What is the effect of reducing the pressure in a gas orifice plate measurement system? The pressure will be reduced from around 50 Bar to 5 Bar. I understand that reducing pressure will speed up the velocity of the gas thus raising the DP although the mass flow rate will reduce due to the lower density.

Is this correct?

 

The density changes by a ratio of BarA Out/BarA plus changes in temperature and compressibility ratios, so you are basically correct.

The flow is governed by the pressure drops in the system.

 

Don't forget about choked or critical flow. After a pressure drop of about 50% or around 25 bar orifice outlet pressure the flow will not increase with drop in outlet pressure. The flow will be choked.