26 API STANDARD 520, PART II—INSTALLATION Typically the inlet isolation valves for spare relief devices are closed and the outlet isolation valves are open. The outlet isolation valve for spare relief devices can be closed during operation if exposure to the fluid is a concern, system fluids without closing the outlet isolation valve can also be achieved either by providing a purge or by installing a discharge rupture disk. Three-way changeover valves are acceptable in spare relief capacity applications provided the installation meets the size and inlet pressure drop requirements (see 8.3.4). To closed (isolation valving required) or atmospheric dischargesystem Nonrecoverable pressure losses not more than 3 % of set pressure (see Note) 山 Typical bleed valve Vessel NOTE See7.3fordetaileddiscussionon inlet pressure drop. Figure 11—Typical PRD Installation for 100 % Spare Relieving Capacity 27 SIZING, SELECTION, AND INSTALLATION OF PRESSURE-RELIEVING DEVICES To closed (isolation valving required) or atmospheric discharge system X Typical Typical isolation valve with bleed provision for car sealing or valve iocking open (see Note 1) Nonrecoverable pressure losses not more than 3 % of set pressure (see Note 2) Vessel NOTE 1 See Section 8. NOTE 2 See 7.3 for detailed discussion on inlet pressure drop. Figure 12—Alternate PRD Arrangement for 100 % Spare Relieving Capacity 8.3.4Three-Way Changeover Valves for Dual PRD Installations Three-way changeover valves are available, which are designed specifically for isolation valve service of dual PRD installations. Such installations provide 100 % of the design relieving capacity with one PRD while a relief device is out of service. The second PRD may be permanently mounted on the three-way changeover valve or may be stored until neededto preserveits integrityand allow benchtesting justpriortoinstallation.Three types of changeover valves are available: a three-way ball valve combined with piping on the inlet and outlet, the shuttle type (see Figure 14) and the rotor type (see Figure 15). a) Three-way changeover valves, used for PRV installations, shall be sized to assure compliance with the inlet loss 28 API STANDARD 520, PART II—INSTALLATION To closed (isolation valving required) or atmospheric discharge system Nonrecoverable pressure losses Typicalbleedvalve not more than 3 % of set pressure (see note) Positive indication of activerelievingdevice Vessel NOTE See7.3fordetailed discussion on inlet pressure drop. Figure 13Alternate PRD Installation Arrangement for 100 % Spare Relieving Capacity SIZING, SELECTION, AND INSTALLATION OF PRESSURE-RELIEVING DEVICES 29 Figure 14—Three-Way Changeover Valve—Shuttle Type Figure 15—Three-Way Changeover Valve—Rotor Type 30 API STANDARD 520, PART II—INSTALLATION greater than the inlet area of a PRV designed for that line size. Other designs, however, may have to be specified one line size larger than the PRV to minimize inlet pressure losses. b) The three-way changeover valve should be designed to prevent both PRDs from being isolated at any time during its switchover operation. c) A positive indication of which PRD is active should be a required accessory for the three-way changeover valve. d) A bleed valve shall be installed between the inlet isolation valve and an isolated PRD to enable the inlet to the e) Individual isolation valves may be used on the outlet side of PRDs that are mounted on an inlet three-way changeover. When using individual outlet isolation valves the recommendations of 8.3.2 should be followed. f) Three-way valves may also be used for outlet isolation. Designs are available that will minimize the effects of built- recommendationsof8.3.2shouldbefollowed. g) Isolation valves shall have the capability of being locked or car-sealed in position. Only an authorized person may break the seal and operate the valve (see 8.3.2 b). Mechanical interlocks and/or management control procedures shall be provided which will assure the proper ope