Pressure Relief Device Sizing

PRD sizing equations arc derived based on isentropic flow of ideal gas or liquid through a nozzle and cmrected by using various correction factors. Interested reader should read reference 1, appendix B, 

Rupture pin PRV sizing should be the same as PRV sizing. Consult vendor to confirm. 

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NFPA 30 and API Standard 2000 provide guidance for design of overpressure protection involving storage tanks that operate at or near atmospheric pressure. In particular, NFPA 30 focuses on flammability issues, while API 2000 addresses both pressure and vacuum requirements. The ASME code (Sections I and VIII) and API RP 520 are the primary references for pressure relief device sizing requirements. 

VILl.l Vapor pressure relief device sizing at sonic flow Calculate required PRV oriflce area or rupture disk area by follow equation ... 

V11.2 Liquid pressure relief device sizing There are two different type of sizing depending on whether the PRD to be certified or not (by ASME code). 

V1L2.1 Certified liquid pressure relief device sizing ASME code requires that PRV used for liquid relief should have capacity certification, guaranty PRV full open at 10% overpressure. Following equations should be used for liquid PRD sizing ... 

VIT.2,2 Non-cetlified liquid pressure relief device sizing This applies to liquid PRV before ASME code set capacity certification requirement. Interested reader can find its sizing requirement in API 520, section 5.9. [1]... 

This section describes the various pressure relief devices that are commonly used, with their characteristics and criteria for selection. Basic calculation procedures for sizing PR valves are covered in subsequent discussions. 

The method used for the safe installation of pressure relief devices is illustrated in Figure 8-1. The first step in the procedure is to specify where relief devices must be installed. Definitive guidelines are available. Second, the appropriate relief device type must be selected. The type depends mostly on the nature of the material relieved and the relief characteristics required. Third, scenarios are developed that describe the various ways in which a relief can occur. The motivation is to determine the material mass flow rate through the relief and the physical state of the material (liquid, vapor, or two phases). Next, data are collected on the relief process, including physical properties of the ejected material, and the relief is sized. Finally, the worst-case scenario is selected and the final relief design is achieved. 

The pressure relief device should be evaluated and sized in accordance with the latest industry standards (i.e., DIERS, API, NFPA). An open passage (nozzle or line) from the separator to the atmosphere may be considered as the pressure relief device provided that it meets the relieving capacity requirements of the ASME Boiler and Pressure Vessel Code (BPVC). 

Within this Workbook, the maximum pressure required to fully open the pressure relief device will be referred to as the "relief pressure". (Caution some papers on relief sizing refer to "set pressure" but mean "relief pressure"). For a bursting disc, the relief pressure will be the maximum specified bursting pressure and for a safety valve, it will be the set pressure plus 10% overpressure (or whatever percentage overpressure the valve has been certified at). 

API Recommended Practice 520 Part I, Sizing and Selection This API design manual includes basic definitions and information about the operational characteristics and applications of various pressure relief devices. It also includes sizing procedures and methods based on steady state flow of Newtonian fluids. This RP covers equipment that has a maximum allowable pressure of 15 psig (1.03 barg) or greater. 

The usual approach in design is to prevent explosions from occurring, for example, by not allowing flammable mixtures to form in the process. If internal explosion is a possibility, then it must be considered as a pressure-relief scenario and the pressure-relief devices must be sized to prevent detonation. This will usually require the use of large bursting disks. Flame arrestors should also be specified on process piping to... 

Selection and sizing of the relief device are the responsibility of the end user of the pressure vessel. Rules for the selection and sizing of pressure-relief devices are given in the ASME BPV Code Sec. VIII D.l Parts UG-125 to UG-137 and D.2 Part AR. 

If a rupture disk is used as the primary pressure-relief device, then when it bursts the operators have no option but to shut down the plant so that the disk can be replaced before the vessel is repressured. Rupture disks are therefore most commonly used at the inlets of relief valves or as secondary relief devices. Rupture disks can be sized using equation 13.105 for compressible gases in sonic flow, with a value of Kd = 0.62. 

A Pressure Relief Device (not shown) is required by NFPA 86 (sect. 4-4.3.12) whenever it is possible to have an event leading to an oxygen supply pressure that is greater than the MAWP (maximum allowable working pressure) of any of the components or piping within the oxygen pipeline. The pressure relief device must be sized properly per manufacturer and industry standards. The outlet of the pressure relief device must be piped to a safe location per NFPA 86 (sect. 4-4.3.7). 

Pressure relief devices must be properly sized (capacity), and discharge location is critical. These devices are generally sized for the most likely pressure increasing event, e.g., external fire or internal process upset such as an uncontrolled exothermic reaction. The engineer must calculate the temperature and pressure increases associated with the event as well as the expected release volume. He or she must also account for pressure drops across the relief device as well as for friction losses in the lines. A decision must also be made about whether to discharge to the atmosphere or to a closed system that includes a scrubber, a flare, or even as simple as a water tank, such as is sometimes used for venting anhydrous ammonia. Some of these design decision issues are addressed in more detail by Crowl and Louvar. ... 

New process equipment installed within 7.6 meters from the grade/ground level will require pressure relief devices due to fire scenario. For revamp cases, it may be advantageous to place vessels on platforms at >7.6 meters if fire case relief load is most credible and bottlenecks the existing flare system. Fire circle or zone is defined as the maximum affected area during any equipment fire in the facility. API 521 (2014) defines its area as 230 to 460 m. Addition of process equipment inside an existing fire circle may increase the fire circle size. Hence, care shall be taken to review the fire circle size with each equipment addition. It will impact the peak relief load during the fire scenario. 

To determine the maximum possible flow through an actuated pressure relief device, use of the phenomenon that a maximum mass flux exists which only depends on the state in the vessel (see Figure 14.4) is made. For a given pressure Po in the vessel, the mass flow is determined by the size of the narrowest cross-flow area A] in the line. When the outlet pressure Pi is decreased, the mass flow increases. However, if Pi falls below a certain value, no further increase of the mass flow takes place (Figure 14.5). The maximum mass flow is called the critical mass flow merit ... 

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