API St 520-2:2020 pdf free download.Sizing, Selection, and Installation of Pressure-relieving Devices Part II—Installation
5.7 Inlet Stresses that Originate from Static Loads in the Discharge Piping
5.7.1 General Improper design or construction of the discharge piping from a PRD can set up stresses that will be transferred to the PRD and its inlet piping. Static loads are stresses that occur during normal operation while the relief device remains closed. These stresses may cause a PRV to leak or malfunction, may change the burst pressure of a rupture disk, or may cause connected equipment flanges to leak. The PRD manufacturer should be consulted about permissible loads.
5.7.2 Thermal Stresses Fluid flowing from the discharge of a PRD may cause a change in the temperature of the discharge piping. A change in temperature may also be caused by prolonged exposure to the sun or to heat radiated from nearby equipment. Any change in the temperature of the discharge piping will cause a change in the length of the piping and may cause stresses that will be transmitted to the PRD and its inlet piping. The PRD should be isolated from piping stresses through proper support, anchoring, or flexibility of the discharge piping.
5.7.3 Mechanical Stresses Discharge piping should be independently supported and aligned. Discharge piping that is supported by only the PRD will induce stresses in the PRD and the inlet piping. Forced alignment of the discharge piping will also induce such stresses.
5.8 Inlet Stresses that Originate from Discharge Reaction Forces
5.8.1 General
The discharge of a PRD will impose a reaction force as a result of the flowing fluid (see Figure 6). This force will be transmitted into the PRD and also into the mounting nozzle and adjacent supporting vessel shell unless designed otherwise. The precise magnitude of the loading and resulting stresses will depend on the reaction force and the configuration 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 components in the system. A method for the design of piping systems to withstand reaction forces from PRDs is given in Appendix II of ASME B31.1 [5] .
The magnitude of the reaction force will differ substantially depending on whether the installation is open or closed discharge. When an elbow is installed in the discharge system to direct the fluid up into a vent pipe, the location of the elbow and any supports is an important consideration in the analysis of the bending moments. 5.8.2 Determining Reaction Forces in an Open Discharge System
5.8.2.1 Vapor Discharge The following formula is based on a condition of critical steady state flow of a compressible fluid that discharges to the atmosphere through an elbow and a vertical discharge pipe. The reaction force (F) includes the effects of both momentum and static pressure; thus, for any gas, vapor, or steam:
where
F is the reaction force at the point of discharge to the atmosphere in lbf [N];
W is the flow of any gas or vapor in lbm/hr [kg/s];
k is the ratio of specific heats (C p /C v ) at the outlet conditions;
C p is the specific heat at constant pressure;
C v is the specific heat at constant volume;
T is the stagnation temperature at the pipe outlet, in °R [K]. Note that the stagnation temperature is often not available. If this is the case, a suitable approximation is the relieving temperature. The user is cautioned that while this approximation is conservative for determination of reaction forces, the relieving temperature shall not be used for other discharge piping design aspects as this does not consider auto-refrigeration or Joule- Thompson cooling effects.API St 520-2 pdf download.API St 520-2:2020 pdf free download
