Overpressure protection requirements

RCS and steam system overpressure protection during power operation are provided by the pressurizer safety valves and the steam generator safety valves, in conjunction with the action of the reactor protection system. Combinations of these systems provide compliance with the overpressure protection requirements of the NRC for PWR... 

ATOMIC ENERGY CONTROL BOARD, Overpressure Protection Requirements for Primary Heat Transport Systems in CANDU Power Reactors Fitted with Two Shutdown Systems, AECB Regulatory Policy Statement R-77, AECB, Ottawa (1987). 

For flammable and/or toxic materials all of the precautions for a pressurized system should be considered. For example, when a centrifuge is pressurized, overpressure protection is required, even if the pressurization is an inert gas. Relieving of the pressure to a closed system or safe location must be considered. 

Now let us consider utility failure as a cause of overpressure. Failure of the utility supphes (e.g., electric power, cooling water, steam, instrument air or instrument power, or fuel) to refinery plant facihties wiU in many instances result in emergency conditions with potential for overpressuring equipment. Although utility supply systems are designed for reliability by the appropriate selection of multiple generation and distribution systems, spare equipment, backup systems, etc., the possibility of failure still remains. Possible failure mechanisms of each utility must, therefore, be examined and evaluated to determine the associated requirements for overpressure protection. The basic rules for these considerations are as follows ... 

Vessels should be provided with overpressure protection as required. Vents and relief valve vent piping should be so arranged Uiat Uie vented vapors will not constitute a liazard. Relief valves must be kept free from corrosion or fouling and should be operable at all Umes. 

Turbine pumps are positive-displacement pumps They cannot be started with a closed discharge valve and require the fitting of a pressure relief valve to provide overpressure protection. 

The ASME code provides the basic requirements for overpressure protection. Section I, Power Boilers, covers fired and unfired steam boilers. All other vessels including exchanger shells and similar pressure-containing equipment fall under Section VIII, Pressure Vessels. API RP 520 and lesser API documents supplement the ASME code. These codes specify allowable accumulation, which is the difference between relieving pressure at which the valve reaches full rated flow and set pressure at which the valve starts to open. Accumulation is expressed as percentage of set pressure in Table 1-9. 

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. 

Some method of pressure relief is required on all pressure vessels and for other process equipment where increasing pressure might rupture the vessel. Much of the piping used in modern chemical operations also requires overpressure protection. Safety relief valves or rupture discs are employed for pressure relief. In many cases, either a rupture disc or a safety relief valve can be used. Safety relief valves are usually used for process protection and rupture discs are used for vessel protection. The safety relief valve or rupture disc must be designed to operate at a known pressure and prevent the pressure within the system from increasing. Therefore, it is important to consider the flowrate the valve can handle. 

Overpressure protection is one of the major requirements to prevent accidents from occurring in modem chemical operations. The two common devices used for overpressure protection are mpture disks and relief valves. Relief valves are usually used for process protection and mpture disks for vessel protection. 

Collective Protection. To achieve collective protection requires 0.5 inches of water overpressure for softwalled shelters such as tents and 0.2 inches for tight concrete shelters and well-sealed rooms. The requirement for air volume flow (in cubic feet per minute) to achieve such pressures is 0.0367 times the room volume in cubic feet for 0.2 inches and approximately 0.07 times the room volume for 0.5 inches. The M20 blower unit can provide 200 cubic feet per minute of airflow. Use plastic sheets and 100 mph tape to seal all cracks, windows, ducts, false ceilings, electric outlets, etc. to create an airtight environment. (This information was provided by Army document M 27APPE-219, 76-332-219). 

Figure 5.3-5a shows that, for heatup, a temperature of 309 F is required before the pressurizer safety valves can be used for overpressure protection without violating any P-T Limits. Figure 5.3-5b shows that, for cooldown, the safety valves cannot be used below a temperature of 225 F. For the temperature interval 140 F above T rpop heatup, as defined by Branch Technical Position RSB S-V, and for the 66 F above op for cooldown, administrative controls would be required ro protect some P-T Limits, particularly the design limit of loo°F/hr. Rather than relying on administrative controls during these temperature intervals, the Tj-. p enable and disable temperatures for System 80+ are defined by the intersection of the controlling P-T Limit (100 F/hr) curve and the pressurizer safety valve setpoint. 

Heatup and cooldown rates from 0 F/hr ("Isothermal" pressurization) to the design limit of 100°F/hr at 10 F/hr intervals are examined in determining the allowable heatup and cooldown rates as a function of temperature to meet low-temperature overpressure protection (LTOP) requirements for the Reactor Coolant system. LTOP considerations are discussed in Section 5.2.2.10. 

Overpressurization of the Reactor Coolant System (RCS) and steam generators is precluded by means of primary safety valves, secondary safety valves and the Reactor Protection System (RPS). Pressure relief capacity for the steam generators and RCS is conservatively sized to satisfy the overpressure requirements of the ASME Boiler and Pressure Vessel Code, Section III. The safety valves in conjunction with the RPS, are designed to provide overpressure protection for a loss-of-load incident with a delayed reactor trip. 

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. 

The ICI procedures required the use of a "safety assessment" guide sheet, which discussed overpressure protection equipment, electrical area classification, changes in alarms and trips, and other categories that might diminish the safety of the system. The guide sheet also had questions on the relevant codes of practice and specifications, questions on the materials of construction... 

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 System 80+ Standard Design utilizes the pressurizer safety valves to protect the RCS from overpressurization as required by the ASME B PV code. Section III (see CESSAR-DC, Section... 

On French plants, cold overpressure protection is provided by pilot operated safety valves on the pressurizer when the RHR system is isolated. When the RHR system is operating, the protection is provided by pilot operated safety valves on the discharge lines from the RHR pumps. All these valves are qualified to operate in water conditions. In addition, specific requirements have been included in the Technical Specifications. 

The US primary system safety and relief valves have been qualified by testing for the design-basis fluid inlet conditions In addition to hot steam conditions, the PORVs were tested for representative low temperature liquid flow, and the safety valves were tested at representative hot liquid flow The NRC required that the reliability of the PORVs be improved to provide better low temperature overpressure protection (LTOP) and better operation at operating temperature Better maintenance and improved Technical Specifications were implemented... 

The TMI-2 accident focused attention on the reliability of the PORVs and the block valves and requirements were established regarding electrical power backup, environmentally qualified position indication and verification operation under all expected flow condition including water flow (see PC 1, Overpressure protection of the RPV at water solid conditions and at low temperatures). Analyses showed that the role of the PORVs and the block valves had changed such that the valves are now relied upon in certain safety related functions. 

Reactor coolant system overpressure protection during power operation is provided by the pressuriser safety valves, discussed in Section 6.3.2. The provided overpressure protection is compliant with the requirements of the ASME III Paragraphs NB-7300 and NC-7300 for pressurised water reactor systems. Low temperature overpressure protection is provided by a relief valve in the suction line of the normal residual heat removal system, as described in Section 6.5.3.3. 

ANS 56.3 ANS 54.6 ANS 56.8 ANS 56.9 Overpressure protection of low-pressure system connected to the reactor coolant pressure boundary Pressure/temperature transient analysis for LWR containments Reactor containment leakage testing requirements Environmental envelopes to be considered in safety-related equipment... 

The reactor vessel is invariably jacketed. In the absence of especial requirements, the jacket is designed to the same specifications as the vessel. The jacket is covered with chloride-free fiberglass insulation which is fully enclosed in a protective shroud as shown in Eig. 18. The jacket is provided with overpressure protection through a relief valve located on the jacket or its associated piping. 

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