Equivalent Flow Rate (pipe flush)

I am working on a problem that I can't seem to figure out. I have a pipe that is required to be flushed by water at 41gpm through a pipe (diameter = 1.25in). I'm trying to figure out what an equivalent flush would be using compressed air. The air flush needs to be from a compressed air tank (pressure unknown) using a .25in connection at the discharge. The tank will be a higher pressure than required using a regulator set to the calculated pressure. I used the equation for mass velocity head to calculate an equivalent velocity using air, but can't figure out how to calculate the required pressure for the tank using a .25in connection. So basically, I need to find the required tank pressure for a known exit velocity and known exit area.