Relief standard sizes

Regardless of how carefully the relief is sized, specified, and tested, a poor installation can result in completely unsatisfactory relief performance. Some installation guidelines are illustrated in Figure 8-11. During field construction, sometimes expediency or construction convenience leads to modifications and deviations from acceptable practice. The engineer must take the responsibility for adhering to standard practices, especially when installing relief systems. 

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. 

Safety features for the tank include a pressure relief-valve system on the tank roof, to be opened when draining or adding to the tank contents. A bursting disc system also allows for pressure release in the event of pressure build-up. Amanhole providesaccessto the tank for internal maintenance. The product inlet line is standard nominal pipe size 1.5, schedule number 40S. This inlet is sized for the maximum production flowrates. The product outlet line is of standard nominal pipe size 6, schedule number 120. This line is sized such that a standard-size road tanker may be filled in 15 minutes. 

The relief valve selected should be one with equal or greater area than calculated using equation 13.105. Relief valve sizes are given in API Standard 526. Sizing equations for subcritical flow of vapors, liquids, steam, and two-phase mixtures are given in API RP 520. 

The chart given in Figure 16 can be used in the following manner in order to size a relief valve for liquid service. First, determine the area required, A, without any viscosity correction (i.e., for K = 1). Then select the next larger standard orifice size from manufacturer s literature. Determine the Reynolds number, based on the following definition ... 

Relief valves are most often sold using the standard orifice sizes shown in Table 13-3. Once a required area is calculated from the applicable formulas, the next larger standard orifice size is specified. 

When a relief valve is sized for viscous liquid service, it is suggested that it be sized first as for nonviscous type application in order to obtain a preliminary required discharge area, A. From manufacturer s standard orifice sizes, the next larger orifice size should be used in determining the Reynold number R, from either of the following relationships ... 

Rupture disk, 455 Safety relief valves, 454, 467, 481 Specifications, centrifugal pumps, 209 Spray nozzle particle size, 225 Standards and Codes, 31, 32, 33 Static electricity, 536 Static mixing, 332 Applications, 336 Calculations, 337, 338 Materials of construction, 337 Principles of operation, 335 Type of equipment 334-338... 

Standard Orifice Sizes for Flanged Steel Safety Relief Valves... 

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). 

This must now be corrected for the effects of friction. It is useful to have an approximate idea of the relief diameter in order to carry out the correction. Using the uncorrected value of G, A=W/G = 329.3/32704 = 0.010 m2. The equivalent diameter is 0.113 m. The next largest standard pipe size will be 0.15 m diameter (6 inches). 

The final safety requirement is a relief line with a relief valve protected by a bursting disc. This disc must be sized according to the Australian Standard AS1 210. 

API 520 Definition A nominal or computed area used with an effective discharge coefficient to calculate the minimum required reheving capacity for a pressure relief valve per the preliminary sizing equations contained in API Standard 526. API Standard 526 provides effective discharge area for a range of sizes in terms of letter designations D through T. 

Codes and standards relevant to safety relief valves (SRVs) can vary quite considerably in format around the world, and many are sections within codes relevant to boilers or pressure-containing vessels. Some will only oudine performance requirements, tolerances and essential constructional detail, but give no guidance on dimensions, orifice sizes and so forth. Others will be related to installation and application. It is quite common within many markets to use several codes in conjunction with one another and it is not uncommon that specifications call for sections taken from several codes, which makes compliance by manufacturers complex and uneconomical. An overview of most common worldwide codes and standards is given in Appendix M. 

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... 

The most commonly used relief device is the conventional spring-loaded relief valve shown in Figure 13.42. This design of valve is available in the widest range of sizes and materials (API Standard 526). 

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