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.
Subsection 6.16.2 recognizes that there are times when hydrostatic relief valves must be removed for maintenance. Sometimes this happens on a long section of pipe and would result in evacuating or releasing a large quantity of product. This section allows the installation of a three-way isolation valve between the piping and two hydrostatic relief valves. This valve is always open to some degree in at least one discharge direction. The hydrostatic relief valve in need of maintenance may be isolated while the other remains active and communicating with the piping it protects.
Please note that this 500 psi rated 3-way valve does NOT appear to be based on the piping MAWP or the set point of the Hydrostatic Relief Valve. Even 6.16 Hydrostatic Relief Valve Installation does NOT specify the set point for the Hydrostatic RVs.
If liquid LP-Gas is trapped in a length of pipe between two closed valves, there is no room for expansion of the liquid if the liquid temperature increases. The pressure developed can be very high [thousands of psig (MPag)] and can result in pipe or valve failures. Operation of a hydrostatic relief valve prevents this pressure from developing by discharging liquid. Liquid, however, represents a greater quantity of discharged flammable gas than vapor, and its presence can introduce hazards such as fire and freezing of personnel’s skin (i.e., as the liquid vaporizes it absorbs heat). Over a temperature range of 30°F to 90°F (0°C to 32°C), liquid propane expands an average of about 1.6 percent for each 10 Fahrenheit degrees (5.5 Celsius degrees). Because liquid propane and most other liquids are not compressible, this thermal expansion results in a substantial and quick pressure rise. Hydrostatic relief valves are usually small [about ¼ in. (6 mm) pipe size] because only a small amount of liquid has to be released to prevent an overpressure hazard. This section was not intended to allow a maintenance valve (locked in the open position) to be placed between a hydrostatic relief valve and the pipe upon which it is installed.