My chemical engineering is a little rusty after 25 years and I could use a little help with this real world problem. This isn't my project but I've been asked to advise. Sorry for the limited information. We have a 30 gallon tank with 75% hydrogen/25% nitrogen mixture at 100 psig. From the tank there is a copper tube 0.93 inch I.D. running to a pressure regulator set to 20 psig. From the regulator there is a 25 foot copper tube(same dims) connecting to a weld box. Assume atmospheric pressure is 14.7 psia and no change in elevation from tank to final pipe end. The problem is to find the flow rate of the gas through the 25 foot tube in the case there is an accident and the box at the end is broken loose from the pipe and the gas vents to atmosphere. I need to know the volumetric flow rate of the gas inside the last tube in order to size an appropriate emergency cut-off valve which we need to install in between the regulator and the end of the 25 foot tube. Attached is a simple diagram of the system. I need to know the calculations so I can explain them to my co-workers. Any help would be appreciated.