Flow Rate through a gas pipe

Hi I'm trying to work out the following problem

The pressure at the start of a 180Km stainless steel natural gas pipeline is 4MPa and at the end is 2MPa.  If the diameter of the pipe is 0.4m, what will be the gas flow rate through the system? (ignore losses other than friction)

(for natural gas, density = 40 Kg/m3, viscosity = 11x10-6)

This is from an exam paper and at the end of the paper there's a moody diagram with roughness values, the closest to stainless steel is commercial steel and has a k value of 0.045 mm.

 I'm not sure how to handle this problem so I used the following formula

Q = ( (PI d4)/(128μL) ) * (P1 - P2)

    = ( ( PI * 0.44 ) / (128 * 11x10-6 * 180 * 103) ) * 2x10^6

    = 634.66 m3/s

This seems to easy to get away with and that seems way too much flow for such a narrow pipe.  And the fact that I'm not using the roughness is bugging me too.

Am I right that I'm wrong? 

I don't know how else to do this since I don't have the velocity. Any better ideas on how to solve this problem?

Any help very much appreciated.  Thanks

BobbyBob