Relief valves Code requirements

The ASME B PV Code, Section III controls the design, fabrication, and initial testing of boundary and relief valves. During operation, the ASME B PV Code, Section XI, specifies boundary and relief valve testing requirements to assure continued valve integrity. 

The basis for design overpressure described in this section is related to the ASME Boiler and Pressure Vessel Codes and ANSI B31.3, Code for Petroleum Refinery Piping. Compliance with these codes is a requirement, or is recognized as the equivalent of a requirement in many locations. Where more stringent codes apply, the local requirements must be met. Therefore, local codes must be checked to determine their requirements. For example, some countries do not permit the use of block valves underneath pressure relief valves, unless dual valves with interlocks are installed. Also, in some cases, 20% accumulation under fire exposure conditions is not permitted, and accumulation allowed may be lower than the ASME Codes. In the United States, the ASME Code is mandatory, since it is a requirement under the Occupational Safety and Health... 

A PR valve is not required for protection against fire on any vessel which normally contains little or no liquid, since failure of the shell from overheating would occur even if a PR valve where provided. Examples are fuel gas knockout drums and compressor suction knockout drums. (Note Some local codes require pressure relief valve protection for "dry drum" situations.)... 

The ASME code requires every pressure vessel that can be blocked in to have a relief valve to alleviate pressure build up due to thermal expan sion of trapped gases or liquids. In addition, the American Petroleum Institute Recommended Practice (API RP) 14C, Analysis, Design, Installation and Testing of Basic Surface Safety Systems on Offshore Production Platforms, recommends that relief valves be installed at vari ous locations in the production system and API RP 520, Design and Installation of Pressure Relieving Systems in Refineries, recommends various conditions for sizing relief valves. 

Rupture disks may be used to satisfy the requirements of the code for conditions such as corrosion and polymer formations, which might make the safety/relief valve inoperative, or where small leakage by a safety valve cannot be tolerated. They are particularly helpful for internal explosion pressure release. 

Safety and relief valves must be the direct spring loaded type, and for pressure ranges noted below the code [1] requires ... 

When the rupture disk is installed on the inlet side of the pressure relief valve (see Figures 7-10, 7-11 and 7-12), the ASME code requires that for untested disk-valve combinations that the relieving capacity of the combination be reduced to 80% of the rated relieving capacity of the pressure relief valve [ 1 ]. 

Ejectors, steam/water requirements, 371 Electrical charge on tanks, 537 Electrical precipaiaiors, 280 Applications, 280, 282 Concept of operation, 281 Emergency relief, 450 Engineering, plant development, 46 Equipment symbols, 19—2 L Abbreviations, 25 Instruments, 21, 26. 29 Piping, 22 Valve codes, 26 Equivalent feel (flow), 86 Estimated design calculation time,... 

Venting, dusts, (deflagration), nomograph, API, 514-520 Relief valves, 400 Code requirements, 415, 420 Installation, 422, 429-434 Safety-relief, 400 Relieving pressure, 411-417 Resistance coefficient, K, (flow), 71, 72 Flow coefficient, Q, 81, 83 Pipe sizing, 83, 84, 86 Sudden contraction, 80 Sudden enlargement, 80 Tables/charts, 73-76, 77, 78-80 Valves, 81... 

Pressure Design of Metallic Components External-pressure stress evaluation of piping is the same as for pressure vessels. But an important difference exists when one is establishing design pressure and wall thickness for internal pressure as a result of the ASME Boiler and Pressure Vessel Codes requirement that the relief-valve setting be not higher than the design pressure. For vessels this means... 

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. 

Relief devices safety relief valves. Safety relief valves should be mounted in the vertical position. The ASME Code requires that the space between a combination rupture disc mounted below an SRV must have a pressure gauge or other device to warn if the rupture disc is leaking. 

If the plant safety shutdown is not rapid enough and an overpressure situation develops, then the pressure relief system is activated. Pressure vessel design codes such as the ASME Boiler and Pressure Vessel Code require relief devices to be fitted on all pressure vessels (see Section 13.17). If the relief system has been properly designed and maintained, then in the event of an overpressure incident, the plant contents will be vented via relief valves or bursting disks into the relief system, where liquids are recovered for treatment and vapors are sent to flare stacks or discharged to the atmosphere if it is safe to do so. The pressure relief system should allow the plant to be relieved of any source of overpressure before damage to process equipment (leaks, bursting, or explosion) can occur. 

A relief valve or burst disk with a rating equivalent to or less than the rating of the vessel is not only a prudent accessory, but is also required by most safety codes. ASME recommends that the burst rating of a safety rupture should not exceed 110% of the MAWP. Rupture disks may be of a prebulged or scored type. Prebulged disks can be operated repeatedly to pressures up to 70% of their rating. Scored disks can be used repeatedly to pressures up to 90% of their rating. 

The design relief capacity of each of two SCS relief valves (shown in P ID Figure 6.3.2-lB) as supplied by the valve manufacturer meets the minimum required relief capacity of 4000 gpm which contains sufficient margin in relieving capacity for even the worst transient. The SCS relief valves are Safety Class 2, designed to Section III of the ASME Code. 

Figure 12.19 A typical safety relief valve, SRV, and rupture disc combination for a chemical process vessel. The rupture disk protects against the metal-to-metal seats on the SRV from seeping. The pressure gage lets you know if the rupture disk has popped or is leaking. The space between the rupture disk and SRV is required to be monitored by the ASME Code.

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