USCG Marine Safety Information Bulletin

A “non-return device” functions in the Inert Gas System (IGS) to prevent hazardous or flammable cargo vapors
from flowing from cargo areas back towards machinery spaces and possible ignition sources. The double block
and bleed valve arrangement isolates or blocks the return of gas from the cargo area to the engine room and permits the bleeding of any residual gas in the IGS.


https://www.dco.uscg.mil/Portals/9/DCO%20Documents/5p/MSIB/2022/MSIB-05-22-Inert-Gas-System.pdf?ver=mQ6soPDlXfiKLuz8HoPgGA%3d%3d