Protection systems isolation valves

The 1997 edition of the API RP 521 extends the two-thirds rule to include the upstream and downstream system. At a minimum, the inlet and outlet piping up to and including isolation valves must be designed for the two-thirds rule to be able to block in the exchanger. If the upstream and downstream equipment is not designed for the two-thirds rule, relief devices may be required on both the inlet and outlet piping to protect the piping and adjaeent equipment. 

Accident Mitigation 28 Detection of leaks/ruptures 29 Emergency shutdown switch locations 30 Accessibility of isolation valves 31 Potential for fire/explosion in unit affecting other equipment 32 Critical controls, mitigation, communication, and fire protection sy stems functional and accessible after initial explosion or release 33 Back-up power supply/redundant feeds for critical electrical systems 34 Water supply for fire fighting 35 Routing of utilities... 

A chemical engineer may have a choice of inherent safety variables, such as quantity stored or process temperatures and pressures, or process safety measures such as emergency isolation valves or containment systems, all of which may greatly reduce the vulnerabilities or the consequences of intentional loss. These are in addition to traditional security measures, which may include physical security, background checks, administrative controls, access controls, or other protective measures. For a more complete discussion of the options, refer to the AIChE Center for Chemical Process Safety Guidelines for Analyzing and Managing the Security Vulnerabilities of Fixed Chemical Sites and other references.f... 

Red a. Stop buttons or electrical switches used for emergency stopping of machines. b. Emergency stop handles or bars on machines. c. Hazardous operation indicating lights on control, alarm panels, or in the installation. d. Fire protection equipment and systems (e g., fire hydrants, monitors, reels, alarms, etc.). e. Portable flammable liquid containers. f. Stop condition. g. Identification of ESD isolation valves and actuators. 

A reaction quench is a system where an inhibiting substance (quench solution stored in a separate container vessel) can be quickly and effectively fed into the reactor via a pipe which is protected with appropriate isolation valves. This action is independent of other process actions that may be required. The reaction quench can be manually initiated or automatically when certain process parameters are exceeded. 

An active fire protection system requires some action to occur before it functions per its design intent. This action may be taken by either a person or control system. Examples of active fire protection systems are monitors, water spray systems, foam systems, emergency isolation valves, and ESD systems. 

Possible remedies for PTS events are automatic overpressure protection which disable systems that can pressurize the RPV, adnodnistrative measures to isolate high pressure systems from the primary circuit, and dedicated outage safety valves. It is also possible to mitigate PTS by recovering the material properties of the RPV through thermal annealing. 

USNRC RG 1.96 (June 1976) Design of main steam isolation valve leakage control systems for boiling water reactor nuclear power plants . USNRC RG 1.106 (March 1977) Thermal overload protection for electric motors on motor-operated valves . 

Overpressure protection for the secondary side of the steam generators is provided by spring-loaded ASME Code safety valves located in the main steam system upstream of the steam line isolation valves. 

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. 

A. The Main Steam Isolation Valves are capable of isolating the steam generators within 5.0 seconds after receiving a signal g from the Engineered Safety Features Actuation System. In the event of a steam line break, this action prevents continuous uncontrolled steam release from more than one steam generator. Protection is offered for breaks inside or outside the containment. 

Leak-tight protective enclosure (containment) with filters, bubblers, systems for pressure self-limitation, as well as with isolation valves in the pipelines penetrating the containment. 

The remaining safety-grade functions are performed by the reactor protection system (it initiates opening of the scram valves to achieve a reactor scram), the containment isolation system (it initiates isolation of the containment by closing isolation valves), the reactor vessel safety valves (based on pressure-activated components), and the passive reactor pool cooling function. These functions are not needed for the protection of the core, however. 

The System 80+ Standard Design is designed to preclude water spray from the fire protection system onto safety-related equipment. The sprinkler systems protecting the safety-related equipment is of the automatic sprinkler type. Actuation of these sprinkler systems requires the opening of the fusible link sprinkler heads and detection by combustible-products and/or heat detectors. In addition, the operator has the capability of isolating flow from the control room by isolating the Sub-sphere Building headers or, locally by manual isolation valves. 

The second principal cause of loss of RHR capability affected a number of plants having an autoclosure interlock for the RHR suction isolation valves, which are intended to protect the RHR system from RCS pressure during operation at power. Correct operation of the interlock causes the valves to close when the RCS pressure increases above the design pressure of the RHR system. NSAC-52 (Reference 3) reported on a considerable number of events that involved loss of RHR flow during cooldown, due to inadvertent actuation of the isolation valve autoclosure interlock. However in all cases but one, operators successfully reopened the valves immediately, and for this reason the NRC considers this type of event of somewhat less concern than air binding of the RHR system. 

The System 80+ Standard Design has a Main Feedwater Isolation System (see CESSAR-DC Section 10.4.7.2.2) to protect the SGs from overfill. The system includes redundant remotely operated isolation valves in each main feedwater line to each SG. The valve actuation system (see CESSAR-DC Section 7.3.1.1.10.3) is composed of redundant trains A and B, and each train s instrumentation and controls are physically and electrically separate from and independent of those of the other train. A failure of one train will not impair the action of the other. The main feedwater isolation valves are automatically actuated by a Main Steam Isolation Signal (MSIS) from the Engineered Safety Features Actuation System (ESFAS, see CESSAR-DC Section 7.3.1). High SG water level, in a 2-out-of-4 logic, is one of the initiators for the MSIS. The main feedwater isolation valves can be in-service tested in accordance with ASME Code Section XI, Subsection IWV. A Technical Specification (CESSAR-DC Chapter 16) will establish testing requirements for the valve actuation system. These requirements will also be incorporated into the plant maintenance procedures. 

Each steam line has two containment isolation valves, one inside and one outside the containment barrier. The isolation valves are spring-loaded pneumatic piston-operated globe valves designed to fail closed on loss of pneumatic pressure or loss of power to the pilot valves. Each valve has an air accumulator to assist in the closiue of the valve upon loss of the air supply, electrical power to the pilot valves, and failure of the loaded spring. Each valve has an independent position switch initiating a signal into the reactor protection system scram trip circuit when the valve closes. 

The nuclear system protection system provides for the closing of valves to isolate the containment, thereby preventing the release of sfeam and process fluids. The logic and equipment required for fhose valves, which are required to be open during ECC are part of each of the separate ECCSs. 

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