Pressure Relief Device Selection

While the above permitted variations are not used for selecting the pressure setting of pressure relief devices, these variations may be used for piping considerations such as startup, shutdown and emergency conditions, provided that the pressure vessel limits are not exceeded and that such variations are permitted by local codes. 

Selection of Pressure Relief Device - From the range of available pressure rehef valves and other devices, selection is made of the appropriate type for each item of equipment subject to overpressure. Instrumentation, check valves, and similar devices are generally not acceptable as means of overpressure protection. 

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. 

Donat, C., Selecting and Dimensioning of Pressure Relief Devices for Dust Explosions, Staub-Reinhaltung der Luft, 31, No. 4, 1971, pp. 17-25. 

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. 

Although many pressure relief devices are called SRVs, not every SRV has the same characteristics or operational precision. Only the choice of the correct pressure safety device for the right application will assure the safety of the system and allow the user to maximize process output and minimize downtime for maintenance purposes. Making the correct choice also means avoiding interference between the process instrumentation set points in the control loop and the pressure relief device limits selected. These SRV operational limits can vary greatly even when all are complying with the codes. 

Each instrument in this control loop has its tolerances, for instance +/ — 5%. To ensure smooth operation, tolerances should never interfere with each other. Also, the SRV should be selected so that it does not start to open under the highest pressure switch setting plus its tolerance. Therefore, it is important to know the tolerance of the pressure relief device, or in this case the SRV, and the same applies for the SRV closure. In short, tolerances should never interfere... 

Generally spoken, the use of non-redosing pressure rdief devices will offer, in most cases, the lower cost solution but requires that the process is shut down or redirected through alternative safety systems to allow for replacement of the burst device. Subsequendy non-redosing pressure relief devices will only be selected as primary relief solutions in cases where loss of process media or shutdown for repair is tolerated or possible (see Section 5.4). 

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. 

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. 

CGA S-7, Method for Selecting Pressure Relief Devices for Compressed Gas Mixtures in Cylinders, describes the method for selecting pressure relief devices for compressed industrial gas mixtures, while CGA S-1.1 applies to pure gases and some medical gas mixtures [7]. 

T. No pressure relief device required. This device is to be selected if a pressure relief device is used. 

In selecting pressure relief devices for cylinders, it should be noted that the length of the package, its DOT specification, fill pressure and lading are some of the parameters that must be taken into consideration. Additionally, knowl-... 

Depending on the contents of a mixture, one or more pressure relief devices may be required on the container. Most pressure relief devices are an integral part of the valve. However, some devices are installed directly into the bottom or top of the container. For an in-depth discussion on pressure relief devices for mixtures, see CGA S-7, Method for Selecting Pressure Relief Devices for Compressed Gas Mixtures in Cylinders [13]. Also consult Chapter 8 of this handbook for a presentation on pressure relief de-... 

At least one pressure relief device is required on any compressed gas cylinder or trailer tube offered for transport of oxygen. A pressure relief device must be selected in accordance with CGA S-1.1, Pressure Relief Device Standards—Part 1—Cylinders for Compressed Gases, [14]... 

Pressure relief device (PRD), such as pressure relief valve (PRV), rupture disk and rupture pm device, are used to protect equipment and lines from overpressure. It is process engineer s responsibility to show the requirement of a PRD at an equipment or line on the P ID drawings. It is also process engineer s responsibility to select the PRD type and its set pressure, and to do a contingency analysis to check out under what conditions the equipment or line will be over pressured, and what the relief rate will be. 

In this chapter, following topics will be discussed type of pressure relief devices, their characteristics, ASME code PRD set pressure, maximum operating pressure, contingency analysts, pressure reliefvalve and rupture disk sizing, pressure relief valve inlet/outlet piping sizing, Eind PRD selection. 

The following subsections describe the guidelines often used in selecting the type of pressure relief device. 

Design and selection philosophy for use of safety devices for pressure relief and alarm. 

Figure 7-7A. Pressure level relationship conditions for pressure relief valve installed on a pressure vessel (vapor phase). Single valves (or more) used for process or supplemental valves for external fire (see labeling on chart). Reprinted by permission, Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part 1 Sizing and Selection, API RP-520, 5th Ed., July 1990, American Petroleum Institute.

Note, most manufacturers tables or charts give SCFH capacities at 14.7 psia and 60°F, and these must be corrected by the gas laws to the artwa/volume at flowng conditions in order to represent the actual performance of the system. The tables or charts of the manufacturers read in SCFH for selected relief device setting and for tank pressure, expressed as air at SCFH (see Figures 7-37A and 37B). 

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