With a pressure drop of 50 mbar, what is the typical gas velocity that can be achieved?

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Gentlemen, I’m facing a problem right now: I have a micro-pressure nitrogen sealing tank with a volume of around 6 m3. The operating pressure is approximately 50 mbarg. There are two control valves in the pressure controller, which are used to regulate the amount of exhaust and intake air respectively (layered control)； It has been found that the intake speed is relatively fast, but the exhaust speed is relatively slow ； If the pressure inside the tank reaches 100 mbarg, even with the exhaust control valve fully open, it will take some time for the pressure to drop back to 50 mbarg ； It is now suspected that the exhaust capacity of the vapor pipeline is insufficient; the diameter of the vapor pipeline is DN50, while the diameter of the control valve is DN25. Please help determine whether the size of this pipeline meets the requirements, and what size would be appropriate? Additionally, what can be the linear velocity of the gas phase in such low-pressure pipes? m/s, how to calculate it ； Thank you!