Sherwood Pressure Relief Device GSV PRD; Ni; 4000 PSI CG4

Almost all compressed gas containers are fitted with pressure relief devices. A pressure relief device is a pressure and/or temperature-activated device used to prevent the pressure from rising above a predetermined maximum, and thereby prevent rupture of a normally charged cylinder when subjected to a standard fire test as required by Title 49 of the US Code of Federal Regulations (49CFR 173.34(d)), or equivalent regulations of Transport Canada. 

The Compressed Gas Association, in pamphlet CGA S-1.1, has classified pressure relief devices according to type using the letter designation CG followed by a numeral.