Sizing plumbing for bulk nitrogen tank - Pressure drop?
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  1. #1
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    Default Sizing plumbing for bulk nitrogen tank - Pressure drop?

    Hello!

    I am trying to figure out if I am going to have a problem with the nitrogen supply line for a 6KW fiber laser.

    The required flow rate from the manufacturer is 3,000L/min at 35 bar. Or 500 psi and 105CFM. I am trying to cut mild steel plate up to 1/2" thick, but I am only using about half of the flow/pressure requirements at max, ~45SFM, 20 bar, 3mm orifice.


    We had 3/4" copper run for the supply, about 300ft worth. The installer swears this is enough for 8 lasers, but I find it exceptionally hard to believe.

    300ft of 3/4 pipe is only 26L of volume, and at 3,000L/min, I figure I'll have velocities higher than 500fps. A "pressure drop" calculator found online, populated with densities and viscosities for N2, has me expecting ~300PSI of drop at 3,000L/min. This run was supposed to be sized for two of these lasers!


    I also figure that 1.25" pipe would get me down to ~110FPS flow velocities, and running the pressure drop calculator tells me to expect to loose about 30 psi. This seems much more reasonable, and ought to be able to run two of these machines at my ~45SFM/machine rate, right? I expect 500PSI supply pressure.

    I am concerned because I see about 50PSI of drop across about 25ft of 3/4" braded stainless, estimated at 5/8" ID, while cutting at the 20bar/45SFM rate. I am using temporary dewers that don't have the vaporization capacity and the duty cycle I can run sucks! Damn near 15% loss here is killing me!

    Finally, 15-25m/sec for gas flow through a pipe is pretty reasonable, right?

    Thanks in advance for any and all advise!

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    First of all, 500psi in copper pipe????

    2nd in calculating your pipe velocity dont forget that 105 cfm is more than likely cfm at atmospheric pressure so at 500 psi the actual flow is much less while under pressure.

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    Bear in mind that I know nothing about using nitrogen in a laser cutter, but:- 3 cubic metres per minute to cut half inch plate sounds like a hell of a lot unless the travel speed is bloody fast. Is a 3mm orifice really the right size for that thickness of plate?

    Be that as it may, for that sort of flow, (at STP) a 3/4" pipe is close to the maximum you'd want at 20 bar.

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    That is a very thick piece to cut with Nitrogen. Curious why not Oxygen.

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    Quote Originally Posted by Scruffy887 View Post
    That is a very thick piece to cut with Nitrogen. Curious why not Oxygen.
    I was wondering the same thing.

    Our 4k we run 1" line for nitrogen. We see drop around the 3/8" stainless range.

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    Quote Originally Posted by JP Machining View Post
    First of all, 500psi in copper pipe????

    2nd in calculating your pipe velocity dont forget that 105 cfm is more than likely cfm at atmospheric pressure so at 500 psi the actual flow is much less while under pressure.
    450/500 PSI is a common pressure relief setting for liquid nitrogen tanks for laser cutting. If not for laser cutting, it's much more common to have the relief set at 250-350 PSI. But since I actually need near the set pressure in the pipe, I figure I actually need that volume of flow at that specific pressure.

    I think this specific type of copper tube (Type K) is rated for ~850psi at 3/4" so it shouldn't immediately blow up!

    Also, as I am actually needing 45CFM at 20 bar at the 3mm orifice, I do believe the flow rate is at that specific pressure. The laser will not run if it doesn't have appropriate pressure in the cutting head. Without enough flow, you won't ever have enough pressure.

    Quote Originally Posted by Mark Rand View Post
    Bear in mind that I know nothing about using nitrogen in a laser cutter, but:- 3 cubic metres per minute to cut half inch plate sounds like a hell of a lot unless the travel speed is bloody fast. Is a 3mm orifice really the right size for that thickness of plate?

    Be that as it may, for that sort of flow, (at STP) a 3/4" pipe is close to the maximum you'd want at 20 bar.
    I do think I can make things more efficient, but I need extra equipment to make that happen and that's not in the cards right now. With lower pressure and volume I do get good cuts, but the burr becomes much larger, enough so a handheld orbital won't remove it.


    It would be much, much better to cut with oxygen on this material but I have to remove all oxides for paint. I don't have much spare labor, so I chose to cut with nitrogen for now.

