References overpressure protection

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. 

The main steam safety valves are direct acting, spring loaded, carbon steel valves. The valves are mounted on each of the main steam lines upstream of the steam line isolation valves, and outside containment. A schematic drawing of the main steam safety valves is given in Figure 5.4.13-2. The valve parameters are given in Table 5.4.13-2. For a description of overpressure protection equipment and components for the main steam system refer to Section 10.3.2. 

Overpressure protection of the Reactor Coolant System is provided by four ASME Code spring-loaded safety valves. Refer to Section 5.4.12 and 5.4.13. 

Full scale, full pressure prototypical testing of pressurizer safety valves was performed by EPRI in 1981 (Reference 1). The blowdown settings required to insure stable valve operation during the blowdown from the set pressure were above the 5% setting specified in the ASME Code. In order to insure that the extended blowdown would not adversely affect overpressure protection or plant operation, analyses were performed to evaluate the NSSS response. The analyses described below demonstrate that a blowdown setting, including associated uncertainties, of 18.5% is acceptable. 

Specifically, the acceptance criteria for the overpressure protection system are based on meeting the intent of the relevant guidance identified in SRP Section 5.2.2 Rev. 2 (Reference 5). 

B. The low temperature, overpressure protection (LTOP) system shall be designed in accordance with the requirements of Branch Technical Position RSB 5-2 (Reference 5). The LTOP system shall be operable during startup and shutdown conditions below the enable temperature. 

The relief capacities of the pressuriser safety valve is determined from the postulated overpressure transient conditions in conjunction with the action of the reactor protection system. An overpressure protection report is prepared according to Article NB-7300 of Section III of the ASME code. Reference 6.2 describes the analytical model used in the analysis ofthe overpressure protection system and the basis for its validity. 

The European standard EN 12186 (formerly the DIN G491) and more specific the EN 14382 (formerly DIN 3381) has been used for the past decades in (mechanically) instrumented overpressure protection systems. These standards prescribe the requirements for the overpressure protection systems, and their components, in gas plants. Not only the response time and accuracy of the loop but also safety factors for oversizing of the actuator of the final element are dictated by these standards. Independent design verification and testing to prove compliance to the EN 14382 standard is mandatory. Therefore, the users often refer to this standard for HIPPS design. 

The autoclave is not the only component of an LDPE plant which may be exposed to a decomposition. Local hot spots in a secondary compressor may initiate a decomposition reaction consequendy it is necessary to protect these units from serious overpressure by pressure relieving devices and to release the products of the decomposition reactions safely. The problem of the aerial decomposition referred to eadier has been largely overcome by rapidly quenching the decomposition products as they enter the vent stack. 

LOCA, is presented in Table 3.4.5-1. In preparing the event tree, reference to the reactor s design determines the effect of the failure of the various systems. Following the pipe break, the system should scram (Figure 3.4.5-2, node 1). If scram is successful, the line following the node goes up. Successful initial steam condensation (node 2 up) protects the containment from initial overpressure. Continuing success in these events traverses the upper line of the event tree to state 1 core cooled. Any failures cause a traversal of other paths in the evL-nl tree. 

When pressure relief devices are intended primarily for protection against overpressure due to external fire or heat, have no permanent supply connection, and are used for storage at ambient temperature of non-refrigerated liquefied compressed gases, they are excluded from requirements of Par. UG-125c (1) and C (2), with specific provisions. See ASME code [1] for detailed references and conditions. 

An SRV is a safety device designed to protect a pressurized vessel or system during an overpressure event. An overpressure event refers to any condition which would cause pressure in a vessel or system to increase beyond the specified design pressure or maximum allowable working pressure (MAWP) (Section 3.6). 

On LNG liquefaction or re-gasification plants, many SRVs will be installed to protect equipment and personnel against the dangers of the same overpressures discussed earlier in this book. The valves considered here, however, must operate on cold cryogenic gas or liquefied gas. Just for reference, cold is arbitrarily defined as any service below -30°C, and cryogenic as any service below -100°C. 

Back pressure regulators provide steady-state control relief valves provide on-off protection from overpressure or vacuum conditions. When the set pressure threshold is exceeded, the valve opens either to atmosphere or to an auxiliary line (where the fluid may go to a flare or storage tank or even to recycling). Once the vessel pressure drops to a predetermined pressure, the valve reseats. The difference in pressure between when the valve relieves and when the valve reseats is referred to as the blowdown it is typically 2-20% lower than the relief pressure. 

The contamination-protected zirconia electrode incorporates a second closed-end zirconia tube within the alumina shaft which is ionically contacted by the electrolyte bridge through zirconia grit and contains the platinum reference electrode, which is thus protected from melt vapors when the overpressure of the reference gas accidentally drops to the pressure of the tank atmosphere [15],... 

If the roll anchors fail, the plenum could lift and stretch the plenum seal, which then may rupture. The rupture would provide pressure relief to protect other components. The roll anchors, plenum inlet nozzles, and plenum seals were not originally designed specifically for overpressurization of the gas plenum. However, modifications have been made to the roll anchors to prevent uplift of the plenum in the event of an ECS actuation, and they are now adequate for the uplift force due to gas plenum overpressurization (Reference 49). 

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