Ok: get your calculator, pi times radius squared times length= area cubed
I use Inches, believe me it is easier this way so follow along slowly!!!
For example:
6" pipe 10' long {120"}
so 3.14 x 6" x 120"= 2260.8" cubed
now regulators read CFH-a cubic foot =12"x12"x12"=1728" cubed so that number times your flow rate say {30 CFH} divided by 60{as in minutes per hour}
so 1728x30 divided by 60= a flow of 864cuin
now divide 2260.8 by 864= 2.61666 minutes at 30CFH
Believe me this is higher geometry than normal but the formula is easier to use after you've done it awhile.
For the mm to in I use 25.4mm=1 inch
I hope this helps more than it harms,
Peace,
pjs
ps: this is too much for a slide chart, but it could be done on a slide rule but that is difficult also! Trust Me!!???!!!
Thanks very much for that formula,it will help me calculate the amount of gas to be use on a job.But what I need to know is what formula to use to adjust the purge gauge prussure to the correct amount when welding various sizes and schedule pipe.eg.(if you are welding 50mm-sch.10) how do I arrive at the right purge pressure, when welding 250mm-sch.40 what purge pressure to use and so on to acheive a perfect root?And what effect does high or low pressure have on your root.Thanks again.
P.s. That's what I love about this forum,you can learn something new every day.Keep it up guys.
pjseaman-
The formula you quote at the top of your post is correct, however when you apply it you use the diameter of the pipe instead of the radius squared. What you computed is the area of the outside of the pipe. The correct line to compute the volume would be 3.14 * 3**2 *120 or 3.14 * 9 *120 or 3391.2 cubic inches. The rest of your method is correct and the result with the correct volume would be 3.925 minutes.
Argon is somewhat heavier than air so I would suppose that if practical the argon would be introduced at the lower end and the exhaust be at the higher end. I would also suppose that some effort to avoid turbulence as the argon is let in would reduce mixing and produce a quicker result. Some extra time to allow for purging out this air/argon mixture would be needed. Medicinehawk's method measures the outflowing gas and removes guesswork here, although one who only does this occasionally might find it cheaper to waste a little gas than to buy another gadget.
bill
Bill:
I did assume that some one would use the ID for the dimensioning, Thanks for clarifing though. I've used this only a few times since I don't regularly weld pipe. I actually have a copy of the mathmatical computation on my shop calculators back side just so I don't have to remember all the parts.
Thanks again,
Rather than do all that math, see if you can get an O2 sensor.In normal atmospheric conditions, the air we breath contains 20.9 percent Oxygen, which is plently for us to breath and to "Sugar" your stainless butt weld so if you crank-up the Argon (Top the ball out) on your flow meter and slam-up the fit for about 2 minutes (on say a 6" joint) than you can put in 4 tacks without em becoming contaminated. You then check with the O2 meter (at the outlet "bleeder hole") how much Oxygen is in the pipe. It will usually drop to 4 or 3 (percent) rapidly and then with in a minute it should drop to .3 to .2 percent Oxygen.....this is what you are looking for. You can then safely weld the joint without surgaring the root as long as you keep tape on 3/4 of the joint, then 1/2, then 1/4 etc.
I have been doing this a long time and the key is to have a really "tight" purge. Use a plastic pipe opening cover with a swagelok union and a 1/4" purge line with tape to secure it to the pipe (if you have room) as it will melt if it is too close to the heat affected zone. Use the same thing for the outlet purge hole and nashua metal (aluminum) tape around the joint.
If your procedure allows.....slope the bevel to about 40 degrees included angle & a knife edge and use a 1/16" gap....this WILL close-up as you start quartering the joint, but use a 1/8" grinding disc and 1/16" filler wire to thin-out where the joint has closed up maintaining a bevel while keeping the joint (where it has slammed-up) thin. You'd be surprised, but the ID (Root) can be just as slivery as the OD of the pipe after you brush it.
If you use this method, you will only need to blast the purge when you tack it up...you can then drop to 40 CFH for the 1st quarter, 30CFH 2nd quarter, 20 CFH for the 3rd quarter and 10-15 CFH for the final quarter. This is with a 1/4" diameter bleed hole. You end up using alot less Argon but you have to be Quick in putting in the root, NOT hurried, but deliberate.
For large diameter pipe, you need more volume so you'd need a 3/8" or 1/2" hose to feed the purge or you can double the inlet with 2 or more 1/4" poly hoses to get the same results.
If you use a 92/8% mixture of Argon/Hydrogen in your tig torch, you will use less AMps to get the same results. Let us know how it comes out.
Be well.
Hawk
if you are in the US then contct COB industries, this is their business, very knowledgable and very nice people. If not I can send you some infor regarding purging of pipes and how you can reduce your gas useage quite dramatically.
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