Selection of pressure relief valves

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

Conventional PRVs are the most widely used relief devices in chemical industries, particularly in a system in which the superimposed back pressure is relatively low. Conventional PRVs are not very suitable for high back pressure. 

Process engineering and design using Visual Basic 

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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. 

Conventional Flare System - The majority of pressure relief valve discharges which must be routed to a closed system are manifolded into a conventional blowdown drum and flare system. The blowdown drum serves to separate liquid and vapor so that the vapor portion can be safely flared, and the separated liquid is pumped to appropriate disposal facilities. The blowdown drum may be of the condensible or noncondensible type, according to the characteristics of the streams entering the system. Selection criteria, as well as the design basis for each type of blowdown drum, are detailed later in this volume. The design of flares, including seal drums and other means of flashback protection, is described later. 

American Petroleum Institute Washington www.api.org API Recommended Practice 520 Part 1 - Sizing and Selection API Recommended Practice 520 Part 2 - Installation API Recommended Practice 521 - Guide for Pressure Relief and Depressurizing Systems API Standard 526 - Flanged Steel Pressure Relief Valves API Standard 527 - Seat Tightness of Pressure Relief Valves API Recommended Practice 576- Inspection of Pressure Relieving Devices... 

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.

In most materials selection processes, it is virtually impossible to make materials choices independent of the product shape. This includes not only the macroscopic, or bulk, shape of the object such as hammer or pressure relief valve, but also the internal or microscopic shape, such as a honeycomb structure or a continuous-fiber-reinforced composite. Shape is so important because in order to achieve it, the material must be subjected to a specific processing step. In Chapter 7, we saw how even simple objects made from a single-phase metal alloy could be formed by multiple processes such as casting or forging, and how these processing steps can affect the ultimate properties of the material. As illustrated in Figure 8.6, function dictates the choice of... 

Once installed and as the process changes, it might be wise to take a dose look at the original selection of SRVs and evaluate whether this selection, made for different process criteria, is still valid. Then, organize compliance audits of pressure relief systems and particularly on individual valves and other devices and re-evaluate the potential overpressure scenarios described earlier in this book. The typical current standard internal audits might not catch the piece of equipment that, for instance, does not have a valve but should. ... 

The selection of an actual pressure relief valve to use when the area is known is not a trivial problem, since there are many vendor catalogs and each can be... 

Confirm relief protection philosophy, select location of major relief valves, and determine vessel and exchanger design pressure and temperatures. 

Figure 7 Schematic diagram of semipreperative-scale SFC chromatograph 1 carbon dioxide supply 1 a regulator 2 prechiller/heat exchanger 3 SD-1 Varian pump with (7) 200-mLpump head with special check valves 4 modifier reservoir 5 SD-1 modifier pump with (6) 200-mL standard pump heads 8 check valve 9 inlet pressure transducer 10 injection valve 11 check valve to prevent blow-back 12 mixer 13 fluid temperature preconditioner 14 column 15 column oven 16 uv detector 17 outlet pressure transducer 18 back-pressure regulator 1 9 evaporator 20 restrictor 21 trim heater 22 selection valve 23 peak detector 24 bank of collection vessels (or cassette) 25 individual collection tubes/bottles 26 pressure relief valves 27 waste container, waste vent. The manual cassette can be replaced with an automated cassette fed by a robot holding 128, 25 x 150-mm or 338, 16 x 150-mm test tubes, or with 7 large bottles.

Pressure-relief valve Pressvue- or temperature-activated device used to prevent buildup of pressure beyond a predetermined maximum that could cause a system to rupture. The design and selection of the device are dictated by codes and standards governing the safe design of the vessel or S3 tem to which it is connected. Also referred to as a relief valve, safety valve, or safety relief valve. 

Pressure-relief valves protect life and property. They are installed on pressurized systems in almost all types of industries such as refineries, oil and gas production facilities, pipelines, electric power generating plants, paper mills, chemical plants, and other industrial facilities worldwide. Pressure-relief designs are available for both compressible and noncompressible fluid applications in a wide range of sizes, material selections, pressure classes and temperature ranges. 

Sizing and selection of relief valves must comply with criteria specified in applicable codes and standards as well as the manufacturer s data concerning the flow characteristics for each design type. Pressure-relief valve manufacturers provide catalogs and computer sizing programs to aid in the sizing and selection of their specific products (Fig. 5.186). 

Pressure-relief valves are offered in many material combinations. Manufacturers recommend materials for specific applications. Proper selection of materials requires that service conditions be specified temperature, pressure, media contained within the system, and any external environmental conditions that need consideration. 

High Pressure is a cause of many serious events. Some of the causes of High Pressure are listed in Table 17.1. For High Pressure to create an incident, it is likely that a protective interlock system failed and/or that the system s pressure relief valves did not function properly. The worst case is selected for the design of the relief device. 

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 calculation method can be selected by application of the decision tree in Figure 9.2. The liquid temperature is believed to be about 339 K, which is the temperature equivalent to the relief valve set pressure. The superheat limit temperatures of propane and butane, the constituents of LPG, can be found in Table 6.1. For propane, T, = 326 K, and for butane, T i = 377 K. The figure specifies that, if the liquid is above its critical superheat limit temperature, the explosively flashing liquid method must be chosen. However, because the temperature of the LPG is below the superheat limit temperature (T i) for butane and above it for propane, it is uncertain whether the liquid will flash. Therefore, the calculation will first be performed with the inclusion of vapor energy only, then with the combined energy of vapor and liquid. 

WThen the pressure rise in a system is gradual and not explosive in nature, a safety or safety relief valve is the proper detdce, but when it is critical to completely depressure a system or the rate of pressure increase might be expected to be rapid, then a rupture disk is the proper device. Properly designed a pilot operated valve may be selected after checking its performance with the manufacturer. 

Instrument Failure assume instrument control valves freeze or fail in open position (or closed, which ever is worse), determine capacity for relief based on flows, temperatures, or pressures possible under these circumstances. The judicious selection of instrument failure sequence may eliminate or greatly reduce relief valve requirements. 

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