Processes that handle Liquified Pressurized Gases are required to have Hydrostatic Relief Valves on liquid piping where the liquid can be trapped. Much like vapor/gas phase Pressure Safety Valves, it is frowned upon when we have an intervening valve under the PSV. However, many codes today ALLOW us to install a 3-way valve below the Hydrostatic/Vapor/Gas PSV. Interestingly enough, NFPA 58 allows this, with one serious stipulation.