How fast the pressure change
 

Hi

I have a pressurized pipe (230bar) which is opened to a way lower pressure tube (1 bar) through a valve. the pipe is filled with 2phase flow (gas and oil). I am looking for an explanation or formulation on how fast the pressure drops upstream the valve. I know pressure equation is of parabolic nature and parabolic equation has the property that the information propagates at arbitrary high speed.
Any reference or explanation is very much appreciated.

Regards,
Hatef