicely.
We use 300 psi fittings on fire sprinkler systems and you can "see" the difference to Sch 40 stuff. Sch 80 is even nicer with beefy walls.
01-10-2005, 10:47 AM #15
I personally wouldn't use Sch 40 with 400 PSI, even if the book says it will do it.
But I also quetion the adage that air pressure in PVC is "death in the making". I worked in a hsop with air/PVC supply and saw it break probably 7 times and never once saw any sort of shrapnel.
Sometimes it broke because someone hit into it, sometimes the stuff in the rafters got so hot in summer it degraded the plastic to the point that it failed. But whether new or old pipe, all it ever did was break in half, no energy in the few scraps.
01-10-2005, 01:37 PM #16
The retired compressor mechanic I bought my unit from told me that copper made the best distribution system. Easy to cut and fit, cooler air, less moisture to filter out at the point of use. That was all they installed in the shops he worked in. He unfortunately didn't provide any detail of pipe type (K, L, ??) or fitting type.
01-10-2005, 06:02 PM #17
I was on a job site many years age where the whole shop was piped for air in PVC.
Within a couple of weeks the 2" main header exploded and a piece of shrapnal about 2" x 6" went to the far end of the shop. It's not like steel flying; but it could still really ruin your day.
01-10-2005, 06:24 PM #18
I have seen PVC pipe degrade from UV exposure, like from flouresent lights, and become brittle. I was replacing some drain pipes in basement that were brittle below the ceiling and fine above the ceiling. This topic comes up all the time and even if PVC was ok there are some many alternatives that it does not make any sense to use it. Plain old half inch copper with sweat joints is cheap enough and easy to put in and will never brake in a hundred years. Alternately I have used 1/2 nylon tubing with puch in fittings, real easy but a bit more expensive.
One thing I will mention while I am at it, make your air lines into a loop so that every drop is fed from both directions, this has the effect of giving you a larger line and cutting down on pressure drops from use. THis doesn't neccessarly increase the amount of piping since you probably are running a loop anyways but just not connecting the far end back into the compressor.