The construction of relief valves must meet local area requirements. Valves are commonly selected on the following basis:
• Use steel, bolted bonnet, thermal relief valves (Crosby Series 900, Teledyne Farris 2740 or 2741, or equal) with flanged or seal welded screwed inlet and outlet connections for all LPG liquid services. It should be noted that seal welding of screwed inlet and outlet connections presents a problem when it is necessary to take the relief valves into the shop for testing and setting. For this reason, flanged ends are preferred (see Figure 1200-23).
• Use the less expensive steel, screwed bonnet, screwed end, thermal relief valves (Crosby Series 900 or equal) for all other liquid process services except in corrosive liquid services or in piping subject to vibration. In these services, use the bolted bonnet style above. The bolted bonnet style may also be preferable in severely corrosive atmospheres where external corrosion could make disassembly of the screwed bonnet difficult
• Use screwed, bronze relief valves (or equal) for water services