API RP 576-2009 pdf download.Inspection of Pressure-relieving Devices.
3.4.7 cold differential test pressure CDTP The pressure at which a pressure-relief valve is adjusted to open on the test stand.The CDTP includes corrections for the service conditions of backpressure or temperatures or both. 3.4.8 leak-test pressure The specified inlet static pressure at which a seat leak test is performed. 3.4.9 manufacturing design range The pressure range at which the rupture disk shall be marked. Manufacturing design ranges are usually catalogued by the manufacturer as a percentage of the specified burst pressure. Catalogued manufacturing ranges may be modified by agreement between the user and the manufacturer. 3.4.10 marked burst pressure or rated burst pressure The marked burst pressure or rated burst pressure of a rupture disk is the burst pressure established by tests for the specified temperature and marked on the disk tag by the manufacturer. The marked burst pressure may be any pressure within the manufacturing design range unless otherwise specified by the customer. The marked burst pressure is applied to all the rupture disks of the same lot. 3.4.11 opening pressure The value of increasing inlet static pressure whereby there is a measurable lift of the disk or at which discharge of the fluid becomes continuous, as determined by seeing, feeling or hearing. 3.4.12 set pressure The inlet gauge pressure at which a pressure-relief valve is set to open under service conditions. 3.4.13 simmer The audible or visible escape of compressible fluid between the seat and disc, which may occur at an inlet static pressure below the set pressure prior to opening. 3.4.14 specified burst pressure The specified burst pressure of a rupture disk is the burst pressure specified by the user. The marked burst pressure may be greater than or less than the specified burst pressure but shall be within the manufacturing design range.
A pressure-relief device is actuated by inlet static pressure and designed to open during emergency or abnormal conditions to prevent a rise of internal fluid pressure in excess of a specified design value. The device may also be designed to prevent excessive internal vacuum. The device may be a pressure-relief valve, a non-reclosing pressure- relief device, or a vacuum-relief valve. Common examples include direct spring-loaded pressure-relief valves, pilot-operated pressure-relief valves, rupture disks, weight-loaded devices, and pressure- and/or vacuum-vent valves. 4.2 Pressure-relief Valve A pressure-relief valve is designed to open for the relief of excess pressure and reclose thereby preventing further flow of fluid after normal conditions have been restored. A pressure-relief valve opens when its upstream pressure reaches the opening pressure. It then allows fluid to flow until its upstream pressure falls to the closing pressure. It then closes, preventing further flow. Examples of specific types of pressure-relief valves include: safety valve, relief valve, conventional safety-relief valve, balanced safety-relief valve, and pilot-operated pressure-relief valve. 4.3 Safety Valve 4.3.1 General A safety valve is a direct spring-loaded pressure-relief valve that is actuated by the static pressure upstream of the valve and characterized by rapid opening or pop action. When the static inlet pressure reaches the set pressure, it will increase the pressure upstream of the disk and overcome the spring force on the disk. Fluid will then enter the huddling chamber, providing additional opening force. This will cause the disk to lift and provide full opening at minimal overpressure. The closing pressure will be less than the set pressure and will be reached after the blowdown phase is completed. The spring of a safety valve is usually fully exposed outside of the valve bonnet to protect it from degradation due to the temperature of the relieving medium.