It was said purging is not suggested. But how do we remove hydrogen from the piping (turnaround of H2 plant) or after depressurization?

Purging is not recommended as a continuous part of vent stack operation. However, maintenance activity is a transient event and it’s prudent and recommended to purge a vent system prior to performing maintenance. It’s always possible that hydrogen could be leaking internally from a valve or other component and therefore create a hazard. Of particular note, care must be taken that proper isolation of the vent system is performed such that the vent system can’t be inadvertently used during maintenance. Since vent systems and stacks rarely have isolation valves to prevent unintended isolation of relief devices, proper maintenance on the vent system may require the entire system or plant to be taken offline.