If a high pressure system with high flow is discharging into atmosphere through a 2" gate valve and a small length of pipe .
Will the pressure drop take place across the valve or the flow will choke at the end of the pipe.Pressure drop will take place across the valve.  Choking is most likely to occur at the point of minimum inside diameter.  Most gate valves are considered full bore.  therefore choking could reasonably be expected at the end of the pipe.
As long as the valve backpressure is below the valve's choke pressure, it will have no effect on the depresssuring rate.  Initially the backpressure could be as high as 4.5 bar and not affect the flow (assuming gas).  As the equipment is depressured, the flow will decrease and along with it, the backpressure will decrease.  Given that this is a short run of pipe, there should not be a problem.  But a quick sanity check of the backpressure should be done.

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