    Quote Originally Posted by Scruffy887 View Post
    That is a very thick piece to cut with Nitrogen. Curious why not Oxygen.
    I have several quotes for what I believe are very cheap timesavers and laser oxide removal machines. I guess we can't afford them, I don't see how we can't afford to NOT have one with what we are trying to do...

    Quote Originally Posted by Spinks View Post
    I was wondering the same thing.

    Our 4k we run 1" line for nitrogen. We see drop around the 3/8" stainless range.
    I am curious, what sort of orifice and pressure are you running on stainless? I think it has less thermal conductivity than mild, so it should cut with less pressure and flow?

    Thank you all for the advise. Think I'm going to have the contractor come back and install 1" like he quoted...

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    Quote Originally Posted by Moonie223 View Post



    I am curious, what sort of orifice and pressure are you running on stainless? I think it has less thermal conductivity than mild, so it should cut with less pressure and flow?

    Thank you all for the advise. Think I'm going to have the contractor come back and install 1" like he quoted...
    Its been a while since we've cut it but i believe it was setup at 16 bar and possible a 3mm nozzle(might be wrong on the nozzle) I believe we adjust the gas to keep it cutting constantly.

    In past we have cut 1/4" steel with nitrogen but it eats up nitrogen. So we don't do it unless its required or we happen to run low on oxygen.

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    If the contractor is recommending and quoting 1" pipe, let them do it.

    As long as your paperwork shows that they are responsible for making it work.

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    Hello,

    I have not posted in a long while but thought I could offer some help.

    I used to work with customers on system requirements for laser installs. The Fiber Lasers use a lot of assist gas compared to the C02 stuff.
    You say the Laser Manufacture requires 3000 L/min at 35 bar which I convert over to 6356 CFH at 514 psi. That flow rate matched what I was told from Mazak, Mitts, etc. as flow rates for new Fiber Lasers. I am not sure of your supply system but typically for a laser if you are going to be doing any amount of sizable or hourly volume you will be in a bulk tank, outside tank in the range of 3000 to 6000 gal capacity with a trifecta(pressure builder)to run your machine. The white bulk tanks are typically a 250 psi max working pressure unless you have a HIGH pressure tank, but they are not used much anymore from what I have seen. You could have a Microbulk(Stainless Tank) installed which could be a range of sizes but if you are using any volume of gas you will drain that very quick. For example, a 3000L liter Microbulk is about 66,000 CF of gas and if you are cutting at the top of your capacity then that would only give you a days’ worth of cutting, give or take.

    Looking at my piping book for laser installs I show that 3/4" K Copper is good for about 7500 CFH at up to 200'. At 300 ft it shows to use 1" for 7500 CFH or 1-1/4 to get 15,000 CFH. I show the pressure rating for K Copper as 852 psi for ¾”, 655 psi for 1”, and 532 psi for 1-1/4”. So 1-1/4” would not have the pressure rating that you need but I have seen done on longer piping runs was to double up on a smaller size to get the volume up and maintain pressure. The (2) pipes would then be looped together at the end to allow combined flow.

    The other thing that if you are seeing a large pressure drop you need to look at the piping system for a reduction in pipe size. I typically see that the install will look real good from the bulk tank to just behind the laser. Then a regulator is installed with a High Pressure fitting(CGA580 for 2500 psi) which is what you would find on a cylinder bottle. The nipple on those have about a 3/16 diameter orifice which is a huge restriction. If a regulator kit was purchased from the Laser Manufacture, they typically come set up for running off High Pressure 12 paks. These regulators have a nipple installed but it can be removed to give access to a ½” NPT, which will work. Remember when you have a regulator in the line you need to have a pressure change across the regulator in order to make it work correctly. The laser has another regulator inside the machine to set the working pressure. You can cause issues with installing too many regulators in the system. Again, the bulk system outside has an adjustable pressure pop off to protect the piping system from being over changed.

    Over the years of helping customers it was very hard to help size a new install for new business. Customer would purchase a machine and expect to cut a range of sizes, then a large job comes along and the flows and volumes double or triple in a week and causing issues.

    I will be happy to help how ever I can, hopefully this will help some.

    Thanks Brian


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