API 520 PT II – 2003 pdf free download

API 520 PT II – 2003 pdf free download.Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries Part II—Installation.
4.3.2 Mechanical Stresses Discharge piping should be independently supported and carefully aligned. Discharge piping that is supported by only the pressure-relief device will induce stresses in the pressure- relief device and the inlet piping. Forced alignment of the dis- charge piping will also induce such stresses. 4.4 INLET STRESSES THAT ORIGINATE FROM DISCHARGE REACTION FORCES The discharge of a pressure-relief device will impose a reaction force as a result of the ßowing ßuid (see Figure 7). This force will be transmitted into the pressure-relief device and also into the mounting nozzle and adjacent supporting vessel shell unless designed otherwise. The precise magni- tude of the loading and resulting stresses will depend on the reaction force and the conÞguration of the piping system. The designer is responsible for analyzing the discharge system to determine if the reaction forces and the associated bending moments will cause excessive stresses on any of the compo- nents in the system.
4.6 RUPTURE DISK DEVICES IN COMBINATION WITH PRESSURE-RELIEF VALVES A rupture disk device may be used as the sole pressure- relief device, or it may be installed between a pressure-relief valve and the vessel or on the downstream side of a pressure- relief valve (see Figure 8). When a rupture disk device is used between the pressure- relief valve and the protected vessel, the space between the rupture disk and the pressure-relief valve shall have a free vent, pressure gauge, trycock, or other suitable telltale indica- tor. A non-vented space with a pressure gage without alarms or other indication devices is not recommended as a suitable telltale indicator. Users are warned that a rupture disk will not burst in tolerance if back pressure builds up in a non-vented space between the rupture disk and the pressure-relief valve, which will occur should leakage develop in the rupture disk due to corrosion or other cause. Only non-fragmenting rup- ture disk devices may be used beneath a pressure-relief valve. Rupture disks may not be available in all sizes at lower pressures; therefore, for these low-pressure applications the available rupture disk may have to be larger than the nominal size of the inlet piping and pressure-relief valve. Refer to API RP 520, Part I for additional information related to the combination capacity factor when a rupture disk is installed in combination with a pressure-relief valve. 4.7 PROCESS LATERALS CONNECTED TO INLET PIPING OF PRESSURE-RELIEF VALVES Process laterals should generally not be connected to the inlet piping of pressure-relief valves (see Figure 9). Excep- tions should be analyzed carefully to ensure that the allow- able pressure drop at the inlet of the pressure-relief valve is not exceeded under simultaneous conditions of rated ßow through the pressure-relief valve and maximum possible ßow through the process lateral.
5.3 BACK PRESSURE LIMITATIONS AND SIZING OF PIPE When discharge piping for pressure-relief valves is designed, consideration should be given to the combined effect of superimposed and built-up back pressure on the operating characteristics of the pressure-relief devices. The discharge piping system should be designed so that the back pressure does not exceed an acceptable value for any pres- sure-relief device in the system. See API RP 520 Part I for limitations on back pressure. When rupture disks are used as the sole relieving device and the discharge is to a closed system, the effect of the superimposed back pressure on the bursting pressure for the disk must be considered. The rated capacity of a conventional spring loaded, bal- anced spring loaded or pop action pilot-operated pressure- relief valve should typically be used to size the atmospheric vent piping or the discharge line from the pressure-relief valve to the relief header. Common relief header piping in closed discharge systems should be sized using the protected systemÕs required relieving capacity. For a modulating pilot-operated pressure-relief valve, the discharge piping can be sized using the required relieving capacity of the system that the valve is protecting. Whenever the atmospheric vent, discharge piping or com- mon relief header piping is sized using the systemÕs required relieving capacity instead of the rated capacity of the valve, the back pressure should be re-checked whenever changes are made to the process that effect the required relieving capacity of the system the valve is protecting. Additional information on sizing of discharge piping systems for vapor or gas service is covered in API RP 521.